jsonschema-validation.xml

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<rfc category="info" docName="draft-handrews-json-schema-validation-00" ipr="trust200902">
    <front>
        <title abbrev="JSON Schema Validation">
            JSON Schema Validation: A Vocabulary for Structural Validation of JSON
        </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>

        <author fullname="Geraint Luff" initials="G" surname="Luff">
            <address>
                <postal>
                    <street></street>
                    <city>Cambridge</city>
                    <country>UK</country>
                </postal>
                <email>luffgd@gmail.com</email>
            </address>
        </author>

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

        <abstract>
            <t>
                JSON Schema (application/schema+json) has several purposes, one of which is JSON
                instance validation.
                This document specifies a vocabulary for JSON Schema to describe the meaning of JSON
                documents, provide hints for user interfaces working with JSON data, and to make
                assertions about what a valid document must look like.
            </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 can be used to require that a given JSON document (an instance)
                satisfies a certain number of criteria. These criteria are asserted by using
                keywords described in this specification. In addition, a set of keywords
                is also defined to assist in interactive user interface instance generation.
            </t>
            <t>
                This specification will use the concepts, syntax, and terminology defined
                by the <xref target="json-schema">JSON Schema core</xref> specification.
            </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>
                This specification uses the term "container instance" to refer to both array and
                object instances. It uses the term "children instances" to refer to array elements
                or object member values.
            </t>
            <t>
                Elements in an array value are said to be unique if no two elements of this array
                are <xref target="json-schema">equal</xref>.
            </t>
        </section>

        <section title="Overview">
            <t>
                JSON Schema validation applies schemas to locations within the instance,
                and asserts constraints on the structure of the data at each location.
                An instance location that satisfies all asserted constraints is then
                annotated with any keywords that contain non-assertion information,
                such as descriptive metadata and usage hints.  If all locations within
                the instance satisfy all asserted constraints, then the instance is
                said to be valid against the schema.
            </t>
            <t>
                Each schema object is independently evaluated against each instance location
                to which it applies.  This greatly simplifies the implementation requirements
                for validators by ensuring that they do not need to maintain state across
                the document-wide validation process.
            </t>
            <section title="Applicability">
                <t>
                    Validation begins by applying the root schema to the complete instance
                    document.  From there, various keywords are used to determine which additional
                    subschemas are applied to either the current location, or a child location.
                    These keywords also define whether and how subschema assertion results are
                    modified and/or combined.  Such keywords do not assert conditions on their
                    own.  Rather, they control how assertions are applied and evaluated.
                </t>
                <t>
                    The keywords in the <xref target="logic">boolean logic</xref> and
                    <xref target="conditional">conditional</xref> sections of this specification
                    apply subschemas to the same location as the parent schema.  The former group
                    defines boolean operations on the subschema assertion results, while the
                    latter evaluates one subschema and uses its assertion results to determine
                    which of two other subschemas to apply as well.
                </t>
                <t>
                    Several keywords determine which subschemas are applied to array items,
                    object property values, and object property names.  They are:
                    "items", "additionalItems", "contains", "properties", "patternProperties",
                    "additionalProperties", and "propertyNames".  The "contains" keyword only
                    requires its subschema to be valid against at least one child instance, while
                    the other keywords require that all subschemas are valid against all child
                    instances to which they apply.
                </t>
                <section title="Keyword Independence">
                    <t>
                        Validation keywords typically operate independently, without
                        affecting each other's outcomes.
                    </t>
                    <t>
                        For schema author convenience, there are some exceptions among the
                        keywords that control subschema applicability:
                        <list>
                            <t>
                                "additionalProperties", whose behavior is defined in terms of
                                "properties" and "patternProperties"; and
                            </t>
                            <t>
                                "additionalItems", whose behavior is defined in terms of "items".
                            </t>
                        </list>
                    </t>
                </section>
            </section>
            <section title="Assertions" anchor="assertions">
                <t>
                    Validation is a process of checking assertions.  Each assertion adds
                    constraints that an instance must satisfy in order to successfully validate.
                </t>
                <t>
                    Assertion keywords that are absent never restrict validation.
                    In some cases, this no-op behavior is identical to a keyword that exists with
                    certain values, and these values are noted where known.
                </t>
                <t>
                    All of the keywords in the <xref target="general">general</xref>,
                    <xref target="numeric">numeric</xref>, and <xref target="string">string</xref>
                    sections are assertions, as well as "minItems", "maxItems", "uniqueItems",
                    "minProperties", "maxProperties", and "required".  Additionally, "dependencies"
                    is shorthand for a combination of conditional and assertion keywords.
                </t>
                <t>
                    The "format", "contentType", and "contentEncoding" keywords can also be
                    implemented as assertions, although that functionality is an optional part
                    of this specification, and the keywords convey additional non-assertion
                    information.
                </t>
                <section title="Assertions and Instance Primitive Types">
                    <t>
                        Most validation assertions only constrain values within a certain
                        primitive type.  When the type of the instance is not of the type
                        targeted by the keyword, the instance is considered to conform
                        to the assertion.
                    </t>
                    <t>
                        For example, the "maxLength" keyword will only restrict certain strings
                        (that are too long) from being valid.  If the instance is a number,
                        boolean, null, array, or object, then it is valid against this assertion.
                    </t>
                </section>
            </section>
            <section title="Annotations" anchor="annotations">
                <t>
                    In addition to assertions, this specification provides a small vocabulary
                    of metadata keywords that can be used to annotate the JSON instance with
                    useful information.  The <xref target="format" /> and <xref target="content" />
                    keywords are also useful as annotations as well as being optional assertions,
                    as they convey additional usage guidance for the instance data.
                </t>
                <t>
                    A schema that is applicable to a particular location in the instance, against
                    which the instance location is valid, attaches its annotations to that location
                    in the instance.  Since many subschemas can be applicable to any single
                    location, annotation keywords need to specify any unusual handling of
                    multiple applicable occurrences of the keyword with different values.
                    The default behavior is simply to collect all values.
                </t>
                <t>
                    Additional vocabularies SHOULD make use of this mechanism for applying
                    their own annotations to instances.
                </t>
                <section title="Negated Schemas">
                    <t>
                        Annotations in a subschema contained within a "not", at any depth,
                        including any number of intervening additional "not" subschemas, MUST be
                        ignored.  Similarly, annotations within a failing branch of a "oneOf",
                        "anyOf", "then", or "else" MUST be ignored.
                    </t>
                </section>
                <section title="Annotations and Short-Circuit Validation">
                    <t>
                        Annotation keywords MUST be applied to all possible sub-instances.
                        Even if such application can be short-circuited when only assertion
                        evaluation is needed.  For instance, the "contains" keyword need only
                        be checked for assertions until at least one array item proves valid.
                        However, when working with annotations, all items in the array must
                        be evaluated to determine all items with which the annotations should
                        be associated.
                    </t>
                </section>
            </section>
        </section>

