JSON Hyper-Schema: A Vocabulary for Hypermedia Annotation of JSON
draft-handrews-json-schema-hyperschema-00

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

   JSON Hyper-Schema: A Vocabulary for Hypermedia Annotation of JSON
               draft-handrews-json-schema-hyperschema-00

Abstract

   JSON Schema is a JSON-based format for describing JSON data using
   various vocabularies.  This document specifies a vocabulary for
   annotating JSON documents with hyperlinks.  These hyperlinks include
   attributes describing how to manipulate and interact with remote
   resources through hypermedia environments such as HTTP, as well as
   determining whether the link is usable based on the instance value.
   The hyperlink serialization format described in this document is also
   usable independent of JSON Schema.

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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   material or to cite them other than as "work in progress."

   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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Functionality . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Meta-Schemas and Output Schema  . . . . . . . . . . . . . . .   7
   5.  Schema Keywords . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  base  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  links . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Link Description Object . . . . . . . . . . . . . . . . . . .   8
     6.1.  Link Context  . . . . . . . . . . . . . . . . . . . . . .   9
       6.1.1.  anchor  . . . . . . . . . . . . . . . . . . . . . . .   9
       6.1.2.  anchorPointer . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Link Relation Type  . . . . . . . . . . . . . . . . . . .  10
       6.2.1.  rel . . . . . . . . . . . . . . . . . . . . . . . . .  10
       6.2.2.  "self" Links  . . . . . . . . . . . . . . . . . . . .  10
       6.2.3.  "collection" and "item" Links . . . . . . . . . . . .  11
       6.2.4.  Using Extension Relation Types  . . . . . . . . . . .  11
     6.3.  Link Target . . . . . . . . . . . . . . . . . . . . . . .  11
       6.3.1.  href  . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.4.  Adjusting URI Template Resolution . . . . . . . . . . . .  12
       6.4.1.  templatePointers  . . . . . . . . . . . . . . . . . .  12
       6.4.2.  templateRequired  . . . . . . . . . . . . . . . . . .  12
     6.5.  Link Target Attributes  . . . . . . . . . . . . . . . . .  12
       6.5.1.  title . . . . . . . . . . . . . . . . . . . . . . . .  13
       6.5.2.  description . . . . . . . . . . . . . . . . . . . . .  13
       6.5.3.  targetMediaType . . . . . . . . . . . . . . . . . . .  13
       6.5.4.  targetSchema  . . . . . . . . . . . . . . . . . . . .  14
       6.5.5.  targetHints . . . . . . . . . . . . . . . . . . . . .  14
     6.6.  Link Input  . . . . . . . . . . . . . . . . . . . . . . .  15
       6.6.1.  hrefSchema  . . . . . . . . . . . . . . . . . . . . .  15

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       6.6.2.  headerSchema  . . . . . . . . . . . . . . . . . . . .  16
       6.6.3.  Manipulating the Target Resource Representation . . .  16
       6.6.4.  Submitting Data for Processing  . . . . . . . . . . .  17
   7.  Implementation Requirements . . . . . . . . . . . . . . . . .  18
     7.1.  Link Discovery and Look-Up  . . . . . . . . . . . . . . .  19
     7.2.  URI Templating  . . . . . . . . . . . . . . . . . . . . .  19
       7.2.1.  Populating Template Data From the Instance  . . . . .  21
       7.2.2.  Accepting Input for Template Data . . . . . . . . . .  21
       7.2.3.  Encoding Data as Strings  . . . . . . . . . . . . . .  22
     7.3.  Providing Access to LDO Keywords  . . . . . . . . . . . .  23
     7.4.  Requests  . . . . . . . . . . . . . . . . . . . . . . . .  23
     7.5.  Responses . . . . . . . . . . . . . . . . . . . . . . . .  24
     7.6.  Streaming Parsers . . . . . . . . . . . . . . . . . . . .  25
   8.  JSON Hyper-Schema and HTTP  . . . . . . . . . . . . . . . . .  25
     8.1.  One Link Per Target and Relation Type . . . . . . . . . .  26
     8.2.  "targetSchema" and HTTP . . . . . . . . . . . . . . . . .  26
     8.3.  HTTP POST and the "submission*" keywords  . . . . . . . .  27
     8.4.  Optimizing HTTP Discoverability With "targetHints"  . . .  27
     8.5.  Advertising HTTP Features With "headerSchema" . . . . . .  28
     8.6.  Creating Resources Through Collections  . . . . . . . . .  28
     8.7.  Content Negotiation and Schema Evolution  . . . . . . . .  29
   9.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.1.  Entry Point Links, No Templates . . . . . . . . . . . . .  29
     9.2.  Individually Identified Resources . . . . . . . . . . . .  31
     9.3.  Submitting a Payload and Accepting URI Input  . . . . . .  32
     9.4.  "anchor", "base" and URI Template Resolution  . . . . . .  35
     9.5.  Collections . . . . . . . . . . . . . . . . . . . . . . .  38
       9.5.1.  Pagination  . . . . . . . . . . . . . . . . . . . . .  43
       9.5.2.  Creating the First Item . . . . . . . . . . . . . . .  46
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  47
     10.1.  Target Attributes  . . . . . . . . . . . . . . . . . . .  47
     10.2.  "self" Links . . . . . . . . . . . . . . . . . . . . . .  48
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  48
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  49
     12.2.  Informative References . . . . . . . . . . . . . . . . .  50
   Appendix A.  Using JSON Hyper-Schema in APIs  . . . . . . . . . .  51
     A.1.  Resource Evolution With Hyper-Schema  . . . . . . . . . .  51
     A.2.  Responses and Errors  . . . . . . . . . . . . . . . . . .  51
     A.3.  Static Analysis of an API's Hyper-Schemas . . . . . . . .  52
   Appendix B.  ChangeLog  . . . . . . . . . . . . . . . . . . . . .  52
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54

1.  Introduction

   JSON Hyper-Schema is a JSON Schema vocabulary for annotating JSON
   documents with hyperlinks and instructions for processing and

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   manipulating remote JSON resources through hypermedia environments
   such as HTTP.

   The term JSON Hyper-Schema is used to refer to a JSON Schema that
   uses these keywords.  The term "hyper-schema" on its own refers to a
   JSON Hyper-Schema within the scope of this specification.

   The primary mechanism introduced for specifying links is the Link
   Description Object (LDO), which is a serialization of the abstract
   link model defined in RFC 8288, section 2 [RFC8288].

   This specification will use the concepts, syntax, and terminology
   defined by the JSON Schema core [json-schema] and JSON Schema
   validation [json-schema-validation] specifications.  It is advised
   that readers have a copy of these specifications.

2.  Notational Conventions

   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].

3.  Overview

   JSON Hyper-Schema makes it possible to build hypermedia systems from
   JSON documents by describing how to construct hyperlinks from
   instance data.

   The combination of a JSON instance document and a valid application/
   schema+json hyper-schema for that instance behaves as a single
   hypermedia representation.  By allowing this separation, hyper-
   schema-based systems can gracefully support applications that expect
   plain JSON, while providing full hypermedia capabilities for hyper-
   schema-aware applications and user agents.

   User agents can detect the presence of hyper-schema by looking for
   the application/schema+json media type and a "$schema" value that
   indicates the presence of the hyper-schema vocabulary.  A user agent
   can then use an implementation of JSON Hyper-Schema to provide an
   interface to the combination of the schema and instance documents as
   a single logical representation of a resource, just as with any
   single-document hypermedia representation format.

   Hyper-schemas allow representations to take up fewer bytes on the
   wire, and distribute the burden of link construction from the server
   to each client.  A user agent need not construct a link unless a
   client application requests that link.  JSON Hyper-Schema can also be
   used on the server side to generate other link serializations or

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   representation formats at runtime, or pre-emptively follow links to
   facilitate server push usage.

   Here is an example hyper-schema that adds a single link, with the
   IANA-registered link relation type "self", that is built from an
   instance with one known object field named "id":

   {
       "type": "object",
       "properties": {
           "id": {
               "type": "number",
               "readOnly": true
           }
       },
       "links": [
           {
               "rel": "self",
               "href": "thing/{id}"
           }
       ]
   }

   If the instance is {"id": 1234}, and its base URI according to RFC
   3986 section 5.1 [RFC3986], is "https://api.example.com/", then
   "https://api.example.com/thing/1234" is the resulting link's target
   URI.

3.1.  Terminology

   The terms "schema", "instance", and "meta-schema" are to be
   interpreted as defined in the JSON Schema core specification
   [json-schema].

   The terms "applicable" and "attached" are to be interpreted as
   defined in Section 3 of the JSON Schema validation specification
   [json-schema-validation].

   The terms "link", "link context" (or "context"), "link target" (or
   "target"), and "target attributes" are to be interpreted as defined
   in Section 2 of RFC 8288 [RFC8288].

   The term "user agent" is to be interpreted as defined in Section 2.1
   of RFC 7230 [RFC7230], generalized to apply to any protocol that may
   be used in a hypermedia system rather than specifically being an HTTP
   client.

   This specification defines the following terms:

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   JSON Hyper-Schema  A JSON Schema using the keywords defined by this
      specification.

   hyper-schema  Within this document, the term "hyper-schema" always
      refers to a JSON Hyper-Schema

   link validity  A valid link for an instance is one that is applicable
      to that instance and does not fail any requirement imposed by the
      keywords in the Link Description Object.

   generic user agent  A user agent which can be used to interact with
      any resource, from any server, from among the standardized link
      relations, media types, URI schemes, and protocols that it
      supports; though it may be extendible to specially handle
      particular profiles of media types.

   client application  An application which uses a hypermedia system for
      a specific purpose.  Such an application may also be its own user
      agent, or it may be built on top of a generic user agent.  A
      client application is programmed with knowledge of link relations,
      media types, URI schemes, protocols, and data structures that are
      specific to the application's domain.

   client input  Data provided through a user agent, and most often also
      through a client application.  Such data may be requested from a
      user interactively, or provided before interaction in forms such
      as command-line arguments, configuration files, or hardcoded
      values in source code.

   operation  A specific use of a hyperlink, such as making a network
      request (for a URI with a scheme such as "http://" that indicates
      a protocol) or otherwise taking action based on a link (reading
      data from a "data:" URI, or constructing an email message based on
      a "mailto:" link).  For protocols such as HTTP that support
      multiple methods, each method is considered to be a separate
      operation on the same link.

