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Internet Engineering Task Force                              K. Zyp, Ed.
Internet-Draft                                             SitePen (USA)
Intended status: Informational                                  G. Court
Expires: May 26, 2011                                  November 22, 2010


   A JSON Media Type for Describing the Structure and Meaning of JSON
                               Documents
                        draft-zyp-json-schema-03

Abstract

   JSON (JavaScript Object Notation) Schema defines the media type
   "application/schema+json", a JSON based format for defining the
   structure of JSON data.  JSON Schema provides a contract for what
   JSON data is required for a given application and how to interact
   with it.  JSON Schema is intended to define validation,
   documentation, hyperlink navigation, and interaction control of JSON
   data.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 26, 2011.

Copyright Notice

   Copyright (c) 2010 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must



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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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Design Considerations  . . . . . . . . . . . . . . . . . .  6
   4.  Schema/Instance Association  . . . . . . . . . . . . . . . . .  6
     4.1.  Self-Descriptive Schema  . . . . . . . . . . . . . . . . .  7
   5.  Core Schema Definition . . . . . . . . . . . . . . . . . . . .  7
     5.1.  type . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.2.  properties . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.3.  patternProperties  . . . . . . . . . . . . . . . . . . . .  9
     5.4.  additionalProperties . . . . . . . . . . . . . . . . . . .  9
     5.5.  items  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.6.  additionalItems  . . . . . . . . . . . . . . . . . . . . . 10
     5.7.  required . . . . . . . . . . . . . . . . . . . . . . . . . 10
     5.8.  dependencies . . . . . . . . . . . . . . . . . . . . . . . 10
     5.9.  minimum  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.10. maximum  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.11. exclusiveMinimum . . . . . . . . . . . . . . . . . . . . . 11
     5.12. exclusiveMaximum . . . . . . . . . . . . . . . . . . . . . 11
     5.13. minItems . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.14. maxItems . . . . . . . . . . . . . . . . . . . . . . . . . 11
     5.15. uniqueItems  . . . . . . . . . . . . . . . . . . . . . . . 11
     5.16. pattern  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.17. minLength  . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.18. maxLength  . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.19. enum . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.20. default  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.21. title  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.22. description  . . . . . . . . . . . . . . . . . . . . . . . 13
     5.23. format . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     5.24. divisibleBy  . . . . . . . . . . . . . . . . . . . . . . . 14
     5.25. disallow . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.26. extends  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     5.27. id . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.28. $ref . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.29. $schema  . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Hyper Schema . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.1.  links  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
       6.1.1.  Link Description Object  . . . . . . . . . . . . . . . 16
     6.2.  fragmentResolution . . . . . . . . . . . . . . . . . . . . 20
       6.2.1.  slash-delimited fragment resolution  . . . . . . . . . 20



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       6.2.2.  dot-delimited fragment resolution  . . . . . . . . . . 21
     6.3.  readonly . . . . . . . . . . . . . . . . . . . . . . . . . 21
     6.4.  contentEncoding  . . . . . . . . . . . . . . . . . . . . . 22
     6.5.  pathStart  . . . . . . . . . . . . . . . . . . . . . . . . 22
     6.6.  mediaType  . . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  Registry of Link Relations . . . . . . . . . . . . . . . . 24
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 24
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 24
   Appendix A.  Change Log  . . . . . . . . . . . . . . . . . . . . . 26
   Appendix B.  Open Issues . . . . . . . . . . . . . . . . . . . . . 27






































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1.  Introduction

   JSON (JavaScript Object Notation) Schema is a JSON media type for
   defining the structure of JSON data.  JSON Schema provides a contract
   for what JSON data is required for a given application and how to
   interact with it.  JSON Schema is intended to define validation,
   documentation, hyperlink navigation, and interaction control of JSON
   data.

2.  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 Schema defines the media type "application/schema+json" for
   describing the structure of other JSON documents.  JSON Schema is
   JSON-based and includes facilities for describing the structure of
   JSON documents in terms of allowable values, descriptions, and
   interpreting relations with other resources.

   JSON Schema format is organized into several separate definitions.
   The first definition is the core schema specification.  This
   definition is primary concerned with describing a JSON structure and
   specifying valid elements in the structure.  The second definition is
   the Hyper Schema specification which is intended define elements in a
   structure that can be interpreted as hyperlinks.  Hyper Schema builds
   on JSON Schema to describe the hyperlink structure of other JSON
   documents and elements of interaction.  This allows user agents to be
   able to successfully navigate JSON documents based on their schemas.

   Cumulatively JSON Schema acts as a meta-document that can be used to
   define the required type and constraints on property values, as well
   as define the meaning of the property values for the purpose of
   describing a resource and determining hyperlinks within the
   representation.













