JSON Schema: A Media Type for Describing JSON Documents
draft-handrews-json-schema-02

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Internet Engineering Task Force                           A. Wright, Ed.
Internet-Draft
Intended status: Informational                           H. Andrews, Ed.
Expires: March 19, 2020
                                                          B. Hutton, Ed.
                                               Wellcome Sanger Institute
                                                               G. Dennis
                                                      September 16, 2019

        JSON Schema: A Media Type for Describing JSON Documents
                     draft-handrews-json-schema-02

Abstract

   JSON Schema defines the media type "application/schema+json", a JSON-
   based format for describing the structure of JSON data.  JSON Schema
   asserts what a JSON document must look like, ways to extract
   information from it, and how to interact with it.  The "application/
   schema-instance+json" media type provides additional feature-rich
   integration with "application/schema+json" beyond what can be offered
   for "application/json" documents.

Note to Readers

   The issues list for this draft can be found at <https://github.com/
   json-schema-org/json-schema-spec/issues>.

   For additional information, see <https://json-schema.org/>.

   To provide feedback, use this issue tracker, the communication
   methods listed on the homepage, or email the document editors.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on March 19, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  JSON Document . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Instance  . . . . . . . . . . . . . . . . . . . . . . . .   6
       4.2.1.  Instance Data Model . . . . . . . . . . . . . . . . .   6
       4.2.2.  Instance Media Types  . . . . . . . . . . . . . . . .   7
       4.2.3.  Instance Equality . . . . . . . . . . . . . . . . . .   7
     4.3.  JSON Schema Documents . . . . . . . . . . . . . . . . . .   8
       4.3.1.  JSON Schema Objects and Keywords  . . . . . . . . . .   8
       4.3.2.  Boolean JSON Schemas  . . . . . . . . . . . . . . . .   9
       4.3.3.  Schema Vocabularies . . . . . . . . . . . . . . . . .   9
       4.3.4.  Meta-Schemas  . . . . . . . . . . . . . . . . . . . .   9
       4.3.5.  Root Schema and Subschemas and Resources  . . . . . .  10
   5.  Fragment Identifiers  . . . . . . . . . . . . . . . . . . . .  10
   6.  General Considerations  . . . . . . . . . . . . . . . . . . .  11
     6.1.  Range of JSON Values  . . . . . . . . . . . . . . . . . .  11
     6.2.  Programming Language Independence . . . . . . . . . . . .  11
     6.3.  Mathematical Integers . . . . . . . . . . . . . . . . . .  11
     6.4.  Regular Expressions . . . . . . . . . . . . . . . . . . .  12
     6.5.  Extending JSON Schema . . . . . . . . . . . . . . . . . .  12
   7.  Keyword Behaviors . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Lexical Scope and Dynamic Scope . . . . . . . . . . . . .  13
     7.2.  Keyword Interactions  . . . . . . . . . . . . . . . . . .  14
     7.3.  Default Behaviors . . . . . . . . . . . . . . . . . . . .  15
     7.4.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  15
     7.5.  Applicators . . . . . . . . . . . . . . . . . . . . . . .  15
       7.5.1.  Referenced and Referencing Schemas  . . . . . . . . .  16

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     7.6.  Assertions  . . . . . . . . . . . . . . . . . . . . . . .  16
       7.6.1.  Assertions and Instance Primitive Types . . . . . . .  17
     7.7.  Annotations . . . . . . . . . . . . . . . . . . . . . . .  17
       7.7.1.  Collecting Annotations  . . . . . . . . . . . . . . .  18
     7.8.  Reserved Locations  . . . . . . . . . . . . . . . . . . .  22
   8.  The JSON Schema Core Vocabulary . . . . . . . . . . . . . . .  22
     8.1.  Meta-Schemas and Vocabularies . . . . . . . . . . . . . .  23
       8.1.1.  The "$schema" Keyword . . . . . . . . . . . . . . . .  24
       8.1.2.  The "$vocabulary" Keyword . . . . . . . . . . . . . .  25
       8.1.3.  Updates to Meta-Schema and Vocabulary URIs  . . . . .  26
       8.1.4.  Detecting a Meta-Schema . . . . . . . . . . . . . . .  26
     8.2.  Base URI, Anchors, and Dereferencing  . . . . . . . . . .  27
       8.2.1.  Initial Base URI  . . . . . . . . . . . . . . . . . .  27
       8.2.2.  The "$id" Keyword . . . . . . . . . . . . . . . . . .  28
       8.2.3.  Defining location-independent identifiers with
               "$anchor" . . . . . . . . . . . . . . . . . . . . . .  30
       8.2.4.  Schema References . . . . . . . . . . . . . . . . . .  31
       8.2.5.  Schema Re-Use With "$defs"  . . . . . . . . . . . . .  36
     8.3.  Comments With "$comment"  . . . . . . . . . . . . . . . .  36
   9.  A Vocabulary for Applying Subschemas  . . . . . . . . . . . .  37
     9.1.  Keyword Independence  . . . . . . . . . . . . . . . . . .  37
     9.2.  Keywords for Applying Subschemas in Place . . . . . . . .  38
       9.2.1.  Keywords for Applying Subschemas With Boolean Logic .  38
       9.2.2.  Keywords for Applying Subschemas Conditionally  . . .  39
     9.3.  Keywords for Applying Subschemas to Child Instances . . .  41
       9.3.1.  Keywords for Applying Subschemas to Arrays  . . . . .  41
       9.3.2.  Keywords for Applying Subschemas to Objects . . . . .  43
   10. Output Formatting . . . . . . . . . . . . . . . . . . . . . .  45
     10.1.  Format . . . . . . . . . . . . . . . . . . . . . . . . .  46
     10.2.  Output Formats . . . . . . . . . . . . . . . . . . . . .  46
     10.3.  Minimum Information  . . . . . . . . . . . . . . . . . .  46
       10.3.1.  Keyword Relative Location  . . . . . . . . . . . . .  47
       10.3.2.  Keyword Absolute Location  . . . . . . . . . . . . .  47
       10.3.3.  Instance Location  . . . . . . . . . . . . . . . . .  47
       10.3.4.  Error or Annotation  . . . . . . . . . . . . . . . .  47
       10.3.5.  Nested Results . . . . . . . . . . . . . . . . . . .  48
     10.4.  Output Structure . . . . . . . . . . . . . . . . . . . .  48
       10.4.1.  Flag . . . . . . . . . . . . . . . . . . . . . . . .  50
       10.4.2.  Basic  . . . . . . . . . . . . . . . . . . . . . . .  50
       10.4.3.  Detailed . . . . . . . . . . . . . . . . . . . . . .  51
       10.4.4.  Verbose  . . . . . . . . . . . . . . . . . . . . . .  53
       10.4.5.  Output validation schemas  . . . . . . . . . . . . .  55
   11. Usage for Hypermedia  . . . . . . . . . . . . . . . . . . . .  55
     11.1.  Linking to a Schema  . . . . . . . . . . . . . . . . . .  55
     11.2.  Identifying a Schema via a Media Type Parameter  . . . .  55
     11.3.  Usage Over HTTP  . . . . . . . . . . . . . . . . . . . .  57
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  57
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  58

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     13.1.  application/schema+json  . . . . . . . . . . . . . . . .  58
     13.2.  application/schema-instance+json . . . . . . . . . . . .  59
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  59
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  59
     14.2.  Informative References . . . . . . . . . . . . . . . . .  60
   Appendix A.  Schema identification examples . . . . . . . . . . .  62
   Appendix B.  Manipulating schema documents and references . . . .  64
     B.1.  Bundling schema resources into a single document  . . . .  64
     B.2.  Reference removal is not always safe  . . . . . . . . . .  64
   Appendix C.  Example of recursive schema extension  . . . . . . .  65
   Appendix D.  Working with vocabularies  . . . . . . . . . . . . .  66
     D.1.  Best practices for vocabulary and meta-schema authors . .  66
     D.2.  Example meta-schema with vocabulary declarations  . . . .  67
   Appendix E.  References and generative use cases  . . . . . . . .  70
   Appendix F.  Acknowledgments  . . . . . . . . . . . . . . . . . .  71
   Appendix G.  ChangeLog  . . . . . . . . . . . . . . . . . . . . .  71
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  74

1.  Introduction

   JSON Schema is a JSON media type for defining the structure of JSON
   data.  JSON Schema is intended to define validation, documentation,
   hyperlink navigation, and interaction control of JSON data.

   This specification defines JSON Schema core terminology and
   mechanisms, including pointing to another JSON Schema by reference,
   dereferencing a JSON Schema reference, specifying the vocabulary
   being used, and defining the expected output.

   Other specifications define the vocabularies that perform assertions
   about validation, linking, annotation, navigation, and interaction.

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The terms "JSON", "JSON text", "JSON value", "member", "element",
   "object", "array", "number", "string", "boolean", "true", "false",
   and "null" in this document are to be interpreted as defined in RFC
   8259 [RFC8259].

3.  Overview

   This document proposes a new media type "application/schema+json" to
   identify a JSON Schema for describing JSON data.  It also proposes a
   further optional media type, "application/schema-instance+json", to

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   provide additional integration features.  JSON Schemas are themselves
   JSON documents.  This, and related specifications, define keywords
   allowing authors to describe JSON data in several ways.

   JSON Schema uses keywords to assert constraints on JSON instances or
   annotate those instances with additional information.  Additional
   keywords are used to apply assertions and annotations to more complex
   JSON data structures, or based on some sort of condition.

   To facilitate re-use, keywords can be organized into vocabularies.  A
   vocabulary consists of a list of keywords, together with their syntax
   and semantics.

   JSON Schema can be extended either by defining additional
   vocabularies, or less formally by defining additional keywords
   outside of any vocabulary.  Unrecognized individual keywords are
   ignored, while the behavior with respect to an unrecognized
   vocabulary can be controlled when declaring which vocabularies are in
   use.

   This document defines a core vocabulary that MUST be supported by any
   implementation, and cannot be disabled.  Its keywords are each
   prefixed with a "$" character to emphasize their required nature.
   This vocabulary is essential to the functioning of the "application/
   schema+json" media type, and is used to bootstrap the loading of
   other vocabularies.

   Additionally, this document defines a RECOMMENDED vocabulary of
   keywords for applying subschemas conditionally, and for applying
   subschemas to the contents of objects and arrays.  Either this
   vocabulary or one very much like it is required to write schemas for
   non-trivial JSON instances, whether those schemas are intended for
   assertion validation, annotation, or both.  While not part of the
   required core vocabulary, for maximum interoperability this
   additional vocabulary is included in this document and its use is
   strongly encouraged.

   Further vocabularies for purposes such as structural validation or
   hypermedia annotation are defined in other documents.

4.  Definitions

4.1.  JSON Document

   A JSON document is an information resource (series of octets)
   described by the application/json media type.

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   In JSON Schema, the terms "JSON document", "JSON text", and "JSON
   value" are interchangeable because of the data model it defines.

   JSON Schema is only defined over JSON documents.  However, any
   document or memory structure that can be parsed into or processed
   according to the JSON Schema data model can be interpreted against a
   JSON Schema, including media types like CBOR [RFC7049].

4.2.  Instance

   A JSON document to which a schema is applied is known as an
   "instance".

4.2.1.  Instance Data Model

   JSON Schema interprets documents according to a data model.  A JSON
   value interpreted according to this data model is called an
   "instance".

   An instance has one of six primitive types, and a range of possible
   values depending on the type:

   null:  A JSON "null" production

   boolean:  A "true" or "false" value, from the JSON "true" or "false"
      productions

   object:  An unordered set of properties mapping a string to an
      instance, from the JSON "object" production

   array:  An ordered list of instances, from the JSON "array"
      production

   number:  An arbitrary-precision, base-10 decimal number value, from
      the JSON "number" production

   string:  A string of Unicode code points, from the JSON "string"
      production

   Whitespace and formatting concerns, including different lexical
   representations of numbers that are equal within the data model, are
   thus outside the scope of JSON Schema.  JSON Schema vocabularies
   (Section 8.1) that wish to work with such differences in lexical
   representations SHOULD define keywords to precisely interpret
   formatted strings within the data model rather than relying on having
   the original JSON representation Unicode characters available.

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   Since an object cannot have two properties with the same key,
   behavior for a JSON document that tries to define two properties (the
   "member" production) with the same key (the "string" production) in a
   single object is undefined.

   Note that JSON Schema vocabularies are free to define their own
   extended type system.  This should not be confused with the core data
   model types defined here.  As an example, "integer" is a reasonable
   type for a vocabulary to define as a value for a keyword, but the
   data model makes no distinction between integers and other numbers.

4.2.2.  Instance Media Types

   JSON Schema is designed to fully work with "application/json"
   documents, as well as media types using the "+json" structured syntax
   suffix.

   Some functionality that is useful for working with schemas is defined
   by each media type, namely media type parameters and URI fragment
   identifier syntax and semantics.  These features are useful in
   content negotiation and in calculating URIs for specific locations
   within an instance, respectively.

   This specification defines the "application/schema-instance+json"
   media type in order to allow instance authors to take full advantage
   of parameters and fragment identifiers for these purposes.

4.2.3.  Instance Equality

   Two JSON instances are said to be equal if and only if they are of
   the same type and have the same value according to the data model.
   Specifically, this means:

      both are null; or

      both are true; or

      both are false; or

      both are strings, and are the same codepoint-for-codepoint; or

      both are numbers, and have the same mathematical value; or

      both are arrays, and have an equal value item-for-item; or

      both are objects, and each property in one has exactly one
      property with a key equal to the other's, and that other property
      has an equal value.

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   Implied in this definition is that arrays must be the same length,
   objects must have the same number of members, properties in objects
   are unordered, there is no way to define multiple properties with the
   same key, and mere formatting differences (indentation, placement of
   commas, trailing zeros) are insignificant.

4.3.  JSON Schema Documents

   A JSON Schema document, or simply a schema, is a JSON document used
   to describe an instance.  A schema can itself be interpreted as an
   instance, but SHOULD always be given the media type "application/
   schema+json" rather than "application/schema-instance+json".  The
   "application/schema+json" media type is defined to offer a superset
   of the media type parameter and fragment identifier syntax and
   semantics provided by "application/schema-instance+json".