        <section title="Interoperability Considerations">

            <section title="Validation of String Instances">
                <t>
                    It should be noted that the nul character (\u0000) is valid in a JSON string. An
                    instance to validate may contain a string value with this character, regardless
                    of the ability of the underlying programming language to deal with such data.
                </t>
            </section>

            <section title="Validation of Numeric Instances">
                <t>
                    The JSON specification allows numbers with arbitrary precision, and JSON Schema
                    does not add any such bounds.
                    This means that numeric instances processed by JSON Schema can be arbitrarily large and/or
                    have an arbitrarily long decimal part, regardless of the ability of the
                    underlying programming language to deal with such data.
                </t>
            </section>

            <section title="Regular Expressions" anchor="regexInterop">
                <t>
                    Two validation keywords, "pattern" and "patternProperties", use regular
                    expressions to express constraints, and the "regex" value for the
                    "format" keyword constrains the instance value to be a regular expression.
                     These regular expressions SHOULD be valid according to the
                    <xref target="ecma262">ECMA 262</xref> regular expression dialect.
                </t>
                <t>
                    Furthermore, given the high disparity in regular expression constructs support,
                    schema authors SHOULD limit themselves to the following regular expression
                    tokens:

                    <list>
                        <t>individual Unicode characters, as defined by the <xref
                        target="RFC7159">JSON specification</xref>;</t>
                        <t>simple character classes ([abc]), range character classes ([a-z]);</t>
                        <t>complemented character classes ([^abc], [^a-z]);</t>
                        <t>simple quantifiers: "+" (one or more), "*" (zero or more), "?" (zero or
                        one), and their lazy versions ("+?", "*?", "??");</t>
                        <t>range quantifiers: "{x}" (exactly x occurrences), "{x,y}" (at least x, at
                        most y, occurrences), {x,} (x occurrences or more), and their lazy
                        versions;</t>
                        <t>the beginning-of-input ("^") and end-of-input ("$") anchors;</t>
                        <t>simple grouping ("(...)") and alternation ("|").</t>
                    </list>
                </t>
                <t>
                    Finally, implementations MUST NOT take regular expressions to be
                    anchored, neither at the beginning nor at the end. This means, for instance,
                    the pattern "es" matches "expression".
                </t>
            </section>

        </section>

        <section title="Meta-Schema">
            <t>
                The current URI for the JSON Schema Validation is
                <eref target="http://json-schema.org/draft-07/schema#"/>.
            </t>
        </section>

        <section title="Validation Keywords">
            <t>
                Validation keywords in a schema impose requirements for successful validation of an
                instance.
            </t>

            <section title="Validation Keywords for Any Instance Type" anchor="general">
                <section title="type">
                    <t>
                        The value of this keyword MUST be either a string or an array. If it is
                        an array, elements of the array MUST be strings and MUST be unique.
                    </t>
                    <t>
                        String values MUST be one of the six primitive types
                        ("null", "boolean", "object", "array", "number", or "string"),
                        or "integer" which matches any number with a zero fractional part.
                    </t>
                    <t>
                        An instance validates if and only if the instance is in any of the sets listed
                        for this keyword.
                    </t>
                </section>

                <section title="enum">
                    <t>
                        The value of this keyword MUST be an array. This array SHOULD have at
                        least one element. Elements in the array SHOULD be unique.
                    </t>
                    <t>
                        An instance validates successfully against this keyword if its value is
                        equal to one of the elements in this keyword's array value.
                    </t>
                    <t>
                        Elements in the array might be of any value, including null.
                    </t>
                </section>

                <section title="const">
                    <t>
                        The value of this keyword MAY be of any type, including null.
                    </t>
                    <t>
                        An instance validates successfully against this keyword if its value is
                        equal to the value of the keyword.
                    </t>
                </section>
            </section>