3.2.  Functionality

   A JSON Hyper-Schema implementation is able to take a hyper-schema, an
   instance, and in some cases client input, and produce a set of fully
   resolved valid links.  As defined by RFC 8288, section 2 [RFC8288], a
   link consists of a context, a typed relation, a target, and
   optionally additional target attributes.

   The relation type and target attributes are taken directly from each
   link's Link Description Object.  The context and target identifiers

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   are constructed from some combination of URI Templates, instance
   data, and (in the case of the target identifier) client input.

   The target is always fully identified by a URI.  Due to the lack of a
   URI fragment identifier syntax for application/json and many other
   media types that can be used with JSON Hyper-Schema, the context may
   be only partially identified by a URI.  In such cases, the remaining
   identification will be provided as a JSON Pointer.

   A few IANA-registered link relation types are given specific
   semantics in a JSON Hyper-Schema document.  A "self" link is used to
   interact with the resource that the instance document represents,
   while "collection" and "item" links identify resources for which
   collection-specific semantics can be assumed.

4.  Meta-Schemas and Output Schema

   [[CREF1: The "draft-07-wip" is a placeholder.]]

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

   The link description format (Section 6) can be used without JSON
   Schema, and use of this format can be declared by referencing the
   normative link description schema as the schema for the data
   structure that uses the links.  The URI of the normative link
   description schema is: <http://json-schema.org/draft-07/links#>.

   JSON Hyper-Schema implementations are free to provide output in any
   format.  However, a specific format is defined for use in the
   conformance test suite, which is also used to illustrate points in
   the "Implementation Requirements" (Section 7), and to show the output
   generated by examples (Section 9).  It is RECOMMENDED that
   implementations be capable of producing output in this format to
   facilitated testing.  The URI of the JSON Schema describing the
   recommended output format is <http://json-schema.org/draft-07/hyper-
   schema-output#>.

5.  Schema Keywords

   Hyper-schema keywords from all schemas that are applicable to a
   position in an instance, as defined by Section 3 of JSON Schema
   validation [json-schema-validation], can be used with that instance.

   When multiple subschemas are applicable to a given sub-instance, all
   "link" arrays MUST be combined, in any order, into a single set.
   Each object in the resulting set MUST retain its own list of

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   applicable "base" values, in resolution order, from the same schema
   and any parent schemas.

   As with all JSON Schema keywords, all keywords described in this
   section are optional.  The minimal valid JSON Hyper-schema is the
   blank object.

5.1.  base

   If present, this keyword MUST be first resolved as a URI Template
   (Section 7.2), and then MUST be resolved as a URI Reference against
   the current URI base of the instance.  The result MUST be set as the
   new URI base for the instance while processing the sub-schema
   containing "base" and all sub-schemas within it.

   The process for resolving the "base" template can be different when
   being resolved for use with "anchor" than when being resolved for use
   with "href", which is explained in detail in the URI Templating
   section.

5.2.  links

   The "links" property of schemas is used to associate Link Description
   Objects with instances.  The value of this property MUST be an array,
   and the items in the array must be Link Description Objects, as
   defined below.

6.  Link Description Object

   A Link Description Object (LDO) is a serialization of the abstract
   link model defined in RFC 8288, section 2 [RFC8288].  As described in
   that document, a link consists of a context, a relation type, a
   target, and optionally target attributes.  JSON Hyper-Schema's LDO
   provides all of these, along with additional features using JSON
   Schema to describe input for use with the links in various ways.

   Due to the use of URI Templates to identify link contexts and
   targets, as well as optional further use of client input when
   identifying targets, an LDO is a link template that may resolve to
   multiple links when used with a JSON instance document.

   A specific use of an LDO, typically involving a request and response
   across a protocol, is referred to as an operation.  For many
   protocols, multiple operations are possible on any given link.  The
   protocol is indicated by the target's URI scheme.  Note that not all
   URI schemes indicate a protocol that can be used for communications,
   and even resources with URI schemes that do indicate such protocols
   need not be available over that protocol.

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   A Link Description Object MUST be an object, and the "href"
   (Section 6.3.1) and "rel" (Section 6.2.1) properties MUST be present.
   Each keyword is covered briefly in this section, with additional
   usage explanation and comprehensive examples given later in the
   document.

6.1.  Link Context

   In JSON Hyper-Schema, the link's context resource is, by default, the
   sub-instance to which it is attached (as defined by Section 3 of the
   JSON Schema validation specification [json-schema-validation]).  This
   is often not the entire instance document.  This default context can
   be changed using the keywords in this section.

   Depending on the media type of the instance, it may or may not be
   possible to assign a URI to the exact default context resource.  In
   particular, application/json does not define a URI fragment
   resolution syntax, so properties or array elements within a plain
   JSON document cannot be fully identified by a URI.  When it is not
   possible to produce a complete URI, the position of the context
   SHOULD be conveyed by the URI of the instance document, together with
   a separate plain-string JSON Pointer.

   Implementations MUST be able to construct the link context's URI, and
   (if necessary for full identification), a JSON Pointer in string
   representation form as per RFC 6901, section 5 [RFC6901] in place of
   a URI fragment.  The process for constructing a URI based on a URI
   template is given in the URI Templating (Section 7.2) section.

6.1.1.  anchor

   This property sets the context URI of the link.  The value of the
   property is a URI Template [RFC6570], and the resulting URI-reference
   [RFC3986] MUST be resolved against the base URI of the instance.

   The URI is computed from the provided URI template using the same
   process described for the "href" (Section 6.3.1) property, with the
   exception that "hrefSchema" (Section 6.6.1) MUST NOT be applied.
   Unlike target URIs, context URIs do not accept user input.

6.1.2.  anchorPointer

   This property changes the point within the instance that is
   considered to be the context resource of the link.  The value of the
   property MUST be a valid JSON Pointer in JSON String representation
   form, or a valid Relative JSON Pointer [relative-json-pointer] which
   is evaluated relative to the default context.

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   While an alternate context with a known URI is best set with the
   "anchor" (Section 6.1.1) keyword, the lack of a fragment identifier
   syntax for application/json means that it is usually not possible to
   change the context within a JSON instance using a URI.

   Even in "+json" media types that define JSON Pointer as a fragment
   identifier syntax, if the default context is nested within an array,
   it is not possible to obtain the index of the default context's
   position in that array in order to construct a pointer to another
   property in that same nested JSON object.  This will be demonstrated
   in the examples.

   The result of processing this keyword SHOULD be a URI fragment if the
   media type of the instance allows for such a fragment.  Otherwise it
   MUST be a string-encoded JSON Pointer.

6.2.  Link Relation Type

   The link's relation type identifies its semantics.  It is the primary
   means of conveying how an application can interact with a resource.

   Relationship definitions are not normally media type dependent, and
   users are encouraged to utilize the most suitable existing accepted
   relation definitions.

6.2.1.  rel

   The value of this property MUST be a string, and MUST be a single
   Link Relation Type as defined in RFC 8288, Section 2.1.

   This property is required.

6.2.2.  "self" Links

   A "self" link, as originally defined by Section 4.2.7.2 of RFC 4287
   [RFC4287], indicates that the target URI identifies a resource
   equivalent to the link context.  In JSON Hyper-Schema, a "self" link
   MUST be resolvable from the instance, and therefore "hrefSchema" MUST
   NOT be present.

   Hyper-schema authors SHOULD use "templateRequired" to ensure that the
   "self" link has all instance data that is needed for use.

   A hyper-schema implementation MUST recognize that a link with
   relation type "self" that has the entire current instance document as
   its context describes how a user agent can interact with the resource
   represented by that instance document.

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6.2.3.  "collection" and "item" Links

   RFC 6573 [RFC6573] defines and registers the "item" and "collection"
   link relation types.  JSON Hyper-Schema imposes additional semantics
   on collection resources indicated by these types.

   Implementations MUST recognize the target of a "collection" link and
   the context of an "item" link as collections.

   A well-known design pattern in hypermedia is to use a collection
   resource to create a member of the collection and give it a server-
   assigned URI.  If the protocol indicated by the URI scheme defines a
   specific method that is suited to creating a resource with a server-
   assigned URI, then a collection resource, as identified by these link
   relation types, MUST NOT define semantics for that method that
   conflict with the semantics of creating a collection member.
   Collection resources MAY implement item creation via such a protocol
   method, and user agents MAY assume that any such operation, if it
   exists, has item creation semantics.

   As such a method would correspond to JSON Hyper-Schema's data
   submission concept, the "submissionSchema" (Section 6.6.4.2) field
   for the link SHOULD be compatible with the schema of the
   representation of the collection's items, as indicated by the "item"
   link's target resource or the "self" link of the "collection" link's
   context resource.

6.2.4.  Using Extension Relation Types

   When no registered relation (aside from "related") applies, users are
   encouraged to mint their own extension relation types, as described
   in section 2.1.2 of RFC 8288 [RFC8288].  The simplest approaches for
   choosing link relation type URIs are to either use a URI scheme that
   is already in use to identify the system's primary resources, or to
   use a human-readable, non-dereferenceable URI scheme such as "tag",
   defined by RFC 4151 [RFC4151].

   Extension relation type URIs need not be dereferenceable, even when
   using a scheme that allows it.

6.3.  Link Target

   The target URI template is used to identify the link's target,
   potentially making use of instance data.  Additionally, with
   "hrefSchema" (Section 6.6.1), this template can identify a set of
   possible target resources to use based on client input.  The full
   process of resolving the URI template, with or without client input,
   is covered in the URI Templating (Section 7.2) section.

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6.3.1.  href

   The value of the "href" link description property is a template used
   to determine the target URI of the related resource.  The value of
   the instance property MUST be resolved as a URI-reference [RFC3986]
   against the base URI of the instance.

   This property is REQUIRED.

6.4.  Adjusting URI Template Resolution

   The keywords in this section are used when resolving all URI
   Templates involved in hyper-schema: "base", "anchor", and "href".
   See the URI Templating (Section 7.2) section for the complete
   template resolution algorithm.