   A JSON Schema MUST be an object or a boolean.

4.3.1.  JSON Schema Objects and Keywords

   Object properties that are applied to the instance are called
   keywords, or schema keywords.  Broadly speaking, keywords fall into
   one of four categories:

   identifiers:  control schema identification through setting the
      schema's canonical URI and/or changing how the base URI is
      determined

   assertions:  produce a boolean result when applied to an instance

   annotations:  attach information to an instance for application use

   applicators:  apply one or more subschemas to a particular location
      in the instance, and combine or modify their results

   reserved locations:  do not directly affect results, but reserve a
      place for a specific purpose to ensure interoperability

   Keywords may fall into multiple categories, although applicators
   SHOULD only produce assertion results based on their subschemas'
   results.  They should not define additional constraints independent
   of their subschemas.

   Extension keywords, meaning those defined outside of this document
   and its companions, are free to define other behaviors as well.

   A JSON Schema MAY contain properties which are not schema keywords.
   Unknown keywords SHOULD be ignored.

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   An empty schema is a JSON Schema with no properties, or only unknown
   properties.

4.3.2.  Boolean JSON Schemas

   The boolean schema values "true" and "false" are trivial schemas that
   always produce themselves as assertions results, regardless of the
   instance value.  They never produce annotation results.

   These boolean schemas exist to clarify schema author intent and
   facilitate schema processing optimizations.  They behave identically
   to the following schema objects (where "not" is part of the subschema
   application vocabulary defined in this document).

   true:  Always passes validation, as if the empty schema {}

   false:  Always fails validation, as if the schema { "not": {} }

   While the empty schema object is unambiguous, there are many possible
   equivalents to the "false" schema.  Using the boolean values ensures
   that the intent is clear to both human readers and implementations.

4.3.3.  Schema Vocabularies

   A schema vocabulary, or simply a vocabulary, is a set of keywords,
   their syntax, and their semantics.  A vocabulary is generally
   organized around a particular purpose.  Different uses of JSON
   Schema, such as validation, hypermedia, or user interface generation,
   will involve different sets of vocabularies.

   Vocabularies are the primary unit of re-use in JSON Schema, as schema
   authors can indicate what vocabularies are required or optional in
   order to process the schema.  Since vocabularies are identified by
   URIs in the meta-schema, generic implementations can load extensions
   to support previously unknown vocabularies.  While keywords can be
   supported outside of any vocabulary, there is no analogous mechanism
   to indicate individual keyword usage.

4.3.4.  Meta-Schemas

   A schema that itself describes a schema is called a meta-schema.
   Meta-schemas are used to validate JSON Schemas and specify which
   vocabularies they are using.

   Typically, a meta-schema will specify a set of vocabularies, and
   validate schemas that conform to the syntax of those vocabularies.
   However, meta-schemas and vocabularies are separate in order to allow
   meta-schemas to validate schema conformance more strictly or more

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   loosely than the vocabularies' specifications call for.  Meta-schemas
   may also describe and validate additional keywords that are not part
   of a formal vocabulary.

4.3.5.  Root Schema and Subschemas and Resources

   A JSON Schema resource is a schema which is canonically [RFC6596]
   identified by an absolute URI [RFC3986].

   The root schema is the schema that comprises the entire JSON document
   in question.  The root schema is always a schema resource, where the
   URI is determined as described in section 8.2.1.

   Some keywords take schemas themselves, allowing JSON Schemas to be
   nested:

   {
       "title": "root",
       "items": {
           "title": "array item"
       }
   }

   In this example document, the schema titled "array item" is a
   subschema, and the schema titled "root" is the root schema.

   As with the root schema, a subschema is either an object or a
   boolean.

   As discussed in section 8.2.2, a JSON Schema document can contain
   multiple JSON Schema resources.  When used without qualification, the
   term "root schema" refers to the document's root schema.  In some
   cases, resource root schemas are discussed.  A resource's root schema
   is its top-level schema object, which would also be a document root
   schema if the resource were to be extracted to a standalone JSON
   Schema document.

5.  Fragment Identifiers

   In accordance with section 3.1 of [RFC6839], the syntax and semantics
   of fragment identifiers specified for any +json media type SHOULD be
   as specified for "application/json".  (At publication of this
   document, there is no fragment identification syntax defined for
   "application/json".)

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   Additionally, the "application/schema+json" media type supports two
   fragment identifier structures: plain names and JSON Pointers.  The
   "application/schema-instance+json" media type supports one fragment
   identifier structure: JSON Pointers.

   The use of JSON Pointers as URI fragment identifiers is described in
   RFC 6901 [RFC6901].  For "application/schema+json", which supports
   two fragment identifier syntaxes, fragment identifiers matching the
   JSON Pointer syntax, including the empty string, MUST be interpreted
   as JSON Pointer fragment identifiers.

   Per the W3C's best practices for fragment identifiers
   [W3C.WD-fragid-best-practices-20121025], plain name fragment
   identifiers in "application/schema+json" are reserved for referencing
   locally named schemas.  All fragment identifiers that do not match
   the JSON Pointer syntax MUST be interpreted as plain name fragment
   identifiers.

   Defining and referencing a plain name fragment identifier within an
   "application/schema+json" document are specified in the "$anchor"
   keyword (Section 8.2.3) section.

6.  General Considerations

6.1.  Range of JSON Values

   An instance may be any valid JSON value as defined by JSON [RFC8259].
   JSON Schema imposes no restrictions on type: JSON Schema can describe
   any JSON value, including, for example, null.

6.2.  Programming Language Independence

   JSON Schema is programming language agnostic, and supports the full
   range of values described in the data model.  Be aware, however, that
   some languages and JSON parsers may not be able to represent in
   memory the full range of values describable by JSON.

6.3.  Mathematical Integers

   Some programming languages and parsers use different internal
   representations for floating point numbers than they do for integers.

   For consistency, integer JSON numbers SHOULD NOT be encoded with a
   fractional part.

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6.4.  Regular Expressions

   Keywords MAY use regular expressions to express constraints, or
   constrain the instance value to be a regular expression.  These
   regular expressions SHOULD be valid according to the regular
   expression dialect described in ECMA 262, section 15.10.1 [ecma262].

   Furthermore, given the high disparity in regular expression
   constructs support, schema authors SHOULD limit themselves to the
   following regular expression tokens:

      individual Unicode characters, as defined by the JSON
      specification [RFC8259];

      simple character classes ([abc]), range character classes ([a-z]);

      complemented character classes ([^abc], [^a-z]);

      simple quantifiers: "+" (one or more), "*" (zero or more), "?"
      (zero or one), and their lazy versions ("+?", "*?", "??");

      range quantifiers: "{x}" (exactly x occurrences), "{x,y}" (at
      least x, at most y, occurrences), {x,} (x occurrences or more),
      and their lazy versions;

      the beginning-of-input ("^") and end-of-input ("$") anchors;

      simple grouping ("(...)") and alternation ("|").

   Finally, implementations MUST NOT take regular expressions to be
   anchored, neither at the beginning nor at the end.  This means, for
   instance, the pattern "es" matches "expression".

6.5.  Extending JSON Schema

   Additional schema keywords and schema vocabularies MAY be defined by
   any entity.  Save for explicit agreement, schema authors SHALL NOT
   expect these additional keywords and vocabularies to be supported by
   implementations that do not explicitly document such support.
   Implementations SHOULD ignore keywords they do not support.

   Implementations MAY provide the ability to register or load handlers
   for vocabularies that they do not support directly.  The exact
   mechanism for registering and implementing such handlers is
   implementation-dependent.

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7.  Keyword Behaviors

   JSON Schema keywords fall into several general behavior categories.
   Assertions validate that an instance satisfies constraints, producing
   a boolean result.  Annotations attach information that applications
   may use in any way they see fit.  Applicators apply subschemas to
   parts of the instance and combine their results.

   Extension keywords SHOULD stay within these categories, keeping in
   mind that annotations in particular are extremely flexible.  Complex
   behavior is usually better delegated to applications on the basis of
   annotation data than implemented directly as schema keywords.
   However, extension keywords MAY define other behaviors for
   specialized purposes.

   Evaluating an instance against a schema involves processing all of
   the keywords in the schema against the appropriate locations within
   the instance.  Typically, applicator keywords are processed until a
   schema object with no applicators (and therefore no subschemas) is
   reached.  The appropriate location in the instance is evaluated
   against the assertion and annotation keywords in the schema object,
   and their results are gathered into the parent schema according to
   the rules of the applicator.

   Evaluation of a parent schema object can complete once all of its
   subschemas have been evaluated, although in some circumstances
   evaluation may be short-circuited due to assertion results.  When
   annotations are being collected, some assertion result short-
   circuiting is not possible due to the need to examine all subschemas
   for annotation collection, including those that cannot further change
   the assertion result.

7.1.  Lexical Scope and Dynamic Scope

   While most JSON Schema keywords can be evaluated on their own, or at
   most need to take into account the values or results of adjacent
   keywords in the same schema object, a few have more complex behavior.

   The lexical scope of a keyword is determined by the nested JSON data
   structure of objects and arrays.  The largest such scope is an entire
   schema document.  The smallest scope is a single schema object with
   no subschemas.

   Keywords MAY be defined with a partial value, such as a URI-
   reference, which must be resolved against another value, such as
   another URI-reference or a full URI, which is found through the
   lexical structure of the JSON document.  The "$id" core keyword and
   the "base" JSON Hyper-Schema keyword are examples of this sort of

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   behavior.  Additionally, "$ref" and "$recursiveRef" from this
   specification resolve their values in this way, although they do not
   change how further values are resolved.

   Note that some keywords, such as "$schema", apply to the lexical
   scope of the entire schema document, and therefore MUST only appear
   in a schema resource's root schema.

   Other keywords may take into account the dynamic scope that exists
   during the evaluation of a schema, typically together with an
   instance document.  The outermost dynamic scope is the root schema of
   the schema document in which processing begins.  The path from this
   root schema to any particular keyword (that includes any "$ref" and
   "$recursiveRef" keywords that may have been resolved) is considered
   the keyword's "validation path."  [[CREF1: Or should this be the
   schema object at which processing begins, even if it is not a root?
   This has some implications for the case where "$recursiveAnchor" is
   only allowed in the root schema but processing begins in a subschema.
   ]]

   Lexical and dynamic scopes align until a reference keyword is
   encountered.  While following the reference keyword moves processing
   from one lexical scope into a different one, from the perspective of
   dynamic scope, following reference is no different from descending
   into a subschema present as a value.  A keyword on the far side of
   that reference that resolves information through the dynamic scope
   will consider the originating side of the reference to be their
   dynamic parent, rather than examining the local lexically enclosing
   parent.

   The concept of dynamic scope is primarily used with "$recursiveRef"
   and "$recursiveAnchor", and should be considered an advanced feature
   and used with caution when defining additional keywords.  It also
   appears when reporting errors and collected annotations, as it may be
   possible to revisit the same lexical scope repeatedly with different
   dynamic scopes.  In such cases, it is important to inform the user of
   the dynamic path that produced the error or annotation.

7.2.  Keyword Interactions

   Keyword behavior MAY be defined in terms of the annotation results of
   subschemas (Section 4.3.5) and/or adjacent keywords.  Such keywords
   MUST NOT result in a circular dependency.  Keywords MAY modify their
   behavior based on the presence or absence of another keyword in the
   same schema object (Section 4.3).

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7.3.  Default Behaviors

   A missing keyword MUST NOT produce a false assertion result, MUST NOT
   produce annotation results, and MUST NOT cause any other schema to be
   evaluated as part of its own behavioral definition.  However, given
   that missing keywords do not contribute annotations, the lack of
   annotation results may indirectly change the behavior of other
   keywords.

   In some cases, the missing keyword assertion behavior of a keyword is
   identical to that produced by a certain value, and keyword
   definitions SHOULD note such values where known.  However, even if
   the value which produces the default behavior would produce
   annotation results if present, the default behavior still MUST NOT
   result in annotations.

   Because annotation collection can add significant cost in terms of
   both computation and memory, implementations MAY opt out of this
   feature.  Keywords known to an implementation to have assertion or
   applicator behavior that depend on annotation results MUST then be
   treated as errors, unless an alternate implementation producing the
   same behavior is available.  Keywords of this sort SHOULD describe
   reasonable alternate approaches when appropriate.  This approach is
   demonstrated by the "additionalItems" and "additionalProperties"
   keywords in this document.

7.4.  Identifiers

   Identifiers set the canonical URI of a schema, or affect how such
   URIs are resolved in references (Section 8.2.4), or both.  The Core
   vocabulary defined in this document defines several identifying
   keywords, most notably "$id".

   Canonical schema URIs MUST NOT change while processing an instance,
   but keywords that affect URI-reference resolution MAY have behavior
   that is only fully determined at runtime.

   While custom identifier keywords are possible, vocabulary designers
   should take care not to disrupt the functioning of core keywords.
   For example, the "$recursiveAnchor" keyword in this specification
   limits its URI resolution effects to the matching "$recursiveRef"
   keyword, leaving "$ref" undisturbed.

7.5.  Applicators

   Applicators allow for building more complex schemas than can be
   accomplished with a single schema object.  Evaluation of an instance
   against a schema document (Section 4.3) begins by applying the root

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   schema (Section 4.3.5) to the complete instance document.  From
   there, keywords known as applicators are used to determine which
   additional schemas are applied.  Such schemas may be applied in-place
   to the current location, or to a child location.

   The schemas to be applied may be present as subschemas comprising all
   or part of the keyword's value.  Alternatively, an applicator may
   refer to a schema elsewhere in the same schema document, or in a
   different one.  The mechanism for identifying such referenced schemas
   is defined by the keyword.

   Applicator keywords also define how subschema or referenced schema
   boolean assertion (Section 7.6) results are modified and/or combined
   to produce the boolean result of the applicator.  Applicators may
   apply any boolean logic operation to the assertion results of
   subschemas, but MUST NOT introduce new assertion conditions of their
   own.