            <section title="Validation Keywords for Numeric Instances (number and integer)"
                     anchor="numeric">
                <section title="multipleOf">
                    <t>
                        The value of "multipleOf" MUST be a number, strictly greater than 0.
                    </t>
                    <t>
                        A numeric instance is valid only if division by this keyword's value results in
                        an integer.
                    </t>
                </section>

                <section title="maximum">
                    <t>
                        The value of "maximum" MUST be a number, representing an inclusive upper limit
                        for a numeric instance.
                    </t>
                    <t>
                        If the instance is a number, then this keyword validates only if the instance is
                        less than or exactly equal to "maximum".
                    </t>
                </section>

                <section title="exclusiveMaximum">
                    <t>
                        The value of "exclusiveMaximum" MUST be number, representing an exclusive upper
                        limit for a numeric instance.
                    </t>
                    <t>
                        If the instance is a number, then the instance is valid only if it has a value
                        strictly less than (not equal to) "exclusiveMaximum".
                    </t>
                </section>

                <section title="minimum">
                    <t>
                        The value of "minimum" MUST be a number, representing an inclusive lower limit
                        for a numeric instance.
                    </t>
                    <t>
                        If the instance is a number, then this keyword validates only if the instance is
                        greater than or exactly equal to "minimum".
                    </t>
                </section>

                <section title="exclusiveMinimum">
                    <t>
                        The value of "exclusiveMinimum" MUST be number, representing an exclusive lower
                        limit for a numeric instance.
                    </t>
                    <t>
                        If the instance is a number, then the instance is valid only if it has a value
                        strictly greater than (not equal to) "exclusiveMinimum".
                    </t>
                </section>
            </section>

            <section title="Validation Keywords for Strings" anchor="string">
                <section title="maxLength">
                    <t>
                        The value of this keyword MUST be a non-negative integer.</t>
                    <t>
                        A string instance is valid against this keyword if its
                        length is less than, or equal to, the value of this keyword.
                    </t>
                    <t>
                        The length of a string instance is defined as the number of its
                        characters as defined by <xref target="RFC7159">RFC 7159</xref>.
                    </t>
                </section>

                <section title="minLength">
                    <t>
                        The value of this keyword MUST be a non-negative integer.
                    </t>
                    <t>
                        A string instance is valid against this keyword if its
                        length is greater than, or equal to, the value of this keyword.
                    </t>

                    <t>
                        The length of a string instance is defined as the number of its
                        characters as defined by <xref target="RFC7159">RFC 7159</xref>.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as a value of 0.
                    </t>
                </section>

                <section title="pattern">
                    <t>
                        The value of this keyword MUST be a string. This string SHOULD be a
                        valid regular expression, according to the ECMA 262 regular expression
                        dialect.
                    </t>
                    <t>
                        A string instance is considered valid if the regular
                        expression matches the instance successfully. Recall: regular
                        expressions are not implicitly anchored.
                    </t>
                </section>
            </section>

            <section title="Validation Keywords for Arrays">
                <section title="items">
                    <t>
                        The value of "items" MUST be either a valid JSON Schema or an array of valid
                        JSON Schemas.
                    </t>
                    <t>
                        This keyword determines how child instances validate for arrays,
                        and does not directly validate the immediate instance itself.
                    </t>
                    <t>
                        If "items" is a schema, validation succeeds if all elements
                        in the array successfully validate against that schema.
                    </t>
                    <t>
                        If "items" is an array of schemas, validation succeeds if
                        each element of the instance validates against the schema at the
                        same position, if any.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty schema.
                    </t>
                </section>

                <section title="additionalItems">
                    <t>
                        The value of "additionalItems" MUST be a valid JSON Schema.
                    </t>
                    <t>
                        This keyword determines how child instances validate for arrays,
                        and does not directly validate the immediate instance itself.
                    </t>
                    <t>
                        If "items" is an array of schemas, validation succeeds
                        if every instance element at a position greater than the size
                        of "items" validates against "additionalItems".
                    </t>
                    <t>
                        Otherwise, "additionalItems" MUST be ignored, as the "items"
                        schema (possibly the default value of an empty schema) is
                        applied to all elements.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty schema.
                    </t>
                </section>

                <section title="maxItems">
                    <t>
                        The value of this keyword MUST be a non-negative integer.
                    </t>
                    <t>
                        An array instance is valid against "maxItems" if its size is
                        less than, or equal to, the value of this keyword.
                    </t>
                </section>

                <section title="minItems">
                    <t>
                        The value of this keyword MUST be a non-negative integer.
                    </t>
                    <t>
                        An array instance is valid against "minItems" if its size is
                        greater than, or equal to, the value of this keyword.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as a value of 0.
                    </t>
                </section>

                <section title="uniqueItems">
                    <t>
                        The value of this keyword MUST be a boolean.
                    </t>
                    <t>
                        If this keyword has boolean value false, the instance validates
                        successfully. If it has boolean value true, the instance validates
                        successfully if all of its elements are unique.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as a value of false.
                    </t>
                </section>

                <section title="contains">
                    <t>
                        The value of this keyword MUST be a valid JSON Schema.
                    </t>
                    <t>
                        An array instance is valid against "contains" if at least one of
                        its elements is valid against the given schema.
                    </t>
                </section>
            </section>

            <section title="Validation Keywords for Objects">
                <section title="maxProperties">
                    <t>
                        The value of this keyword MUST be a non-negative integer.
                    </t>
                    <t>
                        An object instance is valid against "maxProperties" if its
                        number of properties is less than, or equal to, the value of this
                        keyword.
                    </t>
                </section>