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   An example JSON Schema that describes products might look like:

   {
     "name":"Product",
     "properties":{
       "id":{
         "type":"number",
         "description":"Product identifier",
         "required":true
       },
       "name":{
         "description":"Name of the product",
         "type":"string",
         "required":true
       },
       "price":{
         "required":true,
         "type": "number",
         "minimum":0,
         "required":true
       },
       "tags":{
         "type":"array",
         "items":{
           "type":"string"
         }
       }
     },
     "links":[
       {
         "rel":"full",
         "href":"{id}"
       },
       {
         "rel":"comments",
         "href":"comments/?id={id}"
       }
     ]
   }

   This schema defines the properties of the instance JSON documents,
   the required properties (id, name, and price), as well as an optional
   property (tags).  This also defines the link relations of the
   instance JSON documents.







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3.1.  Terminology

   For this specification, *schema* will be used to denote a JSON Schema
   definition, and an *instance* refers to a JSON value that the schema
   will be describing and validating.

3.2.  Design Considerations

   The JSON Schema media type does not attempt to dictate the structure
   of JSON representations that contain data, but rather provides a
   separate format for flexibly communicating how a JSON representation
   should be interpreted and validated, such that user agents can
   properly understand acceptable structures and extrapolate hyperlink
   information with the JSON document.  It is acknowledged that JSON
   documents come in a variety of structures, and JSON is unique in that
   the structure of stored data structures often prescribes a non-
   ambiguous definite JSON representation.  Attempting to force a
   specific structure is generally not viable, and therefore JSON Schema
   allows for a great flexibility in the structure of the JSON data that
   it describes.

   This specification is protocol agnostic.  The underlying protocol
   (such as HTTP) should sufficiently define the semantics of the
   client-server interface, the retrieval of resource representations
   linked to by JSON representations, and modification of those
   resources.  The goal of this format is to sufficiently describe JSON
   structures such that one can utilize existing information available
   in existing JSON representations from a large variety of services
   that leverage a representational state transfer architecture using
   existing protocols.

4.  Schema/Instance Association

   JSON Schema instances are correlated to their schema by the
   "describedby" relation, where the schema is defined to be the target
   of the relation.  Instance representations may be of the
   "application/json" media type or any other subtype.  Consequently,
   dictating how an instance representation should specify the relation
   to the schema is beyond the normative scope of this document (since
   this document specifically defines the JSON Schema media type, and no
   other), but it is recommended that instances specify their schema so
   that user agents can interpret the instance representation and
   messages may retain the self-descriptive characteristic, avoiding the
   need for out-of-band information about instance data.  Two approaches
   are recommended for declaring the relation to the schema that
   describes the meaning of a JSON instance's (or collection of
   instances) structure.  A MIME type parameter named "profile" or a
   relation of "describedby" (which could be defined by a Link header)



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   may be used:

   Content-Type: application/my-media-type+json;
                 profile=http://json.com/my-hyper-schema

   or if the content is being transferred by a protocol (such as HTTP)
   that provides headers, a Link header can be used:

   Link: <http://json.com/my-hyper-schema>; rel="describedby"

   Instances MAY specify multiple schemas, to indicate all the schemas
   that are applicable to the data, and the data SHOULD be valid by all
   the schemas.  The instance data MAY have multiple schemas that it is
   defined by (the instance data SHOULD be valid for those schemas).  Or
   if the document is a collection of instances, the collection MAY
   contain instances from different schemas.  When collections contain
   heterogeneous instances, the "pathStart" attribute MAY be specified
   in the schema to disambiguate which schema should be applied for each
   item in the collection.  However, ultimately, the mechanism for
   referencing a schema is up to the media type of the instance
   documents (if they choose to specify that schemas can be referenced).

4.1.  Self-Descriptive Schema

   JSON Schemas can themselves be described using JSON Schemas.  A self-
   describing JSON Schema for the core JSON Schema can be found at
   http://json-schema.org/schema for the latest version or
   http://json-schema.org/draft-03/schema for the draft-03 version.  The
   hyper schema self-description can be found at
   http://json-schema.org/hyper-schema or
   http://json-schema.org/draft-03/hyper-schema.  All schemas used
   within a protocol with media type definitions SHOULD include a MIME
   parameter that refers to the self-descriptive hyper schema or another
   schema that extends this hyper schema:

   Content-Type: application/json;
                 profile=http://json-schema.org/draft-03/hyper-schema

5.  Core Schema Definition

   A JSON Schema is a JSON Object that defines various attributes
   (including usage and valid values) of a JSON value.  JSON Schema has
   recursive capabilities; there are a number of elements in the
   structure that allow for nested JSON Schemas.