   Note that when resolving a "base" template, the attachment point from
   which resolution begins is the attachment point of the "href" or
   "anchor" keyword being resolved which requires "base" templates to be
   resolved, not the attachment point of the "base" keyword itself.

6.4.1.  templatePointers

   The value of the "templatePointers" link description property MUST be
   an object.  Each property value in the object MUST be a valid JSON
   Pointer [RFC6901], or a valid Relative JSON Pointer
   [relative-json-pointer] which is evaluated relative to the attachment
   point of the link for which the template is being resolved.

   For each property name in the object that matches a variable name in
   the template being resolved, the value of that property adjusts the
   starting position of variable resolution for that variable.
   Properties which do not match template variable names in the template
   being resolved MUST be ignored.

6.4.2.  templateRequired

   The value of this keyword MUST be an array, and the elements MUST be
   unique.  Each element SHOULD match a variable in the link's URI
   Template, without percent-encoding.  After completing the entire URI
   Template resolution process, if any variable that is present in this
   array does not have a value, the link MUST NOT be used.

6.5.  Link Target Attributes

   All properties in this section are advisory only.  While keywords
   such as "title" and "description" are used primarily to present the
   link to users, those keywords that predict the nature of a link

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   interaction or response MUST NOT be considered authoritative.  The
   runtime behavior of the target resource MUST be respected whenever it
   conflicts with the target attributes in the LDO.

6.5.1.  title

   This property defines a title for the link.  The value MUST be a
   string.

   User agents MAY use this title when presenting the link to the user.

6.5.2.  description

   This property provides additional information beyond what is present
   in the title.  The value MUST be a string.  While a title is
   preferably short, a description can be used to go into more detail
   about the purpose and usage of the link.

   User agents MAY use this description when presenting the link to the
   user.

6.5.3.  targetMediaType

   The value of this property represents the media type RFC 2046
   [RFC2046], that is expected to be returned when fetching this
   resource.  This property value MAY be a media range instead, using
   the same pattern defined in RFC 7231, section 5.3.2 - HTTP "Accept"
   header [RFC7231].

   This property is analogous to the "type" property of other link
   serialization formats.  User agents MAY use this information to
   inform the interface they present to the user before the link is
   followed, but MUST NOT use this information in the interpretation of
   the resulting data.  Instead, a user agent MUST use the media type
   given by the response for run-time interpretation.  See the section
   on "Security Concerns" (Section 10) for a detailed examination of
   mis-use of "targetMediaType".

   For protocols supporting content-negotiation, implementations MAY
   choose to describe possible target media types using protocol-
   specific information in "headerSchema" (Section 6.6.2).  If both
   protocol-specific information and "targetMediaType" are present, then
   the value of "targetMediaType" MUST be compatible with the protocol-
   specific information, and SHOULD indicate the media type that will be
   returned in the absence of content negotiation.

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   When no such protocol-specific information is available, or when the
   implementation does not recognize the protocol involved, then the
   value SHOULD be taken to be "application/json".

6.5.4.  targetSchema

   This property provides a schema that is expected to describe the link
   target's representation.  Depending on the protocol, the schema may
   or may not describe the request or response to any particular
   operation performed with the link.  See the JSON Hyper-Schema and
   HTTP (Section 8) section for an in-depth discussion of how this
   keyword is used with HTTP.

6.5.5.  targetHints

   [[CREF2: This section attempts to strike a balance between
   comprehensiveness and flexibility by deferring most of its structure
   to the protocol indicated by the URI scheme.  Note that a resource
   can be identified by a URI with a dereferenceable scheme, yet not be
   accessible over that protocol.  While currently very loose, this
   section is expected to become more well-defined based on draft
   feedback, and may change significantly in future drafts.  ]]

   The value of this property is advisory only.  It represents
   information that is expected to be discoverable through interacting
   with the target resource, typically in the form of protocol-specific
   control information or meta-data such as headers returned in response
   to an HTTP HEAD or OPTIONS request.  The protocol is determined by
   the "href" URI scheme, although note that resources are not
   guaranteed to be accessible over such a protocol.

   The value of this property SHOULD be an object.  The keys to this
   object SHOULD be lower-cased forms of the control data field names.
   Each value SHOULD be an array, in order to uniformly handle multi-
   valued fields.  Multiple values MUST be presented as an array, and
   not as a single string.

   Protocols with control information not suitable for representation as
   a JSON object MAY be represented by another data type, such as an
   array.

   Values that cannot be understood as part of the indicated protocol
   MUST be ignored by a JSON Hyper-Schema implementation.  Applications
   MAY make use of such values, but MUST NOT assume interoperability
   with other implementations.

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   Implementations MUST NOT assume that all discoverable information is
   accounted for in this object.  Client applications MUST properly
   handle run-time responses that contradict this property's values.

   Client applications MUST NOT assume that an implementation will
   automatically take any action based on the value of this property.

   See "JSON Hyper-Schema and HTTP" (Section 8) for guidance on using
   this keyword with HTTP and analogous protocols.

6.6.  Link Input

   There are four ways to use client input with a link, and each is
   addressed by a separate link description object keyword.  When
   performing operations, user agents SHOULD ignore schemas that are not
   relevant to their semantics.

6.6.1.  hrefSchema

   The value of the "hrefSchema" link description property MUST be a
   valid JSON Schema.  This schema is used to validate user input or
   other user agent data for filling out the URI Template in "href"
   (Section 6.3.1).

   Omitting "hrefSchema" or setting the entire schema to "false"
   prevents any user agent data from being accepted.

   Setting any subschema that applies to a particular variable to the
   JSON literal value "false" prevents any user agent data from being
   accepted for that single variable.

   For template variables that can be resolved from the instance data,
   if the instance data is valid against all applicable subschemas in
   "hrefSchema", then it MUST be used to pre-populate the input data set
   for that variable.

   Note that even when data is pre-populated from the instance, the
   validation schema for that variable in "hrefSchema" need not be
   identical to the validation schema(s) that apply to the instance
   data's location.  This allows for different validation rules for user
   agent data, such as supporting spelled-out months for date-time
   input, but using the standard date-time format for storage.

   After input is accepted, potentially overriding the pre-populated
   instance data, the resulting data set MUST successfully validate
   against the value of "hrefSchema".  If it does not then the link MUST
   NOT be used.  If it is valid, then the process given in the "URI
   Templating" section continues with this updated data set.

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6.6.2.  headerSchema

   [[CREF3: As with "targetHints", this keyword is somewhat under-
   specified to encourage experimentation and feedback as we try to
   balance flexibility and clarity.  ]]

   If present, this property is a schema for protocol-specific request
   headers or analogous control and meta-data.  The value of this object
   MUST be a valid JSON Schema.  The protocol is determined by the
   "href" URI scheme, although note that resources are not guaranteed to
   be accessible over such a protocol.  The schema is advisory only; the
   target resource's behavior is not constrained by its presence.

   The purpose of this keyword is to advertise target resource
   interaction features, and indicate to user agents and client
   applications what headers and header values are likely to be useful.
   User agents and client applications MAY use the schema to validate
   relevant headers, but MUST NOT assume that missing headers or values
   are forbidden from use.  While schema authors MAY set
   "additionalProperties" to false, this is NOT RECOMMENDED and MUST NOT
   prevent client applications or user agents from supplying additional
   headers when requests are made.

   The exact mapping of the JSON data model into the headers is
   protocol-dependent.  However, in most cases this schema SHOULD
   specify a type of "object", and the property names SHOULD be lower-
   cased forms of the control data field names.  See the "JSON Hyper-
   Schema and HTTP" (Section 8) section for detailed guidance on using
   this keyword with HTTP and analogous protocols.

   "headerSchema" is applicable to any request method or command that
   the protocol supports.  When generating a request, user agents and
   client applications SHOULD ignore schemas for headers that are not
   relevant to that request.

6.6.3.  Manipulating the Target Resource Representation

   In JSON Hyper-Schema, "targetSchema" (Section 6.5.4) supplies a non-
   authoritative description of the target resource's representation.  A
   client application can use "targetSchema" to structure input for
   replacing or modifying the representation, or as the base
   representation for building a patch document based on a patch media
   type.

   Alternatively, if "targetSchema" is absent or if the client
   application prefers to only use authoritative information, it can
   interact with the target resource to confirm or discover its
   representation structure.

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   "targetSchema" is not intended to describe link operation responses,
   except when the response semantics indicate that it is a
   representation of the target resource.  In all cases, the schema
   indicated by the response itself is authoritative.  See "JSON Hyper-
   Schema and HTTP" (Section 8) for detailed examples.

6.6.4.  Submitting Data for Processing

   The "submissionSchema" (Section 6.6.4.2) and "submissionMediaType"
   (Section 6.6.4.1) keywords describe the domain of the processing
   function implemented by the target resource.  Otherwise, as noted
   above, the submission schema and media type are ignored for
   operations to which they are not relevant.

6.6.4.1.  submissionMediaType

   If present, this property indicates the media type format the client
   application and user agent should use for the request payload
   described by "submissionSchema" (Section 6.6.4.2).

   Omitting this keyword has the same behavior as a value of
   application/json.

   Note that "submissionMediaType" and "submissionSchema" are not
   restricted to HTTP URIs.  [[CREF4: This statement might move to
   wherever the example ends up.]]

6.6.4.2.  submissionSchema

   This property contains a schema which defines the acceptable
   structure of the document to be encoded according to the
   "submissionMediaType" property and sent to the target resource for
   processing.  This can be viewed as describing the domain of the
   processing function implemented by the target resource.

   This is a separate concept from the "targetSchema" (Section 6.5.4)
   property, which describes the target information resource (including
   for replacing the contents of the resource in a PUT request), unlike
   "submissionSchema" which describes the user-submitted request data to
   be evaluated by the resource.  "submissionSchema" is intended for use
   with requests that have payloads that are not necessarily defined in
   terms of the target representation.

   Omitting "submissionSchema" has the same behavior as a value of
   "true".

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7.  Implementation Requirements

   At a high level, a conforming implementation will meet the following
   requirements.  Each of these requirements is covered in more detail
   in the individual keyword sections and keyword group overviews.

   Note that the requirements around how an implementation MUST
   recognize "self", "collection", and "item" links are thoroughly
   covered in the link relation type (Section 6.2) section and are not
   repeated here.