                <section title="minProperties">
                    <t>
                        The value of this keyword MUST be a non-negative integer.
                    </t>
                    <t>
                        An object instance is valid against "minProperties" if its
                        number of properties is greater than, or equal to, the value of this
                        keyword.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as a value of 0.
                    </t>
                </section>

                <section title="required">
                    <t>
                        The value of this keyword MUST be an array.
                        Elements of this array, if any, MUST be strings, and MUST be unique.
                    </t>
                    <t>
                        An object instance is valid against this keyword if every item in the array is
                        the name of a property in the instance.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty array.
                    </t>
                </section>

                <section title="properties">
                    <t>
                        The value of "properties" MUST be an object.
                        Each value of this object MUST be a valid JSON Schema.
                    </t>
                    <t>
                        This keyword determines how child instances validate for objects,
                        and does not directly validate the immediate instance itself.
                    </t>
                    <t>
                        Validation succeeds if, for each name that appears in both
                        the instance and as a name within this keyword's value, the child
                        instance for that name successfully validates against the
                        corresponding schema.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty object.
                    </t>
                </section>

                <section title="patternProperties">
                    <t>
                        The value of "patternProperties" MUST be an object. Each property name
                        of this object SHOULD be a valid regular expression, according to the
                        ECMA 262 regular expression dialect. Each property value of this object
                        MUST be a valid JSON Schema.
                    </t>
                    <t>
                        This keyword determines how child instances validate for objects,
                        and does not directly validate the immediate instance itself.
                        Validation of the primitive instance type against this keyword
                        always succeeds.
                    </t>
                    <t>
                        Validation succeeds if, for each instance name that matches any
                        regular expressions that appear as a property name in this keyword's value,
                        the child instance for that name successfully validates against each
                        schema that corresponds to a matching regular expression.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty object.
                    </t>
                </section>

                <section title="additionalProperties">
                    <t>
                        The value of "additionalProperties" MUST be a valid JSON Schema.
                    </t>
                    <t>
                        This keyword determines how child instances validate for objects,
                        and does not directly validate the immediate instance itself.
                    </t>
                    <t>
                        Validation with "additionalProperties" applies only to the child
                        values of instance names that do not match any names in "properties",
                        and do not match any regular expression in "patternProperties".
                    </t>
                    <t>
                        For all such properties, validation succeeds if the child instance
                        validates against the "additionalProperties" schema.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty schema.
                    </t>
                </section>

                <section title="dependencies">
                    <t>
                        <cref>
                            Now that "if", "then", and "else" are keywords, it is not clear whether
                            there is any benefit in keeping the schema form of "dependencies".
                            It is frequently misunderstood, and having a form that takes a subschema
                            as well as a form that takes a primitive value is particularly
                            confusing.  And it seems to be rarely used.  Depending on feedback
                            with "if", "then", and "else", the schema form of this keyword may
                            well be removed in a future draft.  Current thought is that the string
                            form (giving property names in an array) is a useful shortcut.
                        </cref>
                    </t>
                    <t>
                        This keyword specifies rules that are evaluated if the instance is an object and
                        contains a certain property.
                    </t>
                    <t>
                        This keyword's value MUST be an object. Each property specifies a dependency.
                        Each dependency value MUST be an array or a valid JSON Schema.
                    </t>
                    <t>
                        If the dependency value is a subschema, and the dependency key is a property
                        in the instance, the entire instance must validate against the dependency value.
                    </t>
                    <t>
                        If the dependency value is an array, each element in the array,
                        if any, MUST be a string, and MUST be unique. If the dependency key is
                        a property in the instance, each of the items in the dependency
                        value must be a property that exists in the instance.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty object.
                    </t>
                </section>

                <section title="propertyNames">
                    <t>
                        The value of "propertyNames" MUST be a valid JSON Schema.
                    </t>
                    <t>
                        If the instance is an object, this keyword validates if every property name in
                        the instance validates against the provided schema.
                        Note the property name that the schema is testing will always be a string.
                    </t>
                    <t>
                        Omitting this keyword has the same behavior as an empty schema.
                    </t>
                </section>
            </section>

            <section title="Keywords for Applying Subschemas Conditionally" anchor="conditional">
                <t>
                    These keywords work together to implement conditional
                    application of a subschema based on the outcome of
                    another subschema.
                </t>
                <t>
                    These keywords MUST NOT interact with each other across
                    subschema boundaries.  In other words, an "if" in one
                    branch of an "allOf" MUST NOT have an impact on a "then"
                    or "else" in another branch.
                </t>
                <section title="if">
                    <t>
                        This keyword's value MUST be a valid JSON Schema.
                    </t>
                    <t>
                        Instances that successfully validate against this
                        keyword's subschema MUST also be valid against
                        the subschema value of the "then" keyword, if
                        present.
                    </t>
                    <t>
                        Instances that fail to validate against this
                        keyword's subschema MUST also be valid against
                        the subschema value of the "else" keyword.
                    </t>
                    <t>
                        Validation of the instance against this keyword
                        on its own always succeeds, regardless of the
                        validation outcome of against its subschema.
                    </t>
                </section>
                <section title="then">
                    <t>
                        This keyword's value MUST be a valid JSON Schema.
                    </t>
                    <t>
                        When present alongside of "if", the instance
                        successfully validates against this keyword if
                        it validates against both the "if"'s subschema
                        and this keyword's subschema.
                    </t>
                    <t>
                        When "if" is absent, or the instance fails to
                        validate against its subschema, validation against
                        this keyword always succeeds.  Implementations
                        SHOULD avoid attempting to validate against
                        the subschema in these cases.
                    </t>
                </section>
                <section title="else">
                    <t>
                        This keyword's value MUST be a valid JSON Schema.
                    </t>
                    <t>
                        When present alongside of "if", the instance
                        successfully validates against this keyword if
                        it fails to validate against the "if"'s
                        subschema, and successfully validates against
                        this keyword's subschema.
                    </t>
                    <t>
                        When "if" is absent, or the instance successfully
                        validates against its subschema, validation against
                        this keyword always succeeds.  Implementations
                        SHOULD avoid attempting to validate against
                        the subschema in these cases.
                    </t>
                </section>
            </section>

