Internet Engineering Task Force A. Wright, Ed.
Internet-Draft October 13, 2016
Intended status: Informational
Expires: April 16, 2017

JSON Schema: A Media Type for Describing JSON Documents
draft-wright-json-schema-00

Abstract

JSON Schema defines the media type "application/schema+json", a JSON based format for describing the structure of JSON data. JSON Schema asserts what a JSON document must look like, ways to extract information from it, and how to interact with it, ideal for annotating existing JSON APIs that would not otherwise have hypermedia controls or be machine-readable.

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Table of Contents

1. Introduction

JSON Schema is a JSON media type for defining the structure of JSON data. 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, including pointing to another JSON Schema by reference, dereferencing a JSON Schema reference, specifying the vocabulary being used, and declaring the minimum functionality necessary for processing an instance against a schema.

Other specifications define the vocabularies that perform assertions about validation, linking, annotation, navigation, and interaction.

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 7159 [RFC7159].

3. 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.

3.1. Validation

JSON Schema describes the structure of a JSON document (for instance, required properties and length limitations). Applications can use this information to validate instances (check that constraints are met), or inform interfaces to collect user input such that the constraints are satisfied.

Validation behaviour and keywords are specified in a separate document [json-schema-validation].

3.2. Hypermedia and linking

JSON Hyper-Schema describes the hypertext structure of a JSON document. This includes link relations from the instance to other resources, interpretation of instances as multimedia data, and submission data required to use an API.

Hyper-schema behaviour and keywords are specified in a separate document [json-hyper-schema].

4. Definitions

4.1. JSON document

A JSON document is an information resource (series of octets) described by the application/json media type.

In JSON Schema, the terms "JSON document", "JSON text", and "JSON value" are interchangable because of the data model it defines.

4.2. instance

JSON Schema interperts documents according to a data model. A JSON value interperted according to this data model is called an "instance".

An instance has one of six primitive types, and a range of possible values depending on the type:

null
A JSON "null" production
boolean
A "true" or "false" value, from the JSON "true" or "false" productions
object
An unordered set of properties mapping a string to an instance, from the JSON "object" production
array
An ordered list of instances, from the JSON "array" production
number
An arbitrary-precision, base-10 decimal number value, from the JSON "number" production
string
A string of Unicode code points, from the JSON "string" production

Whitespace and formatting conserns are thus outside the scope of JSON Schema.

Since an object cannot have two properties with the same key, behavior for a JSON document that tries to define two properties (the "member" production) with the same key (the "string" production) in a single object is undefined.

4.3. instance equality

Two JSON instances are said to be equal if and only if they are of the same type and have the same value according to the data model. Specifically, this means:

Implied in this definition is that arrays must be the same length, objects must have the same number of members, properties in objects are unordered, there is no way to define multiple properties with the same key, and mere formatting differences (indentation, placement of commas, trailing zeros) are insignificant.

4.4. JSON Schema document

A JSON Schema document, or simply a schema, is a JSON document used to describe an instance. A schema is itself interperted as an instance. A JSON schema MUST be an object.

Properties that are used to describe the instance are called keywords, or schema keywords. The meaning of properties is specified by the vocabulary that the schema is using.

A JSON Schema MAY contain properties which are not schema keywords. Unknown keywords SHOULD be ignored.

A schema that itself describes a schema is called a meta-schema. Meta-schemas are used to validate JSON Schemas and specify which vocabulary it is using.

An empty schema is a JSON Schema with no properties, or only unknown properties.

4.5. Root schema and subschemas

The root schema is the schema that comprises the entire JSON document in question.

Some keywords take schemas themselves, allowing JSON Schemas to be nested:


{
    "title": "Some gnomes",
    "items": {
        "title": "A gnome"
    }
}

                    

In this example document, the schema titled "array item" is a subschema, and the schema titled "root" is the root schema.

5. General considerations

5.1. Range of JSON values

An instance may be any valid JSON value as defined by JSON [RFC7159]. JSON Schema imposes no restrictions on type: JSON Schema can describe any JSON value, including, for example, null.

5.2. Programming language independence

JSON Schema is programming language agnostic, and supports the full range of values described in the data model. Be aware, however, that some languages and JSON parsers may not be able to represent in memory the full range of values describable by JSON.

5.3. Mathematical integers

Some programming languages and parsers use different internal representations for floating point numbers than they do for integers.

For constistency, integer JSON numbers SHOULD NOT be encoded with a fractional part.

5.4. Extending JSON Schema

Implementations MAY 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.

Authors of extensions to JSON Schema are encouraged to write their own meta-schemas, which extend the existing meta-schemas using "allOf". This extended meta-schema SHOULD be referenced using the "$schema" keyword, to allow tools to follow the correct behaviour.

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.

