Internet Engineering Task Force F. Galiegue, Ed.
Internet-Draft
Intended status: Informational K. Zyp, Ed.
Expires: August 4, 2013 SitePen (USA)
G. Court
January 31, 2013
JSON Schema: core definitions and terminology
draft-zyp-json-schema-04
Abstract
JSON 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Core terminology . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Property, item . . . . . . . . . . . . . . . . . . . . . . 3
3.2. JSON Schema, keywords . . . . . . . . . . . . . . . . . . 3
3.3. Empty schema . . . . . . . . . . . . . . . . . . . . . . . 3
3.4. Root schema, subschema . . . . . . . . . . . . . . . . . . 4
3.5. JSON Schema primitive types . . . . . . . . . . . . . . . 4
3.6. JSON value equality . . . . . . . . . . . . . . . . . . . 5
3.7. Instance . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Validation . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Hypermedia and linking . . . . . . . . . . . . . . . . . . 6
5. General considerations . . . . . . . . . . . . . . . . . . . . 6
5.1. Applicability to all JSON values . . . . . . . . . . . . . 6
5.2. Programming language independence . . . . . . . . . . . . 6
5.3. JSON Schema and HTTP . . . . . . . . . . . . . . . . . . . 6
5.4. JSON Schema and other protocols . . . . . . . . . . . . . 6
5.5. Mathematical integers . . . . . . . . . . . . . . . . . . 7
5.6. Extending JSON Schema . . . . . . . . . . . . . . . . . . 7
5.7. Security considerations . . . . . . . . . . . . . . . . . 7
6. The "$schema" keyword . . . . . . . . . . . . . . . . . . . . 7
6.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Customization . . . . . . . . . . . . . . . . . . . . . . 8
7. URI resolution scopes and dereferencing . . . . . . . . . . . 8
7.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . 8
7.2. URI resolution scope alteration with the "id" keyword . . 8
7.2.1. Valid values . . . . . . . . . . . . . . . . . . . . . 8
7.2.2. Usage . . . . . . . . . . . . . . . . . . . . . . . . 9
7.2.3. Canonical dereferencing and inline dereferencing . . . 10
7.2.4. Inline dereferencing and fragments . . . . . . . . . . 11
7.3. Interoperability considerations . . . . . . . . . . . . . 11
8. Recommended correlation mechanisms for use with the HTTP
protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Correlation by means of the "Content-Type" header . . . . 11
8.2. Correlation by means of the "Link" header . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. ChangeLog . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
JSON 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.
This specification defines JSON Schema core terminology and
mechanisms; related specifications build upon this specification and
define different applications of JSON Schema.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The terms "JSON", "JSON text", "JSON value", "member", "element",
"object", "array", "number", "string", "boolean", "true", "false",
and "null" in this document are to be interpreted as defined in RFC
4627 [RFC4627].
3. Core terminology
3.1. Property, item
When refering to a JSON Object, as defined by [RFC4627], the terms
"member" and "property" may be used interchangeably.
When refering to a JSON Array, as defined by [RFC4627], the terms
"element" and "item" may be used interchangeably.
3.2. JSON Schema, keywords
A JSON Schema is a JSON document, and that document MUST be an
object. Object members (or properties) defined by JSON Schema (this
specification, or related specifications) are called keywords, or
schema keywords.
A JSON Schema MAY contain properties which are not schema keywords.
3.3. Empty schema
An empty schema is a JSON Schema with no properties, or with
properties which are not schema keywords.
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3.4. Root schema, subschema
This example of a JSON Schema has no subschemas:
{
"title": "root"
}
JSON Schemas can also be nested, as in this example:
{
"title": "root",
"otherSchema": {
"title": "nested",
"anotherSchema": {
"title": "alsoNested"
}
}
}
In this example, "nested" and "alsoNested" are subschemas, and "root"
is a root schema.
3.5. JSON Schema primitive types
JSON Schema defines seven primitive types for JSON values:
array A JSON array.
boolean A JSON boolean.
integer A JSON number without a fraction or exponent part.
number Any JSON number. Number includes integer.
null The JSON null value.
object A JSON object.
string A JSON string.
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3.6. JSON value equality
Two JSON values are said to be equal if and only if:
both are nulls; or
both are booleans, and have the same value; or
both are strings, and have the same value; or
both are numbers, and have the same mathematical value; or
both are arrays, and:
have the same number of items; and
items at the same index are equal according to this definition;
or
both are objects, and:
have the same set of property names; and
values for a same property name are equal according to this
definition.
3.7. Instance
An instance is any JSON value. An instance may be described by one
or more schemas.
An instance may also be referred to as "JSON instance", or "JSON
data".
4. Overview
This document proposes a new media type "application/schema+json" to
identify JSON Schema for describing JSON data. JSON Schemas are
themselves written in JSON. This, and related specifications, define
keywords allowing to describe this data in terms of allowable values,
textual descriptions and interpreting relations with other resources.
The following sections are a summary of features defined by related
specifications.