            <section title="Keywords for Applying Subschemas With Boolean Logic" anchor="logic">
                <section title="allOf">
                    <t>
                        This keyword's value MUST be a non-empty array.
                        Each item of the array MUST be a valid JSON Schema.
                    </t>
                    <t>
                        An instance validates successfully against this keyword if it validates
                        successfully against all schemas defined by this keyword's value.
                    </t>
                </section>

                <section title="anyOf">
                    <t>
                        This keyword's value MUST be a non-empty array.
                        Each item of the array MUST be a valid JSON Schema.
                    </t>
                    <t>
                        An instance validates successfully against this keyword if it validates
                        successfully against at least one schema defined by this keyword's value.
                    </t>
                </section>

                <section title="oneOf">
                    <t>
                        This keyword's value MUST be a non-empty array.
                        Each item of the array MUST be a valid JSON Schema.
                    </t>
                    <t>
                        An instance validates successfully against this keyword if it validates
                        successfully against exactly one schema defined by this keyword's value.
                    </t>
                </section>

                <section title="not">
                    <t>
                        This keyword's value MUST be a valid JSON Schema.
                    </t>
                    <t>
                        An instance is valid against this keyword if it fails to validate
                        successfully against the schema defined by this keyword.
                    </t>
                </section>
            </section>
        </section>

        <section title='Semantic Validation With "format"' anchor="format">

            <section title="Foreword">
                <t>
                    Structural validation alone may be insufficient to validate that an instance
                    meets all the requirements of an application. The "format" keyword is defined to
                    allow interoperable semantic validation for a fixed subset of values which are
                    accurately described by authoritative resources, be they RFCs or other external
                    specifications.
                </t>

                <t>
                    The value of this keyword is called a format attribute. It MUST be a string. A
                    format attribute can generally only validate a given set of instance types. If
                    the type of the instance to validate is not in this set, validation for this
                    format attribute and instance SHOULD succeed.
                </t>

            </section>

            <section title="Implementation Requirements">
                <t>
                    The "format" keyword functions as both an annotation
                    (<xref target="annotations" />) and as an assertion
                    (<xref target="assertions" />).  While no special effort is required to
                    implement it as an annotation conveying semantic meaning, implementing
                    validation is non-trivial.
                </t>
                <t>
                    Implementations MAY support the "format" keyword as a validation assertion.
                    Should they choose to do so:

                    <list>
                        <t>they SHOULD implement validation for attributes defined below;</t>
                        <t>they SHOULD offer an option to disable validation for this keyword.</t>
                    </list>

                </t>

                <t>
                    Implementations MAY add custom format attributes. Save for agreement between
                    parties, schema authors SHALL NOT expect a peer implementation to support this
                    keyword and/or custom format attributes.
                </t>
            </section>

            <section title="Defined Formats">

                <section title="Dates and Times">
                    <t>
                        These attributes apply to string instances.
                    </t>
                    <t>
                        Date and time format names are derived from
                        <xref target="RFC3339">RFC 3339, section 5.6</xref>.
                    </t>
                    <t>
                        Implementations supporting formats SHOULD implement support for
                        the following attributes:
                        <list style="hanging">
                            <t hangText="date-time">
                                A string instance is valid against this attribute if it is
                                a valid representation according to the "date-time" production.
                            </t>
                            <t hangText="date">
                                A string instance is valid against this attribute if it is
                                a valid representation according to the "full-date" production.
                            </t>
                            <t hangText="time">
                                A string instance is valid against this attribute if it is
                                a valid representation according to the "full-time" production.
                            </t>
                        </list>
                    </t>
                    <t>
                        Implementations MAY support additional attributes using the other
                        production names defined in that section.  If "full-date" or "full-time"
                        are implemented, the corresponding short form ("date" or "time"
                        respectively) MUST be implemented, and MUST behave identically.
                        Implementations SHOULD NOT define extension attributes
                        with any name matching an RFC 3339 production unless it validates
                        according to the rules of that production.
                        <cref>
                            There is not currently consensus on the need for supporting
                            all RFC 3339 formats, so this approach of reserving the
                            namespace will encourage experimentation without committing
                            to the entire set.  Either the format implementation requirements
                            will become more flexible in general, or these will likely
                            either be promoted to fully specified attributes or dropped.
                        </cref>
                    </t>
                </section>