   An example JSON Schema definition could look like:





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   {
     "description":"A person",
     "type":"object",

     "properties":{
       "name":{"type":"string"},
       "age" :{
           "type":"integer",
           "maximum":125
       }
     }
   }

   A JSON Schema object may have any of the following properties, called
   schema attributes (all attributes are optional):

5.1.  type

   This attribute defines what the primitive type or the schema of the
   instance MUST be in order to validate.  This attribute can take one
   of two forms:

   Simple Types  A string indicating a primitive or simple type.  The
      following are acceptable string values:

      string  Value MUST be a string.

      number  Value MUST be a number, floating point numbers are
         allowed.

      integer  Value MUST be an integer, no floating point numbers are
         allowed.  This is a subset of the number type.

      boolean  Value MUST be a boolean.

      object  Value MUST be an object.

      array  Value MUST be an array.

      null  Value MUST be null.  Note this is mainly for purpose of
         being able use union types to define nullability.  If this type
         is not included in a union, null values are not allowed (the
         primitives listed above do not allow nulls on their own).

      any  Value MAY be of any type including null.

      If the property is not defined or is not in this list, then any
      type of value is acceptable.  Other type values MAY be used for



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      custom purposes, but minimal validators of the specification
      implementation can allow any instance value on unknown type
      values.

   Union Types  An array of two or more simple type definitions.  Each
      item in the array MUST be a simple type definition or a schema.
      The instance value is valid if it is of the same type as one of
      the simple type definitions, or valid by one of the schemas, in
      the array.

   For example, a schema that defines if an instance can be a string or
   a number would be:

   {"type":["string","number"]}

5.2.  properties

   This attribute is an object with property definitions that define the
   valid values of instance object property values.  When the instance
   value is an object, the property values of the instance object MUST
   conform to the property definitions in this object.  In this object,
   each property definition's value MUST be a schema, and the property's
   name MUST be the name of the instance property that it defines.  The
   instance property value MUST be valid according to the schema from
   the property definition.  Properties are considered unordered, the
   order of the instance properties MAY be in any order.

5.3.  patternProperties

   This attribute is an object that defines the schema for a set of
   property names of an object instance.  The name of each property of
   this attribute's object is a regular expression pattern in the ECMA
   262/Perl 5 format, while the value is a schema.  If the pattern
   matches the name of a property on the instance object, the value of
   the instance's property MUST be valid against the pattern name's
   schema value.

5.4.  additionalProperties

   This attribute defines a schema for all properties that are not
   explicitly defined in an object type definition.  If specified, the
   value MUST be a schema or a boolean.  If false is provided, no
   additional properties are allowed beyond the properties defined in
   the schema.  The default value is an empty schema which allows any
   value for additional properties.






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5.5.  items

   This attribute defines the allowed items in an instance array, and
   MUST be a schema or an array of schemas.  The default value is an
   empty schema which allows any value for items in the instance array.

   When this attribute value is a schema and the instance value is an
   array, then all the items in the array MUST be valid according to the
   schema.

   When this attribute value is an array of schemas and the instance
   value is an array, each position in the instance array MUST conform
   to the schema in the corresponding position for this array.  This
   called tuple typing.  When tuple typing is used, additional items are
   allowed, disallowed, or constrained by the "additionalItems"
   (Section 5.6) attribute using the same rules as
   "additionalProperties" (Section 5.4) for objects.

5.6.  additionalItems

   This provides a definition for additional items in an array instance
   when tuple definitions of the items is provided.  This can be false
   to indicate additional items in the array are not allowed, or it can
   be a schema that defines the schema of the additional items.

5.7.  required

   This attribute indicates if the instance must have a value, and not
   be undefined.  This is false by default, making the instance
   optional.

5.8.  dependencies

   This attribute is an object that defines the requirements of a
   property on an instance object.  If an object instance has a property
   with the same name as a property in this attribute's object, then the
   instance must be valid against the attribute's property value
   (hereafter referred to as the "dependency value").

   The dependency value can take one of two forms:

   Simple Dependency  If the dependency value is a string, then the
      instance object MUST have a property with the same name as the
      dependency value.  If the dependency value is an array of strings,
      then the instance object MUST have a property with the same name
      as each string in the dependency value's array.





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   Schema Dependency  If the dependency value is a schema, then the
      instance object MUST be valid against the schema.

5.9.  minimum

   This attribute defines the minimum value of the instance property
   when the type of the instance value is a number.

5.10.  maximum

   This attribute defines the maximum value of the instance property
   when the type of the instance value is a number.

5.11.  exclusiveMinimum

   This attribute indicates if the value of the instance (if the
   instance is a number) can not equal the number defined by the
   "minimum" attribute.  This is false by default, meaning the instance
   value can be greater then or equal to the minimum value.

5.12.  exclusiveMaximum

   This attribute indicates if the value of the instance (if the
   instance is a number) can not equal the number defined by the
   "maximum" attribute.  This is false by default, meaning the instance
   value can be less then or equal to the maximum value.