   While it is not a mandatory format for implementations, the output
   format used in the test suite summarizes what needs to be computed
   for each link before it can be used:

   contextUri  The fully resolved URI (with scheme) of the context
      resource.  If the context is not the entire resource and there is
      a usable fragment identifier syntax, then the URI includes a
      fragment.  Note that there is no such syntax for application/json.

   contextPointer  The JSON Pointer for the location within the instance
      of the context resource.  If the instance media type supports JSON
      Pointers as fragment identifiers, this pointer will be the same as
      the one encoded in the fragment of the "contextUri" field.

   rel  The link relation type, as it appears in the LDO.

   targetUri  The fully resolved URI (with a scheme) of the target
      resource.  If the link accepts input, this can only be produced
      once the input has been supplied.

   hrefInputTemplates  The list of partially resolved URI references for
      a link that accepts input.  The first entry in the list is the
      partially resolved "href".  The additional entries, if any, are
      the partially resolved "base" values ordered from the most
      immediate out to the root of the schema.  Template variables that
      are pre-populated in the input are not resolved at this stage, as
      the pre-populated value can be overridden.

   hrefPrepopulatedInput  The data set that the user agent should use to
      prepopulate any input mechanism before accepting client input.  If
      input is to be accepted but no fields are to be pre-populated,
      then this will be an empty object.

   attachmentPointer  The JSON Pointer for the location within the
      instance to which the link is attached.  By default, "contextUri"
      and "attachmentUri" are the same, but "contextUri" can be changed
      by LDO keywords, while "attachmentUri" cannot.

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   Other LDO keywords that are not involved in producing the above
   information are included exactly as they appear when producing output
   for the test suite.  Those fields will not be further discussed here
   unless specifically relevant.

7.1.  Link Discovery and Look-Up

   Before links can be used, they must be discovered by applying the
   hyper-schema to the instance and finding all applicable and valid
   links.  Note that in addition to collecting valid links, any "base"
   (Section 5.1) values necessary to resolve each LDO's URI Templates
   must also be located and associated with the LDO through whatever
   mechanism is most useful for the implementation's URI Template
   resolution process.

   And implementation MUST support looking up links by either their
   attachment pointer or context pointer, either by performing the look-
   up or by providing the set of all links with both pointers determined
   so that user agents can implement the look-up themselves.

   When performing look-ups by context pointer, links that are attached
   to elements of the same array MUST be returned in the same order as
   the array elements to which they are attached.

7.2.  URI Templating

   Three hyper-schema keywords are URI Templates [RFC6570]: "base",
   "anchor", and "href".  Each are resolved separately to URI-
   references, and then the anchor or href URI-reference is resolved
   against the base (which is itself resolved against earlier bases as
   needed, each of which was first resolved from a URI Template to a
   URI-reference).

   All three keywords share the same algorithm for resolving variables
   from instance data, which makes use of the "templatePointers" and
   "templateRequired" keywords.  When resolving "href", both it and any
   "base" templates needed for resolution to an absolute URI, the
   algorithm is modified to optionally accept user input based on the
   "hrefSchema" keyword.

   For each URI Template (T), the following pseudocode describes an
   algorithm for resolving T into a URI-reference (R).  For the purpose
   of this algorithm:

   o  "ldo.templatePointers" is an empty object if the keyword was not
      present and "ldo.templateRequired" is likewise an empty array.

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   o  "attachmentPointer" is the absolute JSON Pointer for the
      attachment location of the LDO.

   o  "getApplicableSchemas()" returns an iterable set of all
      (sub)schemas that apply to the attachment point in the instance.

   This algorithm should be applied first to either "href" or "anchor",
   and then as needed to each successive "base".  The order is
   important, as it is not always possible to tell whether a template
   will resolve to a full URI or a URI-reference.

   In English, the high-level algorithm is:

   1.  Populate template variable data from the instance

   2.  If input is desired, accept input

   3.  Check that all required variables have a value

   4.  Encode values into strings and fill out the template

   This is the high-level algorithm as pseudocode.  "T" comes from
   either "href" or "anchor" within the LDO, or from "base" in a
   containing schema.  Pseudocode for each step follows.
   "initialTemplateKeyword" indicates which of the two started the
   process (since "base" is always resolved in order to finish resolving
   one or the other of those keywords).

   templateData = populateDataFromInstance(T, ldo, instance)

   if initialTemplateKeyword == "href" and ldo.hrefSchema exists:
       inputData = acceptInput(ldo, instance, templateData)
       for varname in inputData:
           templateData[varname] = inputData[varname]

   for varname in ldo.templateRequired:
       if not exists templateData[varname]
           fatal("Missing required variable(s)")

   templateData = stringEncode(templateData)
   R = rfc6570ResolutionAlgorithm(T, templateData)

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7.2.1.  Populating Template Data From the Instance

   This step looks at various locations in the instance for variable
   values.  For each variable:

   1.  Use "templatePointers" to find a value if the variable appears in
       that keyword's value

   2.  Otherwise, look for a property name matching the variable in the
       instance location to which the link is attached

   3.  In either case, if there is a value at the location, put it in
       the template resolution data set

   for varname in T:
       varname = rfc3986PercentDecode(varname)
       if varname in ldo.templatePointers:
           valuePointer = templatePointers[varname]
           if valuePointer is relative:
               valuePointer = resolveRelative(attachmentPointer,
                                              valuePointer)
       else
           valuePointer = attachmentPointer + "/" + varname

       value = instance.valueAt(valuePointer)
       if value is defined:
           templateData[varname] = value

7.2.2.  Accepting Input for Template Data

   This step is relatively complex, as there are several cases to
   support.  Some variables will forbid input and some will allow it.
   Some will have initial values that need to be presented in the input
   interface, and some will not.

   1.  Determine which variables can accept input

   2.  Pre-populate the input data set if the template resolution data
       set has a value

   3.  Accept input (present a web form, make a callback, etc.)

   4.  Validate the input data set, (not the template resolution data
       set)

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   5.  Put the input in the template resolution data set, overriding any
       existing values

   "InputForm" represents whatever sort of input mechanism is
   appropriate.  This may be a literal web form, or may be a more
   programmatic construct such as a callback function accepting specific
   fields and data types, with the given initial values, if any.

   form = new InputForm()
   for varname in T:
       useField = true
       useInitialData = true
       for schema in getApplicableSchemas(ldo.hrefSchema,
                                          "/" + varname):
           if schema is false:
               useField = false
               break

           if varname in templateData and
              not isValid(templateData[varname], schema)):
               useInitialData = false
               break

       if useField:
           if useInitialData:
               form.addInputFieldFor(varname, ldo.hrefSchema,
                                     templateData[varname])
           else:
               form.addInputFieldFor(varname, ldo.hrefSchema)

   inputData = form.acceptInput()

   if not isValid(inputData, hrefSchema):
       fatal("Input invalid, link is not usable")

   return inputData:

7.2.3.  Encoding Data as Strings

   This section is straightforward, converting literals to their names
   as strings, and converting numbers to strings in the most obvious
   manner, and percent-encoding as needed for use in the URI.

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   for varname in templateData:
       value = templateData[varname]
       if value is true:
           templateData[varname] = "true"
       else if value is false:
           temlateData[varname] = "false"
       else if value is null:
           templateData[varname] = "null"
       else if value is a number:
           templateData[varname] =
               bestEffortOriginalJsonString(value)
       else:
           templateData[varname] = rfc3986PercentEncode(value)

   In some software environments the original JSON representation of a
   number will not be available (there is no way to tell the difference
   between 1.0 and 1), so any reasonable representation should be used.
   Schema and API authors should bear this in mind, and use other types
   (such as string or boolean) if the exact representation is important.
   If the number was provide as input in the form of a string, the
   string used as input SHOULD be used.

7.3.  Providing Access to LDO Keywords

   For a given link, an implementation MUST make the values of all
   target attribute keywords directly available to the user agent.
   Implementations MAY provide additional interfaces for using this
   information, as discussed in each keyword's section.

   For a given link, an implementation MUST make the value of each input
   schema keyword directly available to the user agent.

   To encourage encapsulation of the URI Template resolution process,
   implementations MAY omit the LDO keywords that are used only to
   construct URIs.  However, implementations MUST provide access to the
   link relation type.

   Unrecognized keywords SHOULD be made available to the user agent, and
   MUST otherwise be ignored.

7.4.  Requests

   A hyper-schema implementation SHOULD provide access to all
   information needed to construct any valid request to the target
   resource.

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   The LDO can express all information needed to perform any operation
   on a link.  This section explains what hyper-schema fields a user
   agent should examine to build requests from any combination of
   instance data and client input.  A hyper-schema implementation is not
   itself expected to construct and send requests.

   Target URI construction rules, including "hrefSchema" for accepting
   input, are identical for all possible requests.

   Requests that do not carry a body payload do not require additional
   keyword support.

   Requests that take a target representation as a payload SHOULD use
   the "targetSchema" and "targetMediaType" keywords for input
   description and payload validation.  If a protocol allows an
   operation taking a payload that is based on the representation as
   modified by a media type (such as a patch media type), then such a
   media type SHOULD be indicated through "targetHints" in a protocol-
   specific manner.

   Requests that take a payload that is not derived from the target
   resource's representation SHOULD use the "submissionSchema" and
   "submissionMediaType" keywords for input description and payload
   validation.  Protocols used in hypermedia generally only support one
   such non-representation operation per link.

   RPC systems that pipe many application operations with arbitrarily
   different request structures through a single hypermedia protocol
   operation are outside of the scope of a hypermedia format such as
   JSON Hyper-Schema.

7.5.  Responses

   As a hypermedia format, JSON Hyper-Schema is concerned with
   describing a resource, including describing its links in sufficient
   detail to make all valid requests.  It is not concerned with directly
   describing all possible responses for those requests.

   As in any hypermedia system, responses are expected to be self-
   describing.  In the context of hyper-schema, this means that each
   response MUST link its own hyper-schema(s).  While responses that
   consist of a representation of the target resource are expected to be
   valid against "targetSchema" and "targetMediaType", those keywords
   are advisory only and MUST be ignored if contradicted by the response
   itself.

