jsonschema-core.xml

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]>
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>
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<rfc category="info" docName="draft-handrews-json-schema-00" ipr="trust200902">
    <front>
        <title abbrev="JSON Schema">JSON Schema: A Media Type for Describing JSON Documents</title>

        <author fullname="Austin Wright" initials="A" surname="Wright" role="editor">
            <address>
                <email>aaa@bzfx.net</email>
            </address>
        </author>

        <author fullname="Henry Andrews" initials="H" surname="Andrews" role="editor">
            <organization>Cloudflare, Inc.</organization>
            <address>
                <postal>
                    <street></street>
                    <city>San Francisco</city>
                    <region>CA</region>
                    <country>USA</country>
                </postal>
                <email>henry@cloudflare.com</email>
            </address>
        </author>

        <date year="2018"/>
        <workgroup>Internet Engineering Task Force</workgroup>
        <keyword>JSON</keyword>
        <keyword>Schema</keyword>
        <keyword>Hyper Schema</keyword>
        <keyword>Hypermedia</keyword>

        <abstract>
            <t>
                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.
                The "application/schema-instance+json" media type provides additional
                feature-rich integration with "application/schema+json" beyond what can be offered
                for "application/json" documents.
            </t>
        </abstract>
        <note title="Note to Readers">
            <t>
                The issues list for this draft can be found at
                <eref target="https://github.com/json-schema-org/json-schema-spec/issues"/>.
            </t>
            <t>
                For additional information, see <eref target="http://json-schema.org/"/>.
            </t>
            <t>
                To provide feedback, use this issue tracker, the communication methods listed on the
                homepage, or email the document editors.
            </t>
        </note>
    </front>

    <middle>
        <section title="Introduction">
            <t>
                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.
            </t>
            <t>
                This specification defines JSON Schema core terminology and mechanisms, including
                pointing to another JSON Schema by reference,
                dereferencing a JSON Schema reference,
                and specifying the vocabulary being used.
            </t>
            <t>
                Other specifications define the vocabularies that perform assertions about validation,
                linking, annotation, navigation, and interaction.
            </t>
        </section>

        <section title="Conventions and Terminology">
            <t>
                <!-- The text in this section has been copied from the official boilerplate,
                and should not be modified.-->

                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 <xref target="RFC2119">RFC 2119</xref>.
            </t>

            <t>
                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 <xref target="RFC7159">RFC 7159</xref>.
            </t>
        </section>

        <section title="Overview">
            <t>
                This document proposes a new media type "application/schema+json" to identify a JSON
                Schema for describing JSON data.
                It also proposes a further optional media type, "application/schema-instance+json",
                to provide additional integration features.
                JSON Schemas are themselves JSON documents.
                This, and related specifications, define keywords allowing authors to describe JSON
                data in several ways.
            </t>

            <section title="Validation">
                <t>
                    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.
                </t>
                <t>
                    Validation behaviour and keywords are specified in
                    <xref target="json-schema-validation">a separate document</xref>.
                </t>
            </section>

            <section title="Hypermedia and Linking">
                <t>
                    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.
                </t>
                <t>
                    Hyper-schema behaviour and keywords are specified in
                    <xref target="json-hyper-schema">a separate document</xref>.
                </t>
            </section>
        </section>

        <section title="Definitions">

            <section title="JSON Document">
                <t>
                    A JSON document is an information resource (series of octets) described by the
                    application/json media type.
                </t>
                <t>
                    In JSON Schema, the terms "JSON document", "JSON text", and "JSON value" are
                    interchangeable because of the data model it defines.
                </t>
                <t>
                    JSON Schema is only defined over JSON documents. However, any document or memory
                    structure that can be parsed into or processed according to the JSON Schema data
                    model can be interpreted against a JSON Schema, including media types like
                    <xref target="RFC7049">CBOR</xref>.
                </t>
            </section>

            <section title="Instance">
                <t>
                    A JSON document to which a schema is applied is known as an "instance".
                </t>

                <section title="Instance Data Model">
                    <t>
                        JSON Schema interprets documents according to a data model. A JSON value
                        interpreted according to this data model is called an "instance".
                    </t>
                    <t>
                        An instance has one of six primitive types, and a range of possible values
                        depending on the type:

                        <list style="hanging">
                            <t hangText="null">A JSON "null" production</t>
                            <t hangText="boolean">A "true" or "false" value, from the JSON "true" or "false" productions</t>
                            <t hangText="object">An unordered set of properties mapping a string to an instance, from the JSON "object" production</t>
                            <t hangText="array">An ordered list of instances, from the JSON "array" production</t>
                            <t hangText="number">An arbitrary-precision, base-10 decimal number value, from the JSON "number" production</t>
                            <t hangText="string">A string of Unicode code points, from the JSON "string" production</t>
                        </list>
                    </t>
                    <t>
                        Whitespace and formatting concerns, including different lexical
                        representations of numbers that are equal within the data model, are thus
                        outside the scope of JSON Schema.  JSON Schema
                        <xref target="vocabulary">vocabularies</xref> that wish
                        to work with such differences in lexical representations SHOULD define
                        keywords to precisely interpret formatted strings within the data model
                        rather than relying on having the original JSON representation Unicode
                        characters available.
                    </t>
                    <t>
                        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.
                    </t>
                    <t>
                        Note that JSON Schema vocabularies are free to define their own extended
                        type system.  This should not be confused with the core data model types
                        defined here.  As an example, "integer" is a reasonable type for a
                        vocabulary to define as a value for a keyword, but the data model
                        makes no distinction between integers and other numbers.
                    </t>
                </section>

                <section title="Instance Media Types">
                    <t>
                        JSON Schema is designed to fully work with "application/json" documents,
                        as well as media types using the "+json" structured syntax suffix.
                    </t>
                    <t>
                        Some functionality that is useful for working with schemas is
                        defined by each media type, namely media type parameters and
                        URI fragment identifier syntax and semantics.  These features are
                        useful in content negotiation and in calculating URIs for specific
                        locations within an instance, respectively.
                    </t>
                    <t>
                        This specification defines the "application/schema-instance+json"
                        media type in order to allow instance authors to take full advantage
                        of parameters and fragment identifiers for these purposes.
                    </t>
                </section>

                <section title="Instance Equality">
                    <t>
                        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:

                        <list>
                            <t>both are null; or</t>
                            <t>both are true; or</t>
                            <t>both are false; or</t>
                            <t>both are strings, and are the same codepoint-for-codepoint; or</t>
                            <t>both are numbers, and have the same mathematical value; or</t>
                            <t>both are arrays, and have an equal value item-for-item; or</t>
                            <t>both are objects, and each property in one has exactly one property with
                                a key equal to the other's, and that other property has an equal
                                value.</t>
                        </list>
                    </t>
                    <t>
                        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.
                    </t>
                </section>
            </section>