The root schema of a JSON Schema document SHOULD use this keyword. The value of this keyword MUST be a URI [RFC3986] (an "absolute" URI), and this URI MUST be normalized. The current schema MUST be valid against the meta-schema identified by this URI.

Values for this property are defined in other documents and by other parties. JSON Schema implementations SHOULD implement support for current and previous published drafts of JSON Schema vocabularies as deemed reasonable.

7. Schema references with $ref

Any time a subschema is expected, a schema may instead use an object containing a "$ref" property. The value of the $ref is a URI Reference. Resolved against the current URI base, it identifies the URI of a schema to use. All other properties in a "$ref" object MUST be ignored.

The URI is not a network locator, only an identifier. A schema need not be downloadable from the address if it is a network-addressible URL, and implementations SHOULD NOT assume they should perform a network operation when they encounter a network-addressible URI.

A schema MUST NOT be run into an infinite loop against a schema. For example, if two schemas "#alice" and "#bob" both have an "allOf" property that refers to the other, a naive validator might get stuck in an infinite recursive loop trying to validate the instance. Schemas SHOULD NOT make use of infinite recursive nesting like this, the behavior is undefined.

8. Base URI and dereferencing

8.1. Initial base URI

RFC3986 Section 5.1 [RFC3986] defines how to determine the default base URI of a document.

Informatively, the initial base URI of a schema is the URI it was found at, or a suitable substitute URI if none is known.

8.2. The "id" keyword

The "id" keyword defines a URI for the schema, and the base URI that other URI references within the schema are resolved against. The "id" keyword itself is resolved against the base URI that the object as a whole appears in.

If present, the value for this keyword MUST be a string, and MUST represent a valid URI-reference [RFC3986]. This value SHOULD be normalized, and SHOULD NOT be an empty fragment <#> or an empty string <>.

The root schema of a JSON Schema document SHOULD contain an "id" keyword with an absolute-URI (containing a scheme, but no fragment).

To name subschemas in a JSON Schema document, subschemas can use "id" to give themselves a document-local identifier. This form of "id" keyword MUST begin with a hash ("#") to identify it as a fragment URI reference, followed by a letter ([A-Za-z]), followed by any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons (":"), or periods (".").


{
    "id": "http://example.com/root.json",
    "definitions": {
        "A": { "id": "#foo" },
        "B": {
            "id": "other.json",
            "definitions": {
                "X": { "id": "#bar" },
                "Y": { "id": "t/inner.json" }
            }
        },
        "C": {
            "id": "urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f"
        }
    }
}

                        

For example:

The schemas at the following URI-encoded JSON Pointers [RFC6901] (relative to the root schema) have the following base URIs, and are identifiable by either URI:

# (document root)
http://example.com/root.json#
#/definitions/A
http://example.com/root.json#foo
#/definitions/B
http://example.com/other.json
#/definitions/B/definitions/X
http://example.com/other.json#bar
#/definitions/B/definitions/Y
http://example.com/t/inner.json
#/definitions/C
urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f

8.2.1. Internal references

Schemas can be identified by any URI that has been given to them, including a JSON Pointer or their URI given directly by "id".

Tools SHOULD take note of the URIs that schemas, including subschemas, provide for themselves using "id". This is known as "Internal referencing".

For example, consider this schema:


{
    "id": "http://example.net/root.json",
    "items": {
        "type": "array",
        "items": { "$ref": "#item" }
    },
    "definitions": {
        "single": {
            "id": "#item",
            "type": "integer"
        },
    }
}

                        

When an implementation encounters the <#/definitions/single> schema, it resolves the "id" URI reference against the current base URI to form <http://example.net/root.json#item>.

When an implementation then looks inside the <#/items> schema, it encounters the <#item> reference, and resolves this to <http://example.net/root.json#item> which is understood as the schema defined elsewhere in the same document.

8.2.2. External references

To differentiate schemas between each other in a vast ecosystem, schemas are identified by URI. As specified above, this does not necessarially mean anything is downloaded, but instead JSON Schema implementations SHOULD already understand the schemas they will be using, including the URIs that identify them.

Implementations SHOULD be able to associate arbritrary URIs with an arbritrary schema and/or automatically associate a schema's "id"-given URI, depending on the trust that the the validator has in the schema.

A schema MAY (and likely will) have multiple URIs, but there is no way for a URI to identify more than one schema. When multiple schemas try to identify with the same URI, validators SHOULD raise an error condition.

9. Usage for hypermedia

One of the largest adoptors of JSON has been HTTP servers for automated APIs and robots. This section describes how to enhance processing of JSON documents in a more RESTful manner when used with protocols that support media types and Web linking [RFC5988].