   Other responses, including error responses, complex redirections, and
   processing status representations SHOULD also link to their own

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   schemas and use appropriate media types (e.g. "application/
   problem+json" [RFC7807] for errors).  Certain errors might not link a
   schema due to being generated by an intermediary that is not aware of
   hyper-schema, rather than by the origin.

   User agents are expected to understand protocol status codes and
   response media types well enough to handle common situations, and
   provide enough information to client applications to handle domain-
   specific responses.

   Statically mapping all possible responses and their schemas at design
   time is outside of the scope of JSON Hyper-Schema, but may be within
   the scope of other JSON Schema vocabularies which build on hyper-
   schema (see Appendix A.3).

7.6.  Streaming Parsers

   The requirements around discovering links based on their context, or
   using the context of links to identify collections, present unique
   challenges when used with streaming parsers.  It is not possible to
   authoritatively fulfill these requirements without processing the
   entire schema and instance documents.

   Such implementations MAY choose to return non-authoritative answers
   based on data processed to date.  When offering this approach,
   implementations MUST be clear on the nature of the response, and MUST
   offer an option to block and wait until all data is processed and an
   authoritative answer can be returned.

8.  JSON Hyper-Schema and HTTP

   While JSON Hyper-Schema is a hypermedia format and therefore
   protocol-independent, it is expected that its most common use will be
   in HTTP systems, or systems using protocols such as CoAP that are
   explicitly analogous to HTTP.

   This section provides guidance on how to use each common HTTP method
   with a link, and how collection resources impose additional
   constraints on HTTP POST.  Additionally, guidance is provided on
   hinting at HTTP response header values and describing possible HTTP
   request headers that are relevant to the given resource.

   Section 11 of the JSON Schema core specification [json-schema]
   provides guidance on linking instances in a hypermedia system to
   their schemas.  This may be done with network-accessible schemas, or
   may simply identify schemas which were pre-packaged within the client
   application.  JSON Hyper-Schema intentionally does not constrain this

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   mechanism, although it is RECOMMENDED that the techniques outlined in
   the core specification be used to whatever extent is possible.

8.1.  One Link Per Target and Relation Type

   Link Description Objects do not directly indicate what operations,
   such as HTTP methods, are supported by the target resource.  Instead,
   operations should be inferred primarily from link relation types
   (Section 6.2.1) and URI schemes.

   This means that for each target resource and link relation type pair,
   schema authors SHOULD only define a single LDO.  While it is possible
   to use "allow" with "targetHints" to repeat a relation type and
   target pair with different HTTP methods marked as allowed, this is
   NOT RECOMMENDED and may not be well-supported by conforming
   implementations.

   All information necessary to use each HTTP method can be conveyed in
   a single LDO as explained in this section.  The "allow" field in
   "targetHints" is intended simply to hint at which operations are
   supported, not to separately define each operation.

   Note, however, that a resource may always decline an operation at
   runtime, for instance due to authorization failure, or due to other
   application state that controls the operation's availability.

8.2.  "targetSchema" and HTTP

   "targetSchema" describes the resource on the target end of the link,
   while "targetMediaType" defines that resource's media type.  With
   HTTP links, "headerSchema" can also be used to describe valid values
   for use in an "Accept" request header, which can support multiple
   media types or media ranges.  When both ways of indicating the target
   media type are present, "targetMediaType" SHOULD indicate the default
   representation media type, while the schema for "accept" in
   "headerSchema" SHOULD include the default as well as any alternate
   media types or media ranges that can be requested.

   Since the semantics of many HTTP methods are defined in terms of the
   target resource, "targetSchema" is used for requests and/or responses
   for several HTTP methods.  In particular, "targetSchema" suggests
   what a client application can expect for the response to an HTTP GET
   or any response for which the "Content-Location" header is equal to
   the request URI, and what a client application should send if it
   replaces the resource in an HTTP PUT request.  These correlations are
   defined by RFC 7231, section 4.3.1 - "GET", section 4.3.4 "PUT", and
   section 3.1.4.2, "Content-Location" [RFC7231].

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   Per RFC 5789 [RFC5789], the request structure for an HTTP PATCH is
   determined by the combination of "targetSchema" and the request media
   type, which is conveyed by the "Accept-Patch" header, which may be
   included in "targetHints".  Media types that are suitable for PATCH-
   ing define a syntax for expressing changes to a document, which can
   be applied to the representation described by "targetSchema" to
   determine the set of syntactically valid request payloads.  Often,
   the simplest way to validate a PATCH request is to apply it and
   validate the result as a normal representation.

8.3.  HTTP POST and the "submission*" keywords

   JSON Hyper-Schema allows for resources that process arbitrary data in
   addition to or instead of working with the target's representation.
   This arbitrary data is described by the "submissionSchema" and
   "submissionMediaType" keywords.  In the case of HTTP, the POST method
   is the only one that handles such data.  While there are certain
   conventions around using POST with collections, the semantics of a
   POST request are defined by the target resource, not HTTP.

   In addition to the protocol-neutral "submission*" keywords (see
   Section 9.3 for a non-HTTP example), the "Accept-Post" header can be
   used to specify the necessary media type, and MAY be advertised via
   the "targetHints" field.  [[CREF5: What happens if both are used?
   Also, "submissionSchema" is a MUST to support, while "targetHints"
   are at most a SHOULD.  But forbidding the use of "Accept-Post" in
   "targetHints" seems incorrect.  ]]

   Successful responses to POST other than a 201 or a 200 with "Content-
   Location" set likewise have no HTTP-defined semantics.  As with all
   HTTP responses, any representation in the response should link to its
   own hyper-schema to indicate how it may be processed.  As noted in
   Appendix A.2, connecting hyperlinks with all possible operation
   responses is not within the scope of JSON Hyper-Schema.

8.4.  Optimizing HTTP Discoverability With "targetHints"

   [[CREF6: It would be good to also include a section with CoAP
   examples.]]

   JSON serializations of HTTP response header information SHOULD follow
   the guidelines established by the work in progress "A JSON Encoding
   for HTTP Header Field Values" [I-D.reschke-http-jfv].  Approaches
   shown in that document's examples SHOULD be applied to other
   similarly structured headers wherever possible.

   Headers for all possible HTTP method responses all share
   "headerSchema".  In particular, both headers that appear in a HEAD

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   response and those that appear in an OPTIONS response can appear.  No
   distinction is made within "headerSchema" as to which method response
   contains which header.

   It is RECOMMENDED that schema authors provide hints for the values of
   the following types of HTTP headers whenever applicable:

   o  Method allowance

   o  Method-specific request media types

   o  Authentication challenges

   In general, headers that are likely to have different values at
   different times SHOULD NOT be included in "targetHints".

8.5.  Advertising HTTP Features With "headerSchema"

   Schemas SHOULD be written to describe JSON serializations that follow
   guidelines established by the work in progress "A JSON Encoding for
   HTTP Header Field Values" [I-D.reschke-http-jfv] Approaches shown in
   that document's examples SHOULD be applied to other similarly
   structured headers wherever possible.

   It is RECOMMENDED that schema authors describe the available usage of
   the following types of HTTP headers whenever applicable:

   o  Content negotiation

   o  Authentication and authorization

   o  Range requests

   o  The "Prefer" header

   Headers such as cache control and conditional request headers are
   generally implemented by intermediaries rather than the resource, and
   are therefore not generally useful to describe.  While the resource
   must supply the information needed to use conditional requests, the
   runtime handling of such headers and related responses is not
   resource-specific.

8.6.  Creating Resources Through Collections

   When using HTTP, or a protocol such as CoAP that is explicitly
   analogous to HTTP, this is done by POST-ing a representation of the
   individual resource to be created to the collection resource.  The

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   process for recognizing collection and item resources is described in
   Section 6.2.3.

8.7.  Content Negotiation and Schema Evolution

   JSON Hyper-Schema facilitates HTTP content negotiation, and allows
   for a hybrid of the proactive and reactive strategies.  As mentioned
   above, a hyper-schema can include a schema for HTTP headers such as
   "Accept", "Accept-Charset", "Accept-Language", etc with the
   "headerSchema" keyword.  A user agent or client application can use
   information in this schema, such as an enumerated list of supported
   languages, in lieu of making an initial request to start the reactive
   negotiation process.

   In this way, the proactive content negotiation technique of setting
   these headers can be informed by server information about what values
   are possible, similar to examining a list of alternatives in reactive
   negotiation.

   For media types that allow specifying a schema as a media type
   parameter, the "Accept" values sent in a request or advertised in
   "headerSchema" can include the URI(s) of the schema(s) to which the
   negotiated representation is expected to conform.  One possible use
   for schema parameters in content negotiation is if the resource has
   conformed to several different schema versions over time.  The client
   application can indicate what version(s) it understands in the
   "Accept" header in this way.

9.  Examples

   This section shows how the keywords that construct URIs and JSON
   Pointers are used.  The results are shown in the format used by the
   test suite.  [[CREF7: Need to post that and link it, but it should be
   pretty self-explanatory to those of you reviewing things at this
   stage.  ]]

   Most other keywords are either straightforward ("title" and
   "description"), apply validation to specific sorts of input,
   requests, or responses, or have protocol-specific behavior.  Examples
   demonstrating HTTP usage are available in an Appendix (Section 8).

9.1.  Entry Point Links, No Templates

   For this example, we will assume an example API with a documented
   entry point URI of https://example.com, which is an empty JSON object
   with a link to a schema.  Here, the entry point has no data of its
   own and exists only to provide an initial set of links:

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   GET https://api.example.com HTTP/1.1

   200 OK
   Content-Type: application/json
   Link: <https://schema.example.com/entry> rel=describedBy
   {}

   The linked hyper-schema defines the API's base URI and provides two
   links: an "about" link to API documentation, and a "self" link
   indicating that this is a schema for the base URI.  In this case the
   base URI is also the entry point URI.

