JSON Schema Validation: A Vocabulary for Structural Validation of JSON
draft-handrews-json-schema-validation-00

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Internet Engineering Task Force                           A. Wright, Ed.
Internet-Draft
Intended status: Informational                           H. Andrews, Ed.
Expires: May 23, 2018                                   Cloudflare, Inc.
                                                                 G. Luff
                                                       November 19, 2017

 JSON Schema Validation: A Vocabulary for Structural Validation of JSON
                draft-handrews-json-schema-validation-00

Abstract

   JSON Schema (application/schema+json) has several purposes, one of
   which is JSON instance validation.  This document specifies a
   vocabulary for JSON Schema to describe the meaning of JSON documents,
   provide hints for user interfaces working with JSON data, and to make
   assertions about what a valid document must look like.

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 <http://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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 23, 2018.

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Copyright Notice

   Copyright (c) 2017 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
     3.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Keyword Independence  . . . . . . . . . . . . . . . .   5
     3.2.  Assertions  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Assertions and Instance Primitive Types . . . . . . .   6
     3.3.  Annotations . . . . . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  Negated Schemas . . . . . . . . . . . . . . . . . . .   7
       3.3.2.  Annotations and Short-Circuit Validation  . . . . . .   7
   4.  Interoperability Considerations . . . . . . . . . . . . . . .   7
     4.1.  Validation of String Instances  . . . . . . . . . . . . .   7
     4.2.  Validation of Numeric Instances . . . . . . . . . . . . .   7
     4.3.  Regular Expressions . . . . . . . . . . . . . . . . . . .   7
   5.  Meta-Schema . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Validation Keywords . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Validation Keywords for Any Instance Type . . . . . . . .   8
       6.1.1.  type  . . . . . . . . . . . . . . . . . . . . . . . .   8
       6.1.2.  enum  . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.1.3.  const . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Validation Keywords for Numeric Instances (number and
           integer)  . . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.1.  multipleOf  . . . . . . . . . . . . . . . . . . . . .   9
       6.2.2.  maximum . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.3.  exclusiveMaximum  . . . . . . . . . . . . . . . . . .   9
       6.2.4.  minimum . . . . . . . . . . . . . . . . . . . . . . .   9
       6.2.5.  exclusiveMinimum  . . . . . . . . . . . . . . . . . .  10
     6.3.  Validation Keywords for Strings . . . . . . . . . . . . .  10
       6.3.1.  maxLength . . . . . . . . . . . . . . . . . . . . . .  10
       6.3.2.  minLength . . . . . . . . . . . . . . . . . . . . . .  10
       6.3.3.  pattern . . . . . . . . . . . . . . . . . . . . . . .  10