                <section title="Email Addresses">
                    <t>
                        These attributes apply to string instances.
                    </t>
                    <t>
                        A string instance is valid against these attributes if it is a valid
                        Internet email address as follows:
                        <list style="hanging">
                            <t hangText="email">
                                As defined by <xref target="RFC5322">RFC 5322, section 3.4.1</xref>.
                            </t>
                            <t hangText="idn-email">
                                As defined by <xref target="RFC6531">RFC 6531</xref>
                            </t>
                        </list>
                        Note that all strings valid against the "email" attribute are also
                        valid against the "idn-email" attribute.
                    </t>
                </section>
                <section title="Hostnames">
                    <t>
                        These attributes apply to string instances.
                    </t>
                    <t>
                        A string instance is valid against these attributes if it is a valid
                        representation for an Internet hostname as follows:
                        <list style="hanging">
                            <t hangText="hostname">
                                As defined by <xref target="RFC1034">RFC 1034, section 3.1</xref>,
                                including host names produced using the Punycode algorithm
                                specified in <xref target="RFC5891">RFC 5891, section 4.4</xref>.
                            </t>
                            <t hangText="idn-hostname">
                                As defined by either RFC 1034 as for hostname, or an
                                internationalized hostname as defined by
                                <xref target="RFC5890">RFC 5890, section 2.3.2.3</xref>.
                            </t>
                        </list>
                        Note that all strings valid against the "hostname" attribute are also
                        valid against the "idn-hostname" attribute.
                    </t>
                </section>

                <section title="IP Addresses">
                    <t>
                        These attributes apply to string instances.
                    </t>
                    <t>
                        A string instance is valid against these attributes if it is a valid
                        representation of an IP address as follows:
                        <list style="hanging">
                            <t hangText="ipv4">
                                An IPv4 address according to the "dotted-quad" ABNF
                                syntax as defined in
                                <xref target="RFC2673">RFC 2673, section 3.2</xref>.
                            </t>
                            <t hangText="ipv6">
                                An IPv6 address as defined in
                                <xref target="RFC4291">RFC 4291, section 2.2</xref>.
                            </t>
                        </list>
                    </t>
                </section>

                <section title="Resource Identifiers">
                    <t>
                        These attributes apply to string instances.
                    </t>
                    <t>
                        <list style="hanging">
                            <t hangText="uri">
                                A string instance is valid against this attribute if it is
                                a valid URI, according to <xref target="RFC3986"/>.
                            </t>
                            <t hangText="uri-reference">
                                A string instance is valid against this attribute if it is a valid URI
                                Reference (either a URI or a relative-reference),
                                according to <xref target="RFC3986"/>.
                            </t>
                            <t hangText="iri">
                                A string instance is valid against this attribute if it is
                                a valid IRI, according to <xref target="RFC3987"/>.
                            </t>
                            <t hangText="iri-reference">
                                A string instance is valid against this attribute if it is a valid IRI
                                Reference (either an IRI or a relative-reference),
                                according to <xref target="RFC3987"/>.
                            </t>
                        </list>
                        Note that all valid URIs are valid IRIs, and all valid URI References are
                        also valid IRI References.
                    </t>
                </section>

                <section title="uri-template">
                    <t>
                        This attribute applies to string instances.
                    </t>
                    <t>
                        A string instance is valid against this attribute if it is a valid URI Template
                        (of any level), according to <xref target="RFC6570"/>.
                    </t>
                    <t>
                        Note that URI Templates may be used for IRIs; there is no separate
                        IRI Template specification.
                    </t>
                </section>

                <section title="JSON Pointers">
                    <t>
                        These attributes apply to string instances.
                    </t>
                    <t>
                        <list style="hanging">
                            <t hangText="json-pointer">
                                A string instance is valid against this attribute if it
                                is a valid JSON string representation of a JSON Pointer,
                                according to <xref target="RFC6901">RFC 6901, section 5</xref>.
                            </t>
                            <t hangText="relative-json-pointer">
                                A string instance is valid against this attribute if it is a valid
                                <xref target="relative-json-pointer">Relative JSON Pointer</xref>.
                            </t>
                        </list>
                        To allow for both absolute and relative JSON Pointers, use "anyOf" or
                        "oneOf" to indicate support for either format.
                    </t>
                </section>
                <section title="regex">
                    <t>
                        This attribute applies to string instances.
                    </t>
                    <t>
                        A regular expression, which SHOULD be valid according to the
                        <xref target="ecma262">ECMA 262</xref> regular expression dialect.
                    </t>
                    <t>
                        Implementations that validate formats MUST accept at least the subset of
                        ECMA 262 defined in the <xref target="regexInterop">Regular Expressions</xref>
                        section of this specification, and SHOULD accept all valid ECMA 262 expressions.
                    </t>
                </section>
            </section>
        </section>

        <section title='String-Encoding Non-JSON Data' anchor="content">

            <section title="Foreword">
                <t>
                    Properties defined in this section indicate that an instance contains
                    non-JSON data encoded in a JSON string.
                    They describe the type of content and how it is encoded.
                </t>
                <t>
                    These properties provide additional information required to interpret JSON data
                    as rich multimedia documents.
                </t>
            </section>

            <section title="Implementation Requirements">
                <t>
                    The content keywords function as both annotations
                    (<xref target="annotations" />) and as assertions
                    (<xref target="assertions" />).
                    While no special effort is required to implement them as annotations conveying
                    how applications can interpret the data in the string, implementing
                    validation of conformance to the media type and encoding is non-trivial.
                </t>
                <t>
                    Implementations MAY support the "contentMediaType" and "contentEncoding"
                    keywords as validation assertions.
                    Should they choose to do so, they SHOULD offer an option to disable validation
                    for these keywords.
                </t>
            </section>