   {
       "$id": "https://schema.example.com/entry",
       "$schema": "http://json-schema.org/draft-07-wip/hyper-schema#",
       "base": "https://api.example.com",
       "links": [
           {
               "rel": "self",
               "href": ""
           }, {
               "rel": "about",
               "href": "/docs"
           }
       ]
   }

   These are the simplest possible links, with only a relation type and
   an "href" with no template variables.  They resolve as follows:

   [
       {
           "contextUri": "https://api.example.com",
           "contextPointer": "",
           "rel": "self",
           "targetUri": "https://api.example.com",
           "attachmentPointer": ""
       },
       {
           "contextUri": "https://api.example.com",
           "contextPointer": "",
           "rel": "about",
           "targetUri": "https://api.example.com/docs",
           "attachmentPointer": ""
       }
   ]

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   The attachment pointer is the root pointer (the only possibility with
   an empty object for the instance).  The context URI is the default,
   which is the requested document.  Since application/json does not
   allow for fragments, the context pointer is necessary to fully
   describe the context.  Its default behavior is to be the same as the
   attachment pointer.

9.2.  Individually Identified Resources

   Let's add "things" to our system, starting with an individual thing:

   {
       "$id": "https://schema.example.com/thing",
       "$schema": "http://json-schema.org/draft-07-wip/hyper-schema#",
       "base": "https://api.example.com",
       "type": "object",
       "required": ["data"],
       "properties": {
           "id": {"$ref": "#/definitions/id"},
           "data": true
       },
       "links": [
           {
               "rel": "self",
               "href": "things/{id}",
               "templateRequired": ["id"],
               "targetSchema": {"$ref": "#"}
           }
       ],
       "definitions": {
           "id": {
               "type": "integer",
               "minimum": 1,
               "readOnly": true
           }
       }
   }

   Our "thing" has a server-assigned id, which is required in order to
   construct the "self" link.  It also has a "data" field which can be
   of any type.  The reason for the "definitions" section will be clear
   in the next example.

   Note that "id" is not required by the validation schema, but is
   required by the self link.  This makes sense: a "thing" only has a
   URI if it has been created, and the server has assigned an id.
   However, you can use this schema with an instance containing only the

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   data field, which allows you to validate "thing" instances that you
   are about to create.

   Let's add a link to our entry point schema that lets you jump
   directly to a particular thing if you can supply it's id as input.
   To save space, only the new LDO is shown.  Unlike "self" and "about",
   there is no IANA-registered relationship about hypothetical things,
   so an extension relationship is defined using the "tag:" URI scheme
   [RFC4151]:

   {
       "rel": "tag:rel.example.com,2017:thing",
       "href": "things/{id}",
       "hrefSchema": {
           "required": ["id"],
           "properties": {
               "id": {"$ref": "thing#/definitions/id"}
           }
       },
       "targetSchema": {"$ref": "thing#"}
   }

   The "href" value here is the same, but everything else is different.
   Recall that the instance is an empty object, so "id" cannot be
   resolved from instance data.  Instead it is required as client input.
   This LDO could also have used "templateRequired" but with "required"
   in "hrefSchema" it is not strictly necessary.  Providing
   "templateRequired" without marking "id" as required in "hrefSchema"
   would lead to errors, as client input is the only possible source for
   resolving this link.

9.3.  Submitting a Payload and Accepting URI Input

   This example covers using the "submission" fields for non-
   representation input, as well as using them alongside of resolving
   the URI Template with input.  Unlike HTML forms, which require either
   constructing a URI or sending a payload, but do not allow not both at
   once, JSON Hyper-Schema can describe both sorts of input for the same
   operation on the same link.

   The "submissionSchema" and "submissionMediaType" fields are for
   describing payloads that are not representations of the target
   resource.  When used with "http(s)://" URIs, they generally refer to
   a POST request payload, as seen in the appendix on HTTP usage
   (Section 8).

   In this case, we use a "mailto:" URI, which, per RFC 6068, Section 3"
   [RFC6068], does not provide any operation for retrieving a resource.

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   It can only be used to construct a message for sending.  Since there
   is no concept of a retrievable, replaceable, or deletable target
   resource, "targetSchema" and "targetMediaType" are not used.  Non-
   representation payloads are described by "submissionSchema" and
   "submissionMediaType".

   We use "submissionMediaType" to indicate a multipart/alternative
   payload format, providing two representations of the same data (HTML
   and plain text).  Since a multipart/alternative message is an ordered
   sequence (the last part is the most preferred alternative), we model
   the sequence as an array in "submissionSchema".  Since each part is
   itself a document with a media type, we model each item in the array
   as a string, using "contentMediaType" to indicate the format within
   the string.

   Note that media types such as multipart/form-data, which associate a
   name with each part and are not ordered, should be modeled as JSON
   objects rather than arrays.

   Note that some lines are wrapped to fit this document's width
   restrictions.

   {
       "$id": "https://schema.example.com/interesting-stuff",
       "$schema": "http://json-schema.org/draft-07-wip/hyper-schema#",
       "required": ["stuffWorthEmailingAbout", "email", "title"],
       "properties": {
           "title": {
               "type": "string"
           },
           "stuffWorthEmailingAbout": {
               "type": "string"
           },
           "email": {
               "type": "string",
               "format": "email"
           },
           "cc": false
       },
       "links": [
           {
               "rel": "author",
               "href": "mailto:{email}?subject={title}{&cc}",
               "templateRequired": ["email"],
               "hrefSchema": {
                   "required": ["title"],
                   "properties": {
                       "title": {

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                           "type": "string"
                       },
                       "cc": {
                           "type": "string",
                           "format": "email"
                       },
                       "email": false
                   }
               },
               "submissionMediaType":
                       "multipart/alternative; boundary=ab2",
               "submissionSchema": {
                   "type": "array",
                   "items": [
                       {
                           "type": "string",
                           "contentMediaType":
                                   "text/plain; charset=utf8"
                       },
                       {
                           "type": "string",
                           "contentMediaType": "text/html"
                       }
                   ],
                   "minItems": 2
               }
           }
       ]
   }

   For the URI parameters, each of the three demonstrates a different
   way of resolving the input:

   email:  This variable's presence in "templateRequired" means that it
      must be resolved for the template to be used.  Since the "false"
      schema assigned to it in "hrefSchema" excludes it from the input
      data set, it must be resolved from the instance.

   title:  The instance field matching this variable is required, and it
      is also allowed in the input data.  So its instance value is used
      to pre-populate the input data set before accepting client input.
      The client application can opt to leave the instance value in
      place.  Since this field is required in "hrefSchema", the client
      application cannot delete it (although it could set it to an empty
      string).

   cc:  The "false" schema set for this in the main schema prevents this
      field from having an instance value.  If it is present at all, it

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      must come from client input.  As it is not required in
      "hrefSchema", it may not be used at all.

   So, given the following instance retrieved from
   "https://api.example.com/stuff":

   {
       "title": "The Awesome Thing",
       "stuffWorthEmailingAbout": "Lots of text here...",
       "email": "someone@exapmle.com"
   }

   We can partially resolve the link as follows, before asking the
   client application for input.

   {
       "contextUri": "https://api.example.com/stuff",
       "contextPointer": "",
       "rel": "author",
       "hrefInputTemplates": [
         "mailto:someone@example.com?subject={title}{&cc}"
       ],
       "hrefPrepopulatedInput": {
           "title": "The Really Awesome Thing"
       },
       "attachmentPointer": ""
   }

   Notice the "href*" keywords in place of "targetUri".  These are three
   possible kinds of "targetUri" values covering different sorts of
   input.  Here are examples of each:

   No additional or changed input:  "mailto:someone@example.com?subject=
      The%20Awesome%20Thing"

   Change "title" to "your work":  "mailto:someone@example.com?subject=y
      our%20work"

   Change title and add a "cc" of "other@elsewhere.org":
      "mailto:someone@example.com?subject=your%20work&cc=other@elsewhere
      .org"

9.4.  "anchor", "base" and URI Template Resolution

   A link is a typed connection from a context resource to a target
   resource.  Older link serializations support a "rev" keyword that
   takes a link relation type as "rel" does, but reverses the semantics.
   This has long been deprecated, so JSON Hyper-Schema does not support

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   it.  Instead, "anchor"'s ability to change the context URI can be
   used to reverse the direction of a link.  It can also be used to
   describe a link between two resources, neither of which is the
   current resource.

   As an example, there is an IANA-registered "up" relation, but there
   is no "down".  In an HTTP Link header, you could implement "down" as
   ""rev": "up"".

   First let's look at how this could be done in HTTP, showing a "self"
   link and two semantically identical links, one with "rev": "up" and
   the other using "anchor" with "rel": "up" (line wrapped due to
   formatting limitations).

   GET https://api.example.com/trees/1/nodes/123 HTTP/1.1

   200 OK
   Content-Type: application/json
   Link: <https://api.example.com/trees/1/nodes/123> rel=self
   Link: <https://api.example.com/trees/1/nodes/123> rel=up
           anchor=<https://api.example.com/trees/1/nodes/456>
   Link: <https://api.example.com/trees/1/nodes/456> rev=up
   {
       "id": 123,
       "treeId": 1,
       "childIds": [456]
   }

   Note that the "rel=up" link has a target URI identical to the
   "rel=self" link, and sets "anchor" (which identifies the link's
   context) to the child's URI.  This sort of reversed link is easily
   detectable by tools when a "self" link is also present.

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   The following hyper-schema, applied to the instance in the response
   above, would produce the same "self" link and "up" link with
   "anchor".  It also shows the use of a templated "base" URI, plus both
   absolute and relative JSON Pointers in "templatePointers".

       "base": "trees/{treeId}",
       "properties": {
           "id": {"type": "integer"},
           "treeId": {"type": "integer"},
           "childIds": {
               "type": "array",
               "items": {
                   "type": "integer",
                   "links": [
                       {
                           "anchor": "nodes/{thisNodeId}",
                           "rel": "up",
                           "href": "nodes/{childId}",
                           "templatePointers": {
                               "thisNodeId": "/id",
                               "childId": "0"
                           }
                       }
                   ]
               }
           }
       },
       "links": [
           {
               "rel": "self",
               "href": "nodes/{id}"
           }
       ]
   }

   The "base" template is evaluated identically for both the target
   ("href") and context ("anchor") URIs.

   Note the two different sorts of templatePointers used.  "thisNodeId"
   is mapped to an absolute JSON Pointer, "/id", while "childId" is
   mapped to a relative pointer, "0", which indicates the value of the
   current item.  Absolute JSON Pointers do not support any kind of
   wildcarding, so there is no way to specify a concept like "current
   item" without a relative JSON Pointer.