   Annotation (Section 7.7) results are combined according to the rules
   specified by each annotation keyword.

7.5.1.  Referenced and Referencing Schemas

   As noted in Section 7.5, an applicator keyword may refer to a schema
   to be applied, rather than including it as a subschema in the
   applicator's value.  In such situations, the schema being applied is
   known as the referenced schema, while the schema containing the
   applicator keyword is the referencing schema.

   While root schemas and subschemas are static concepts based on a
   schema's position within a schema document, referenced and
   referencing schemas are dynamic.  Different pairs of schemas may find
   themselves in various referenced and referencing arrangements during
   the evaluation of an instance against a schema.

   For some by-reference applicators, such as "$ref" (Section 8.2.4.1),
   the referenced schema can be determined by static analysis of the
   schema document's lexical scope.  Others, such as "$recursiveRef" and
   "$recursiveAnchor", may make use of dynamic scoping, and therefore
   only be resolvable in the process of evaluating the schema with an
   instance.

7.6.  Assertions

   JSON Schema can be used to assert constraints on a JSON document,
   which either passes or fails the assertions.  This approach can be
   used to validate conformance with the constraints, or document what
   is needed to satisfy them.

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   JSON Schema implementations produce a single boolean result when
   evaluating an instance against schema assertions.

   An instance can only fail an assertion that is present in the schema.

7.6.1.  Assertions and Instance Primitive Types

   Most assertions only constrain values within a certain primitive
   type.  When the type of the instance is not of the type targeted by
   the keyword, the instance is considered to conform to the assertion.

   For example, the "maxLength" keyword from the companion validation
   vocabulary [json-schema-validation]: will only restrict certain
   strings (that are too long) from being valid.  If the instance is a
   number, boolean, null, array, or object, then it is valid against
   this assertion.

   This behavior allows keywords to be used more easily with instances
   that can be of multiple primitive types.  The companion validation
   vocabulary also includes a "type" keyword which can independently
   restrict the instance to one or more primitive types.  This allows
   for a concise expression of use cases such as a function that might
   return either a string of a certain length or a null value:

   {
       "type": ["string", "null"],
       "maxLength": 255
   }

   If "maxLength" also restricted the instance type to be a string, then
   this would be substantially more cumbersome to express because the
   example as written would not actually allow null values.  Each
   keyword is evaluated separately unless explicitly specified
   otherwise, so if "maxLength" restricted the instance to strings, then
   including "null" in "type" would not have any useful effect.

7.7.  Annotations

   JSON Schema can annotate an instance with information, whenever the
   instance validates against the schema object containing the
   annotation, and all of its parent schema objects.  The information
   can be a simple value, or can be calculated based on the instance
   contents.

   Annotations are attached to specific locations in an instance.  Since
   many subschemas can be applied to any single location, annotation

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   keywords need to specify any unusual handling of multiple applicable
   occurrences of the keyword with different values.

   Unlike assertion results, annotation data can take a wide variety of
   forms, which are provided to applications to use as they see fit.
   JSON Schema implementations are not expected to make use of the
   collected information on behalf of applications.

   Unless otherwise specified, the value of an annotation keyword's
   annotation is the keyword's value.  However, other behaviors are
   possible.  For example, JSON Hyper-Schema's [json-hyper-schema]
   "links" keyword is a complex annotation that produces a value based
   in part on the instance data.

   While "short-circuit" evaluation is possible for assertions,
   collecting annotations requires examining all schemas that apply to
   an instance location, even if they cannot change the overall
   assertion result.  The only exception is that subschemas of a schema
   object that has failed validation MAY be skipped, as annotations are
   not retained for failing schemas.

7.7.1.  Collecting Annotations

   Annotations are collected by keywords that explicitly define
   annotation-collecting behavior.  Note that boolean schemas cannot
   produce annotations as they do not make use of keywords.

   A collected annotation MUST include the following information:

      The name of the keyword that produces the annotation

      The instance location to which it is attached, as a JSON Pointer

      The schema location path, indicating how reference keywords such
      as "$ref" were followed to reach the absolute schema location.

      The absolute schema location of the attaching keyword, as a URI.
      This MAY be omitted if it is the same as the schema location path
      from above.

      The attached value(s)

   If the same keyword attaches values from multiple schema locations to
   the same instance location, and the annotation defines a process for
   combining such values, then the combined value MUST also be
   associated with the instance location.  The output formats
   (Section 10) described in this specification that include annotation
   information meet this requirement.

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7.7.1.1.  Distinguishing Among Multiple Values

   Applications MAY make decisions on which of multiple annotation
   values to use based on the schema location that contributed the
   value.  This is intended to allow flexible usage.  Collecting the
   schema location facilitates such usage.

   For example, consider this schema, which uses annotations and
   assertions from the Validation specification
   [json-schema-validation]:

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   Note that some lines are wrapped for clarity.

   {
       "title": "Feature list",
       "type": "array",
       "items": [
           {
               "title": "Feature A",
               "properties": {
                   "enabled": {
                       "$ref": "#/$defs/enabledToggle",
                       "default": true
                   }
               }
           },
           {
               "title": "Feature B",
               "properties": {
                   "enabled": {
                       "description": "If set to null, Feature B
                                       inherits the enabled
                                       value from Feature A",
                       "$ref": "#/$defs/enabledToggle"
                   }
               }
           }
       ],
       "$defs": {
           "enabledToggle": {
               "title": "Enabled",
               "description": "Whether the feature is enabled (true),
                               disabled (false), or under
                               automatic control (null)",
               "type": ["boolean", "null"],
               "default": null
           }
       }
   }

   In this example, both Feature A and Feature B make use of the re-
   usable "enabledToggle" schema.  That schema uses the "title",
   "description", and "default" annotations, none of which define
   special behavior for handling multiple values.  Therefore the
   application has to decide how to handle the additional "default"
   value for Feature A, and the additional "description" value for
   Feature B.

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   The application programmer and the schema author need to agree on the
   usage.  For this example, let's assume that they agree that the most
   specific "default" value will be used, and any additional, more
   generic "default" values will be silently ignored.  Let's also assume
   that they agree that all "description" text is to be used, starting
   with the most generic, and ending with the most specific.  This
   requires the schema author to write descriptions that work when
   combined in this way.

   The application can use the schema location path to determine which
   values are which.  The values in the feature's immediate "enabled"
   property schema are more specific, while the values under the re-
   usable schema that is referenced to with "$ref" are more generic.
   The schema location path will show whether each value was found by
   crossing a "$ref" or not.

   Feature A will therefore use a default value of true, while Feature B
   will use the generic default value of null.  Feature A will only have
   the generic description from the "enabledToggle" schema, while
   Feature B will use that description, and also append its locally
   defined description that explains how to interpret a null value.

   Note that there are other reasonable approaches that a different
   application might take.  For example, an application may consider the
   presence of two different values for "default" to be an error,
   regardless of their schema locations.

7.7.1.2.  Annotations and Assertions

   Schema objects that produce a false assertion result MUST NOT produce
   any annotation results, whether from their own keywords or from
   keywords in subschemas.

   Note that the overall schema results may still include annotations
   collected from other schema locations.  Given this schema:

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   {
       "oneOf": [
           {
               "title": "Integer Value",
               "type": "integer"
           },
           {
               "title": "String Value",
               "type": "string"
           }
       ]
   }

   And the instance ""This is a string"", the title annotation "Integer
   Value" is discarded because the type assertion in that schema object
   fails.  The title annotation "String Value" is kept, as the instance
   passes the string type assertions.

7.7.1.3.  Annotations and Applicators

   In addition to possibly defining annotation results of their own,
   applicator keywords aggregate the annotations collected in their
   subschema(s) or referenced schema(s).  The rules for aggregating
   annotation values are defined by each annotation keyword, and are not
   directly affected by the logic used for combining assertion results.

7.8.  Reserved Locations

   A fourth category of keywords simply reserve a location to hold re-
   usable components or data of interest to schema authors that is not
   suitable for re-use.  These keywords do not affect validation or
   annotation results.  Their purpose in the core vocabulary is to
   ensure that locations are available for certain purposes and will not
   be redefined by extension keywords.

   While these keywords do not directly affect results, as explained in
   section 8.2.4.4 unrecognized extension keywords that reserve
   locations for re-usable schemas may have undesirable interactions
   with references in certain circumstances.

8.  The JSON Schema Core Vocabulary

   Keywords declared in this section, which all begin with "$", make up
   the JSON Schema Core vocabulary.  These keywords are either required
   in order process any schema or meta-schema, including those split
   across multiple documents, or exist to reserve keywords for purposes
   that require guaranteed interoperability.

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   The Core vocabulary MUST be considered mandatory at all times, in
   order to bootstrap the processing of further vocabularies.  Meta-
   schemas that use the "$vocabulary" (Section 8.1) keyword to declare
   the vocabularies in use MUST explicitly list the Core vocabulary,
   which MUST have a value of true indicating that it is required.

   The behavior of a false value for this vocabulary (and only this
   vocabulary) is undefined, as is the behavior when "$vocabulary" is
   present but the Core vocabulary is not included.  However, it is
   RECOMMENDED that implementations detect these cases and raise an
   error when they occur.  It is not meaningful to declare that a meta-
   schema optionally uses Core.

   Meta-schemas that do not use "$vocabulary" MUST be considered to
   require the Core vocabulary as if its URI were present with a value
   of true.

   The current URI for the Core vocabulary is: <https://json-schema.org/
   draft/2019-09/vocab/core>.

   The current URI for the corresponding meta-schema is: <https://json-
   schema.org/draft/2019-09/meta/core>.

   While the "$" prefix is not formally reserved for the Core
   vocabulary, it is RECOMMENDED that extension keywords (in
   vocabularies or otherwise) begin with a character other than "$" to
   avoid possible future collisions.

8.1.  Meta-Schemas and Vocabularies

   Two concepts, meta-schemas and vocabularies, are used to inform an
   implementation how to interpret a schema.  Every schema has a meta-
   schema, which can be declared using the "$schema" keyword.

   The meta-schema serves two purposes:

   Declaring the vocabularies in use  The "$vocabulary" keyword, when it
      appears in a meta-schema, declares which vocabularies are
      available to be used in schemas that refer to that meta-schema.
      Vocabularies define keyword semantics, as well as their general
      syntax.

   Describing valid schema syntax  A schema MUST successfully validate
      against its meta-schema, which constrains the syntax of the
      available keywords.  The syntax described is expected to be
      compatible with the vocabularies declared; while it is possible to
      describe an incompatible syntax, such a meta-schema would be
      unlikely to be useful.

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   Meta-schemas are separate from vocabularies to allow for vocabularies
   to be combined in different ways, and for meta-schema authors to
   impose additional constraints such as forbidding certain keywords, or
   performing unusually strict syntactical validation, as might be done
   during a development and testing cycle.  Each vocabulary typically
   identifies a meta-schema consisting only of the vocabulary's
   keywords.

   Meta-schema authoring is an advanced usage of JSON Schema, so the
   design of meta-schema features emphasizes flexibility over
   simplicity.

8.1.1.  The "$schema" Keyword

   The "$schema" keyword is both used as a JSON Schema feature set
   identifier and as the identifier of a resource which is itself a JSON
   Schema, which describes the set of valid schemas written for this
   particular feature set.

   The value of this keyword MUST be a URI [RFC3986] (containing a
   scheme) and this URI MUST be normalized.  The current schema MUST be
   valid against the meta-schema identified by this URI.

   If this URI identifies a retrievable resource, that resource SHOULD
   be of media type "application/schema+json".

   The "$schema" keyword SHOULD be used in a resource root schema.  It
   MUST NOT appear in resource subschemas.  If absent from the root
   schema, the resulting behavior is implementation-defined.

   If multiple schema resources are present in a single document, then
   all schema resources SHOULD Have the same value for "$schema".  The
   result of differing values for "$schema" within the same schema
   document is implementation-defined.  [[CREF2: Using multiple
   "$schema" keywords in the same document would imply that the feature
   set and therefore behavior can change within a document.  This would
   necessitate resolving a number of implementation concerns that have
   not yet been clearly defined.  So, while the pattern of using
   "$schema" only in root schemas is likely to remain the best practice
   for schema authoring, implementation behavior is subject to be
   revised or liberalized in future drafts.  ]] [[CREF3: The exception
   made for embedded schema resources is to allow bundling multiple
   schema resources into a single schema document without needing to
   change their contents, as described later in this specification.  ]]

   Values for this property are defined elsewhere in this and other
   documents, and by other parties.

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8.1.2.  The "$vocabulary" Keyword

   The "$vocabulary" keyword is used in meta-schemas to identify the
   vocabularies available for use in schemas described by that meta-
   schema.  It is also used to indicate whether each vocabulary is
   required or optional, in the sense that an implementation MUST
   understand the required vocabularies in order to successfully process
   the schema.

   The value of this keyword MUST be an object.  The property names in
   the object MUST be URIs (containing a scheme) and this URI MUST be
   normalized.  Each URI that appears as a property name identifies a
   specific set of keywords and their semantics.

   The URI MAY be a URL, but the nature of the retrievable resource is
   currently undefined, and reserved for future use.  Vocabulary authors
   MAY use the URL of the vocabulary specification, in a human-readable
   media type such as text/html or text/plain, as the vocabulary URI.
   [[CREF4: Vocabulary documents may be added in forthcoming drafts.
   For now, identifying the keyword set is deemed sufficient as that,
   along with meta-schema validation, is how the current "vocabularies"
   work today.  Any future vocabulary document format will be specified
   as a JSON document, so using text/html or other non-JSON formats in
   the meantime will not produce any future ambiguity.  ]]

   The values of the object properties MUST be booleans.  If the value
   is true, then implementations that do not recognize the vocabulary
   MUST refuse to process any schemas that declare this meta-schema with
   "$schema".  If the value is false, implementations that do not
   recognize the vocabulary SHOULD proceed with processing such schemas.

   Per 6.5, unrecognized keywords SHOULD be ignored.  This remains the
   case for keywords defined by unrecognized vocabularies.  It is not
   currently possible to distinguish between unrecognized keywords that
   are defined in vocabularies from those that are not part of any
   vocabulary.