            <section title="contentEncoding">

                <t>
                    If the instance value is a string, this property defines that the string
                    SHOULD be interpreted as binary data and decoded using the encoding
                    named by this property.
                    <xref target="RFC2045">RFC 2045, Sec 6.1</xref> lists the possible
                    values for this property.
                </t>

                <t>
                    The value of this property MUST be a string.
                </t>

                <t>
                    The value of this property SHOULD be ignored if the instance described is not a
                    string.
                </t>

            </section>

            <section title="contentMediaType">
                <t>
                    The value of this property must be a media type, as defined by
                    <xref target="RFC2046">RFC 2046</xref>. This property defines the media
                    type of instances which this schema defines.
                </t>

                <t>
                    The value of this property MUST be a string.
                </t>

                <t>
                    The value of this property SHOULD be ignored if the instance described is not a
                    string.
                </t>

                <t>
                    If the "contentEncoding" property is not present, but the instance value is a
                    string, then the value of this property SHOULD specify a text document type,
                    and the character set SHOULD be the character set into which the JSON string
                    value was decoded (for which the default is Unicode).
                </t>
            </section>

            <section title="Example">
                <figure>
                    <preamble>
                        Here is an example schema, illustrating the use of "contentEncoding" and
                        "contentMediaType":
                    </preamble>
                    <artwork>
<![CDATA[
{
    "type": "string",
    "contentEncoding": "base64",
    "contentMediaType": "image/png"
}
]]>
                    </artwork>
                    <postamble>
                        Instances described by this schema should be strings, and their values
                        should be interpretable as base64-encoded PNG images.
                    </postamble>
                </figure>

                <figure>
                    <preamble>
                        Another example:
                    </preamble>
                    <artwork>
<![CDATA[
{
    "type": "string",
    "contentMediaType": "text/html"
}
]]>
                    </artwork>
                    <postamble>
                        Instances described by this schema should be strings containing HTML, using
                        whatever character set the JSON string was decoded into (default is
                        Unicode).
                    </postamble>
                </figure>
            </section>

        </section>

        <section title='Schema Re-Use With "definitions"'>
            <t>
                The "definitions" keywords provides a standardized location for schema
                authors to inline re-usable JSON Schemas into a more general schema.
                The keyword does not directly affect the validation result.
            </t>
            <t>
                This keyword's value MUST be an object.
                Each member value of this object MUST be a valid JSON Schema.
            </t>
            <t>
                As an example, here is a schema describing an array of positive
                integers, where the positive integer constraint is a subschema in
                "definitions":

                <figure>
                    <artwork>
<![CDATA[
{
    "type": "array",
    "items": { "$ref": "#/definitions/positiveInteger" },
    "definitions": {
        "positiveInteger": {
            "type": "integer",
            "exclusiveMinimum": 0
        }
    }
}
]]>
                    </artwork>
                </figure>
            </t>
        </section>

        <section title="Schema Annotations">
            <t>
                Schema validation is a useful mechanism for annotating instance data
                with additional information.  The rules for determining when and how
                annotations are associated with an instance are outlined in this specification's
                overview.
            </t>
            <t>
                These general-purpose annotation keywords provide commonly used information
                for documentation and user interface display purposes.  They are not intended
                to form a comprehensive set of features.  Rather, additional vocabularies
                can be defined for more complex annotation-based applications.
            </t>
            <section title='"title" and "description"'>
                <t>
                    The value of both of these keywords MUST be a string.
                </t>
                <t>
                    Both of these keywords can be used to decorate a user interface with
                    information about the data produced by this user interface. A title will
                    preferably be short, whereas a description will provide explanation about
                    the purpose of the instance described by this schema.
                </t>
            </section>

            <section title='"default"'>
                <t>
                    There are no restrictions placed on the value of this keyword.  When
                    multiple occurrences of this keyword are applicable to a single
                    sub-instance, implementations SHOULD remove duplicates.
                </t>
                <t>
                    This keyword can be used to supply a default JSON value associated with a
                    particular schema. It is RECOMMENDED that a default value be valid against
                    the associated schema.
                </t>
            </section>

            <section title='"readOnly" and "writeOnly"'>
                <t>
                    The value of these keywords MUST be a boolean.  When multiple occurrences
                    of these keywords are applicable to a single sub-instance, the resulting
                    value MUST be true if any occurrence specifies a true value, and MUST
                    be false otherwise.
                </t>
                <t>
                    If "readOnly" has a value of boolean true, it indicates that the value
                    of the instance is managed exclusively by the owning authority, and
                    attempts by an application to modify the value of this property are
                    expected to be ignored or rejected by that owning authority.
                </t>
                <t>
                    An instance document that is marked as "readOnly for the entire document
                    MAY be ignored if sent to the owning authority, or MAY result in an
                    error, at the authority's discretion.
                </t>
                <t>
                    If "writeOnly" has a value of boolean true, it indicates that the value
                    is never present when the instance is retrieved from the owning authority.
                    It can be present when sent to the owning authority to update or create
                    the document (or the resource it represents), but it will not be included
                    in any updated or newly created version of the instance.
                </t>
                <t>
                    An instance document that is marked as "writeOnly" for the entire document
                    MAY be returned as a blank document of some sort, or MAY produce an error
                    upon retrieval, or have the retrieval request ignored, at the authority's
                    discretion.
                </t>
                <t>
                    For example, "readOnly" would be used to mark a database-generated serial
                    number as read-only, while "writeOnly" would be used to mark a password
                    input field.
                </t>
                <t>
                    These keywords can be used to assist in user interface instance generation.
                    In particular, an application MAY choose to use a widget that hides
                    input values as they are typed for write-only fields.
                </t>
                <t>
                    Omitting these keywords has the same behavior as values of false.
                </t>
            </section>