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     6.4.  Validation Keywords for Arrays  . . . . . . . . . . . . .  11
       6.4.1.  items . . . . . . . . . . . . . . . . . . . . . . . .  11
       6.4.2.  additionalItems . . . . . . . . . . . . . . . . . . .  11
       6.4.3.  maxItems  . . . . . . . . . . . . . . . . . . . . . .  11
       6.4.4.  minItems  . . . . . . . . . . . . . . . . . . . . . .  11
       6.4.5.  uniqueItems . . . . . . . . . . . . . . . . . . . . .  12
       6.4.6.  contains  . . . . . . . . . . . . . . . . . . . . . .  12
     6.5.  Validation Keywords for Objects . . . . . . . . . . . . .  12
       6.5.1.  maxProperties . . . . . . . . . . . . . . . . . . . .  12
       6.5.2.  minProperties . . . . . . . . . . . . . . . . . . . .  12
       6.5.3.  required  . . . . . . . . . . . . . . . . . . . . . .  12
       6.5.4.  properties  . . . . . . . . . . . . . . . . . . . . .  13
       6.5.5.  patternProperties . . . . . . . . . . . . . . . . . .  13
       6.5.6.  additionalProperties  . . . . . . . . . . . . . . . .  13
       6.5.7.  dependencies  . . . . . . . . . . . . . . . . . . . .  14
       6.5.8.  propertyNames . . . . . . . . . . . . . . . . . . . .  14
     6.6.  Keywords for Applying Subschemas Conditionally  . . . . .  15
       6.6.1.  if  . . . . . . . . . . . . . . . . . . . . . . . . .  15
       6.6.2.  then  . . . . . . . . . . . . . . . . . . . . . . . .  15
       6.6.3.  else  . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.7.  Keywords for Applying Subschemas With Boolean Logic . . .  16
       6.7.1.  allOf . . . . . . . . . . . . . . . . . . . . . . . .  16
       6.7.2.  anyOf . . . . . . . . . . . . . . . . . . . . . . . .  16
       6.7.3.  oneOf . . . . . . . . . . . . . . . . . . . . . . . .  16
       6.7.4.  not . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Semantic Validation With "format" . . . . . . . . . . . . . .  16
     7.1.  Foreword  . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.2.  Implementation Requirements . . . . . . . . . . . . . . .  17
     7.3.  Defined Formats . . . . . . . . . . . . . . . . . . . . .  17
       7.3.1.  Dates and Times . . . . . . . . . . . . . . . . . . .  17
       7.3.2.  Email Addresses . . . . . . . . . . . . . . . . . . .  18
       7.3.3.  Hostnames . . . . . . . . . . . . . . . . . . . . . .  18
       7.3.4.  IP Addresses  . . . . . . . . . . . . . . . . . . . .  19
       7.3.5.  Resource Identifiers  . . . . . . . . . . . . . . . .  19
       7.3.6.  uri-template  . . . . . . . . . . . . . . . . . . . .  19
       7.3.7.  JSON Pointers . . . . . . . . . . . . . . . . . . . .  19
       7.3.8.  regex . . . . . . . . . . . . . . . . . . . . . . . .  20
   8.  String-Encoding Non-JSON Data . . . . . . . . . . . . . . . .  20
     8.1.  Foreword  . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  Implementation Requirements . . . . . . . . . . . . . . .  20
     8.3.  contentEncoding . . . . . . . . . . . . . . . . . . . . .  21
     8.4.  contentMediaType  . . . . . . . . . . . . . . . . . . . .  21
     8.5.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Schema Re-Use With "definitions"  . . . . . . . . . . . . . .  22
   10. Schema Annotations  . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  "title" and "description"  . . . . . . . . . . . . . . .  23
     10.2.  "default"  . . . . . . . . . . . . . . . . . . . . . . .  23
     10.3.  "readOnly" and "writeOnly" . . . . . . . . . . . . . . .  23

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     10.4.  "examples" . . . . . . . . . . . . . . . . . . . . . . .  24
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  24
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  28
   Appendix B.  ChangeLog  . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   JSON Schema can be used to require that a given JSON document (an
   instance) satisfies a certain number of criteria.  These criteria are
   asserted by using keywords described in this specification.  In
   addition, a set of keywords is also defined to assist in interactive
   user interface instance generation.

   This specification will use the concepts, syntax, and terminology
   defined by the JSON Schema core [json-schema] specification.

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].

   This specification uses the term "container instance" to refer to
   both array and object instances.  It uses the term "children
   instances" to refer to array elements or object member values.

   Elements in an array value are said to be unique if no two elements
   of this array are equal [json-schema].

3.  Overview

   JSON Schema validation applies schemas to locations within the
   instance, and asserts constraints on the structure of the data at
   each location.  An instance location that satisfies all asserted
   constraints is then annotated with any keywords that contain non-
   assertion information, such as descriptive metadata and usage hints.
   If all locations within the instance satisfy all asserted
   constraints, then the instance is said to be valid against the
   schema.

   Each schema object is independently evaluated against each instance
   location to which it applies.  This greatly simplifies the
   implementation requirements for validators by ensuring that they do

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   not need to maintain state across the document-wide validation
   process.

3.1.  Applicability

   Validation begins by applying the root schema to the complete
   instance document.  From there, various keywords are used to
   determine which additional subschemas are applied to either the
   current location, or a child location.  These keywords also define
   whether and how subschema assertion results are modified and/or
   combined.  Such keywords do not assert conditions on their own.
   Rather, they control how assertions are applied and evaluated.

   The keywords in the boolean logic (Section 6.7) and conditional
   (Section 6.6) sections of this specification apply subschemas to the
   same location as the parent schema.  The former group defines boolean
   operations on the subschema assertion results, while the latter
   evaluates one subschema and uses its assertion results to determine
   which of two other subschemas to apply as well.

   Several keywords determine which subschemas are applied to array
   items, object property values, and object property names.  They are:
   "items", "additionalItems", "contains", "properties",
   "patternProperties", "additionalProperties", and "propertyNames".
   The "contains" keyword only requires its subschema to be valid
   against at least one child instance, while the other keywords require
   that all subschemas are valid against all child instances to which
   they apply.

3.1.1.  Keyword Independence

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

   For schema author convenience, there are some exceptions among the
   keywords that control subschema applicability:

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

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

3.2.  Assertions

   Validation is a process of checking assertions.  Each assertion adds
   constraints that an instance must satisfy in order to successfully
   validate.