5.13.  minItems

   This attribute defines the minimum number of values in an array when
   the array is the instance value.

5.14.  maxItems

   This attribute defines the maximum number of values in an array when
   the array is the instance value.

5.15.  uniqueItems

   This attribute indicates that all items in an array instance MUST be
   unique (contains no two identical values).

   Two instance are consider equal if they are both of the same type
   and:

      are null; or





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      are booleans/numbers/strings and have the same value; or

      are arrays, contains the same number of items, and each item in
      the array is equal to the corresponding item in the other array;
      or

      are objects, contains the same property names, and each property
      in the object is equal to the corresponding property in the other
      object.

5.16.  pattern

   When the instance value is a string, this provides a regular
   expression that a string instance MUST match in order to be valid.
   Regular expressions SHOULD follow the regular expression
   specification from ECMA 262/Perl 5

5.17.  minLength

   When the instance value is a string, this defines the minimum length
   of the string.

5.18.  maxLength

   When the instance value is a string, this defines the maximum length
   of the string.

5.19.  enum

   This provides an enumeration of all possible values that are valid
   for the instance property.  This MUST be an array, and each item in
   the array represents a possible value for the instance value.  If
   this attribute is defined, the instance value MUST be one of the
   values in the array in order for the schema to be valid.  Comparison
   of enum values uses the same algorithm as defined in "uniqueItems"
   (Section 5.15).

5.20.  default

   This attribute defines the default value of the instance when the
   instance is undefined.

5.21.  title

   This attribute is a string that provides a short description of the
   instance property.





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5.22.  description

   This attribute is a string that provides a full description of the of
   purpose the instance property.

5.23.  format

   This property defines the type of data, content type, or microformat
   to be expected in the instance property values.  A format attribute
   MAY be one of the values listed below, and if so, SHOULD adhere to
   the semantics describing for the format.  A format SHOULD only be
   used to give meaning to primitive types (string, integer, number, or
   boolean).  Validators MAY (but are not required to) validate that the
   instance values conform to a format.  The following formats are
   predefined:

   date-time  This SHOULD be a date in ISO 8601 format of YYYY-MM-
      DDThh:mm:ssZ in UTC time.  This is the recommended form of date/
      timestamp.

   date  This SHOULD be a date in the format of YYYY-MM-DD.  It is
      recommended that you use the "date-time" format instead of "date"
      unless you need to transfer only the date part.

   time  This SHOULD be a time in the format of hh:mm:ss.  It is
      recommended that you use the "date-time" format instead of "time"
      unless you need to transfer only the time part.

   utc-millisec  This SHOULD be the difference, measured in
      milliseconds, between the specified time and midnight, 00:00 of
      January 1, 1970 UTC.  The value SHOULD be a number (integer or
      float).

   regex  A regular expression, following the regular expression
      specification from ECMA 262/Perl 5.

   color  This is a CSS color (like "#FF0000" or "red"), based on CSS
      2.1 [W3C.CR-CSS21-20070719].

   style  This is a CSS style definition (like "color: red; background-
      color:#FFF"), based on CSS 2.1 [W3C.CR-CSS21-20070719].

   phone  This SHOULD be a phone number (format MAY follow E.123).

   uri  This value SHOULD be a URI..






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   email  This SHOULD be an email address.

   ip-address  This SHOULD be an ip version 4 address.

   ipv6  This SHOULD be an ip version 6 address.

   host-name  This SHOULD be a host-name.

   Additional custom formats MAY be created.  These custom formats MAY
   be expressed as an URI, and this URI MAY reference a schema of that
   format.

5.24.  divisibleBy

   This attribute defines what value the number instance must be
   divisible by with no remainder (the result of the division must be an
   integer.)  The value of this attribute SHOULD NOT be 0.

5.25.  disallow

   This attribute takes the same values as the "type" attribute, however
   if the instance matches the type or if this value is an array and the
   instance matches any type or schema in the array, then this instance
   is not valid.

5.26.  extends

   The value of this property MUST be another schema which will provide
   a base schema which the current schema will inherit from.  The
   inheritance rules are such that any instance that is valid according
   to the current schema MUST be valid according to the referenced
   schema.  This MAY also be an array, in which case, the instance MUST
   be valid for all the schemas in the array.  A schema that extends
   another schema MAY define additional attributes, constrain existing
   attributes, or add other constraints.