   The "$vocabulary" keyword SHOULD be used in the root schema of any
   schema document intended for use as a meta-schema.  It MUST NOT
   appear in subschemas.

   The "$vocabulary" keyword MUST be ignored in schema documents that
   are not being processed as a meta-schema.  This allows validating a
   meta-schema M against its own meta-schema M' without requiring the
   validator to understand the vocabularies declared by M.

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8.1.2.1.  Default vocabularies

   If "$vocabulary" is absent, an implementation MAY determine behavior
   based on the meta-schema if it is recognized from the URI value of
   the referring schema's "$schema" keyword.  This is how behavior (such
   as Hyper-Schema usage) has been recognized prior to the existence of
   vocabularies.

   If the meta-schema, as referenced by the schema, is not recognized,
   or is missing, then the behavior is implementation-defined.  If the
   implementation proceeds with processing the schema, it MUST assume
   the use of the core vocabulary.  If the implementation is built for a
   specific purpose, then it SHOULD assume the use of all of the most
   relevant vocabularies for that purpose.

   For example, an implementation that is a validator SHOULD assume the
   use of all vocabularies in this specification and the companion
   Validation specification.

8.1.2.2.  Non-inheritability of vocabularies

   Note that the processing restrictions on "$vocabulary" mean that
   meta-schemas that reference other meta-schemas using "$ref" or
   similar keywords do not automatically inherit the vocabulary
   declarations of those other meta-schemas.  All such declarations must
   be repeated in the root of each schema document intended for use as a
   meta-schema.  This is demonstrated in the example meta-schema
   (Appendix D.2).  [[CREF5: This requirement allows implementations to
   find all vocabulary requirement information in a single place for
   each meta-schema.  As schema extensibility means that there are
   endless potential ways to combine more fine-grained meta-schemas by
   reference, requiring implementations to anticipate all possibilities
   and search for vocabularies in referenced meta-schemas would be
   overly burdensome.  ]]

8.1.3.  Updates to Meta-Schema and Vocabulary URIs

   Updated vocabulary and meta-schema URIs MAY be published between
   specification drafts in order to correct errors.  Implementations
   SHOULD consider URIs dated after this specification draft and before
   the next to indicate the same syntax and semantics as those listed
   here.

8.1.4.  Detecting a Meta-Schema

   Implementations MUST recognize a schema as a meta-schema if it is
   being examined because it was identified as such by another schema's
   "$schema" keyword.  This means that a single schema document might

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   sometimes be considered a regular schema, and other times be
   considered a meta-schema.

   In the case of examining a schema which is its own meta-schema, when
   an implementation begins processing it as a regular schema, it is
   processed under those rules.  However, when loaded a second time as a
   result of checking its own "$schema" value, it is treated as a meta-
   schema.  So the same document is processed both ways in the course of
   one session.

   Implementations MAY allow a schema to be explicitly passed as a meta-
   schema, for implementation-specific purposes, such as pre-loading a
   commonly used meta-schema and checking its vocabulary support
   requirements up front.  Meta-schema authors MUST NOT expect such
   features to be interoperable across implementations.

8.2.  Base URI, Anchors, and Dereferencing

   To differentiate between schemas in a vast ecosystem, schemas are
   identified by URI [RFC3986], and can embed references to other
   schemas by specifying their URI.

   Several keywords can accept a relative URI-reference [RFC3986], or a
   value used to construct a relative URI-reference.  For these
   keywords, it is necessary to establish a base URI in order to resolve
   the reference.

8.2.1.  Initial Base URI

   RFC3986 Section 5.1 [RFC3986] defines how to determine the default
   base URI of a document.

   Informatively, the initial base URI of a schema is the URI at which
   it was found, whether that was a network location, a local
   filesystem, or any other situation identifiable by a URI of any known
   scheme.

   If a schema document defines no explicit base URI with "$id"
   (embedded in content), the base URI is that determined per RFC 3986
   section 5 [RFC3986].

   If no source is known, or no URI scheme is known for the source, a
   suitable implementation-specific default URI MAY be used as described
   in RFC 3986 Section 5.1.4 [RFC3986].  It is RECOMMENDED that
   implementations document any default base URI that they assume.

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   Unless the "$id" keyword described in the next section is present in
   the root schema, this base URI SHOULD be considered the canonical URI
   of the schema document's root schema resource.

8.2.2.  The "$id" Keyword

   The "$id" keyword identifies a schema resource with its canonical
   [RFC6596] URI.

   Note that this URI is an identifier and not necessarily a network
   locator.  In the case of a network-addressable URL, a schema need not
   be downloadable from its canonical URI.

   If present, the value for this keyword MUST be a string, and MUST
   represent a valid URI-reference [RFC3986].  This URI-reference SHOULD
   be normalized, and MUST resolve to an absolute-URI [RFC3986] (without
   a fragment).  Therefore, "$id" MUST NOT contain a non-empty fragment,
   and SHOULD NOT contain an empty fragment.

   Since an empty fragment in the context of the application/schema+json
   media type refers to the same resource as the base URI without a
   fragment, an implementation MAY normalize a URI ending with an empty
   fragment by removing the fragment.  However, schema authors SHOULD
   NOT rely on this behavior across implementations.  [[CREF6: This is
   primarily allowed because older meta-schemas have an empty fragment
   in their $id (or previously, id).  A future draft may outright forbid
   even empty fragments in "$id".  ]]

   This URI also serves as the base URI for relative URI-references in
   keywords within the schema resource, in accordance with RFC 3986
   section 5.1.1 [RFC3986] regarding base URIs embedded in content.

   The presence of "$id" in a subschema indicates that the subschema
   constitutes a distinct schema resource within a single schema
   document.  Furthermore, in accordance with RFC 3986 section 5.1.2
   [RFC3986] regarding encapsulating entities, if an "$id" in a
   subschema is a relative URI-reference, the base URI for resolving
   that reference is the URI of the parent schema resource.

   If no parent schema object explicitly identifies itself as a resource
   with "$id", the base URI is that of the entire document, as
   established by the steps given in the previous section.
   (Section 8.2.1)

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8.2.2.1.  Identifying the root schema

   The root schema of a JSON Schema document SHOULD contain an "$id"
   keyword with an absolute-URI [RFC3986] (containing a scheme, but no
   fragment).

8.2.2.2.  JSON Pointer fragments and embedded schema resources

   Since JSON Pointer URI fragments are constructed based on the
   structure of the schema document, an embedded schema resource and its
   subschemas can be identified by JSON Pointer fragments relative to
   either its own canonical URI, or relative to the containing
   resource's URI.

   Conceptually, a set of linked schema resources should behave
   identically whether each resource is a separate document connected
   with schema references (Section 8.2.4), or is structured as a single
   document with one or more schema resources embedded as subschemas.

   Since URIs involving JSON Pointer fragments relative to the parent
   schema resource's URI cease to be valid when the embedded schema is
   moved to a separate document and referenced, applications and schemas
   SHOULD NOT use such URIs to identify embedded schema resources or
   locations within them.

   Consider the following schema document that contains another schema
   resource embedded within it:

   {
     "$id": "https://example.com/foo",
     "items": {
       "$id": "https://example.com/bar",
       "additionalProperties": { }
     }
   }

   The URI "https://example.com/foo#/items/additionalProperties" points
   to the schema of the "additionalProperties" keyword in the embedded
   resource.  The canonical URI of that schema, however, is
   "https://example.com/bar#/additionalProperties".

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   Now consider the following two schema resources linked by reference
   using a URI value for "$ref":

   {
     "$id": "https://example.com/foo",
     "items": {
       "$ref": "bar"
     }
   }

   {
     "$id": "https://example.com/bar",
     "additionalProperties": { }
   }

   Here we see that the canonical URI for that "additionalProperties"
   subschema is still valid, while the non-canonical URI with the
   fragment beginning with "#/items/$ref" now resolves to nothing.

   Note also that "https://example.com/foo#/items" is valid in both
   arrangments, but resolves to a different value.  This URI ends up
   functioning similarly to a retrieval URI for a resource.  While
   valid, examining the resolved value and either using the "$id" (if
   the value is a subschema), or resolving the reference and using the
   "$id" of the reference target, is preferable.

   An implementation MAY choose not to support addressing schemas by
   non-canonical URIs.  As such, it is RECOMENDED that schema authors
   only use canonical URIs, as using non-canonical URIs may reduce
   schema interoperability.  [[CREF7: This is to avoid requiring
   implementations to keep track of a whole stack of possible base URIs
   and JSON Pointer fragments for each, given that all but one will be
   fragile if the schema resources are reorganized.  Some have argued
   that this is easy so there is no point in forbidding it, while others
   have argued that it complicates schema identification and should be
   forbidden.  Feedback on this topic is encouraged.  ]]

   Further examples of such non-canonical URIs, as well as the
   appropriate canonical URIs to use instead, are provided in appendix
   A.

8.2.3.  Defining location-independent identifiers with "$anchor"

   Using JSON Pointer fragments requires knowledge of the structure of
   the schema.  When writing schema documents with the intention to
   provide re-usable schemas, it may be preferable to use a plain name

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   fragment that is not tied to any particular structural location.
   This allows a subschema to be relocated without requiring JSON
   Pointer references to be updated.

   The "$anchor" keyword is used to specify such a fragment.  It is an
   identifier keyword that can only be used to create plain name
   fragments.

   If present, the value of this keyword MUST be a string, which MUST
   start with a letter ([A-Za-z]), followed by any number of letters,
   digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), or
   periods (".").  [[CREF8: Note that the anchor string does not include
   the "#" character, as it is not a URI-reference.  An "$anchor": "foo"
   becomes the fragment "#foo" when used in a URI.  See below for full
   examples.  ]]

   The base URI to which the resulting fragment is appended is
   determined by the "$id" keyword as explained in the previous section.
   Two "$anchor" keywords in the same schema document MAY have the same
   value if they apply to different base URIs, as the resulting full
   URIs will be distinct.  However, the effect of two "$anchor" keywords
   with the same value and the same base URI is undefined.
   Implementations MAY raise an error if such usage is detected.

8.2.4.  Schema References

   Several keywords can be used to reference a schema which is to be
   applied to the current instance location. "$ref" and "$recursiveRef"
   are applicator keywords, applying the referenced schema to the
   instance.  "$recursiveAnchor" is an identifier keyword that controls
   how the base URI for resolving the URI-reference value of
   "$recursiveRef is determined.

   As the values of "$ref" and "$recursiveRef" are URI References, this
   allows the possibility to externalise or divide a schema across
   multiple files, and provides the ability to validate recursive
   structures through self-reference.

   The resolved URI produced by these keywords is not necessarily a
   network locator, only an identifier.  A schema need not be
   downloadable from the address if it is a network-addressable URL, and
   implementations SHOULD NOT assume they should perform a network
   operation when they encounter a network-addressable URI.

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8.2.4.1.  Direct References with "$ref"

   The "$ref" keyword is an applicator that is used to reference a
   statically identified schema.  Its results are the results of the
   referenced schema.  [[CREF9: Note that this definition of how the
   results are determined means that other keywords can appear alongside
   of "$ref" in the same schema object.  ]]

   The value of the "$ref" property MUST be a string which is a URI-
   Reference.  Resolved against the current URI base, it produces the
   URI of the schema to apply.

8.2.4.2.  Recursive References with "$recursiveRef" and
          "$recursiveAnchor"

   The "$recursiveRef" and "$recursiveAnchor" keywords are used to
   construct extensible recursive schemas.  A recursive schema is one
   that has a reference to its own root, identified by the empty
   fragment URI reference ("#").

   Simply stated, a "$recursiveRef" behaves identically to "$ref",
   except when its target schema contains "$recursiveAnchor" with a
   value of true.  In that case, the dynamic scope is examined to
   determine a new base URI, and the URI-reference in "$recursiveRef" is
   re-evaluated against that base URI.  Unlike base URI changes with
   "$id", changes with "$recursiveAnchor" are calculated each time a
   "$recursiveRef" is resolved, and do not impact any other keywords.

   For an example using these keyword, see appendix C.  [[CREF10: The
   difference between the hyper-schema meta-schema in previous drafts
   and an this draft dramatically demonstrates the utility of these
   keywords.  ]]

8.2.4.2.1.  Dynamically recursive references with "$recursiveRef"

   The value of the "$recursiveRef" property MUST be a string which is a
   URI-reference.  It is a by-reference applicator that uses a
   dynamically calculated base URI to resolve its value.

   The behavior of this keyword is defined only for the value "#".
   Implementations MAY choose to consider other values to be errors.
   [[CREF11: This restriction may be relaxed in the future, but to date
   only the value "#" has a clear use case.  ]]

   The value of "$recursiveRef" is initially resolved against the
   current base URI, in the same manner as for "$ref".

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   The schema identified by the resulting URI is examined for the
   presence of "$recursiveAnchor", and a new base URI is calculated as
   described for that keyword in the following section.

   Finally, the value of "$recursiveRef" is resolved against the new
   base URI determined according to "$recursiveAnchor" producing the
   final resolved reference URI.

   Note that in the absence of "$recursiveAnchor" (and in some cases
   when it is present), "$recursiveRef"'s behavior is identical to that
   of "$ref".

   As with "$ref", the results of this keyword are the results of the
   referenced schema.

8.2.4.2.2.  Enabling Recursion with "$recursiveAnchor"

   The value of the "$recursiveAnchor" property MUST be a boolean.

   "$recursiveAnchor" is used to dynamically identify a base URI at
   runtime for "$recursiveRef" by marking where such a calculation can
   start, and where it stops.  This keyword MUST NOT affect the base URI
   of other keywords, unless they are explicitly defined to rely on it.

   If set to true, then when the containing schema object is used as a
   target of "$recursiveRef", a new base URI is determined by examining
   the dynamic scope (Section 7.1) for the outermost schema that also
   contains "$recursiveAnchor" with a value of true.  The base URI of
   that schema is then used as the dynamic base URI.

   If no such schema exists, then the base URI is unchanged.

   If this keyword is set to false, the base URI is unchanged.

   Omitting this keyword has the same behavior as a value of false.