            <section title="JSON Schema Documents">
                <t>
                    A JSON Schema document, or simply a schema, is a JSON document used to describe
                    an instance.
                    A schema is itself interpreted as an instance, but SHOULD always be given
                    the media type "application/schema+json" rather than
                    "application/schema-instance+json".  The "application/schema+json" media
                    type is defined to offer a superset of the media type parameter and
                    fragment identifier syntax and semantics provided by
                    "application/schema-instance+json".
                </t>
                <section title="JSON Schema Values and Keywords">
                    <t>
                        A JSON Schema MUST be an object or a boolean.
                    </t>
                    <t>
                        Object properties that are applied to the instance are called keywords,
                        or schema keywords.  Broadly speaking, keywords fall into one or both
                        of two categories:
                        <list style="hanging">
                            <t hangText="assertions">
                                produce a boolean result when applied to an instance
                            </t>
                            <t hangText="annotations">
                                attach information to an instance for application use
                            </t>
                        </list>
                    </t>
                    <t>
                        Keywords may fall into either or both categories.  Extension keywords,
                        meaning those defined outside of this document and its companions,
                        are free to define other behaviors as well.
                    </t>
                    <t>
                        The boolean schema values "true" and "false" are trivial assertions that
                        always return themselves regardless of the instance value.  As an example,
                        in terms of the validation vocabulary, boolean schemas are equivalent to
                        the following behaviors:
                        <list style="hanging">
                            <t hangText="true">
                                Always passes validation, as if the empty schema {}
                            </t>
                            <t hangText="false">
                                Always fails validation, as if the schema { "not":{} }
                            </t>
                        </list>
                    </t>
                    <t>
                        A JSON Schema MAY contain properties which are not schema keywords.
                        Unknown keywords SHOULD be ignored.
                    </t>
                    <t>
                        An empty schema is a JSON Schema with no properties, or only unknown
                        properties.
                    </t>
                </section>
                <section title="JSON Schema Vocabularies" anchor="vocabulary">
                    <t>
                        A JSON Schema vocabulary is a set of keywords defined for a particular
                        purpose.  The vocabulary specifies the meaning of its keywords as
                        assertions, annotations, and/or any vocabulary-defined keyword category.
                        The two companion standards to this document each define a vocabulary:
                        One for instance validation, and one for hypermedia annotations.
                        Vocabularies are the primary mechanism for extensibility within
                        the JSON Schema media type.
                    </t>
                    <t>
                        Vocabularies may be defined by any entity.  Vocabulary authors SHOULD
                        take care to avoid keyword name collisions if the vocabulary is intended
                        for broad use, and potentially combined with other vocabularies.  JSON
                        Schema does not provide any formal namespacing system, but also does
                        not constrain keyword names, allowing for any number of namespacing
                        approaches.
                    </t>
                    <t>
                        Vocabularies may build on each other, such as by defining the behavior
                        of their keywords with respect to the behavior of keywords from another
                        vocabulary, or by using a keyword from another vocabulary with
                        a restricted or expanded set of acceptable values.  Not all such
                        vocabulary re-use will result in a new vocabulary that is compatible
                        with the vocabulary on which it is built.  Vocabulary authors SHOULD
                        clearly document what level of compatibility, if any, is expected.
                    </t>
                    <t>
                        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.
                        <cref>
                            Currently, only a single meta-schema may be specified per schema,
                            meaning that in order to use multiple vocabularies, a meta-schema
                            must be written that encompasses all of them.  The hyper-schema
                            meta-schema is an example of this, as it encompasses the validation
                            vocabulary as well as the hypermedia vocabulary.
                        </cref>
                    </t>
                </section>
                <section title="Root Schema and Subschemas">
                    <t>
                        The root schema is the schema that comprises the entire JSON document
                        in question.
                    </t>
                    <t>
                        Some keywords take schemas themselves, allowing JSON Schemas to be nested:
                    </t>
                    <figure>
                        <artwork>
<![CDATA[
{
    "title": "root",
    "items": {
        "title": "array item"
    }
}
]]>
                        </artwork>
                    </figure>
                    <t>
                        In this example document, the schema titled "array item" is a subschema,
                        and the schema titled "root" is the root schema.
                    </t>
                    <t>
                        As with the root schema, a subschema is either an object or a boolean.
                    </t>
                </section>
            </section>

        </section>

        <section title="Fragment Identifiers" anchor="fragments">
            <t>
                In accordance with section 3.1 of <xref target="RFC6839"></xref>,
                the syntax and semantics of fragment identifiers specified for
                any +json media type SHOULD be as specified for "application/json".
                (At publication of this document, there is no fragment identification
                syntax defined for "application/json".)
            </t>
            <t>
                Additionally, the "application/schema+json" media type supports two
                fragment identifier structures: plain names and JSON Pointers.
                The "application/schema-instance+json" media type supports one
                fragment identifier structure: JSON Pointers.
            </t>
            <t>
                The use of JSON Pointers as URI fragment identifiers is described in
                <xref target="RFC6901">RFC 6901</xref>.
                For "application/schema+json", which supports two fragment identifier syntaxes,
                fragment identifiers matching the JSON Pointer syntax, including the empty string,
                MUST be interpreted as JSON Pointer fragment identifiers.
            </t>
            <t>
                Per the W3C's
                <xref target="W3C.WD-fragid-best-practices-20121025">best practices for fragment identifiers</xref>,
                plain name fragment identifiers in "application/schema+json" are reserved for referencing
                locally named schemas.  All fragment identifiers that do
                not match the JSON Pointer syntax MUST be interpreted as
                plain name fragment identifiers.
            </t>
            <t>
                Defining and referencing a plain name fragment identifier within an
                "application/schema+json" document are specified
                in the <xref target="id-keyword">"$id" keyword</xref> section.
            </t>
            <t>
            </t>
        </section>

        <section title="General Considerations">

            <section title="Range of JSON Values">
                <t>
                    An instance may be any valid JSON value as defined by <xref target="RFC7159">JSON</xref>.
                    JSON Schema imposes no restrictions on type: JSON Schema can describe any JSON
                    value, including, for example, null.
                </t>
            </section>