   Conceptually, the behavior of extends can be seen as validating an
   instance against all constraints in the extending schema as well as
   the extended schema(s).  More optimized implementations that merge
   schemas are possible, but are not required.  An example of using
   "extends":

   {
     "description":"An adult",
     "properties":{"age":{"minimum": 21}},
     "extends":"person"
   }




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   {
     "description":"Extended schema",
     "properties":{"deprecated":{"type": "boolean"}},
     "extends":"http://json-schema.org/draft-03/schema"
   }

5.27.  id

   This attribute defines the current URI of this schema (this attribute
   is effectively a "self" link).  This URI MAY be relative or absolute.
   If the URI is relative it is resolved against the current URI of the
   parent schema it is contained in.  If this schema is not contained in
   any parent schema, the current URI of the parent schema is held to be
   the URI under which this schema was addressed.  If id is missing, the
   current URI of a schema is defined to be that of the parent schema.
   The current URI of the schema is also used to construct relative
   references such as for $ref.

5.28.  $ref

   This attribute defines a URI of a schema that contains the full
   representation of this schema.  When a validator encounters this
   attribute, it SHOULD replace the current schema with the schema
   referenced by the value's URI (if known and available) and re-
   validate the instance.  This URI MAY be relative or absolute, and
   relative URIs SHOULD be resolved against the URI of the current
   schema.

5.29.  $schema

   This attribute defines a URI of a JSON Schema that is the schema of
   the current schema.  When this attribute is defined, a validator
   SHOULD use the schema referenced by the value's URI (if known and
   available) when resolving Hyper Schema (Section 6) links
   (Section 6.1).

   A validator MAY use this attribute's value to determine which version
   of JSON Schema the current schema is written in, and provide the
   appropriate validation features and behavior.  Therefore, it is
   RECOMMENDED that all schema authors include this attribute in their
   schemas to prevent conflicts with future JSON Schema specification
   changes.

6.  Hyper Schema

   The following attributes are specified in addition to those
   attributes that already provided by the core schema with the specific
   purpose of informing user agents of relations between resources based



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   on JSON data.  Just as with JSON schema attributes, all the
   attributes in hyper schemas are optional.  Therefore, an empty object
   is a valid (non-informative) schema, and essentially describes plain
   JSON (no constraints on the structures).  Addition of attributes
   provides additive information for user agents.

6.1.  links

   The value of the links property MUST be an array, where each item in
   the array is a link description object which describes the link
   relations of the instances.

6.1.1.  Link Description Object

   A link description object is used to describe link relations.  In the
   context of a schema, it defines the link relations of the instances
   of the schema, and can be parameterized by the instance values.  The
   link description format can be used on its own in regular (non-schema
   documents), and use of this format can be declared by referencing the
   normative link description schema as the the schema for the data
   structure that uses the links.  The URI of the normative link
   description schema is: http://json-schema.org/links (latest version)
   or http://json-schema.org/draft-03/links (draft-03 version).

6.1.1.1.  href

   The value of the "href" link description property indicates the
   target URI of the related resource.  The value of the instance
   property SHOULD be resolved as a URI-Reference per RFC 3986 [RFC3986]
   and MAY be a relative URI.  The base URI to be used for relative
   resolution SHOULD be the URI used to retrieve the instance object
   (not the schema) when used within a schema.  Also, when links are
   used within a schema, the URI SHOULD be parametrized by the property
   values of the instance object, if property values exist for the
   corresponding variables in the template (otherwise they MAY be
   provided from alternate sources, like user input).

   Instance property values SHOULD be substituted into the URIs where
   matching braces ('{', '}') are found surrounding zero or more
   characters, creating an expanded URI.  Instance property value
   substitutions are resolved by using the text between the braces to
   denote the property name from the instance to get the value to
   substitute.  For example, if an href value is defined:

   http://somesite.com/{id}

   Then it would be resolved by replace the value of the "id" property
   value from the instance object.  If the value of the "id" property



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   was "45", the expanded URI would be:

   http://somesite.com/45

   If matching braces are found with the string "@" (no quotes) between
   the braces, then the actual instance value SHOULD be used to replace
   the braces, rather than a property value.  This should only be used
   in situations where the instance is a scalar (string, boolean, or
   number), and not for objects or arrays.

6.1.1.2.  rel

   The value of the "rel" property indicates the name of the relation to
   the target resource.  The relation to the target SHOULD be
   interpreted as specifically from the instance object that the schema
   (or sub-schema) applies to, not just the top level resource that
   contains the object within its hierarchy.  If a resource JSON
   representation contains a sub object with a property interpreted as a
   link, that sub-object holds the relation with the target.  A relation
   to target from the top level resource MUST be indicated with the
   schema describing the top level JSON representation.