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9.5.  Collections

   In many systems, individual resources are grouped into collections.
   Those collections also often provide a way to create individual item
   resources with server-assigned identifiers.

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   For this example, we will re-use the individual thing schema as shown
   in an earlier section.  It is repeated here for convenience, with an
   added "collection" link with a "targetSchema" reference pointing to
   the collection schema we will introduce next.

   {
       "$id": "https://schema.example.com/thing",
       "$schema": "http://json-schema.org/draft-07-wip/hyper-schema#",
       "base": "https://api.example.com",
       "type": "object",
       "required": ["data"],
       "properties": {
           "id": {"$ref": "#/definitions/id"},
           "data": true
       },
       "links": [
           {
               "rel": "self",
               "href": "things/{id}",
               "templateRequired": ["id"],
               "targetSchema": {"$ref": "#"}
           }, {
               "rel": "collection",
               "href": "/things",
               "targetSchema": {"$ref": "thing-collection#"},
               "submissionSchema": {"$ref": "#"}
           }
       ],
       "definitions": {
           "id": {
               "type": "integer",
               "minimum": 1,
               "readOnly": true
           }
       }
   }

   The "collection" link is the same for all items, so there are no URI
   Template variables.  The "submissionSchema" is that of the item
   itself.  As described in Section 6.2.3, if a "collection" link
   supports a submission mechanism (POST in HTTP) then it MUST implement
   item creation semantics.  Therefore "submissionSchema" is the schema
   for creating a "thing" via this link.

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   Now we want to describe collections of "thing"s.  This schema
   describes a collection where each item representation is identical to
   the individual "thing" representation.  While many collection
   representations only include subset of the item representations, this
   example uses the entirety to minimize the number of schemas involved.
   The actual collection items appear as an array within an object, as
   we will add more fields to the object in the next example.

   {
       "$id": "https://schema.example.com/thing-collection",
       "$schema": "http://json-schema.org/draft-07-wip/hyper-schema#",
       "base": "https://api.example.com",
       "type": "object",
       "required": ["elements"],
       "properties": {
           "elements": {
               "type": "array",
               "items": {
                   "allOf": [{"$ref": "thing#"}],
                   "links": [
                       {
                           "anchorPointer": "",
                           "rel": "item",
                           "href": "things/{id}",
                           "templateRequired": ["id"],
                           "targetSchema": {"$ref": "thing#"}
                       }
                   ]
               }
           }
       },
       "links": [
           {
               "rel": "self",
               "href": "things",
               "targetSchema": {"$ref": "#"},
               "submissionSchema": {"$ref": "thing"}
           }
       ]
   }

   {
       "elements": [
           {"id": 12345, "data": {}},
           {"id": 67890, "data": {}}
       ]
   }

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   Here are all of the links that apply to this instance, including
   those that are defined in the referenced individual "thing" schema:

   [
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "",
           "rel": "self",
           "targetUri": "https://api.example.com/things",
           "attachmentPointer": ""
       },
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "/elements/0",
           "rel": "self",
           "targetUri": "https://api.example.com/things/12345",
           "attachmentPointer": "/elements/0"
       },
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "/elements/1",
           "rel": "self",
           "targetUri": "https://api.example.com/things/67890",
           "attachmentPointer": "/elements/1"
       },
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "",
           "rel": "item",
           "targetUri": "https://api.example.com/things/12345",
           "attachmentPointer": "/elements/0"
       },
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "",
           "rel": "item",
           "targetUri": "https://api.example.com/things/67890",
           "attachmentPointer": "/elements/1"
       },
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "/elements/0",
           "rel": "collection",
           "targetUri": "https://api.example.com/things",
           "attachmentPointer": "/elements/0"
       },
       {
           "contextUri": "https://api.example.com/things",

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           "contextPointer": "/elements/1",
           "rel": "collection",
           "targetUri": "https://api.example.com/things",
           "attachmentPointer": "/elements/1"
       }
   ]

   In all cases, the context URI is shown for an instance of media type
   application/json, which does not support fragments.  If the instance
   media type was application/instance+json, which supports JSON Pointer
   fragments, then the context URIs would contain fragments identical to
   the context pointer field.  For application/json and other media
   types without fragments, it is critically important to consider the
   context pointer as well as the context URI.

   There are three "self" links, one for the collection, and one for
   each item in the "elements" array.  The item "self" links are defined
   in the individual "thing" schema which is referenced with "$ref".
   The three links can be distinguished by their context or attachment
   pointers.  We will revisit the "submissionSchema" of the collection's
   "self" link in Section 9.5.2.

   There are two "item" links, one for each item in the "elements"
   array.  Unlike the "self" links, these are defined only in the
   collection schema.  Each of them have the same target URI as the
   corresponding "self" link that shares the same attachment pointer.
   However, each has a different context pointer.  The context of the
   "self" link is the entry in "elements", while the context of the
   "item" link is always the entire collection regardless of the
   specific item.

   Finally, there are two "collection" links, one for each item in
   "elements".  In the individual item schema, these produce links with
   the item resource as the context.  When referenced from the
   collection schema, the context is the location in the "elements"
   array of the relevant "thing", rather than that "thing"'s own
   separate resource URI.

   The collection links have identical target URIs as there is only one
   relevant collection URI.  While calculating both links as part of a
   full set of constructed links may not seem useful, when constructing
   links on an as-needed basis, this arrangement means that there is a
   "collection" link definition close at hand no matter which "elements"
   entry you are processing.

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9.5.1.  Pagination

   Here we add pagination to our collection.  There is a "meta" section
   to hold the information about current, next, and previous pages.
   Most of the schema is the same as in the previous section and has
   been omitted.  Only new fields and new or (in the case of the main
   "self" link) changed links are shown in full.

   {
       "properties": {
           "elements": {
               ...
           },
           "meta": {
               "type": "object",
               "properties": {
                   "prev": {"$ref": "#/definitions/pagination"},
                   "current": {"$ref": "#/definitions/pagination"},
                   "next": {"$ref": "#/definitions/pagination"}
               }
           }
       },
       "links": [
           {
               "rel": "self",
               "href": "things{?offset,limit}",
               "templateRequired": ["offset", "limit"],
               "templatePointers": {
                   "offset": "/meta/current/offset",
                   "limit": "/meta/current/limit"
               },
               "targetSchema": {"$ref": "#"}
           }, {
               "rel": "prev",
               "href": "things{?offset,limit}",
               "templateRequired": ["offset", "limit"],
               "templatePointers": {
                   "offset": "/meta/prev/offset",
                   "limit": "/meta/prev/limit"
               },
               "targetSchema": {"$ref": "#"}
           }, {
               "rel": "next",
               "href": "things{?offset,limit}",
               "templateRequired": ["offset", "limit"],
               "templatePointers": {
                   "offset": "/meta/next/offset",
                   "limit": "/meta/next/limit"

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               },
               "targetSchema": {"$ref": "#"}
           }
       ],
       "definitions": {
           "pagination": {
               "type": "object",
               "properties": {
                   "offset": {
                       "type": "integer",
                       "minimum": 0,
                       "default": 0
                   },
                   "limit": {
                       "type": "integer",
                       "minimum": 1,
                       "maximum": 100,
                       "default": 10
                   }
               }
           }
       }
   }

   Notice that the "self" link includes the pagination query that
   produced the exact representation, rather than being a generic link
   to the collection allowing selecting the page via input.

   Given this instance:

   {
       "elements": [
           {"id": 12345, "data": {}},
           {"id": 67890, "data": {}}
       ],
       "meta": {
           "current": {
               "offset": 0,
               "limit": 2
           },
           "next": {
               "offset": 3,
               "limit": 2
           }
       }
   }

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   Here are all of the links that apply to this instance that either did
   not appear in the previous example or have been changed with
   pagination added.

   [
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "",
           "rel": "self",
           "targetUri":
               "https://api.example.com/things?offset=20,limit=2",
           "attachmentPointer": ""
       },
       {
           "contextUri": "https://api.example.com/things",
           "contextPointer": "",
           "rel": "next",
           "targetUri":
               "https://api.example.com/things?offset=22,limit=2",
           "attachmentPointer": ""
       }
   ]

   Note that there is no "prev" link in the output, as we are looking at
   the first page.  The lack of a "prev" field under "meta", together
   with the "prev" link's "templateRequired" values, means that the link
   is not usable with this particular instance.

   [[CREF8: It's not clear how pagination should work with the link from
   the "collection" links in the individual "thing" schema.
   Technically, a link from an item to a paginated or filtered
   collection should go to a page/filter that contains the item (in this
   case the "thing") that is the link context.  See GitHub issue #421
   for more discussion.  ]]

   Let's add a link for this collection to the entry point schema
   (Section 9.1), including pagination input in order to allow client
   applications to jump directly to a specific page.  Recall that the
   entry point schema consists only of links, therefore we only show the
   newly added link:

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   {
       "rel": "tag:rel.example.com,2017:thing-collection",
       "href": "/things{?offset,limit}",
       "hrefSchema": {
           "$ref": "thing-collection#/definitions/pagination"
       },
       "submissionSchema": {
           "$ref": "thing#"
       },
       "targetSchema": {
           "$ref": "thing-collection#"
       }
   }

   Now we see the pagination parameters being accepted as input, so we
   can jump to any page within the collection.  The link relation type
   is a custom one as the generic "collection" link can only be used
   with an item as its context, not an entry point or other resource.

9.5.2.  Creating the First Item

   When we do not have any "thing"s, we do not have any resources with a
   relevant "collection" link.  Therefore we cannot use a "collection"
   link's submission keywords to create the first "thing"; hyper-schemas
   must be evaluated with respect to an instance.  Since the "elements"
   array in the collection instance would be empty, it cannot provide us
   with a collection link either.

   However, our entry point link can take us to the empty collection,
   and we can use the presence of "item" links in the hyper-schema to
   recognize that it is a collection.  Since the context of the "item"
   link is the collection, we simply look for a "self" link with the
   same context, which we can then treat as collection for the purposes
   of a creation operation.

   Presumably, our custom link relation type in the entry point schema
   was sufficient to ensure that we have found the right collection.  A
   client application that recognizes that custom link relation type may
   know that it can immediately assume that the target is a collection,
   but a generic user agent cannot do so.  Despite the presence of a
   "-collection" suffix in our example, a generic user agent would have
   no way of knowing whether that substring indicates a hypermedia
   resource collection, or some other sort of collection.