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   Assertion keywords that are absent never restrict validation.  In
   some cases, this no-op behavior is identical to a keyword that exists
   with certain values, and these values are noted where known.

   All of the keywords in the general (Section 6.1), numeric
   (Section 6.2), and string (Section 6.3) sections are assertions, as
   well as "minItems", "maxItems", "uniqueItems", "minProperties",
   "maxProperties", and "required".  Additionally, "dependencies" is
   shorthand for a combination of conditional and assertion keywords.

   The "format", "contentType", and "contentEncoding" keywords can also
   be implemented as assertions, although that functionality is an
   optional part of this specification, and the keywords convey
   additional non-assertion information.

3.2.1.  Assertions and Instance Primitive Types

   Most validation 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 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.

3.3.  Annotations

   In addition to assertions, this specification provides a small
   vocabulary of metadata keywords that can be used to annotate the JSON
   instance with useful information.  The Section 7 and Section 8
   keywords are also useful as annotations as well as being optional
   assertions, as they convey additional usage guidance for the instance
   data.

   A schema that is applicable to a particular location in the instance,
   against which the instance location is valid, attaches its
   annotations to that location in the instance.  Since many subschemas
   can be applicable to any single location, annotation keywords need to
   specify any unusual handling of multiple applicable occurrences of
   the keyword with different values.  The default behavior is simply to
   collect all values.

   Additional vocabularies SHOULD make use of this mechanism for
   applying their own annotations to instances.

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3.3.1.  Negated Schemas

   Annotations in a subschema contained within a "not", at any depth,
   including any number of intervening additional "not" subschemas, MUST
   be ignored.  Similarly, annotations within a failing branch of a
   "oneOf", "anyOf", "then", or "else" MUST be ignored.

3.3.2.  Annotations and Short-Circuit Validation

   Annotation keywords MUST be applied to all possible sub-instances.
   Even if such application can be short-circuited when only assertion
   evaluation is needed.  For instance, the "contains" keyword need only
   be checked for assertions until at least one array item proves valid.
   However, when working with annotations, all items in the array must
   be evaluated to determine all items with which the annotations should
   be associated.

4.  Interoperability Considerations

4.1.  Validation of String Instances

   It should be noted that the nul character (\u0000) is valid in a JSON
   string.  An instance to validate may contain a string value with this
   character, regardless of the ability of the underlying programming
   language to deal with such data.

4.2.  Validation of Numeric Instances

   The JSON specification allows numbers with arbitrary precision, and
   JSON Schema does not add any such bounds.  This means that numeric
   instances processed by JSON Schema can be arbitrarily large and/or
   have an arbitrarily long decimal part, regardless of the ability of
   the underlying programming language to deal with such data.

4.3.  Regular Expressions

   Two validation keywords, "pattern" and "patternProperties", use
   regular expressions to express constraints, and the "regex" value for
   the "format" keyword constrains the instance value to be a regular
   expression.  These regular expressions SHOULD be valid according to
   the ECMA 262 [ecma262] regular expression dialect.

   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 [RFC7159];

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      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".

5.  Meta-Schema

   The current URI for the JSON Schema Validation is <http://json-
   schema.org/draft-07/schema#>.

6.  Validation Keywords

   Validation keywords in a schema impose requirements for successful
   validation of an instance.

6.1.  Validation Keywords for Any Instance Type

6.1.1.  type

   The value of this keyword MUST be either a string or an array.  If it
   is an array, elements of the array MUST be strings and MUST be
   unique.

   String values MUST be one of the six primitive types ("null",
   "boolean", "object", "array", "number", or "string"), or "integer"
   which matches any number with a zero fractional part.

   An instance validates if and only if the instance is in any of the
   sets listed for this keyword.

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6.1.2.  enum

   The value of this keyword MUST be an array.  This array SHOULD have
   at least one element.  Elements in the array SHOULD be unique.

   An instance validates successfully against this keyword if its value
   is equal to one of the elements in this keyword's array value.

   Elements in the array might be of any value, including null.

6.1.3.  const

   The value of this keyword MAY be of any type, including null.

   An instance validates successfully against this keyword if its value
   is equal to the value of the keyword.

6.2.  Validation Keywords for Numeric Instances (number and integer)

6.2.1.  multipleOf

   The value of "multipleOf" MUST be a number, strictly greater than 0.

   A numeric instance is valid only if division by this keyword's value
   results in an integer.

6.2.2.  maximum

   The value of "maximum" MUST be a number, representing an inclusive
   upper limit for a numeric instance.

   If the instance is a number, then this keyword validates only if the
   instance is less than or exactly equal to "maximum".