   Relationship definitions SHOULD NOT be media type dependent, and
   users are encouraged to utilize existing accepted relation
   definitions, including those in existing relation registries (see RFC
   4287 [RFC4287]).  However, we define these relations here for clarity
   of normative interpretation within the context of JSON hyper schema
   defined relations:

   self  If the relation value is "self", when this property is
      encountered in the instance object, the object represents a
      resource and the instance object is treated as a full
      representation of the target resource identified by the specified
      URI.

   full  This indicates that the target of the link is the full
      representation for the instance object.  The object that contains
      this link possibly may not be the full representation.

   describedby  This indicates the target of the link is the schema for
      the instance object.  This MAY be used to specifically denote the
      schemas of objects within a JSON object hierarchy, facilitating
      polymorphic type data structures.

   root  This relation indicates that the target of the link SHOULD be
      treated as the root or the body of the representation for the
      purposes of user agent interaction or fragment resolution.  All
      other properties of the instance objects can be regarded as meta-



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      data descriptions for the data.

   The following relations are applicable for schemas (the schema as the
   "from" resource in the relation):

   instances  This indicates the target resource that represents
      collection of instances of a schema.

   create  This indicates a target to use for creating new instances of
      a schema.  This link definition SHOULD be a submission link with a
      non-safe method (like POST).

   For example, if a schema is defined:

   {
     "links": [
       {
         "rel": "self"
         "href": "{id}"
       },
       {
         "rel": "up"
         "href": "{upId}"
       },
       {
         "rel": "children"
         "href": "?upId={id}"
       }
     ]
   }

   And if a collection of instance resource's JSON representation was
   retrieved:

   GET /Resource/

   [
     {
       "id": "thing",
       "upId": "parent"
     },
     {
       "id": "thing2",
       "upId": "parent"
     }
   ]

   This would indicate that for the first item in the collection, its



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   own (self) URI would resolve to "/Resource/thing" and the first
   item's "up" relation SHOULD be resolved to the resource at
   "/Resource/parent".  The "children" collection would be located at
   "/Resource/?upId=thing".

6.1.1.3.  targetSchema

   This property value is a schema that defines the expected structure
   of the JSON representation of the target of the link.

6.1.1.4.  Submission Link Properties

   The following properties also apply to link definition objects, and
   provide functionality analogous to HTML forms, in providing a means
   for submitting extra (often user supplied) information to send to a
   server.

6.1.1.4.1.  method

   This attribute defines which method can be used to access the target
   resource.  In an HTTP environment, this would be "GET" or "POST"
   (other HTTP methods such as "PUT" and "DELETE" have semantics that
   are clearly implied by accessed resources, and do not need to be
   defined here).  This defaults to "GET".

6.1.1.4.2.  enctype

   If present, this property indicates a query media type format that
   the server supports for querying or posting to the collection of
   instances at the target resource.  The query can be suffixed to the
   target URI to query the collection with property-based constraints on
   the resources that SHOULD be returned from the server or used to post
   data to the resource (depending on the method).  For example, with
   the following schema:

   {
    "links":[
      {
        "enctype":"application/x-www-form-urlencoded",
        "method":"GET",
        "href":"/Product/",
        "properties":{
           "name":{"description":"name of the product"}
        }
      }
    ]
   }




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   This indicates that the client can query the server for instances
   that have a specific name:

   /Product/?name=Slinky

   If no enctype or method is specified, only the single URI specified
   by the href property is defined.  If the method is POST,
   "application/json" is the default media type.

6.1.1.4.3.  schema

   This attribute contains a schema which defines the acceptable
   structure of the submitted request (for a GET request, this schema
   would define the properties for the query string and for a POST
   request, this would define the body).

6.2.  fragmentResolution

   This property indicates the fragment resolution protocol to use for
   resolving fragment identifiers in URIs within the instance
   representations.  This applies to the instance object URIs and all
   children of the instance object's URIs.  The default fragment
   resolution protocol is "slash-delimited", which is defined below.
   Other fragment resolution protocols MAY be used, but are not defined
   in this document.

   The fragment identifier is based on RFC 2396, Sec 5 [RFC2396], and
   defines the mechanism for resolving references to entities within a
   document.

6.2.1.  slash-delimited fragment resolution

   With the slash-delimited fragment resolution protocol, the fragment
   identifier is interpreted as a series of property reference tokens
   that start with and are delimited by the "/" character (\x2F).  Each
   property reference token is a series of unreserved or escaped URI
   characters.  Each property reference token SHOULD be interpreted,
   starting from the beginning of the fragment identifier, as a path
   reference in the target JSON structure.  The final target value of
   the fragment can be determined by starting with the root of the JSON
   structure from the representation of the resource identified by the
   pre-fragment URI.  If the target is a JSON object, then the new
   target is the value of the property with the name identified by the
   next property reference token in the fragment.  If the target is a
   JSON array, then the target is determined by finding the item in
   array the array with the index defined by the next property reference
   token (which MUST be a number).  The target is successively updated
   for each property reference token, until the entire fragment has been



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   traversed.

   Property names SHOULD be URI-encoded.  In particular, any "/" in a
   property name MUST be encoded to avoid being interpreted as a
   property delimiter.