4.1. Validation
JSON Schema allows applications to validate instances, either non
interactively or interactively. For instance, an application may
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collect JSON data and check that this data matches a given set of
constraints; another application may use a JSON Schema to build an
interactive interface in order to collect user input according to
constraints described by JSON Schema.
4.2. Hypermedia and linking
JSON Schema provides a method for extracting link relations from
instances to other resources, as well as describing interpretations
of instances as multimedia data. This allows JSON data to be
interpreted as rich hypermedia documents, placed in the context of a
larger set of related resources.
5. General considerations
5.1. Applicability to all JSON values
It is acknowledged that an instance may be any valid JSON value as
defined by [RFC4627]. As such, JSON Schema does not mandate that an
instance be of a particular type: JSON Schema can describe any JSON
value, including null.
5.2. Programming language independence
JSON Schema is programming language agnostic. The only limitations
are the ones expressed by [RFC4627] and those of the host programming
language.
5.3. JSON Schema and HTTP
This specification acknowledges the role of HTTP [RFC2616] as the
dominant protocol in use on the Internet, and the wealth of official
specifications related to it.
This specification uses a subset of these specifications to recommend
a set of mechanisms, usable by this protocol, to associate JSON
instances to one or more schemas.
5.4. JSON Schema and other protocols
JSON Schema does not define any semantics for the client-server
interface for any other protocols than HTTP. These semantics are
application dependent, or subject to agreement between the parties
involved in the use of JSON Schema for their own needs.
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5.5. Mathematical integers
It is acknowledged by this specification that some programming
languages, and their associated parsers, use different internal
representations for floating point numbers and integers, while others
do not.
As a consequence, for interoperability reasons, JSON values used in
the context of JSON Schema, whether that JSON be a JSON Schema or an
instance, SHOULD ensure that mathematical integers be represented as
integers as defined by this specification.
5.6. Extending JSON Schema
Implementations MAY choose to define additional keywords to JSON
Schema. Save for explicit agreement, schema authors SHALL NOT expect
these additional keywords to be supported by peer implementations.
Implementations SHOULD ignore keywords they do not support.
5.7. Security considerations
Both schemas and instances are JSON values. As such, all security
considerations defined in RFC 4627 [RFC4627] apply.
6. The "$schema" keyword
6.1. Purpose
The "$schema" keyword is both used as a JSON Schema version
identifier and the location of a resource which is itself a JSON
Schema, which describes any schema written for this particular
version.
This keyword MUST be located at the root of a JSON Schema. The value
of this keyword MUST be a URI [RFC3986] and a valid JSON Reference
[json-reference]; this URI MUST be both absolute and normalized. The
resource located at this URI MUST successfully describe itself. It
is RECOMMENDED that schema authors include this keyword in their
schemas.
The following values are predefined:
http://json-schema.org/schema# JSON Schema written against the
current version of the specification.
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http://json-schema.org/hyper-schema# JSON Schema written against the
current version of the specification.
http://json-schema.org/draft-04/schema# JSON Schema written against
this version.
http://json-schema.org/draft-04/hyper-schema# JSON Schema
hyperschema written against this version.
http://json-schema.org/draft-03/schema# JSON Schema written against
JSON Schema, draft v3 [json-schema-03].
http://json-schema.org/draft-03/hyper-schema# JSON Schema
hyperschema written against JSON Schema, draft v3
[json-schema-03].
6.2. Customization
When extending JSON Schema with custom keywords, schema authors
SHOULD define a custom URI for "$schema". This custom URI MUST NOT
be one of the predefined values.
7. URI resolution scopes and dereferencing
7.1. Definition
JSON Schema uses JSON Reference [json-reference] as a mechanism for
schema addressing. It extends this specification in two ways:
JSON Schema offers facilities to alter the base URI against which
a reference must resolve by the means of the "id" keyword;
it defines a specific dereferencing mechanism extending JSON
Reference to accept arbitrary fragment parts.
Altering the URI within a schema is called defining a new resolution
scope. The initial resolution scope of a schema is the URI of the
schema itself, if any, or the empty URI if the schema was not loaded
from a URI.
7.2. URI resolution scope alteration with the "id" keyword
7.2.1. Valid values
The value for this keyword MUST be a string, and MUST be a valid URI.
This URI MUST be normalized, and SHOULD NOT be an empty fragment (#)
or the empty URI.
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7.2.2. Usage
The "id" keyword (or "id", for short) is used to alter the resolution
scope. When an id is encountered, an implementation MUST resolve
this id against the most immediate parent scope. The resolved URI
will be the new resolution scope for this subschema and all its
children, until another id is encountered.
When using "id" to alter resolution scopes, schema authors SHOULD
ensure that resolution scopes are unique within the schema.