6.2.3.  exclusiveMaximum

   The value of "exclusiveMaximum" MUST be number, representing an
   exclusive upper limit for a numeric instance.

   If the instance is a number, then the instance is valid only if it
   has a value strictly less than (not equal to) "exclusiveMaximum".

6.2.4.  minimum

   The value of "minimum" MUST be a number, representing an inclusive
   lower limit for a numeric instance.

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   If the instance is a number, then this keyword validates only if the
   instance is greater than or exactly equal to "minimum".

6.2.5.  exclusiveMinimum

   The value of "exclusiveMinimum" MUST be number, representing an
   exclusive lower limit for a numeric instance.

   If the instance is a number, then the instance is valid only if it
   has a value strictly greater than (not equal to) "exclusiveMinimum".

6.3.  Validation Keywords for Strings

6.3.1.  maxLength

   The value of this keyword MUST be a non-negative integer.

   A string instance is valid against this keyword if its length is less
   than, or equal to, the value of this keyword.

   The length of a string instance is defined as the number of its
   characters as defined by RFC 7159 [RFC7159].

6.3.2.  minLength

   The value of this keyword MUST be a non-negative integer.

   A string instance is valid against this keyword if its length is
   greater than, or equal to, the value of this keyword.

   The length of a string instance is defined as the number of its
   characters as defined by RFC 7159 [RFC7159].

   Omitting this keyword has the same behavior as a value of 0.

6.3.3.  pattern

   The value of this keyword MUST be a string.  This string SHOULD be a
   valid regular expression, according to the ECMA 262 regular
   expression dialect.

   A string instance is considered valid if the regular expression
   matches the instance successfully.  Recall: regular expressions are
   not implicitly anchored.

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6.4.  Validation Keywords for Arrays

6.4.1.  items

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

   This keyword determines how child instances validate for arrays, and
   does not directly validate the immediate instance itself.

   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.

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

6.4.2.  additionalItems

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

   This keyword determines how child instances validate for arrays, and
   does not directly validate the immediate instance itself.

   If "items" is an array of schemas, validation succeeds if every
   instance element at a position greater than the size of "items"
   validates against "additionalItems".

   Otherwise, "additionalItems" MUST be ignored, as the "items" schema
   (possibly the default value of an empty schema) is applied to all
   elements.

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

6.4.3.  maxItems

   The value of this keyword MUST be a non-negative integer.

   An array instance is valid against "maxItems" if its size is less
   than, or equal to, the value of this keyword.

6.4.4.  minItems

   The value of this keyword MUST be a non-negative integer.

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   An array instance is valid against "minItems" if its size is greater
   than, or equal to, the value of this keyword.

   Omitting this keyword has the same behavior as a value of 0.

6.4.5.  uniqueItems

   The value of this keyword MUST be a boolean.

   If this keyword has boolean value false, the instance validates
   successfully.  If it has boolean value true, the instance validates
   successfully if all of its elements are unique.

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

6.4.6.  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.

6.5.  Validation Keywords for Objects

6.5.1.  maxProperties

   The value of this keyword MUST be a non-negative integer.

   An object instance is valid against "maxProperties" if its number of
   properties is less than, or equal to, the value of this keyword.

6.5.2.  minProperties

   The value of this keyword MUST be a non-negative integer.

   An object instance is valid against "minProperties" if its number of
   properties is greater than, or equal to, the value of this keyword.

   Omitting this keyword has the same behavior as a value of 0.

6.5.3.  required

   The value of this keyword MUST be an array.  Elements of this array,
   if any, MUST be strings, and MUST be unique.

   An object instance is valid against this keyword if every item in the
   array is the name of a property in the instance.

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

6.5.4.  properties

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

   This keyword determines how child instances validate for objects, and
   does not directly validate the immediate instance itself.

   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.

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

6.5.5.  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.

   This keyword determines how child instances validate for objects, and
   does not directly validate the immediate instance itself.  Validation
   of the primitive instance type against this keyword always succeeds.

   Validation succeeds if, for each instance name that matches any
   regular expressions that appear as a property name in this keyword's
   value, the child instance for that name successfully validates
   against each schema that corresponds to a matching regular
   expression.

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

6.5.6.  additionalProperties

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

   This keyword determines how child instances validate for objects, and
   does not directly validate the immediate instance itself.

   Validation with "additionalProperties" applies only to the child
   values of instance names that do not match any names in "properties",
   and do not match any regular expression in "patternProperties".