            <section title="Programming Language Independence" anchor="language">
                <t>
                    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.
                </t>
            </section>

            <section title="Mathematical Integers" anchor="integers">
                <t>
                    Some programming languages and parsers use different internal representations
                    for floating point numbers than they do for integers.
                </t>
                <t>
                    For consistency, integer JSON numbers SHOULD NOT be encoded with a fractional
                    part.
                </t>
            </section>

            <section title="Extending JSON Schema">
                <t>
                    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.
                </t>
                <t>
                    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.
                </t>
                <t>
                    Note that the recursive nature of meta-schemas requires re-defining
                    recursive keywords in the extended meta-schema, as can be seen in
                    the JSON Hyper-Schema meta-schema.
                </t>
            </section>

        </section>

        <section title='The "$schema" Keyword'>
            <!-- TODO a custom $schema keyword might also be used to enforce minimum required functionality of a validator -->
            <t>
                The "$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.
            </t>
            <t>
                The value of this keyword MUST be a <xref target="RFC3986">URI</xref>
                (containing a scheme) and this URI MUST be normalized.
                The current schema MUST be valid against the meta-schema identified by this URI.
            </t>
            <t>
                If this URI identifies a retrievable resource, that resource SHOULD be of
                media type "application/schema+json".
            </t>
            <t>
                The "$schema" keyword SHOULD be used in a root schema.
                It MUST NOT appear in subschemas.
            </t>
            <t>
                <cref>
                    Using multiple "$schema" keywords in the same document would imply that the
                    vocabulary and therefore behavior can change within a document.  This would
                    necessitate resolving a number of implementation concerns that have not yet
                    been clearly defined.  So, while the pattern of using "$schema" only in root
                    schemas is likely to remain the best practice for schema authoring,
                    implementation behavior is subject to be revised or liberalized in
                    future drafts.
                </cref>
                <!--
                    In particular, the process of validating an instance, including validating a
                    schema as an instance against its meta-schema, only allows for a single set
                    of rules across the entire instance document.  There is no equivalent of
                    changing the meta-schema partway through the validation for non-schema
                    instances.
                  -->
            </t>
            <t>
                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.
            </t>
        </section>

        <section title='Schema References With "$ref"'>
            <t>
                The "$ref" keyword is used to reference a schema, and provides the ability to
                validate recursive structures through self-reference.
            </t>
            <t>
                An object schema with a "$ref" property MUST be interpreted as a "$ref" reference.
                The value of the "$ref" property MUST be a URI Reference.
                Resolved against the current URI base, it identifies the URI of a schema to use.
                All other properties in a "$ref" object MUST be ignored.
            </t>
            <t>
                The URI is not a network locator, only an identifier. A schema need not be
                downloadable from the address if it is a network-addressable URL, and
                implementations SHOULD NOT assume they should perform a network operation when they
                encounter a network-addressable URI.
            </t>
            <t>
                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.
            </t>
        </section>

        <section title="Base URI and Dereferencing">
            <section title="Initial Base URI">
                <t>
                    <xref target="RFC3986">RFC3986 Section 5.1</xref> defines how to determine the
                    default base URI of a document.
                </t>
                <t>
                    Informatively, the initial base URI of a schema is the URI at which it was
                    found, or a suitable substitute URI if none is known.
                </t>
            </section>