8.2.4.3.  Guarding Against Infinite Recursion

   A schema MUST NOT be run into an infinite loop against an instance.
   For example, if two schemas "#alice" and "#bob" both have an "allOf"
   property that refers to the other, a naive validator might get stuck
   in an infinite recursive loop trying to validate the instance.
   Schemas SHOULD NOT make use of infinite recursive nesting like this;
   the behavior is undefined.

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8.2.4.4.  References to Possible Non-Schemas

   Subschema objects (or booleans) are recognized by their use with
   known applicator keywords or with location-reserving keywords such as
   "$defs" (Section 8.2.5) that take one or more subschemas as a value.
   These keywords may be "$defs" and the standard applicators from this
   document, or extension keywords from a known vocabulary, or
   implementation-specific custom keywords.

   Multi-level structures of unknown keywords are capable of introducing
   nested subschemas, which would be subject to the processing rules for
   "$id".  Therefore, having a reference target in such an unrecognized
   structure cannot be reliably implemented, and the resulting behavior
   is undefined.  Similarly, a reference target under a known keyword,
   for which the value is known not to be a schema, results in undefined
   behavior in order to avoid burdening implementations with the need to
   detect such targets.  [[CREF12: These scenarios are analogous to
   fetching a schema over HTTP but receiving a response with a Content-
   Type other than application/schema+json.  An implementation can
   certainly try to interpret it as a schema, but the origin server
   offered no guarantee that it actually is any such thing.  Therefore,
   interpreting it as such has security implications and may produce
   unpredictable results.  ]]

   Note that single-level custom keywords with identical syntax and
   semantics to "$defs" do not allow for any intervening "$id" keywords,
   and therefore will behave correctly under implementations that
   attempt to use any reference target as a schema.  However, this
   behavior is implementation-specific and MUST NOT be relied upon for
   interoperability.

8.2.4.5.  Loading a referenced schema

   The use of URIs to identify remote schemas does not necessarily mean
   anything is downloaded, but instead JSON Schema implementations
   SHOULD understand ahead of time which schemas they will be using, and
   the URIs that identify them.

   When schemas are downloaded, for example by a generic user-agent that
   doesn't know until runtime which schemas to download, see Usage for
   Hypermedia (Section 11).

   Implementations SHOULD be able to associate arbitrary URIs with an
   arbitrary schema and/or automatically associate a schema's "$id"-
   given URI, depending on the trust that the validator has in the
   schema.  Such URIs and schemas can be supplied to an implementation
   prior to processing instances, or may be noted within a schema

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   document as it is processed, producing associations as shown in
   appendix A.

   A schema MAY (and likely will) have multiple URIs, but there is no
   way for a URI to identify more than one schema.  When multiple
   schemas try to identify as the same URI, validators SHOULD raise an
   error condition.

8.2.4.6.  Dereferencing

   Schemas can be identified by any URI that has been given to them,
   including a JSON Pointer or their URI given directly by "$id".  In
   all cases, dereferencing a "$ref" reference involves first resolving
   its value as a URI reference against the current base URI per RFC
   3986 [RFC3986].

   If the resulting URI identifies a schema within the current document,
   or within another schema document that has been made available to the
   implementation, then that schema SHOULD be used automatically.

   For example, consider this schema:

   {
       "$id": "https://example.net/root.json",
       "items": {
           "type": "array",
           "items": { "$ref": "#item" }
       },
       "$defs": {
           "single": {
               "$anchor": "item",
               "type": "object",
               "additionalProperties": { "$ref": "other.json" }
           }
       }
   }

   When an implementation encounters the <#/$defs/single> schema, it
   resolves the "$id" URI reference against the current base URI to form
   <https://example.net/root.json#item>.

   When an implementation then looks inside the <#/items> schema, it
   encounters the <#item> reference, and resolves this to
   <https://example.net/root.json#item>, which it has seen defined in
   this same document and can therefore use automatically.

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   When an implementation encounters the reference to "other.json", it
   resolves this to <https://example.net/other.json>, which is not
   defined in this document.  If a schema with that identifier has
   otherwise been supplied to the implementation, it can also be used
   automatically.  [[CREF13: What should implementations do when the
   referenced schema is not known?  Are there circumstances in which
   automatic network dereferencing is allowed?  A same origin policy?  A
   user-configurable option?  In the case of an evolving API described
   by Hyper-Schema, it is expected that new schemas will be added to the
   system dynamically, so placing an absolute requirement of pre-loading
   schema documents is not feasible.  ]]

8.2.5.  Schema Re-Use With "$defs"

   The "$defs" keyword reserves a location for schema authors to inline
   re-usable JSON Schemas into a more general schema.  The keyword does
   not directly affect the validation result.

   This keyword's value MUST be an object.  Each member value of this
   object MUST be a valid JSON Schema.

   As an example, here is a schema describing an array of positive
   integers, where the positive integer constraint is a subschema in
   "$defs":

   {
       "type": "array",
       "items": { "$ref": "#/$defs/positiveInteger" },
       "$defs": {
           "positiveInteger": {
               "type": "integer",
               "exclusiveMinimum": 0
           }
       }
   }

8.3.  Comments With "$comment"

   This keyword reserves a location for comments from schema authors to
   readers or maintainers of the schema.

   The value of this keyword MUST be a string.  Implementations MUST NOT
   present this string to end users.  Tools for editing schemas SHOULD
   support displaying and editing this keyword.  The value of this
   keyword MAY be used in debug or error output which is intended for
   developers making use of schemas.

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   Schema vocabularies SHOULD allow "$comment" within any object
   containing vocabulary keywords.  Implementations MAY assume
   "$comment" is allowed unless the vocabulary specifically forbids it.
   Vocabularies MUST NOT specify any effect of "$comment" beyond what is
   described in this specification.

   Tools that translate other media types or programming languages to
   and from application/schema+json MAY choose to convert that media
   type or programming language's native comments to or from "$comment"
   values.  The behavior of such translation when both native comments
   and "$comment" properties are present is implementation-dependent.

   Implementations SHOULD treat "$comment" identically to an unknown
   extension keyword.  They MAY strip "$comment" values at any point
   during processing.  In particular, this allows for shortening schemas
   when the size of deployed schemas is a concern.

   Implementations MUST NOT take any other action based on the presence,
   absence, or contents of "$comment" properties.  In particular, the
   value of "$comment" MUST NOT be collected as an annotation result.

9.  A Vocabulary for Applying Subschemas

   This section defines a vocabulary of applicator keywords that are
   RECOMMENDED for use as the basis of other vocabularies.

   Meta-schemas that do not use "$vocabulary" SHOULD be considered to
   require this vocabulary as if its URI were present with a value of
   true.

   The current URI for this vocabulary, known as the Applicator
   vocabulary, is: <https://json-schema.org/draft/2019-09/vocab/
   applicator>.

   The current URI for the corresponding meta-schema is: <https://json-
   schema.org/draft/2019-09/meta/applicator>.

   Updated vocabulary and meta-schema URIs MAY be published between
   specification drafts in order to correct errors.  Implementations
   SHOULD consider URIs dated after this specification draft and before
   the next to indicate the same syntax and semantics as those listed
   here.

9.1.  Keyword Independence

   Schema keywords typically operate independently, without affecting
   each other's outcomes.

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   For schema author convenience, there are some exceptions among the
   keywords in this vocabulary:

      "additionalProperties", whose behavior is defined in terms of
      "properties" and "patternProperties"

      "unevaluatedProperties", whose behavior is defined in terms of
      annotations from "properties", "patternProperties",
      "additionalProperties" and itself

      "additionalItems", whose behavior is defined in terms of "items"

      "unevaluatedItems", whose behavior is defined in terms of
      annotations from "items", "additionalItems" and itself

9.2.  Keywords for Applying Subschemas in Place

   These keywords apply subschemas to the same location in the instance
   as the parent schema is being applied.  They allow combining or
   modifying the subschema results in various ways.

9.2.1.  Keywords for Applying Subschemas With Boolean Logic

   These keywords correspond to logical operators for combining or
   modifying the boolean assertion results of the subschemas.  They have
   no direct impact on annotation collection, although they enable the
   same annotation keyword to be applied to an instance location with
   different values.  Annotation keywords define their own rules for
   combining such values.

9.2.1.1.  allOf

   This keyword's value MUST be a non-empty array.  Each item of the
   array MUST be a valid JSON Schema.

   An instance validates successfully against this keyword if it
   validates successfully against all schemas defined by this keyword's
   value.

9.2.1.2.  anyOf

   This keyword's value MUST be a non-empty array.  Each item of the
   array MUST be a valid JSON Schema.

   An instance validates successfully against this keyword if it
   validates successfully against at least one schema defined by this
   keyword's value.  Note that when annotations are being collected, all

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   subschemas MUST be examined so that annotations are collected from
   each subschema that validates successfully.

9.2.1.3.  oneOf

   This keyword's value MUST be a non-empty array.  Each item of the
   array MUST be a valid JSON Schema.

   An instance validates successfully against this keyword if it
   validates successfully against exactly one schema defined by this
   keyword's value.

9.2.1.4.  not

   This keyword's value MUST be a valid JSON Schema.

   An instance is valid against this keyword if it fails to validate
   successfully against the schema defined by this keyword.

9.2.2.  Keywords for Applying Subschemas Conditionally

   Three of these keywords work together to implement conditional
   application of a subschema based on the outcome of another subschema.
   The fourth is a shortcut for a specific conditional case.

   "if", "then", and "else" MUST NOT interact with each other across
   subschema boundaries.  In other words, an "if" in one branch of an
   "allOf" MUST NOT have an impact on a "then" or "else" in another
   branch.

   There is no default behavior for "if", "then", or "else" when they
   are not present.  In particular, they MUST NOT be treated as if
   present with an empty schema, and when "if" is not present, both
   "then" and "else" MUST be entirely ignored.

9.2.2.1.  if

   This keyword's value MUST be a valid JSON Schema.

   This validation outcome of this keyword's subschema has no direct
   effect on the overall validation result.  Rather, it controls which
   of the "then" or "else" keywords are evaluated.

   Instances that successfully validate against this keyword's subschema
   MUST also be valid against the subschema value of the "then" keyword,
   if present.

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   Instances that fail to validate against this keyword's subschema MUST
   also be valid against the subschema value of the "else" keyword, if
   present.

   If annotations (Section 7.7) are being collected, they are collected
   from this keyword's subschema in the usual way, including when the
   keyword is present without either "then" or "else".

9.2.2.2.  then

   This keyword's value MUST be a valid JSON Schema.

   When "if" is present, and the instance successfully validates against
   its subschema, then validation succeeds against this keyword if the
   instance also successfully validates against this keyword's
   subschema.

   This keyword has no effect when "if" is absent, or when the instance
   fails to validate against its subschema.  Implementations MUST NOT
   evaluate the instance against this keyword, for either validation or
   annotation collection purposes, in such cases.

9.2.2.3.  else

   This keyword's value MUST be a valid JSON Schema.

   When "if" is present, and the instance fails to validate against its
   subschema, then validation succeeds against this keyword if the
   instance successfully validates against this keyword's subschema.

   This keyword has no effect when "if" is absent, or when the instance
   successfully validates against its subschema.  Implementations MUST
   NOT evaluate the instance against this keyword, for either validation
   or annotation collection purposes, in such cases.

9.2.2.4.  dependentSchemas

   This keyword specifies subschemas that are evaluated if the instance
   is an object and contains a certain property.

   This keyword's value MUST be an object.  Each value in the object
   MUST be a valid JSON Schema.

   If the object key is a property in the instance, the entire instance
   must validate against the subschema.  Its use is dependent on the
   presence of the property.

   Omitting this keyword has the same behavior as an empty object.

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9.3.  Keywords for Applying Subschemas to Child Instances

   Each of these keywords defines a rule for applying its subschema(s)
   to child instances, specifically object properties and array items,
   and combining their results.

9.3.1.  Keywords for Applying Subschemas to Arrays

9.3.1.1.  items

   The value of "items" MUST be either a valid JSON Schema or an array
   of valid JSON Schemas.

   If "items" is a schema, validation succeeds if all elements in the
   array successfully validate against that schema.

   If "items" is an array of schemas, validation succeeds if each
   element of the instance validates against the schema at the same
   position, if any.

   This keyword produces an annotation value which is the largest index
   to which this keyword applied a subschema.  The value MAY be a
   boolean true if a subschema was applied to every index of the
   instance, such as when "items" is a schema.

   Annotation results for "items" keywords from multiple schemas applied
   to the same instance location are combined by setting the combined
   result to true if any of the values are true, and otherwise retaining
   the largest numerical value.

   Omitting this keyword has the same assertion behavior as an empty
   schema.

9.3.1.2.  additionalItems

   The value of "additionalItems" MUST be a valid JSON Schema.

   The behavior of this keyword depends on the presence and annotation
   result of "items" within the same schema object.  If "items" is
   present, and its annotation result is a number, validation succeeds
   if every instance element at an index greater than that number
   validates against "additionalItems".

   Otherwise, if "items" is absent or its annotation result is the
   boolean true, "additionalItems" MUST be ignored.

   If the "additionalItems" subschema is applied to any positions within
   the instance array, it produces an annotation result of boolean true,

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   analogous to the single schema behavior of "items".  If any
   "additionalItems" keyword from any subschema applied to the same
   instance location produces an annotation value of true, then the
   combined result from these keywords is also true.

   Omitting this keyword has the same assertion behavior as an empty
   schema.

   Implementations MAY choose to implement or optimize this keyword in
   another way that produces the same effect, such as by directly
   checking for the presence and size of an "items" array.
   Implementations that do not support annotation collection MUST do so.

9.3.1.3.  unevaluatedItems

   The value of "unevaluatedItems" MUST be a valid JSON Schema.

   The behavior of this keyword depends on the annotation results of
   adjacent keywords that apply to the instance location being
   validated.  Specifically, the annotations from "items" and
   "additionalItems", which can come from those keywords when they are
   adjacent to the "unevaluatedItems" keyword.  Those two annotations,
   as well as "unevaluatedItems", can also result from any and all
   adjacent in-place applicator (Section 9.2) keywords.  This includes
   but is not limited to the in-place applicators defined in this
   document.