9.1. Linking to a schema

It is RECOMMENDED that instances described by a schema/profile provide a link to a downloadable JSON Schema using the link relation "describedby", as defined by Linked Data Protocol 1.0, section 8.1 [W3C.REC-ldp-20150226].

In HTTP, such links can be attached to any response using the Link header [RFC5988]. An example of such a header would be:


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

                    

9.2. Describing a profile of JSON

Instances MAY specify a "profile" as described in The 'profile' Link Relation [RFC6906]. When used as a media-type parameter, HTTP servers gain the ability to perform Content-Type Negotiation based on profile. The media-type parameter MUST be a whitespace-separated list of URIs (i.e. relative references are invalid).

The profile URI is opaque and SHOULD NOT automatically be dereferenced. If the implementation does not understand the semantics of the provided profile, the implementation can instead follow the "describedby" links, if any, which may provide information on how to handle the profile. Since "profile" doesn't necessarily point to a network location, the "describedby" relation is used for linking to a downloadable schema. However, for simplicity, schema authors should make these URIs point to the same resource when possible.

In HTTP, the media-type parameter would be sent inside the Content-Type header:


Content-Type: application/json;
          profile="http://example.com/my-hyper-schema#"

                    

Multiple profiles are whitespace seperated:


Content-Type: application/json;
          profile="http://example.com/alice http://example.com/bob"

                    

HTTP can also send the "profile" in a Link, though this may impact media-type semantics and Content-Type negotiation if this replaces the media-type parameter entirely:


Link: </alice>;rel="profile", </bob>;rel="profile"

                    

9.3. Usage over HTTP

When used for hypermedia systems over a network, HTTP [RFC7231] is frequently the protocol of choice for distributing schemas. Misbehaved clients can pose problems for server maintainers if they pull a schema over the network more frequently than necessary, when it's instead possible to cache a schema for a long period of time.

HTTP servers SHOULD set long-lived caching headers on JSON Schemas. HTTP clients SHOULD observe caching headers and not re-request documents within their freshness period. Distributed systems SHOULD make use of a shared cache and/or caching proxy.


User-Agent: so-cool-json-schema/1.0.2 curl/7.43.0

                        

Clients SHOULD set or prepend a User-Agent header specific to the JSON Schema implementation or software product. Since symbols are listed in decreasing order of significance, the JSON Schema library name/version goes first, then the more generic HTTP library name (if any). For example:

Clients SHOULD be able to make requests with a "From" header so that server operators can contact the owner of a potentially misbehaving script.

10. Security considerations

Both schemas and instances are JSON values. As such, all security considerations defined in RFC 7159 [RFC7159] apply.

Instances and schemas are both frequently witten by untrusted third parties, to be deployed on public Internet servers. Validators should take care that the parsing of schemas doesn't consume excessive system resources. Validators MUST NOT fall into an infinite loop.

Servers need to take care that malicious parties can't change the functionality of existing schemas by uploading a schema with an pre-existing or very similar "id".

Individual JSON Schema vocabularies are liable to also have their own security considerations. Consult the respective specifications for more information.

11. IANA Considerations

The proposed MIME media type for JSON Schema is defined as follows:

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014.
[W3C.REC-ldp-20150226] Speicher, S., Arwe, J. and A. Malhotra, "Linked Data Platform 1.0", World Wide Web Consortium Recommendation REC-ldp-20150226, February 2015.

12.2. Informative References

[RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010.
[RFC6901] Bryan, P., Zyp, K. and M. Nottingham, "JavaScript Object Notation (JSON) Pointer", RFC 6901, DOI 10.17487/RFC6901, April 2013.
[RFC6906] Wilde, E., "The 'profile' Link Relation Type", RFC 6906, DOI 10.17487/RFC6906, March 2013.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014.
[json-schema-validation] Wright, A. and G. Luff, "JSON Schema Validation: A Vocabulary for Structural Validation of JSON", Internet-Draft draft-wright-json-schema-validation-00, October 2016.
[json-hyper-schema] Wright, A. and G. Luff, "JSON Hyper-Schema: A Vocabulary for Hypermedia Annotation of JSON", Internet-Draft draft-wright-json-schema-hyperschema-00, October 2016.

Appendix A. Acknowledgments

Thanks to Gary Court, Francis Galiegue, Kris Zyp, and Geraint Luff for their work on the initial drafts of JSON Schema.

Thanks to Jason Desrosiers, Daniel Perrett, Erik Wilde, Ben Hutton, Evgeny Poberezkin, and Henry H. Andrews for their submissions and patches to the document.

Appendix B. ChangeLog

[CREF1]This section to be removed before leaving Internet-Draft status.

draft-wright-json-schema-00

draft-zyp-json-schema-04

draft-zyp-json-schema-00

Author's Address

Austin Wright (editor) EMail: aaa@bzfx.net