            <section title='The "$id" Keyword' anchor="id-keyword">
                <t>
                    The "$id" keyword defines a URI for the schema, and the base URI that
                    other URI references within the schema are resolved against.
                    A subschema's "$id" is resolved against the base URI of its parent schema.
                    If no parent sets an explicit base with "$id", the base URI is that of the
                    entire document, as determined per
                    <xref target="RFC3986">RFC 3986 section 5</xref>.
                </t>
                <t>
                    If present, the value for this keyword MUST be a string, and MUST represent a
                    valid <xref target="RFC3986">URI-reference</xref>.
                    This value SHOULD be normalized, and SHOULD NOT be an empty fragment &lt;#&gt;
                    or an empty string &lt;&gt;.
                </t>
                <t>
                    The root schema of a JSON Schema document SHOULD contain an "$id" keyword with
                    a URI (containing a scheme).  This URI SHOULD either not have a fragment, or
                    have one that is an empty string.
                    <!-- All of the standard meta-schemas use an empty fragment in their id/$id values. -->
                    <cref>
                        How should an "$id" URI reference containing a fragment with other components
                        be interpreted?  There are two cases:  when the other components match
                        the current base URI and when they change the base URI.
                    </cref>
                </t>
                <t>
                    To name subschemas in a JSON Schema document,
                    subschemas can use "$id" to give themselves a document-local identifier.
                    This is done by setting "$id" to a URI reference consisting
                    only of a plain name fragment (not a JSON Pointer fragment).
                    The fragment identifier MUST begin with a letter ([A-Za-z]), followed by
                    any number of letters, digits ([0-9]), hyphens ("-"), underscores ("_"), colons
                    (":"), or periods (".").
                </t>
                <t>
                    Providing a plain name fragment enables a subschema to be
                    relocated within a schema without requiring that JSON
                    Pointer references are updated.
                </t>
                <t>
                    The effect of defining a fragment-only "$id" URI reference that neither
                    matches the above requirements nor is a valid JSON pointer
                    is not defined.
                </t>
                <t>
                    For example:
                    <figure>
                        <artwork>
<![CDATA[
{
    "$id": "http://example.com/root.json",
    "definitions": {
        "A": { "$id": "#foo" },
        "B": {
            "$id": "other.json",
            "definitions": {
                "X": { "$id": "#bar" },
                "Y": { "$id": "t/inner.json" }
            }
        },
        "C": {
            "$id": "urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f"
        }
    }
}
]]>
                        </artwork>
                    </figure>
                </t>
                <t>
                    The schemas at the following URI-encoded <xref target="RFC6901">JSON
                    Pointers</xref> (relative to the root schema) have the following
                    base URIs, and are identifiable by either URI in accordance with
                    Section <xref target="fragments" format="counter"></xref> above:
                </t>
                <t>
                    <list style="hanging">
                        <t hangText="# (document root)">http://example.com/root.json#</t>
                        <t hangText="#/definitions/A">http://example.com/root.json#foo</t>
                        <t hangText="#/definitions/B">http://example.com/other.json</t>
                        <t hangText="#/definitions/B/definitions/X">http://example.com/other.json#bar</t>
                        <t hangText="#/definitions/B/definitions/Y">http://example.com/t/inner.json</t>
                        <t hangText="#/definitions/C">urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f</t>
                    </list>
                </t>
                <section title="Internal References">
                    <t>
                        Schemas can be identified by any URI that has been given to them, including
                        a JSON Pointer or their URI given directly by "$id".
                    </t>
                    <t>
                        Tools SHOULD take note of the URIs that schemas, including subschemas,
                        provide for themselves using "$id". This is known as "Internal referencing".
                    </t>

                    <t>
                        For example, consider this schema:
                    </t>

                    <figure>
                        <artwork>
<![CDATA[
{
    "$id": "http://example.net/root.json",
    "items": {
        "type": "array",
        "items": { "$ref": "#item" }
    },
    "definitions": {
        "single": {
            "$id": "#item",
            "type": "integer"
        }
    }
}
]]>
                        </artwork>
                    </figure>
                    <t>
                        When an implementation encounters the &lt;#/definitions/single&gt; schema,
                        it resolves the "$id" URI reference against the current base URI to form
                        &lt;http://example.net/root.json#item&gt;.
                    </t>
                    <t>
                        When an implementation then looks inside the &lt;#/items&gt; schema, it
                        encounters the &lt;#item&gt; reference, and resolves this to
                        &lt;http://example.net/root.json#item&gt; which is understood as the schema
                        defined elsewhere in the same document without needing to
                        resolve the fragment against the base URI.
                    </t>
                </section>
                <section title="External References">
                    <t>
                        To differentiate schemas between each other in a vast ecosystem, schemas are
                        identified by URI. As specified above, this does not necessarily mean
                        anything is downloaded, but instead JSON Schema implementations SHOULD
                        already understand the schemas they will be using, including the URIs that
                        identify them.
                    </t>
                    <t>
                        Implementations SHOULD be able to associate arbitrary URIs with an arbitrary
                        schema and/or automatically associate a schema's "$id"-given URI, depending
                        on the trust that the validator has in the schema.
                    </t>
                    <t>
                        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.
                    </t>
                </section>
            </section>
        </section>

        <section title='Comments With "$comment"'>
            <t>
                This keyword is reserved for comments from schema authors to readers or
                maintainers of the schema.