   If an "items" annotation is present, and its annotation result is a
   number, and no "additionalItems" or "unevaluatedItems" annotation is
   present, then validation succeeds if every instance element at an
   index greater than the "items" annotation validates against
   "unevaluatedItems".

   Otherwise, if any "items", "additionalItems", or "unevaluatedItems"
   annotations are present with a value of boolean true, then
   "unevaluatedItems" MUST be ignored.  However, if none of these
   annotations are present, "unevaluatedItems" MUST be applied to all
   locations in the array.

   This means that "items", "additionalItems", and all in-place
   applicators MUST be evaluated before this keyword can be evaluated.
   Authors of extension keywords MUST NOT define an in-place applicator
   that would need to be evaluated before this keyword.

   If the "unevaluatedItems" subschema is applied to any positions
   within the instance array, it produces an annotation result of
   boolean true, analogous to the single schema behavior of "items".  If
   any "unevaluatedItems" keyword from any subschema applied to the same

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   instance location produces an annotation value of true, then the
   combined result from these keywords is also true.

   Omitting this keyword has the same assertion behavior as an empty
   schema.

   Implementations that do not collect annotations MUST raise an error
   upon encountering this keyword.

9.3.1.4.  contains

   The value of this keyword MUST be a valid JSON Schema.

   An array instance is valid against "contains" if at least one of its
   elements is valid against the given schema.  Note that when
   collecting annotations, the subschema MUST be applied to every array
   element even after the first match has been found.  This is to ensure
   that all possible annotations are collected.

9.3.2.  Keywords for Applying Subschemas to Objects

9.3.2.1.  properties

   The value of "properties" MUST be an object.  Each value of this
   object MUST be a valid JSON Schema.

   Validation succeeds if, for each name that appears in both the
   instance and as a name within this keyword's value, the child
   instance for that name successfully validates against the
   corresponding schema.

   The annotation result of this keyword is the set of instance property
   names matched by this keyword.  Annotation results for "properties"
   keywords from multiple schemas applied to the same instance location
   are combined by taking the union of the sets.

   Omitting this keyword has the same assertion behavior as an empty
   object.

9.3.2.2.  patternProperties

   The value of "patternProperties" MUST be an object.  Each property
   name of this object SHOULD be a valid regular expression, according
   to the ECMA 262 regular expression dialect.  Each property value of
   this object MUST be a valid JSON Schema.

   Validation succeeds if, for each instance name that matches any
   regular expressions that appear as a property name in this keyword's

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   value, the child instance for that name successfully validates
   against each schema that corresponds to a matching regular
   expression.

   The annotation result of this keyword is the set of instance property
   names matched by this keyword.  Annotation results for
   "patternProperties" keywords from multiple schemas applied to the
   same instance location are combined by taking the union of the sets.

   Omitting this keyword has the same assertion behavior as an empty
   object.

9.3.2.3.  additionalProperties

   The value of "additionalProperties" MUST be a valid JSON Schema.

   The behavior of this keyword depends on the presence and annotation
   results of "properties" and "patternProperties" within the same
   schema object.  Validation with "additionalProperties" applies only
   to the child values of instance names that do not appear in the
   annotation results of either "properties" or "patternProperties".

   For all such properties, validation succeeds if the child instance
   validates against the "additionalProperties" schema.

   The annotation result of this keyword is the set of instance property
   names validated by this keyword's subschema.  Annotation results for
   "additionalProperties" keywords from multiple schemas applied to the
   same instance location are combined by taking the union of the sets.

   Omitting this keyword has the same assertion behavior as an empty
   schema.

   Implementations MAY choose to implement or optimize this keyword in
   another way that produces the same effect, such as by directly
   checking the names in "properties" and the patterns in
   "patternProperties" against the instance property set.
   Implementations that do not support annotation collection MUST do so.

9.3.2.4.  unevaluatedProperties

   The value of "unevaluatedProperties" MUST be a valid JSON Schema.

   The behavior of this keyword depends on the annotation results of
   adjacent keywords that apply to the instance location being
   validated.  Specifically, the annotations from "properties",
   "patternProperties", and "additionalProperties", which can come from
   those keywords when they are adjacent to the "unevaluatedProperties"

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   keyword.  Those three annotations, as well as
   "unevaluatedProperties", can also result from any and all adjacent
   in-place applicator (Section 9.2) keywords.  This includes but is not
   limited to the in-place applicators defined in this document.

   Validation with "unevaluatedProperties" applies only to the child
   values of instance names that do not appear in the "properties",
   "patternProperties", "additionalProperties", or
   "unevaluatedProperties" annotation results that apply to the instance
   location being validated.

   For all such properties, validation succeeds if the child instance
   validates against the "unevaluatedProperties" schema.

   This means that "properties", "patternProperties",
   "additionalProperties", and all in-place applicators MUST be
   evaluated before this keyword can be evaluated.  Authors of extension
   keywords MUST NOT define an in-place applicator that would need to be
   evaluated before this keyword.

   The annotation result of this keyword is the set of instance property
   names validated by this keyword's subschema.  Annotation results for
   "unevaluatedProperties" keywords from multiple schemas applied to the
   same instance location are combined by taking the union of the sets.

   Omitting this keyword has the same assertion behavior as an empty
   schema.

   Implementations that do not collect annotations MUST raise an error
   upon encountering this keyword.

9.3.2.5.  propertyNames

   The value of "propertyNames" MUST be a valid JSON Schema.

   If the instance is an object, this keyword validates if every
   property name in the instance validates against the provided schema.
   Note the property name that the schema is testing will always be a
   string.

   Omitting this keyword has the same behavior as an empty schema.

10.  Output Formatting

   JSON Schema is defined to be platform-independent.  As such, to
   increase compatibility across platforms, implementations SHOULD
   conform to a standard validation output format.  This section

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   describes the minimum requirements that consumers will need to
   properly interpret validation results.

10.1.  Format

   JSON Schema output is defined using the JSON Schema data instance
   model as described in section 4.2.1.  Implementations MAY deviate
   from this as supported by their specific languages and platforms,
   however it is RECOMMENDED that the output be convertible to the JSON
   format defined herein via serialization or other means.

10.2.  Output Formats

   This specification defines four output formats.  See the "Output
   Structure" section for the requirements of each format.

      Flag - A boolean which simply indicates the overall validation
      result with no further details.

      Basic - Provides validation information in a flat list structure.

      Detailed - Provides validation information in a condensed
      hierarchical structure based on the structure of the schema.

      Verbose - Provides validation information in an uncondensed
      hierarchical structure that matches the exact structure of the
      schema.

   An implementation SHOULD provide at least the "flag", "basic", or
   "detailed" format and MAY provide the "verbose" format.  If it
   provides one or more of the complex formats, it MUST also provide the
   "flag" format.  Implementations SHOULD specify in their documentation
   which formats they support.

10.3.  Minimum Information

   Beyond the simplistic "flag" output, additional information is useful
   to aid in debugging a schema or instance.  Each sub-result SHOULD
   contain the information contained within this section at a minimum.

   A single object that contains all of these components is considered
   an output unit.

   Implementations MAY elect to provide additional information.

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10.3.1.  Keyword Relative Location

   The relative location of the validating keyword that follows the
   validation path.  The value MUST be expressed as a JSON Pointer, and
   it MUST include any by-reference applicators such as "$ref" or
   "$recursiveRef".

   #/properties/width/$ref/minimum

   Note that this pointer may not be resolvable by the normal JSON
   Pointer process due to the inclusion of these by-reference applicator
   keywords.

   The JSON key for this information is "keywordLocation".

10.3.2.  Keyword Absolute Location

   The absolute, dereferenced location of the validating keyword.  The
   value MUST be expressed as an absolute URI using the canonical URI of
   the relevant schema object, and it MUST NOT include by-reference
   applicators such as "$ref" or "$recursiveRef" as non-terminal path
   components.  It MAY end in such keywords if the error or annotation
   is for that keyword, such as an unresolvable reference.

   https://example.com/schemas/common#/$defs/count/minimum

   This information MAY be omitted only if either the relative location
   contains no references or if the schema does not declare an absolute
   URI as its "$id".

   The JSON key for this information is "absoluteKeywordLocation".

10.3.3.  Instance Location

   The location of the JSON value within the instance being validated.
   The value MUST be expressed as a URI fragment-encoded JSON Pointer.

   The JSON key for this information is "instanceLocation".

10.3.4.  Error or Annotation

   The error or annotation that is produced by the validation.

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   For errors, the specific wording for the message is not defined by
   this specification.  Implementations will need to provide this.

   For annotations, each keyword that produces an annotation specifies
   its format.  By default, it is the keyword's value.

   The JSON key for failed validations is "error"; for successful
   validations it is "annotation".

10.3.5.  Nested Results

   For the two hierarchical structures, this property will hold nested
   errors and annotations.

   The JSON key for nested results in failed validations is "errors";
   for successful validations it is "annotations".  Note the plural
   forms, as a keyword with nested results can also have a local error
   or annotation.

10.4.  Output Structure

   The output MUST be an object containing a boolean property named
   "valid".  When additional information about the result is required,
   the output MUST also contain "errors" or "annotations" as described
   below.

      "valid" - a boolean value indicating the overall validation
      success or failure

      "errors" - the collection of errors or annotations produced by a
      failed validation

      "annotations" - the collection of errors or annotations produced
      by a successful validation

   For these examples, the following schema and instance will be used.

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   {
     "$id": "https://example.com/polygon",
     "$schema": "https://json-schema.org/draft/2019-09/schema",
     "$defs": {
       "point": {
         "type": "object",
         "properties": {
           "x": { "type": "number" },
           "y": { "type": "number" }
         },
         "additionalProperties": false,
         "required": [ "x", "y" ]
       }
     },
     "type": "array",
     "items": { "$ref": "#/$defs/point" },
     "minItems": 3
   }

   [
     {
       "x": 2.5,
       "y": 1.3,
     },
     {
       "x": 1,
       "z": 6.7
     }
   ]

   This instance will fail validation and produce errors, but it's
   trivial to deduce examples for passing schemas that produce
   annotations.

   Specifically, the errors it will produce are:

      The second element in the "vertices" property is missing a "y"
      property.

      The second element in the "vertices" property has a disallowed "z"
      property.

      There are only two vertices, but three are required.

   Note that the error message wording as depicted in these examples is
   not a requirement of this specification.  Implementations SHOULD
   craft error messages tailored for their audience or provide a

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   templating mechanism that allows their users to craft their own
   messages.

10.4.1.  Flag

   In the simplest case, merely the boolean result for the "valid" valid
   property needs to be fulfilled.

   {
     "valid": false
   }

   Because no errors or annotations are returned with this format, it is
   RECOMMENDED that implementations use short-circuiting logic to return
   failure or success as soon as the outcome can be determined.  For
   example, if an "anyOf" keyword contains five sub-schemas, and the
   second one passes, there is no need to check the other three.  The
   logic can simply return with success.

10.4.2.  Basic

   The "Basic" structure is a flat list of output units.

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{
  "valid": false,
  "errors": [
    {
      "keywordLocation": "#",
      "instanceLocation": "#",
      "error": "A subschema had errors."
    },
    {
      "keywordLocation": "#/items/$ref",
      "absoluteKeywordLocation":
        "https://example.com/polygon#/$defs/point",
      "instanceLocation": "#/1",
      "error": "A subschema had errors."
    },
    {
      "keywordLocation": "#/items/$ref/required",
      "absoluteKeywordLocation":
        "https://example.com/polygon#/$defs/point/required",
      "instanceLocation": "#/1",
      "error": "Required property 'y' not found."
    },
    {
      "keywordLocation": "#/items/$ref/additionalProperties",
      "absoluteKeywordLocation":
        "https://example.com/polygon#/$defs/point/additionalProperties",
      "instanceLocation": "#/1/z",
      "error": "Additional property 'z' found but was invalid."
    },
    {
      "keywordLocation": "#/minItems",
      "instanceLocation": "#",
      "error": "Expected at least 3 items but found 2"
    }
  ]
}

10.4.3.  Detailed

   The "Detailed" structure is based on the schema and can be more
   readable for both humans and machines.  Having the structure
   organized this way makes associations between the errors more
   apparent.  For example, the fact that the missing "y" property and
   the extra "z" property both stem from the same location in the
   instance is not immediately obvious in the "Basic" structure.  In a
   hierarchy, the correlation is more easily identified.

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   The following rules govern the construction of the results object:

      All applicator keywords ("*Of", "$ref", "if"/"then"/"else", etc.)
      require a node.

      Nodes that have no children are removed.

      Nodes that have a single child are replaced by the child.

   Branch nodes do not require an error message or an annotation.

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{
  "valid": false,
  "keywordLocation": "#",
  "instanceLocation": "#",
  "errors": [
    {
      "valid": false,
      "keywordLocation": "#/items/$ref",
      "absoluteKeywordLocation":
        "https://example.com/polygon#/$defs/point",
      "instanceLocation": "#/1",
      "errors": [
        {
          "valid": false,
          "keywordLocation": "#/items/$ref/required",
          "absoluteKeywordLocation":
            "https://example.com/polygon#/$defs/point/required",
          "instanceLocation": "#/1",
          "error": "Required property 'y' not found."
        },
        {
          "valid": false,
          "keywordLocation": "#/items/$ref/additionalProperties",
          "absoluteKeywordLocation":
            "https://example.com/polygon#/$defs/point/additionalProperties",
          "instanceLocation": "#/1/z",
          "error": "Additional property 'z' found but was invalid."
        }
      ]
    },
    {
      "valid": false,
      "keywordLocation": "#/minItems",
      "instanceLocation": "#",
      "error": "Expected at least 3 items but found 2"
    }
  ]
}

10.4.4.  Verbose

   The "Verbose" structure is a fully realized hierarchy that exactly
   matches that of the schema.  This structure has applications in form
   generation and validation where the error's location is important.