   For example, for the following JSON representation:

   {
     "foo":{
       "anArray":[
         {"prop":44}
       ],
       "another prop":{
         "baz":"A string"
       }
     }
   }

   The following fragment identifiers would be resolved:

   fragment identifier      resolution
   -------------------      ----------
   #                        self, the root of the resource itself
   #/foo                    the object referred to by the foo property
   #/foo/another%20prop     the object referred to by the "another prop"
                            property of the object referred to by the
                            "foo" property
   #/foo/another%20prop/baz the string referred to by the value of "baz"
                            property of the "another prop" property of
                            the object referred to by the "foo" property
   #/foo/anArray/0          the first object in the "anArray" array

6.2.2.  dot-delimited fragment resolution

   The dot-delimited fragment resolution protocol is the same as slash-
   delimited fragment resolution protocol except that the "." character
   (\x2E) is used as the delimiter between property names (instead of
   "/") and the path does not need to start with a ".".  For example,
   #.foo and #foo are a valid fragment identifiers for referencing the
   value of the foo propery.

6.3.  readonly

   This attribute indicates that the instance property SHOULD NOT be
   changed.  Attempts by a user agent to modify the value of this
   property are expected to be rejected by a server.




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6.4.  contentEncoding

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

6.5.  pathStart

   This attribute is a URI that defines what the instance's URI MUST
   start with in order to validate.  The value of the "pathStart"
   attribute MUST be resolved as per RFC 3986, Sec 5 [RFC3986], and is
   relative to the instance's URI.

   When multiple schemas have been referenced for an instance, the user
   agent can determine if this schema is applicable for a particular
   instance by determining if the URI of the instance begins with the
   the value of the "pathStart" attribute.  If the URI of the instance
   does not start with this URI, or if another schema specifies a
   starting URI that is longer and also matches the instance, this
   schema SHOULD NOT be applied to the instance.  Any schema that does
   not have a pathStart attribute SHOULD be considered applicable to all
   the instances for which it is referenced.

6.6.  mediaType

   This attribute defines the media type of the instance representations
   that this schema is defining.

7.  Security Considerations

   This specification is a sub-type of the JSON format, and consequently
   the security considerations are generally the same as RFC 4627
   [RFC4627].  However, an additional issue is that 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 the
   URI used to request the resource representation which contains the
   target URI with the "self" link.  For example, if a hyper schema was
   defined:










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   {
     "links":[
           {
             "rel":"self",
             "href":"{id}"
           }
     ]
   }

   And a resource was requested from somesite.com:

   GET /foo/

   With a response of:

Content-Type: application/json; profile=/schema-for-this-data
[
  {"id":"bar", "name":"This representation can be safely treated \
        as authoritative "},
  {"id":"/baz", "name":"This representation should not be treated as \
        authoritative the user agent should make request the resource\
        from "/baz" to ensure it has the authoritative representation"},
  {"id":"http://othersite.com/something", "name":"This representation\
        should also not be treated as authoritative and the target\
        resource representation should be retrieved for the\
        authoritative representation"}
]

8.  IANA Considerations

   The proposed MIME media type for JSON Schema is "application/
   schema+json".

   Type name: application

   Subtype name: schema+json

   Required parameters: profile

   The value of the profile parameter SHOULD be a URI (relative or
   absolute) that refers to the schema used to define the structure of
   this structure (the meta-schema).  Normally the value would be
   http://json-schema.org/draft-03/hyper-schema, but it is allowable to
   use other schemas that extend the hyper schema's meta- schema.

   Optional parameters: pretty

   The value of the pretty parameter MAY be true or false to indicate if



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   additional whitespace has been included to make the JSON
   representation easier to read.

8.1.  Registry of Link Relations

   This registry is maintained by IANA per RFC 4287 [RFC4287] and this
   specification adds four values: "full", "create", "instances",
   "root".  New assignments are subject to IESG Approval, as outlined in
   RFC 5226 [RFC5226].  Requests should be made by email to IANA, which
   will then forward the request to the IESG, requesting approval.

9.  References

9.1.  Normative References

   [RFC2045]                          Freed, N. and N. Borenstein,
                                      "Multipurpose Internet Mail
                                      Extensions (MIME) Part One: Format
                                      of Internet Message Bodies",
                                      RFC 2045, November 1996.

   [RFC2119]                          Bradner, S., "Key words for use in
                                      RFCs to Indicate Requirement
                                      Levels", BCP 14, RFC 2119,
                                      March 1997.

   [RFC2396]                          Berners-Lee, T., Fielding, R., and
                                      L. Masinter, "Uniform Resource
                                      Identifiers (URI): Generic
                                      Syntax", RFC 2396, August 1998.

   [RFC3339]                          Klyne, G., Ed. and C. Newman,
                                      "Date and Time on the Internet:
                                      Timestamps", RFC 3339, July 2002.