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

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

6.5.7.  dependencies

   [[CREF1: Now that "if", "then", and "else" are keywords, it is not
   clear whether there is any benefit in keeping the schema form of
   "dependencies".  It is frequently misunderstood, and having a form
   that takes a subschema as well as a form that takes a primitive value
   is particularly confusing.  And it seems to be rarely used.
   Depending on feedback with "if", "then", and "else", the schema form
   of this keyword may well be removed in a future draft.  Current
   thought is that the string form (giving property names in an array)
   is a useful shortcut.  ]]

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

   This keyword's value MUST be an object.  Each property specifies a
   dependency.  Each dependency value MUST be an array or a valid JSON
   Schema.

   If the dependency value is a subschema, and the dependency key is a
   property in the instance, the entire instance must validate against
   the dependency value.

   If the dependency value is an array, each element in the array, if
   any, MUST be a string, and MUST be unique.  If the dependency key is
   a property in the instance, each of the items in the dependency value
   must be a property that exists in the instance.

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

6.5.8.  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.

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6.6.  Keywords for Applying Subschemas Conditionally

   These keywords work together to implement conditional application of
   a subschema based on the outcome of another subschema.

   These keywords 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.

6.6.1.  if

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

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

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

   Validation of the instance against this keyword on its own always
   succeeds, regardless of the validation outcome of against its
   subschema.

6.6.2.  then

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

   When present alongside of "if", the instance successfully validates
   against this keyword if it validates against both the "if"'s
   subschema and this keyword's subschema.

   When "if" is absent, or the instance fails to validate against its
   subschema, validation against this keyword always succeeds.
   Implementations SHOULD avoid attempting to validate against the
   subschema in these cases.

6.6.3.  else

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

   When present alongside of "if", the instance successfully validates
   against this keyword if it fails to validate against the "if"'s
   subschema, and successfully validates against this keyword's
   subschema.

   When "if" is absent, or the instance successfully validates against
   its subschema, validation against this keyword always succeeds.

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   Implementations SHOULD avoid attempting to validate against the
   subschema in these cases.

6.7.  Keywords for Applying Subschemas With Boolean Logic

6.7.1.  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.

6.7.2.  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.

6.7.3.  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.

6.7.4.  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.

7.  Semantic Validation With "format"

7.1.  Foreword

   Structural validation alone may be insufficient to validate that an
   instance meets all the requirements of an application.  The "format"
   keyword is defined to allow interoperable semantic validation for a
   fixed subset of values which are accurately described by

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   authoritative resources, be they RFCs or other external
   specifications.

   The value of this keyword is called a format attribute.  It MUST be a
   string.  A format attribute can generally only validate a given set
   of instance types.  If the type of the instance to validate is not in
   this set, validation for this format attribute and instance SHOULD
   succeed.

7.2.  Implementation Requirements

   The "format" keyword functions as both an annotation (Section 3.3)
   and as an assertion (Section 3.2).  While no special effort is
   required to implement it as an annotation conveying semantic meaning,
   implementing validation is non-trivial.

   Implementations MAY support the "format" keyword as a validation
   assertion.  Should they choose to do so:

      they SHOULD implement validation for attributes defined below;

      they SHOULD offer an option to disable validation for this
      keyword.

   Implementations MAY add custom format attributes.  Save for agreement
   between parties, schema authors SHALL NOT expect a peer
   implementation to support this keyword and/or custom format
   attributes.

7.3.  Defined Formats

7.3.1.  Dates and Times

   These attributes apply to string instances.

   Date and time format names are derived from RFC 3339, section 5.6
   [RFC3339].

   Implementations supporting formats SHOULD implement support for the
   following attributes:

   date-time  A string instance is valid against this attribute if it is
      a valid representation according to the "date-time" production.

   date  A string instance is valid against this attribute if it is a
      valid representation according to the "full-date" production.

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   time  A string instance is valid against this attribute if it is a
      valid representation according to the "full-time" production.

   Implementations MAY support additional attributes using the other
   production names defined in that section.  If "full-date" or "full-
   time" are implemented, the corresponding short form ("date" or "time"
   respectively) MUST be implemented, and MUST behave identically.
   Implementations SHOULD NOT define extension attributes with any name
   matching an RFC 3339 production unless it validates according to the
   rules of that production.  [[CREF2: There is not currently consensus
   on the need for supporting all RFC 3339 formats, so this approach of
   reserving the namespace will encourage experimentation without
   committing to the entire set.  Either the format implementation
   requirements will become more flexible in general, or these will
   likely either be promoted to fully specified attributes or dropped.
   ]]

7.3.2.  Email Addresses

   These attributes apply to string instances.

