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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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