   [RFC3986]                          Berners-Lee, T., Fielding, R., and
                                      L. Masinter, "Uniform Resource
                                      Identifier (URI): Generic Syntax",
                                      STD 66, RFC 3986, January 2005.

   [RFC4287]                          Nottingham, M., Ed. and R. Sayre,
                                      Ed., "The Atom Syndication
                                      Format", RFC 4287, December 2005.

9.2.  Informative References

   [RFC2616]                          Fielding, R., Gettys, J., Mogul,
                                      J., Frystyk, H., Masinter, L.,



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                                      Leach, P., and T. Berners-Lee,
                                      "Hypertext Transfer Protocol --
                                      HTTP/1.1", RFC 2616, June 1999.

   [RFC4627]                          Crockford, D., "The application/
                                      json Media Type for JavaScript
                                      Object Notation (JSON)", RFC 4627,
                                      July 2006.

   [RFC5226]                          Narten, T. and H. Alvestrand,
                                      "Guidelines for Writing an IANA
                                      Considerations Section in RFCs",
                                      BCP 26, RFC 5226, May 2008.

   [I-D.hammer-discovery]             Hammer-Lahav, E., "LRDD: Link-
                                      based Resource Descriptor
                                      Discovery",
                                      draft-hammer-discovery-06 (work in
                                      progress), May 2010.

   [I-D.gregorio-uritemplate]         Gregorio, J., Fielding, R.,
                                      Hadley, M., and M. Nottingham,
                                      "URI Template",
                                      draft-gregorio-uritemplate-04
                                      (work in progress), March 2010.

   [I-D.nottingham-http-link-header]  Nottingham, M., "Web Linking", dra
                                      ft-nottingham-http-link-header-10
                                      (work in progress), May 2010.

   [W3C.REC-html401-19991224]         Raggett, D., Hors, A., and I.
                                      Jacobs, "HTML 4.01 Specification",
                                      World Wide Web Consortium Recommen
                                      dation REC-html401-19991224,
                                      December 1999, <http://www.w3.org/
                                      TR/1999/REC-html401-19991224>.

   [W3C.CR-CSS21-20070719]            Hickson, I., Lie, H., Celik, T.,
                                      and B. Bos, "Cascading Style
                                      Sheets Level 2 Revision 1 (CSS
                                      2.1) Specification", World Wide
                                      Web Consortium CR CR-CSS21-
                                      20070719, July 2007, <http://
                                      www.w3.org/TR/2007/
                                      CR-CSS21-20070719>.






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Appendix A.  Change Log

   draft-03

      *  Added example and verbiage to "extends" attribute.

      *  Defined slash-delimited to use a leading slash.

      *  Made "root" a relation instead of an attribute.

      *  Removed address values, and MIME media type from format to
         reduce confusion (mediaType already exists, so it can be used
         for MIME types).

      *  Added more explanation of nullability.

      *  Removed "alternate" attribute.

      *  Upper cased many normative usages of must, may, and should.

      *  Replaced the link submission "properties" attribute to "schema"
         attribute.

      *  Replaced "optional" attribute with "required" attribute.

      *  Replaced "maximumCanEqual" attribute with "exclusiveMaximum"
         attribute.

      *  Replaced "minimumCanEqual" attribute with "exclusiveMinimum"
         attribute.

      *  Replaced "requires" attribute with "dependencies" attribute.

      *  Moved "contentEncoding" attribute to hyper schema.

      *  Added "additionalItems" attribute.

      *  Added "id" attribute.

      *  Switched self-referencing variable substitution from "-this" to
         "@" to align with reserved characters in URI template.

      *  Added "patternProperties" attribute.

      *  Schema URIs are now namespace versioned.

      *  Added "$ref" and "$schema" attributes.




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   draft-02

      *  Replaced "maxDecimal" attribute with "divisibleBy" attribute.

      *  Added slash-delimited fragment resolution protocol and made it
         the default.

      *  Added language about using links outside of schemas by
         referencing its normative URI.

      *  Added "uniqueItems" attribute.

      *  Added "targetSchema" attribute to link description object.

   draft-01

      *  Fixed category and updates from template.

   draft-00

      *  Initial draft.

Appendix B.  Open Issues

      Should we give a preference to MIME headers over Link headers (or
      only use one)?

      Should "root" be a MIME parameter?

      Should "format" be renamed to "mediaType" or "contentType" to
      reflect the usage MIME media types that are allowed?

      How should dates be handled?

Authors' Addresses

   Kris Zyp (editor)
   SitePen (USA)
   530 Lytton Avenue
   Palo Alto, CA 94301
   USA

   Phone: +1 650 968 8787
   EMail: kris@sitepen.com







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   Gary Court
   Calgary, AB
   Canada

   EMail: gary.court@gmail.com














































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