   A string instance is valid against these attributes if it is a valid
   Internet email address as follows:

   email  As defined by RFC 5322, section 3.4.1 [RFC5322].

   idn-email  As defined by RFC 6531 [RFC6531]

   Note that all strings valid against the "email" attribute are also
   valid against the "idn-email" attribute.

7.3.3.  Hostnames

   These attributes apply to string instances.

   A string instance is valid against these attributes if it is a valid
   representation for an Internet hostname as follows:

   hostname  As defined by RFC 1034, section 3.1 [RFC1034], including
      host names produced using the Punycode algorithm specified in RFC
      5891, section 4.4 [RFC5891].

   idn-hostname  As defined by either RFC 1034 as for hostname, or an
      internationalized hostname as defined by RFC 5890, section 2.3.2.3
      [RFC5890].

   Note that all strings valid against the "hostname" attribute are also
   valid against the "idn-hostname" attribute.

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7.3.4.  IP Addresses

   These attributes apply to string instances.

   A string instance is valid against these attributes if it is a valid
   representation of an IP address as follows:

   ipv4  An IPv4 address according to the "dotted-quad" ABNF syntax as
      defined in RFC 2673, section 3.2 [RFC2673].

   ipv6  An IPv6 address as defined in RFC 4291, section 2.2 [RFC4291].

7.3.5.  Resource Identifiers

   These attributes apply to string instances.

   uri  A string instance is valid against this attribute if it is a
      valid URI, according to [RFC3986].

   uri-reference  A string instance is valid against this attribute if
      it is a valid URI Reference (either a URI or a relative-
      reference), according to [RFC3986].

   iri  A string instance is valid against this attribute if it is a
      valid IRI, according to [RFC3987].

   iri-reference  A string instance is valid against this attribute if
      it is a valid IRI Reference (either an IRI or a relative-
      reference), according to [RFC3987].

   Note that all valid URIs are valid IRIs, and all valid URI References
   are also valid IRI References.

7.3.6.  uri-template

   This attribute applies to string instances.

   A string instance is valid against this attribute if it is a valid
   URI Template (of any level), according to [RFC6570].

   Note that URI Templates may be used for IRIs; there is no separate
   IRI Template specification.

7.3.7.  JSON Pointers

   These attributes apply to string instances.

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   json-pointer  A string instance is valid against this attribute if it
      is a valid JSON string representation of a JSON Pointer, according
      to RFC 6901, section 5 [RFC6901].

   relative-json-pointer  A string instance is valid against this
      attribute if it is a valid Relative JSON Pointer
      [relative-json-pointer].

   To allow for both absolute and relative JSON Pointers, use "anyOf" or
   "oneOf" to indicate support for either format.

7.3.8.  regex

   This attribute applies to string instances.

   A regular expression, which SHOULD be valid according to the ECMA 262
   [ecma262] regular expression dialect.

   Implementations that validate formats MUST accept at least the subset
   of ECMA 262 defined in the Regular Expressions (Section 4.3) section
   of this specification, and SHOULD accept all valid ECMA 262
   expressions.

8.  String-Encoding Non-JSON Data

8.1.  Foreword

   Properties defined in this section indicate that an instance contains
   non-JSON data encoded in a JSON string.  They describe the type of
   content and how it is encoded.

   These properties provide additional information required to interpret
   JSON data as rich multimedia documents.

8.2.  Implementation Requirements

   The content keywords function as both annotations (Section 3.3) and
   as assertions (Section 3.2).  While no special effort is required to
   implement them as annotations conveying how applications can
   interpret the data in the string, implementing validation of
   conformance to the media type and encoding is non-trivial.

   Implementations MAY support the "contentMediaType" and
   "contentEncoding" keywords as validation assertions.  Should they
   choose to do so, they SHOULD offer an option to disable validation
   for these keywords.

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8.3.  contentEncoding

   If the instance value is a string, this property defines that the
   string SHOULD be interpreted as binary data and decoded using the
   encoding named by this property.  RFC 2045, Sec 6.1 [RFC2045] lists
   the possible values for this property.

   The value of this property MUST be a string.

   The value of this property SHOULD be ignored if the instance
   described is not a string.

8.4.  contentMediaType

   The value of this property must be a media type, as defined by RFC
   2046 [RFC2046].  This property defines the media type of instances
   which this schema defines.

   The value of this property MUST be a string.

   The value of this property SHOULD be ignored if the instance
   described is not a string.

   If the "contentEncoding" property is not present, but the instance
   value is a string, then the value of this property SHOULD specify a
   text document type, and the character set SHOULD be the character set
   into which the JSON string value was decoded (for which the default
   is Unicode).