   Once we have recognized the "self" link as being for the correct
   collection, we can use its "submissionSchema" and/or
   "submissionMediaType" keywords to perform an item creation operation.
   [[CREF9: This works perfectly if the collection is unfiltered and

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   unpaginated.  However, one should generally POST to a collection that
   will contain the created resource, and a "self" link MUST include any
   filters, pagination, or other query parameters.  Is it still valid to
   POST to such a "self" link even if the resulting item would not match
   the filter or appear within that page?  See GitHub issue #421 for
   further discussion.  ]] [[CREF10: Draft-04 of Hyper-Schema defined a
   "create" link relation that had the schema, rather than the instance,
   as its context.  This did not fit into the instance-based link model,
   and incorrectly used an operation name for a link relation type.
   However, defining a more correctly designed link from the schema to
   the collection instance may be one possible approach to solving this.
   Again, see GitHub issue #421 for more details.  ]]

10.  Security Considerations

   JSON Hyper-Schema defines a vocabulary for JSON Schema core and
   concerns all the security considerations listed there.  As a link
   serialization format, the security considerations of RFC 8288 Web
   Linking [RFC8288] also apply, with appropriate adjustments (e.g.
   "anchor" as an LDO keyword rather than an HTTP Link header
   attribute).

10.1.  Target Attributes

   As stated in Section 6.5, all LDO keywords describing the target
   resource are advisory and MUST NOT be used in place of the
   authoritative information supplied by the target resource in response
   to an operation.  Target resource responses SHOULD indicate their own
   hyper-schema, which is authoritative.

   If the hyper-schema in the target response matches (by "$id") the
   hyper-schema in which the current LDO was found, then the target
   attributes MAY be considered authoritative.  [[CREF11: Need to add
   something about the risks of spoofing by "$id", but given that other
   parts of the specification discourage always re-downloading the
   linked schema, the risk mitigation options are unclear.  ]]

   User agents or client applications MUST NOT use the value of
   "targetSchema" to aid in the interpretation of the data received in
   response to following the link, as this leaves "safe" data open to
   re-interpretation.

   When choosing how to interpret data, the type information provided by
   the server (or inferred from the filename, or any other usual method)
   MUST be the only consideration, and the "targetMediaType" property of
   the link MUST NOT be used.  User agents MAY use this information to
   determine how they represent the link or where to display it (for
   example hover-text, opening in a new tab).  If user agents decide to

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   pass the link to an external program, they SHOULD first verify that
   the data is of a type that would normally be passed to that external
   program.

   This is to guard against re-interpretation of "safe" data, similar to
   the precautions for "targetSchema".

   Protocol meta-data values conveyed in "targetHints" MUST NOT be
   considered authoritative.  Any security considerations defined by the
   protocol that may apply based on incorrect assumptions about meta-
   data values apply.

   Even when no protocol security considerations are directly
   applicable, implementations MUST be prepared to handle responses that
   do not match the link's "targetHints" values.

10.2.  "self" Links

   When link relation of "self" is used to denote a full representation
   of an object, the user agent SHOULD NOT consider the representation
   to be the authoritative representation of the resource denoted by the
   target URI if the target URI is not equivalent to or a sub-path of
   the URI used to request the resource representation which contains
   the target URI with the "self" link.  [[CREF12: It is no longer
   entirely clear what was intended by the "sub-path" option in this
   paragraph.  It may have been intended to allow "self" links for
   embedded item representations in a collection, which usually have
   target URIs that are sub-paths of that collection's URI, to be
   considered authoritative.  However, this is simply a common design
   convention and does not appear to be based in RFC 3986 or any other
   guidance on URI usage.  See GitHub issue #485 for further discussion.
   ]]

11.  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.

12.  References

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12.1.  Normative References

   [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.

   [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-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.

   [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>.

   [RFC4287]  Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
              Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
              December 2005, <https://www.rfc-editor.org/info/rfc4287>.

   [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>.

   [RFC6573]  Amundsen, M., "The Item and Collection Link Relations",
              RFC 6573, DOI 10.17487/RFC6573, April 2012,
              <https://www.rfc-editor.org/info/rfc6573>.

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

12.2.  Informative References

   [I-D.reschke-http-jfv]
              Reschke, J., "A JSON Encoding for HTTP Header Field
              Values", draft-reschke-http-jfv-06 (work in progress),
              June 2017.

   [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>.

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, DOI 10.17487/RFC4151, October 2005,
              <https://www.rfc-editor.org/info/rfc4151>.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, DOI 10.17487/RFC5789, March 2010,
              <https://www.rfc-editor.org/info/rfc5789>.

   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
              <https://www.rfc-editor.org/info/rfc6068>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [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>.

   [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
              APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
              <https://www.rfc-editor.org/info/rfc7807>.

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Appendix A.  Using JSON Hyper-Schema in APIs

   Hypermedia APIs, which follow the constraints of the REST
   architectural style, enable the creation of generic user agents.
   Such a user agent has no application-specific knowledge.  Rather, it
   understands pre-defined media types, URI schemes, protocols, and link
   relations, often by recognizing these and coordinating the use of
   existing software that implements support for them.  Client
   applications can then be built on top of such a user agent, focusing
   on their own semantics and logic rather than the mechanics of the
   interactions.

   Hyper-schema is only concerned with one resource and set of
   associated links at a time.  Just as a web browser works with only
   one HTML page at a time, with no concept of whether or how that page
   functions as part of a "site", a hyper-schema-aware user agent works
   with one resource at a time, without any concept of whether or how
   that resource fits into an API.

   Therefore, hyper-schema is suitable for use within an API, but is not
   suitable for the description of APIs as complete entities in their
   own right.  There is no way to describe concepts at the API scope,
   rather than the resource and link scope, and such descriptions are
   outside of the boundaries of JSON Hyper-Schema.

A.1.  Resource Evolution With Hyper-Schema

   Since a given JSON Hyper-Schema is used with a single resource at a
   single point in time, it has no inherent notion of versioning.
   However, a given resource can change which schema or schemas it uses
   over time, and the URIs of these schemas can be used to indicate
   versioning information.  When used with a media type that supports
   indicating a schema with a media type parameter, these versioned
   schema URIs can be used in content negotiation.

   A resource can indicate that it is an instance of multiple schemas,
   which allows supporting multiple compatible versions simultaneously.
   A client application can then make use of the hyper-schema that it
   recognizes, and ignore newer or older versions.

A.2.  Responses and Errors

   Because a hyper-schema represents a single resource at a time, it
   does not provide for an enumeration of all possible responses to
   protocol operations performed with links.  Each response, including
   errors, is considered its own (possibly anonymous) resource, and
   should identify its own hyper-schema, and optionally use an
   appropriate media type such as RFC 7807's "application/problem+json"

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   [RFC7807], to allow the user agent or client application to interpret
   any information that is provided beyond the protocol's own status
   reporting.

A.3.  Static Analysis of an API's Hyper-Schemas

   It is possible to statically analyze a set of hyper-schemas without
   instance data in order to generate output such as documentation or
   code.  However, the full feature set of both validation and hyper-
   schema cannot be accessed without runtime instance data.

   This is an intentional design choice to provide the maximum runtime
   flexibility for hypermedia systems.  JSON Schema as a media type
   allows for establishing additional vocabularies for static analysis
   and content generation, which are not addressed in this
   specification.  Additionally, individual systems may restrict their
   usage to subsets that can be analyzed statically if full design-time
   description is a goal.  [[CREF13: Vocabularies for API documentation
   and other purposes have been proposed, and contributions are welcome
   at https://github.com/json-schema-org/json-schema-vocabularies ]]

Appendix B.  ChangeLog

   [[CREF14: This section to be removed before leaving Internet-Draft
   status.]]

   draft-handrews-json-schema-hyperschema-00

      *  Top to bottom reorganization and rewrite

      *  Group keywords per RFC 8288 context/relation/target/target
         attributes

      *  Additional keyword groups for template resolution and
         describing input

      *  Clarify implementation requirements with a suggested output
         format

      *  Expanded overview to provide context

      *  Consolidated examples into their own section, illustrate real-
         world patterns

      *  Consolidated HTTP guidance in its own section

      *  Added a subsection on static analysis of hyper-schemas

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      *  Consolidated security concerns in their own section

      *  Added an appendix on usage in APIs

      *  Moved "readOnly" to the validation specification

      *  Moved "media" to validation as
         "contentMediaType"/"contentEncoding"

      *  Renamed "submissionEncType" to "submissionMediaType"

      *  Renamed "mediaType" to "targetMediaType"

      *  Added "anchor" and "anchorPointer"

      *  Added "templatePointers" and "templateRequired"

      *  Clarified how "hrefSchema" is used

      *  Added "targetHints" and "headerSchema"

      *  Added guidance on "self", "collection" and "item" link usage

      *  Added "description" as an LDO keyword

      *  Added "$comment" in LDOs to match the schema keyword

   draft-wright-json-schema-hyperschema-01

      *  Fixed examples

      *  Added "hrefSchema" for user input to "href" URI Templates

      *  Removed URI Template pre-processing

      *  Clarified how links and data submission work

      *  Clarified how validation keywords apply hyper-schema keywords
         and links

      *  Clarified HTTP use with "targetSchema"

      *  Renamed "schema" to "submissionSchema"

      *  Renamed "encType" to "submissionEncType"

      *  Removed "method"

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   draft-wright-json-schema-hyperschema-00

      *  "rel" is now optional

      *  rel="self" no longer changes URI base

      *  Added "base" keyword to change instance URI base

      *  Removed "root" link relation

      *  Removed "create" link relation

      *  Removed "full" link relation

      *  Removed "instances" link relation

      *  Removed special behavior for "describedBy" link relation

      *  Removed "pathStart" keyword

      *  Removed "fragmentResolution" keyword

      *  Updated references to JSON Pointer, HTML

      *  Changed behavior of "method" property to align with hypermedia
         best current practices

   draft-luff-json-hyper-schema-01

      *  Split from main specification.

Authors' Addresses

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

   EMail: henry@cloudflare.com

   Austin Wright (editor)

   EMail: aaa@bzfx.net

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