                The value of this keyword MUST be a string. Implementations MUST NOT present this
                string to end users.  Tools for editing schemas SHOULD support displaying and
                editing this keyword.  The value of this keyword MAY be used in debug or error
                output which is intended for developers making use of schemas.

                Schema vocabularies SHOULD allow "$comment" within any object containing
                vocabulary keywords.  Implementations MAY assume "$comment" is allowed
                unless the vocabulary specifically forbids it.  Vocabularies MUST NOT
                specify any effect of "$comment" beyond what is described in this
                specification.

                Tools that translate other media types or programming languages
                to and from application/schema+json MAY choose to convert that media type or
                programming language's native comments to or from "$comment" values.
                The behavior of such translation when both native comments and "$comment"
                properties are present is implementation-dependent.

                Implementations SHOULD treat "$comment" identically to an unknown extension
                keyword.  They MAY strip "$comment" values at any point during processing.
                In particular, this allows for shortening schemas when the size of deployed
                schemas is a concern.

                Implementations MUST NOT take any other action based on the presence, absence,
                or contents of "$comment" properties.
            </t>
        </section>

        <section title="Usage for Hypermedia">

            <t>
                JSON has been adopted widely by 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
                <xref target="RFC8288">Web linking</xref>.
            </t>

            <section title='Linking to a Schema'>
                <t>
                    It is RECOMMENDED that instances described by a schema provide a link to
                    a downloadable JSON Schema using the link relation "describedby", as defined by
                    <xref target="W3C.REC-ldp-20150226">Linked Data Protocol 1.0, section 8.1</xref>.
                </t>

                <t>
                    In HTTP, such links can be attached to any response using the
                    <xref target="RFC8288">Link header</xref>. An example of such a header would be:
                </t>

                <figure>
                    <artwork>
<![CDATA[
Link: <http://example.com/my-hyper-schema#>; rel="describedby"
]]>
                    </artwork>
                </figure>

            </section>


            <section title='Identifying a Schema via a Media Type Parameter' anchor="parameter">
                <t>
                    Media types MAY allow for a "schema" media type parameter, which gives
                    HTTP servers the ability to perform Content-Type Negotiation based on schema.
                    The media-type parameter MUST be a whitespace-separated list of URIs
                    (i.e. relative references are invalid).
                </t>
                <t>
                    When using the media type application/schema-instance+json, the "schema"
                    parameter MUST be supplied.
                </t>
                <t>
                    The schema URI is opaque and SHOULD NOT automatically be dereferenced.
                    If the implementation does not understand the semantics of the provided schema,
                    the implementation can instead follow the "describedby" links, if any, which may
                    provide information on how to handle the schema.
                    Since "schema" 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.
                </t>

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

                <figure>
                    <artwork>
<![CDATA[
Content-Type: application/json;
          schema="http://example.com/my-hyper-schema#"
]]>
                    </artwork>
                </figure>

                <t>
                    Multiple schemas are whitespace separated:
                </t>

                <figure>
                    <artwork>
<![CDATA[
Content-Type: application/json;
          schema="http://example.com/alice http://example.com/bob"
]]>
                    </artwork>
                </figure>

                <t>
                    <cref>
                        This paragraph assumes that we can register a "schema" link relation.
                        Should we instead specify something like "tag:json-schema.org,2017:schema"
                        for now?
                    </cref>
                    HTTP can also send the "schema" in a Link, though this may impact media-type
                    semantics and Content-Type negotiation if this replaces the media-type parameter
                    entirely:
                </t>

                <figure>
                    <artwork>
<![CDATA[
Link: </alice>;rel="schema", </bob>;rel="schema"
]]>
                    </artwork>
                </figure>

            </section>

             <section title="Usage Over HTTP">
                <t>
                    When used for hypermedia systems over a network,
                    <xref target="RFC7231">HTTP</xref> is frequently the protocol of choice for
                    distributing schemas. Misbehaving 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.
                </t>
                <t>
                    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.
                </t>
                <t>
                    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 should precede the more
                    generic HTTP library name (if any). For example:
                    <figure>
                        <artwork>
<![CDATA[
User-Agent: product-name/5.4.1 so-cool-json-schema/1.0.2 curl/7.43.0
]]>
                        </artwork>
                    </figure>
                </t>
                <t>
                    Clients SHOULD be able to make requests with a "From" header so that server
                    operators can contact the owner of a potentially misbehaving script.
                </t>
            </section>