8.5.  Example

   Here is an example schema, illustrating the use of "contentEncoding"
   and "contentMediaType":

   {
       "type": "string",
       "contentEncoding": "base64",
       "contentMediaType": "image/png"
   }

   Instances described by this schema should be strings, and their
   values should be interpretable as base64-encoded PNG images.

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   Another example:

   {
       "type": "string",
       "contentMediaType": "text/html"
   }

   Instances described by this schema should be strings containing HTML,
   using whatever character set the JSON string was decoded into
   (default is Unicode).

9.  Schema Re-Use With "definitions"

   The "definitions" keywords provides a standardized 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
   "definitions":

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

10.  Schema Annotations

   Schema validation is a useful mechanism for annotating instance data
   with additional information.  The rules for determining when and how
   annotations are associated with an instance are outlined in this
   specification's overview.

   These general-purpose annotation keywords provide commonly used
   information for documentation and user interface display purposes.

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   They are not intended to form a comprehensive set of features.
   Rather, additional vocabularies can be defined for more complex
   annotation-based applications.

10.1.  "title" and "description"

   The value of both of these keywords MUST be a string.

   Both of these keywords can be used to decorate a user interface with
   information about the data produced by this user interface.  A title
   will preferably be short, whereas a description will provide
   explanation about the purpose of the instance described by this
   schema.

10.2.  "default"

   There are no restrictions placed on the value of this keyword.  When
   multiple occurrences of this keyword are applicable to a single sub-
   instance, implementations SHOULD remove duplicates.

   This keyword can be used to supply a default JSON value associated
   with a particular schema.  It is RECOMMENDED that a default value be
   valid against the associated schema.

10.3.  "readOnly" and "writeOnly"

   The value of these keywords MUST be a boolean.  When multiple
   occurrences of these keywords are applicable to a single sub-
   instance, the resulting value MUST be true if any occurrence
   specifies a true value, and MUST be false otherwise.

   If "readOnly" has a value of boolean true, it indicates that the
   value of the instance is managed exclusively by the owning authority,
   and attempts by an application to modify the value of this property
   are expected to be ignored or rejected by that owning authority.

   An instance document that is marked as "readOnly for the entire
   document MAY be ignored if sent to the owning authority, or MAY
   result in an error, at the authority's discretion.

   If "writeOnly" has a value of boolean true, it indicates that the
   value is never present when the instance is retrieved from the owning
   authority.  It can be present when sent to the owning authority to
   update or create the document (or the resource it represents), but it
   will not be included in any updated or newly created version of the
   instance.

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   An instance document that is marked as "writeOnly" for the entire
   document MAY be returned as a blank document of some sort, or MAY
   produce an error upon retrieval, or have the retrieval request
   ignored, at the authority's discretion.

   For example, "readOnly" would be used to mark a database-generated
   serial number as read-only, while "writeOnly" would be used to mark a
   password input field.

   These keywords can be used to assist in user interface instance
   generation.  In particular, an application MAY choose to use a widget
   that hides input values as they are typed for write-only fields.

   Omitting these keywords has the same behavior as values of false.

10.4.  "examples"

   The value of this keyword MUST be an array.  There are no
   restrictions placed on the values within the array.  When multiple
   occurrences of this keyword are applicable to a single sub-instance,
   implementations MUST provide a flat array of all values rather than
   an array of arrays.

   This keyword can be used to provide sample JSON values associated
   with a particular schema, for the purpose of illustrating usage.  It
   is RECOMMENDED that these values be valid against the associated
   schema.

   Implementations MAY use the value(s) of "default", if present, as an
   additional example.  If "examples" is absent, "default" MAY still be
   used in this manner.

11.  Security Considerations

   JSON Schema validation defines a vocabulary for JSON Schema core and
   concerns all the security considerations listed there.

   JSON Schema validation allows the use of Regular Expressions, which
   have numerous different (often incompatible) implementations.  Some
   implementations allow the embedding of arbitrary code, which is
   outside the scope of JSON Schema and MUST NOT be permitted.  Regular
   expressions can often also be crafted to be extremely expensive to
   compute (with so-called "catastrophic backtracking"), resulting in a
   denial-of-service attack.

   Implementations that support validating or otherwise evaluating
   instance string data based on "contentEncoding" and/or
   "contentMediaType" are at risk of evaluating data in an unsafe way

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   based on misleading information.  Applications can mitigate this risk
   by only performing such processing when a relationship between the
   schema and instance is established (e.g., they share the same
   authority).