   The primary difference between this and the "Detailed" structure is
   that all results are returned.  This includes sub-schema validation

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   results that would otherwise be removed (e.g. annotations for failed
   validations, successful validations inside a `not` keyword, etc.).
   Because of this, it is RECOMMENDED that each node also carry a
   `valid` property to indicate the validation result for that node.

   Because this output structure can be quite large, a smaller example
   is given here for brevity.  The URI of the full output structure of
   the example above is: <https://json-schema.org/draft/2019-09/output/
   verbose-example>.

// schema
{
  "$id": "https://example.com/polygon",
  "$schema": "https://json-schema.org/draft/2019-09/schema",
  "type": "object",
  "properties": {
    "validProp": true,
  },
  "additionalProperties": false
}

// instance
{
  "validProp": 5,
  "disallowedProp": "value"
}

// result
{
  "valid": false,
  "keywordLocation": "#",
  "instanceLocation": "#",
  "errors": [
    {
      "valid": true,
      "keywordLocation": "#/type",
      "instanceLocation": "#"
    },
    {
      "valid": true,
      "keywordLocation": "#/properties",
      "instanceLocation": "#"
    },
    {
      "valid": false,
      "keywordLocation": "#/additionalProperties",
      "instanceLocation": "#",

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      "errors": [
        {
          "valid": false,
          "keywordLocation": "#/additionalProperties",
          "instanceLocation": "#/disallowedProp",
          "error": "Additional property 'disallowedProp' found but was invalid."
        }
      ]
    }
  ]
}

10.4.5.  Output validation schemas

   For convenience, JSON Schema has been provided to validate output
   generated by implementations.  Its URI is: <https://json-schema.org/
   draft/2019-09/output/schema>.

11.  Usage for Hypermedia

   JSON has been adopted widely by HTTP servers for automated APIs and
   robots.  This section describes how to enhance processing of JSON
   documents in a more RESTful manner when used with protocols that
   support media types and Web linking [RFC8288].

11.1.  Linking to a Schema

   It is RECOMMENDED that instances described by a schema provide a link
   to a downloadable JSON Schema using the link relation "describedby",
   as defined by Linked Data Protocol 1.0, section 8.1
   [W3C.REC-ldp-20150226].

   In HTTP, such links can be attached to any response using the Link
   header [RFC8288].  An example of such a header would be:

   Link: <https://example.com/my-hyper-schema#>; rel="describedby"

11.2.  Identifying a Schema via a Media Type Parameter

   Media types MAY allow for a "schema" media type parameter, which
   gives HTTP servers the ability to perform Content-Type Negotiation
   based on schema.  The media-type parameter MUST be a whitespace-
   separated list of URIs (i.e. relative references are invalid).

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   When using the media type application/schema-instance+json, the
   "schema" parameter MUST be supplied.

   When using the media type application/schema+json, the "schema"
   parameter MAY be supplied.  If supplied, it SHOULD contain the same
   URI as identified by the "$schema" keyword, and MAY contain
   additional URIs.  The "$schema" URI MUST be considered the schema's
   canonical meta-schema, regardless of the presence of alternative or
   additional meta-schemas as a media type parameter.

   The schema URI is opaque and SHOULD NOT automatically be
   dereferenced.  If the implementation does not understand the
   semantics of the provided schema, the implementation can instead
   follow the "describedby" links, if any, which may provide information
   on how to handle the schema.  Since "schema" doesn't necessarily
   point to a network location, the "describedby" relation is used for
   linking to a downloadable schema.  However, for simplicity, schema
   authors should make these URIs point to the same resource when
   possible.

   In HTTP, the media-type parameter would be sent inside the Content-
   Type header:

   Content-Type: application/json;
             schema="https://example.com/my-hyper-schema#"

   Multiple schemas are whitespace separated, and indicate that the
   instance conforms to all of the listed schemas:

   Content-Type: application/json;
             schema="https://example.com/alice https://example.com/bob"

   Media type parameters are also used in HTTP's Accept request header:

   Accept: application/json;
             schema="https://example.com/qiang https://example.com/li",
           application/json;
             schema="https://example.com/kumar"

   As with Content-Type, multiple schema parameters in the same string
   requests an instance that conforms to all of the listed schemas.

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   Unlike Content-Type, Accept can contain multiple values to indicate
   that the client can accept several media types.  In the above
   example, note that the two media types differ only by their schema
   parameter values.  This requests an application/json representation
   that conforms to at least one of the identified schemas.

   [[CREF14: This paragraph assumes that we can register a "schema" link
   relation.  Should we instead specify something like "tag:json-
   schema.org,2017:schema" for now? ]] HTTP can also send the "schema"
   in a Link, though this may impact media-type semantics and Content-
   Type negotiation if this replaces the media-type parameter entirely:

   Link: </alice>;rel="schema", </bob>;rel="schema"

11.3.  Usage Over HTTP

   When used for hypermedia systems over a network, HTTP [RFC7231] is
   frequently the protocol of choice for distributing schemas.
   Misbehaving clients can pose problems for server maintainers if they
   pull a schema over the network more frequently than necessary, when
   it's instead possible to cache a schema for a long period of time.

   HTTP servers SHOULD set long-lived caching headers on JSON Schemas.
   HTTP clients SHOULD observe caching headers and not re-request
   documents within their freshness period.  Distributed systems SHOULD
   make use of a shared cache and/or caching proxy.

   Clients SHOULD set or prepend a User-Agent header specific to the
   JSON Schema implementation or software product.  Since symbols are
   listed in decreasing order of significance, the JSON Schema library
   name/version should precede the more generic HTTP library name (if
   any).  For example:

   User-Agent: product-name/5.4.1 so-cool-json-schema/1.0.2 curl/7.43.0

   Clients SHOULD be able to make requests with a "From" header so that
   server operators can contact the owner of a potentially misbehaving
   script.

12.  Security Considerations

   Both schemas and instances are JSON values.  As such, all security
   considerations defined in RFC 8259 [RFC8259] apply.

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   Instances and schemas are both frequently written by untrusted third
   parties, to be deployed on public Internet servers.  Validators
   should take care that the parsing and validating against schemas
   doesn't consume excessive system resources.  Validators MUST NOT fall
   into an infinite loop.

   Servers MUST ensure that malicious parties can't change the
   functionality of existing schemas by uploading a schema with a pre-
   existing or very similar "$id".

   Individual JSON Schema vocabularies are liable to also have their own
   security considerations.  Consult the respective specifications for
   more information.

   Schema authors should take care with "$comment" contents, as a
   malicious implementation can display them to end-users in violation
   of a spec, or fail to strip them if such behavior is expected.

   A malicious schema author could place executable code or other
   dangerous material within a "$comment".  Implementations MUST NOT
   parse or otherwise take action based on "$comment" contents.

13.  IANA Considerations

13.1.  application/schema+json

   The proposed MIME media type for JSON Schema is defined as follows:

      Type name: application

      Subtype name: schema+json

      Required parameters: N/A

      Optional parameters:

      schema:  A non-empty list of space-separated URIs, each
         identifying a JSON Schema resource.  The instance SHOULD
         successfully validate against at least one of these meta-
         schemas.  Non-validating meta-schemas MAY be included for
         purposes such as allowing clients to make use of older versions
         of a meta-schema as long as the runtime instance validates
         against that older version.

      Encoding considerations: Encoding considerations are identical to
      those specified for the "application/json" media type.  See JSON
      [RFC8259].

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      Security considerations: See Section 12 above.

      Interoperability considerations: See Sections 6.2, 6.3, and 6.4
      above.

      Fragment identifier considerations: See Section 5

13.2.  application/schema-instance+json

   The proposed MIME media type for JSON Schema Instances that require a
   JSON Schema-specific media type is defined as follows:

      Type name: application

      Subtype name: schema-instance+json

      Required parameters:

      schema:  A non-empty list of space-separated URIs, each
         identifying a JSON Schema resource.  The instance SHOULD
         successfully validate against at least one of these schemas.
         Non-validating schemas MAY be included for purposes such as
         allowing clients to make use of older versions of a schema as
         long as the runtime instance validates against that older
         version.

      Encoding considerations: Encoding considerations are identical to
      those specified for the "application/json" media type.  See JSON
      [RFC8259].

      Security considerations: See Section 12 above.

      Interoperability considerations: See Sections 6.2, 6.3, and 6.4
      above.

      Fragment identifier considerations: See Section 5

14.  References

14.1.  Normative References

   [ecma262]  "ECMA 262 specification", <http://www.ecma-
              international.org/publications/files/ECMA-ST/
              Ecma-262.pdf>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6839]  Hansen, T. and A. Melnikov, "Additional Media Type
              Structured Syntax Suffixes", RFC 6839,
              DOI 10.17487/RFC6839, January 2013,
              <https://www.rfc-editor.org/info/rfc6839>.

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/info/rfc6901>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [W3C.REC-ldp-20150226]
              Speicher, S., Arwe, J., and A. Malhotra, "Linked Data
              Platform 1.0", World Wide Web Consortium Recommendation
              REC-ldp-20150226, February 2015,
              <http://www.w3.org/TR/2015/REC-ldp-20150226>.

14.2.  Informative References

   [json-hyper-schema]
              Andrews, H. and A. Wright, "JSON Hyper-Schema: A
              Vocabulary for Hypermedia Annotation of JSON", draft-
              handrews-json-schema-hyperschema-02 (work in progress),
              November 2017.

   [json-schema-validation]
              Wright, A., Andrews, H., and G. Luff, "JSON Schema
              Validation: A Vocabulary for Structural Validation of
              JSON", draft-handrews-json-schema-validation-02 (work in
              progress), November 2017.

   [RFC6596]  Ohye, M. and J. Kupke, "The Canonical Link Relation",
              RFC 6596, DOI 10.17487/RFC6596, April 2012,
              <https://www.rfc-editor.org/info/rfc6596>.

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   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [W3C.WD-fragid-best-practices-20121025]
              Tennison, J., "Best Practices for Fragment Identifiers and
              Media Type Definitions", World Wide Web Consortium WD WD-
              fragid-best-practices-20121025, October 2012,
              <http://www.w3.org/TR/2012/
              WD-fragid-best-practices-20121025>.

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Appendix A.  Schema identification examples

   Consider the following schema, which shows "$id" being used to
   identify both the root schema and various subschemas, and "$anchor"
   being used to define plain name fragment identifiers.

   {
       "$id": "https://example.com/root.json",
       "$defs": {
           "A": { "$anchor": "foo" },
           "B": {
               "$id": "other.json",
               "$defs": {
                   "X": { "$anchor": "bar" },
                   "Y": {
                       "$id": "t/inner.json",
                       "$anchor": "bar"
                   }
               }
           },
           "C": {
               "$id": "urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f"
           }
       }
   }

   The schemas at the following URI-encoded JSON Pointers [RFC6901]
   (relative to the root schema) have the following base URIs, and are
   identifiable by any listed URI in accordance with sections 5 and
   8.2.2.2 above.

   # (document root)

      canonical absolute-URI (and also base URI)  https://example.com/
         root.json

      canonical URI with pointer fragment  https://example.com/
         root.json#

   #/$defs/A

      base URI  https://example.com/root.json

      canonical URI with plain fragment  https://example.com/
         root.json#foo

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      canonical URI with pointer fragment
      https://example.com/root.json#/$defs/A

   #/$defs/B

      base URI  https://example.com/other.json

      canonical URI with pointer fragment  https://example.com/
         other.json#

      non-canonical URI with fragment relative to root.json
      https://example.com/root.json#/$defs/B

   #/$defs/B/$defs/X

      base URI  https://example.com/other.json

      canonical URI with plain fragment  https://example.com/
         other.json#bar

      canonical URI with pointer fragment
      https://example.com/other.json#/$defs/X

   non-canonical URI with fragment relative to root.json
      https://example.com/root.json#/$defs/B/$defs/X

   #/$defs/B/$defs/Y

      base URI  https://example.com/t/inner.json

      canonical URI with plain fragment  https://example.com/t/
         inner.json#bar

      canonical URI with pointer fragment  https://example.com/t/
         inner.json#

      non-canonical URI with fragment relative to other.json
      https://example.com/other.json#/$defs/Y

   non-canonical URI with fragment relative to root.json
      https://example.com/root.json#/$defs/B/$defs/Y

   #/$defs/C

      base URI  urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f

      canonical URI with pointer fragment  urn:uuid:ee564b8a-
         7a87-4125-8c96-e9f123d6766f#

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      non-canonical URI with fragment relative to root.json
      https://example.com/root.json#/$defs/C

Appendix B.  Manipulating schema documents and references

   Various tools have been created to rearrange schema documents based
   on how and where references ("$ref") appear.  This appendix discusses
   which use cases and actions are compliant with this specification.

B.1.  Bundling schema resources into a single document

   A set of schema resources intended for use together can be organized
   with each in its own schema document, all in the same schema
   document, or any granularity of document grouping in between.

   Numerous tools exist to perform various sorts of reference removal.
   A common case of this is producing a single file where all references
   can be resolved within that file.  This is typically done to simplify
   distribution, or to simplify coding so that various invocations of
   JSON Schema libraries do not have to keep track of and load a large
   number of resources.

   This transformation can be safely and reversibly done as long as all
   static references (e.g. "$ref") use URI-references that resolve to
   canonical URIs, and all schema resources have an absolute-URI as the
   "$id" in their root schema.

   With these conditions met, each external resource can be copied under
   "$defs", without breaking any references among the resources' schema
   objects, and without changing any aspect of validation or annotation
   results.  The names of the schemas under "$defs" do not affect
   behavior, assuming they are each unique, as they do not appear in
   canonical URIs for the embedded resources.

B.2.  Reference removal is not always safe

   Attempting to remove all references and produce a single schema
   document does not, in all cases, produce a schema with identical
   behavior to the original form.

   Since "$ref" is now treated like any other keyword, with other
   keywords allowed in the same schema objects, fully supporting non-
   recursive "$ref" removal in all cases can require relatively complex
   schema manipulations.  It is beyond the scope of this specification
   to determine or provide a set of safe "$ref" removal transformations,
   as they depend not only on the schema structure but also on the
   intended usage.