       </section>

        <section title="Security Considerations" anchor="security">
            <t>
                Both schemas and instances are JSON values. As such, all security considerations
                defined in <xref target="RFC7159">RFC 7159</xref> apply.
            </t>
            <t>
                Instances and schemas are both frequently written 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.
            </t>
            <t>
                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 "$id".
            </t>
            <t>
                Individual JSON Schema vocabularies are liable to also have their own security
                considerations. Consult the respective specifications for more information.
            </t>
            <t>
                Schema authors should take care with "$comment" contents, as a malicious
                implementation can display them to end-users in violation of a spec, or
                fail to strip them if such behavior is expected.
            </t>
            <t>
                A malicious schema author could place executable code or other dangerous
                material within a "$comment".  Implementations MUST NOT parse or otherwise
                take action based on "$comment" contents.
            </t>
        </section>

        <section title="IANA Considerations">
            <section title="application/schema+json">
                <t>
                    The proposed MIME media type for JSON Schema is defined as follows:

                    <list>
                        <t>Type name: application</t>
                        <t>Subtype name: schema+json</t>
                        <t>Required parameters: N/A</t>
                        <t>
                            Encoding considerations: Encoding considerations are
                            identical to those specified for the "application/json"
                            media type.  See <xref target="RFC7159">JSON</xref>.
                        </t>
                        <t>
                            Security considerations: See Section
                            <xref target="security" format="counter"></xref> above.
                        </t>
                        <t>
                            Interoperability considerations: See Sections
                            <xref target="language" format="counter"></xref> and
                            <xref target="integers" format="counter"></xref> above.
                        </t>
                        <t>
                            Fragment identifier considerations: See Section
                            <xref target="fragments" format="counter"></xref>
                        </t>
                    </list>
                </t>
            </section>
            <section title="application/schema-instance+json">
                <t>
                    The proposed MIME media type for JSON Schema Instances that require
                    a JSON Schema-specific media type is defined as follows:

                    <list>
                        <t>Type name: application</t>
                        <t>Subtype name: schema-instance+json</t>
                        <t>
                            Required parameters:
                            <list style="hanging">
                                <t hangText="schema">
                                    A non-empty list of space-separated URIs, each identifying
                                    a JSON Schema resource.  The instance SHOULD successfully
                                    validate against at least one of these schemas.
                                    Non-validating schemas MAY be included for purposes such
                                    as allowing clients to make use of older versions of a schema
                                    as long as the runtime instance validates against that
                                    older version.
                                </t>
                            </list>
                        </t>
                        <t>
                            Encoding considerations: Encoding considerations are
                            identical to those specified for the "application/json"
                            media type.  See <xref target="RFC7159">JSON</xref>.
                        </t>
                        <t>
                            Security considerations: See Section
                            <xref target="security" format="counter"></xref> above.
                        </t>
                        <t>
                            Interoperability considerations: See Sections
                            <xref target="language" format="counter"></xref> and
                            <xref target="integers" format="counter"></xref> above.
                        </t>
                        <t>
                            Fragment identifier considerations: See Section
                            <xref target="fragments" format="counter"></xref>
                        </t>
                    </list>
                </t>
            </section>
        </section>
    </middle>

    <back>
        <!-- References Section -->
        <references title="Normative References">
            &RFC2119;
            &RFC3986;
            &RFC6839;
            &RFC6901;
            &RFC7159;
            &ldp;
        </references>

        <references title="Informative References">
            &RFC7049;
            &RFC7231;
            &RFC8288;
            &fragid-best-practices;
            <reference anchor="json-schema-validation">
                <front>
                    <title>JSON Schema Validation: A Vocabulary for Structural Validation of JSON</title>
                    <author initials="A." surname="Wright">
                        <organization/>
                    </author>
                    <author initials="H." surname="Andrews">
                        <organization>Cloudflare, Inc.</organization>
                    </author>
                    <author initials="G." surname="Luff">
                        <organization/>
                    </author>
                    <date year="2017" month="November"/>
                </front>
                <seriesInfo name="Internet-Draft" value="draft-handrews-json-schema-validation-00" />
            </reference>
            <reference anchor="json-hyper-schema">
                <front>
                    <title>JSON Hyper-Schema: A Vocabulary for Hypermedia Annotation of JSON</title>
                    <author initials="H." surname="Andrews">
                        <organization>Cloudflare, Inc.</organization>
                    </author>
                    <author initials="A." surname="Wright">
                        <organization/>
                    </author>
                    <date year="2017" month="November"/>
                </front>
                <seriesInfo name="Internet-Draft" value="draft-handrews-json-schema-hyperschema-00" />
            </reference>
        </references>