   Processing a media type or encoding is subject to the security
   considerations of that media type or encoding.  For example, the
   security considerations of RFC 4329 Scripting Media Types [RFC4329]
   apply when processing JavaScript or ECMAScript encoded within a JSON
   string.

12.  References

12.1.  Normative References

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

   [json-schema]
              Wright, A. and H. Andrews, "JSON Schema: A Media Type for
              Describing JSON Documents", draft-handrews-json-schema-00
              (work in progress), November 2017.

   [relative-json-pointer]
              Luff, G. and H. Andrews, "Relative JSON Pointers", draft-
              handrews-relative-json-pointer-00 (work in progress),
              November 2017.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [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>.

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   [RFC2673]  Crawford, M., "Binary Labels in the Domain Name System",
              RFC 2673, DOI 10.17487/RFC2673, August 1999,
              <https://www.rfc-editor.org/info/rfc2673>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [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>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/info/rfc3987>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://www.rfc-editor.org/info/rfc5891>.

   [RFC6531]  Yao, J. and W. Mao, "SMTP Extension for Internationalized
              Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
              <https://www.rfc-editor.org/info/rfc6531>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [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>.

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   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

12.2.  Informative References

   [RFC4329]  Hoehrmann, B., "Scripting Media Types", RFC 4329,
              DOI 10.17487/RFC4329, April 2006,
              <https://www.rfc-editor.org/info/rfc4329>.

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Appendix A.  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, Dave
   Finlay, and Denis Laxalde for their submissions and patches to the
   document.

Appendix B.  ChangeLog

   [[CREF3: This section to be removed before leaving Internet-Draft
   status.]]

   draft-handrews-json-schema-validation-00

      *  Added "if"/"then"/"else"

      *  Classify keywords as assertions or annotations per the core
         spec

      *  Warn of possibly removing "dependencies" in the future

      *  Grouped validation keywords into sub-sections for readability

      *  Moved "readOnly" from hyper-schema to validation meta-data

      *  Added "writeOnly"

      *  Added string-encoded media section, with former hyper-schema
         "media" keywords

      *  Restored "regex" format (removal was unintentional)

      *  Added "date" and "time" formats, and reserved additional RFC
         3339 format names

      *  I18N formats: "iri", "iri-reference", "idn-hostname", "idn-
         email"

      *  Clarify that "json-pointer" format means string encoding, not
         URI fragment

      *  Fixed typo that inverted the meaning of "minimum" and
         "exclusiveMinimum"

      *  Move format syntax references into Normative References

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      *  JSON is a normative requirement

   draft-wright-json-schema-validation-01

      *  Standardized on hyphenated format names ("uriref" becomes "uri-
         ref")

      *  Add the formats "uri-template" and "json-pointer"

      *  Changed "exclusiveMaximum"/"exclusiveMinimum" from boolean
         modifiers of "maximum"/"minimum" to independent numeric fields.

      *  Split the additionalItems/items into two sections

      *  Reworked properties/patternProperties/additionalProperties
         definition

      *  Added "examples" keyword

      *  Added "contains" keyword

      *  Allow empty "required" and "dependencies" arrays

      *  Fixed "type" reference to primitive types

      *  Added "const" keyword

      *  Added "propertyNames" keyword

   draft-wright-json-schema-validation-00

      *  Added additional security considerations

      *  Removed reference to "latest version" meta-schema, use numbered
         version instead

      *  Rephrased many keyword definitions for brevity

      *  Added "uriref" format that also allows relative URI references

   draft-fge-json-schema-validation-01

      *  Initial draft.

      *  Salvaged from draft v3.

      *  Redefine the "required" keyword.

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      *  Remove "extends", "disallow"

      *  Add "anyOf", "allOf", "oneOf", "not", "definitions",
         "minProperties", "maxProperties".

      *  "dependencies" member values can no longer be single strings;
         at least one element is required in a property dependency
         array.

      *  Rename "divisibleBy" to "multipleOf".

      *  "type" arrays can no longer have schemas; remove "any" as a
         possible value.

      *  Rework the "format" section; make support optional.

      *  "format": remove attributes "phone", "style", "color"; rename
         "ip-address" to "ipv4"; add references for all attributes.

      *  Provide algorithms to calculate schema(s) for array/object
         instances.

      *  Add interoperability considerations.

Authors' Addresses

   Austin Wright (editor)

   EMail: aaa@bzfx.net

   Henry Andrews (editor)
   Cloudflare, Inc.
   San Francisco, CA
   USA

   EMail: henry@cloudflare.com

   Geraint Luff
   Cambridge
   UK

   EMail: luffgd@gmail.com

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