This schema will be taken as an example:
{
"id": "http://x.y.z/rootschema.json#",
"schema1": {
"id": "#foo"
},
"schema2": {
"id": "otherschema.json",
"nested": {
"id": "#bar"
},
"alsonested": {
"id": "t/inner.json#a"
}
},
"schema3": {
"id": "some://where.else/completely#"
}
}
Subschemas at the following URI-encoded JSON Pointer [json-pointer]s
(starting from the root schema) define the following resolution
scopes:
# (document root) http://x.y.z/rootschema.json#
#/schema1 http://x.y.z/rootschema.json#foo
#/schema2 http://x.y.z/otherschema.json#
#/schema2/nested http://x.y.z/otherschema.json#bar
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#/schema2/alsonested http://x.y.z/t/inner.json#a
#/schema3 some://where.else/completely#
7.2.3. Canonical dereferencing and inline dereferencing
When resolving a URI against a resolution scope, an implementation
may choose two modes of operation:
canonical dereferencing The implementation dereferences all resolved
URIs.
inline dereferencing The implementation chooses to dereference URIs
within the schema.
Implementations MUST support canonical dereferencing, and MAY support
inline dereferencing.
For example, consider this schema:
{
"id": "http://my.site/myschema#",
"definitions": {
"schema1": {
"id": "schema1",
"type": "integer"
},
"schema2", {
"type": "array",
"items": { "$ref": "schema1" }
}
}
}
When an implementation encounters the "schema1" reference, it
resolves it against the most immediate parent scope, leading to URI
"http://my.site/schema1#". The way to process this URI will differ
according to the chosen dereferencing mode:
if canonical dereferencing is used, the implementation will
dereference this URI, and fetch the content at this URI;
if inline dereferencing is used, the implementation will notice
that URI scope "http://my.site/schema1#" is already defined within
the schema, and choose to use the appropriate subschema.
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7.2.4. Inline dereferencing and fragments
When using inline dereferencing, a resolution scope may lead to a URI
which has a non empty fragment part which is not a JSON Pointer, as
in this example:
{
"id": "http://some.site/schema#",
"not": { "$ref": "#inner" },
"definitions": {
"schema1": {
"id": "#inner",
"type": "boolean"
}
}
}
An implementation choosing to support inline dereferencing SHOULD be
able to use this kind of reference. Implementations choosing to use
canonical dereferencing, however, are not required to support it.
7.3. Interoperability considerations
Inline dereferencing can produce canonical URIs which differ from the
canonical URI of the root schema. Schema authors SHOULD ensure that
implementations using canonical dereferencing obtain the same content
as implementations using inline dereferencing.
Extended JSON References using fragments which are not JSON Pointers
are not dereferenceable by implementations choosing not to support
inline dereferencing. This kind of reference is defined for
backwards compatibility, and SHOULD NOT be used in new schemas.
8. Recommended correlation mechanisms for use with the HTTP protocol
It is acknowledged by this specification that the majority of
interactive JSON Schema processing will be over HTTP. This section
therefore gives recommendations for materializing an instance/schema
correlation using mechanisms currently available for this protocol.
An instance is said to be described by one (or more) schema(s).
8.1. Correlation by means of the "Content-Type" header
It is RECOMMENDED that a MIME type parameter by the name of "profile"
be appended to the "Content-Type" header of the instance being
processed. If present, the value of this parameter MUST be a valid
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URI, and this URI SHOULD resolve to a valid JSON Schema. The MIME
type MUST be "application/json", or any other subtype.
An example of such a header would be:
Content-Type: application/my-media-type+json;
profile=http://example.com/my-hyper-schema#
8.2. Correlation by means of the "Link" header
When using the "Link" header, the relation type used MUST be
"describedBy", as defined by RFC 5988, section 5.3 [RFC5988]. The
target URI of the "Link" header MUST be a valid JSON Schema.
An example of such a header would be:
Link: <http://example.com/my-hyper-schema#>; rel="describedBy"
9. IANA Considerations
The proposed MIME media type for JSON Schema is defined as follows:
type name: application;
subtype name: schema+json.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter,
"Uniform Resource Identifier (URI): Generic
Syntax", STD 66, RFC 3986, January 2005.
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[RFC4627] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627,
July 2006.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
October 2010.
[json-reference] Bryan, P. and K. Zyp, "JSON Reference (work in
progress)", September 2012, <http://tools.ietf.org/
html/draft-pbryan-zyp-json-ref-03>.
[json-pointer] Bryan, P. and K. Zyp, "JSON Pointer (work in
progress)", September 2012, <http://tools.ietf.org/
html/draft-ietf-appsawg-json-pointer-07>.
[json-schema-03] Court, G. and K. Zyp, "JSON Schema, draft 3",
September 2012, <http://tools.ietf.org/html/
draft-zyp-json-schema-03>.
Appendix A. ChangeLog
draft-00
* Initial draft.
* Salvaged from draft v3.
* Mandate the use of JSON Reference, JSON Pointer.
* Define the role of "id". Define URI resolution scope.
* Add interoperability considerations.
Authors' Addresses
Francis Galiegue (editor)
EMail: fgaliegue@gmail.com
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Kris Zyp (editor)
SitePen (USA)
530 Lytton Avenue
Palo Alto, CA 94301
USA
Phone: +1 650 968 8787
EMail: kris@sitepen.com
Gary Court
Calgary, AB
Canada
EMail: gary.court@gmail.com
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