        <section title="Acknowledgments">
            <t>
                Thanks to
                Gary Court,
                Francis Galiegue,
                Kris Zyp,
                and Geraint Luff
                for their work on the initial drafts of JSON Schema.
            </t>
            <t>
                Thanks to
                Jason Desrosiers,
                Daniel Perrett,
                Erik Wilde,
                Ben Hutton,
                Evgeny Poberezkin,
                Brad Bowman,
                Gowry Sankar,
                Donald Pipowitch,
                and Dave Finlay
                for their submissions and patches to the document.
            </t>
        </section>

        <section title="ChangeLog">
            <t>
                <cref>This section to be removed before leaving Internet-Draft status.</cref>
            </t>
            <t>
                <list style="hanging">
                    <t hangText="draft-handrews-json-schema-00">
                        <list style="symbols">
                            <t>Make the concept of a schema keyword vocabulary more clear</t>
                            <t>Note that the concept of "integer" is from a vocabulary, not the data model</t>
                            <t>Classify keywords as assertions or annotations and describe their general behavior</t>
                            <t>Explain the boolean schemas in terms of generalized assertions</t>
                            <t>Reserve "$comment" for non-user-visible notes about the schema</t>
                            <t>Wording improvements around "$id" and fragments</t>
                            <t>Note the challenges of extending meta-schemas with recursive references</t>
                            <t>Add "application/schema-instance+json" media type</t>
                            <t>Recommend a "schema" link relation / parameter instead of "profile"</t>
                        </list>
                    </t>
                    <t hangText="draft-wright-json-schema-01">
                        <list style="symbols">
                            <t>Updated intro</t>
                            <t>Allowed for any schema to be a boolean</t>
                            <t>"$schema" SHOULD NOT appear in subschemas, although that may change</t>
                            <t>Changed "id" to "$id"; all core keywords prefixed with "$"</t>
                            <t>Clarify and formalize fragments for application/schema+json</t>
                            <t>Note applicability to formats such as CBOR that can be represented in the JSON data model</t>
                        </list>
                    </t>
                    <t hangText="draft-wright-json-schema-00">
                        <list style="symbols">
                            <t>Updated references to JSON</t>
                            <t>Updated references to HTTP</t>
                            <t>Updated references to JSON Pointer</t>
                            <t>Behavior for "id" is now specified in terms of RFC3986</t>
                            <t>Aligned vocabulary usage for URIs with RFC3986</t>
                            <t>Removed reference to draft-pbryan-zyp-json-ref-03</t>
                            <t>Limited use of "$ref" to wherever a schema is expected</t>
                            <t>Added definition of the "JSON Schema data model"</t>
                            <t>Added additional security considerations</t>
                            <t>Defined use of subschema identifiers for "id"</t>
                            <t>Rewrote section on usage with HTTP</t>
                            <t>Rewrote section on usage with rel="describedBy" and rel="profile"</t>
                            <t>Fixed numerous invalid examples</t>
                        </list>
                    </t>
                    <t hangText="draft-zyp-json-schema-04">
                        <list style="symbols">
                            <t>Salvaged from draft v3.</t>
                            <t>Split validation keywords into separate document.</t>
                            <t>Split hypermedia keywords into separate document.</t>
                            <t>Initial post-split draft.</t>
                            <t>Mandate the use of JSON Reference, JSON Pointer.</t>
                            <t>Define the role of "id". Define URI resolution scope.</t>
                            <t>Add interoperability considerations.</t>
                        </list>
                    </t>
                    <t hangText="draft-zyp-json-schema-00">
                        <list style="symbols">
                            <t>Initial draft.</t>
                        </list>
                    </t>
                </list>
            </t>
        </section>
    </back>
</rfc>