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Appendix C.  Example of recursive schema extension

   Consider the following two schemas describing a simple recursive tree
   structure, where each node in the tree can have a "data" field of any
   type.  The first schema allows and ignores other instance properties.
   The second is more strict and only allows the "data" and "children"
   properties.  An example instance with "data" misspelled as "daat" is
   also shown.

   // tree schema, extensible
   {
       "$schema": "https://json-schema.org/draft/2019-09/schema",
       "$id": "https://example.com/tree",
       "$recursiveAnchor": true,

       "type": "object",
       "properties": {
           "data": true,
           "children": {
               "type": "array",
               "items": {
                   "$recursiveRef": "#"
               }
           }
       }
   }

   // strict-tree schema, guards against misspelled properties
   {
       "$schema": "https://json-schema.org/draft/2019-09/schema",
       "$id": "https://example.com/strict-tree",
       "$recursiveAnchor": true,

       "$ref": "tree",
       "unevaluatedProperties": false
   }

   // instance with misspelled field
   {
       "children": [ { "daat": 1 } ]
   }

   If we apply the "strict-tree" schema to the instance, we will follow
   the "$ref" to the "tree" schema, examine its "children" subschema,
   and find the "$recursiveAnchor" in its "items" subschema.  At this

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   point, the dynamic path is "#/$ref/properties/children/
   items/$recursiveRef".

   The base URI at this point is "https://example.com/tree", so the
   "$recursiveRef" initially resolves to "https://example.com/tree#".
   Since "$recursiveAnchor" is true, we examine the dynamic path to see
   if there is a different base URI to use.  We find "$recursiveAnchor"
   with a true value at the dynamic paths of "#" and "#/$ref".

   The outermost is "#", which is the root schema of the "strict-tree"
   schema, so we use its base URI of "https://example.com/strict-tree",
   which produces a final resolved URI of "https://example.com/strict-
   tree#" for the "$recursiveRef".

   This way, the recursion in the "tree" schema recurses to the root of
   "strict-tree", instead of only applying "strict-tree" to the instance
   root, but applying "tree" to instance children.

Appendix D.  Working with vocabularies

D.1.  Best practices for vocabulary and meta-schema authors

   Vocabulary authors should take care to avoid keyword name collisions
   if the vocabulary is intended for broad use, and potentially combined
   with other vocabularies.  JSON Schema does not provide any formal
   namespacing system, but also does not constrain keyword names,
   allowing for any number of namespacing approaches.

   Vocabularies may build on each other, such as by defining the
   behavior of their keywords with respect to the behavior of keywords
   from another vocabulary, or by using a keyword from another
   vocabulary with a restricted or expanded set of acceptable values.
   Not all such vocabulary re-use will result in a new vocabulary that
   is compatible with the vocabulary on which it is built.  Vocabulary
   authors should clearly document what level of compatibility, if any,
   is expected.

   Meta-schema authors should not use "$vocabulary" to combine multiple
   vocabularies that define conflicting syntax or semantics for the same
   keyword.  As semantic conflicts are not generally detectable through
   schema validation, implementations are not expected to detect such
   conflicts.  If conflicting vocabularies are declared, the resulting
   behavior is undefined.

   Vocabulary authors should provide a meta-schema that validates the
   expected usage of the vocabulary's keywords on their own.  Such meta-
   schemas should not forbid additional keywords, and must not forbid
   any keywords from the Core vocabulary.

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   It is recommended that meta-schema authors reference each
   vocabulary's meta-schema using the "allOf" (Section 9.2.1.1) keyword,
   although other mechanisms for constructing the meta-schema may be
   appropriate for certain use cases.

   The recursive nature of meta-schemas makes the "$recursiveAnchor" and
   "$recursiveRef" keywords particularly useful for extending existing
   meta-schemas, as can be seen in the JSON Hyper-Schema meta-schema
   which extends the Validation meta-schema.

   Meta-schemas may impose additional constraints, including describing
   keywords not present in any vocabulary, beyond what the meta-schemas
   associated with the declared vocabularies describe.  This allows for
   restricting usage to a subset of a vocabulary, and for validating
   locally defined keywords not intended for re-use.

   However, meta-schemas should not contradict any vocabularies that
   they declare, such as by requiring a different JSON type than the
   vocabulary expects.  The resulting behavior is undefined.

   Meta-schemas intended for local use, with no need to test for
   vocabulary support in arbitrary implementations, can safely omit
   "$vocabulary" entirely.

D.2.  Example meta-schema with vocabulary declarations

   This meta-schema explicitly declares both the Core and Applicator
   vocabularies, together with an extension vocabulary, and combines
   their meta-schemas with an "allOf".  The extension vocabulary's meta-
   schema, which describes only the keywords in that vocabulary, is
   shown after the main example meta-schema.

   The main example meta-schema also restricts the usage of the
   Applicator vocabulary by forbidding the keywords prefixed with
   "unevaluated", which are particularly complex to implement.  This
   does not change the semantics or set of keywords defined by the
   Applicator vocabulary.  It just ensures that schemas using this meta-
   schema that attempt to use the keywords prefixed with "unevaluted"
   will fail validation against this meta-schema.

   Finally, this meta-schema describes the syntax of a keyword,
   "localKeyword", that is not part of any vocabulary.  Presumably, the
   implementors and users of this meta-schema will understand the
   semantics of "localKeyword".  JSON Schema does not define any
   mechanism for expressing keyword semantics outside of vocabularies,
   making them unsuitable for use except in a specific environment in
   which they are understood.

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   This meta-schema combines several vocabularies for general use.

  {
    "$schema": "https://json-schema.org/draft/2019-09/schema",
    "$id": "https://example.com/meta/general-use-example",
    "$recursiveAnchor": true,
    "$vocabulary": {
      "https://json-schema.org/draft/2019-09/vocab/core": true,
      "https://json-schema.org/draft/2019-09/vocab/applicator": true,
      "https://json-schema.org/draft/2019-09/vocab/validation": true,
      "https://example.com/vocab/example-vocab": true
    },
    "allOf": [
      {"$ref": "https://json-schema.org/draft/2019-09/meta/core"},
      {"$ref": "https://json-schema.org/draft/2019-09/meta/applicator"},
      {"$ref": "https://json-schema.org/draft/2019-09/meta/validation"},
      {"$ref": "https://example.com/meta/example-vocab",
    ],
    "patternProperties": {
      "^unevaluated.*$": false
    },
    "properties": {
      "localKeyword": {
        "$comment": "Not in vocabulary, but validated if used",
        "type": "string"
      }
    }
  }

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   This meta-schema describes only a single extension vocabulary.

   {
     "$schema": "https://json-schema.org/draft/2019-09/schema",
     "$id": "https://example.com/meta/example-vocab",
     "$recursiveAnchor": true,
     "$vocabulary": {
       "https://example.com/vocab/example-vocab": true,
     },
     "type": ["object", "boolean"],
     "properties": {
       "minDate": {
         "type": "string",
         "pattern": "\d\d\d\d-\d\d-\d\d",
         "format": "date",
       }
     }
   }

   As shown above, even though each of the single-vocabulary meta-
   schemas referenced in the general-use meta-schema's "allOf" declares
   its corresponding vocabulary, this new meta-schema must re-declare
   them.

   The standard meta-schemas that combine all vocabularies defined by
   the Core and Validation specification, and that combine all
   vocabularies defined by those specifications as well as the Hyper-
   Schema specification, demonstrate additional complex combinations.
   These URIs for these meta-schemas may be found in the Validation and
   Hyper-Schema specifications, respectively.

   While the general-use meta-schema can validate the syntax of
   "minDate", it is the vocabulary that defines the logic behind the
   semantic meaning of "minDate".  Without an understanding of the
   semantics (in this example, that the instance value must be a date
   equal to or after the date provided as the keyword's value in the
   schema), an implementation can only validate the syntactic usage.  In
   this case, that means validating that it is a date-formatted string
   (using "pattern" to ensure that it is validated even when "format"
   functions purely as an annotation, as explained in the Validation
   specification [json-schema-validation].

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Appendix E.  References and generative use cases

   While the presence of references is expected to be transparent to
   validation results, generative use cases such as code generators and
   UI renderers often consider references to be semantically
   significant.

   To make such use case-specific semantics explicit, the best practice
   is to create an annotation keyword for use in the same schema object
   alongside of a reference keyword such as "$ref".

   For example, here is a hypothetical keyword for determining whether a
   code generator should consider the reference target to be a distinct
   class, and how those classes are related.  Note that this example is
   solely for illustrative purposes, and is not intended to propose a
   functional code generation keyword.

   {
       "allOf": [
           {
               "classRelation": "is-a",
               "$ref": "classes/base.json"
           },
           {
               "$ref": "fields/common.json"
           }
       ],
       "properties": {
           "foo": {
               "classRelation": "has-a",
               "$ref": "classes/foo.json"
           },
           "date": {
               "$ref": "types/dateStruct.json",
           }
       }
   }

   Here, this schema represents some sort of object-oriented class.  The
   first reference in the "allOf" is noted as the base class.  The
   second is not assigned a class relationship, meaning that the code
   generator should combine the target's definition with this one as if
   no reference were involved.

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   Looking at the properties, "foo" is flagged as object composition,
   while the "date" property is not.  It is simply a field with sub-
   fields, rather than an instance of a distinct class.

   This style of usage requires the annotation to be in the same object
   as the reference, which must be recognizable as a reference.

Appendix F.  Acknowledgments

   Thanks to Gary Court, Francis Galiegue, Kris Zyp, and Geraint Luff
   for their work on the initial drafts of JSON Schema.

   Thanks to Jason Desrosiers, Daniel Perrett, Erik Wilde, Ben Hutton,
   Evgeny Poberezkin, Brad Bowman, Gowry Sankar, Donald Pipowitch, and
   Dave Finlay for their submissions and patches to the document.

Appendix G.  ChangeLog

   [[CREF15: This section to be removed before leaving Internet-Draft
   status.]]

   draft-handrews-json-schema-02

      *  Update to RFC 8259 for JSON specification

      *  Moved "definitions" from the Validation specification here as
         "$defs"

      *  Moved applicator keywords from the Validation specification as
         their own vocabulary

      *  Moved the schema form of "dependencies" from the Validation
         specification as "dependentSchemas"

      *  Formalized annotation collection

      *  Specified recommended output formats

      *  Defined keyword interactions in terms of annotation and
         assertion results

      *  Added "unevaluatedProperties" and "unevaluatedItems"

      *  Define "$ref" behavior in terms of the assertion, applicator,
         and annotation model

      *  Allow keywords adjacent to "$ref"

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      *  Note undefined behavior for "$ref" targets involving unknown
         keywords

      *  Add recursive referencing, primarily for meta-schema extension

      *  Add the concept of formal vocabularies, and how they can be
         recognized through meta-schemas

      *  Additional guidance on initial base URIs beyond network
         retrieval

      *  Allow "schema" media type parameter for "application/
         schema+json"

      *  Better explanation of media type parameters and the HTTP Accept
         header

      *  Use "$id" to establish canonical and base absolute-URIs only,
         no fragments

      *  Replace plain-name-fragment-only form of "$id" with "$anchor"

      *  Clarified that the behavior of JSON Pointers across "$id"
         boundary is unreliable

   draft-handrews-json-schema-01

      *  This draft is purely a clarification with no functional changes

      *  Emphasized annotations as a primary usage of JSON Schema

      *  Clarified $id by use cases

      *  Exhaustive schema identification examples

      *  Replaced "external referencing" with how and when an
         implementation might know of a schema from another document

      *  Replaced "internal referencing" with how an implementation
         should recognized schema identifiers during parsing

      *  Dereferencing the former "internal" or "external" references is
         always the same process

      *  Minor formatting improvements

   draft-handrews-json-schema-00

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      *  Make the concept of a schema keyword vocabulary more clear

      *  Note that the concept of "integer" is from a vocabulary, not
         the data model

      *  Classify keywords as assertions or annotations and describe
         their general behavior

      *  Explain the boolean schemas in terms of generalized assertions

      *  Reserve "$comment" for non-user-visible notes about the schema

      *  Wording improvements around "$id" and fragments

      *  Note the challenges of extending meta-schemas with recursive
         references

      *  Add "application/schema-instance+json" media type

      *  Recommend a "schema" link relation / parameter instead of
         "profile"

   draft-wright-json-schema-01

      *  Updated intro

      *  Allowed for any schema to be a boolean

      *  "$schema" SHOULD NOT appear in subschemas, although that may
         change

      *  Changed "id" to "$id"; all core keywords prefixed with "$"

      *  Clarify and formalize fragments for application/schema+json

      *  Note applicability to formats such as CBOR that can be
         represented in the JSON data model

   draft-wright-json-schema-00

      *  Updated references to JSON

      *  Updated references to HTTP

      *  Updated references to JSON Pointer

      *  Behavior for "id" is now specified in terms of RFC3986

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      *  Aligned vocabulary usage for URIs with RFC3986

      *  Removed reference to draft-pbryan-zyp-json-ref-03

      *  Limited use of "$ref" to wherever a schema is expected

      *  Added definition of the "JSON Schema data model"

      *  Added additional security considerations

      *  Defined use of subschema identifiers for "id"

      *  Rewrote section on usage with HTTP

      *  Rewrote section on usage with rel="describedBy" and
         rel="profile"

      *  Fixed numerous invalid examples

   draft-zyp-json-schema-04

      *  Salvaged from draft v3.

      *  Split validation keywords into separate document.

      *  Split hypermedia keywords into separate document.

      *  Initial post-split draft.

      *  Mandate the use of JSON Reference, JSON Pointer.

      *  Define the role of "id".  Define URI resolution scope.

      *  Add interoperability considerations.

   draft-zyp-json-schema-00

      *  Initial draft.

Authors' Addresses

   Austin Wright (editor)

   EMail: aaa@bzfx.net

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   Henry Andrews (editor)

   EMail: andrews_henry@yahoo.com

   Ben Hutton (editor)
   Wellcome Sanger Institute

   EMail: bh7@sanger.ac.uk
   URI:   https://jsonschema.dev

   Greg Dennis
   Auckland
   NZ

   EMail: gregsdennis@yahoo.com

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