            <section title='"examples"'>
                <t>
                    The value of this keyword MUST be an array.
                    There are no restrictions placed on the values within the array.
                    When multiple occurrences of this keyword are applicable to a single
                    sub-instance, implementations MUST provide a flat array of all
                    values rather than an array of arrays.
                </t>
                <t>
                    This keyword can be used to provide sample JSON values associated with a
                    particular schema, for the purpose of illustrating usage.  It is
                    RECOMMENDED that these values be valid against the associated schema.
                </t>
                <t>
                    Implementations MAY use the value(s) of "default", if present, as
                    an additional example.  If "examples" is absent, "default"
                    MAY still be used in this manner.
                </t>
            </section>
        </section>

        <section title="Security Considerations">
            <t>
                JSON Schema validation defines a vocabulary for JSON Schema core and concerns all
                the security considerations listed there.
            </t>
            <t>
                JSON Schema validation allows the use of Regular Expressions, which have numerous
                different (often incompatible) implementations.
                Some implementations allow the embedding of arbitrary code, which is outside the
                scope of JSON Schema and MUST NOT be permitted.
                Regular expressions can often also be crafted to be extremely expensive to compute
                (with so-called "catastrophic backtracking"), resulting in a denial-of-service
                attack.
            </t>
            <t>
                Implementations that support validating or otherwise evaluating instance
                string data based on "contentEncoding" and/or "contentMediaType" are at
                risk of evaluating data in an unsafe way based on misleading information.
                Applications can mitigate this risk by only performing such processing
                when a relationship between the schema and instance is established
                (e.g., they share the same authority).
            </t>
            <t>
                Processing a media type or encoding is subject to the security considerations
                of that media type or encoding.  For example, the security considerations
                of <xref target="RFC4329">RFC 4329 Scripting Media Types</xref> apply when
                processing JavaScript or ECMAScript encoded within a JSON string.
            </t>
        </section>

        <!--
        <section title="IANA Considerations">
            <t>
                This specification does not have any influence with regards to IANA.
            </t>
        </section>
        -->
    </middle>

    <back>
        <!-- References Section -->
        <references title="Normative References">
            &RFC2119;
            &RFC1034;
            &RFC2045;
            &RFC2046;
            &RFC2673;
            &RFC3339;
            &RFC3986;
            &RFC3987;
            &RFC4291;
            &RFC5322;
            &RFC5890;
            &RFC5891;
            &RFC6570;
            &RFC6531;
            &RFC6901;
            &RFC7159;
            <reference anchor="ecma262"
            target="http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf">
                <front>
                    <title>ECMA 262 specification</title>
                    <author/>
                    <date/>
                </front>
            </reference>
            <reference anchor="relative-json-pointer">
                <front>
                    <title>Relative JSON Pointers</title>
                    <author initials="G." surname="Luff">
                        <organization/>
                    </author>
                    <author initials="H." surname="Andrews">
                        <organization>Cloudflare, Inc.</organization>
                    </author>
                    <date year="2017" month="November"/>
                </front>
                <seriesInfo name="Internet-Draft" value="draft-handrews-relative-json-pointer-00" />
            </reference>
            <reference anchor="json-schema">
                <front>
                    <title>JSON Schema: A Media Type for Describing JSON Documents</title>
                    <author initials="A." surname="Wright">
                        <organization/>
                    </author>
                    <author initials="H." surname="Andrews">
                        <organization>Cloudflare, Inc.</organization>
                    </author>
                    <date year="2017" month="November"/>
                </front>
                <seriesInfo name="Internet-Draft" value="draft-handrews-json-schema-00" />
            </reference>
        </references>

        <references title="Informative References">
            &RFC4329;
        </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,
                Dave Finlay,
                and Denis Laxalde
                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-validation-00">
                        <list style="symbols">
                            <t>Added "if"/"then"/"else"</t>
                            <t>Classify keywords as assertions or annotations per the core spec</t>
                            <t>Warn of possibly removing "dependencies" in the future</t>
                            <t>Grouped validation keywords into sub-sections for readability</t>
                            <t>Moved "readOnly" from hyper-schema to validation meta-data</t>
                            <t>Added "writeOnly"</t>
                            <t>Added string-encoded media section, with former hyper-schema "media" keywords</t>
                            <t>Restored "regex" format (removal was unintentional)</t>
                            <t>Added "date" and "time" formats, and reserved additional RFC 3339 format names</t>
                            <t>I18N formats: "iri", "iri-reference", "idn-hostname", "idn-email"</t>
                            <t>Clarify that "json-pointer" format means string encoding, not URI fragment</t>
                            <t>Fixed typo that inverted the meaning of "minimum" and "exclusiveMinimum"</t>
                            <t>Move format syntax references into Normative References</t>
                            <t>JSON is a normative requirement</t>
                        </list>
                    </t>
                    <t hangText="draft-wright-json-schema-validation-01">
                        <list style="symbols">
                            <t>Standardized on hyphenated format names ("uriref" becomes "uri-ref")</t>
                            <t>Add the formats "uri-template" and "json-pointer"</t>
                            <t>Changed "exclusiveMaximum"/"exclusiveMinimum" from boolean modifiers of "maximum"/"minimum" to independent numeric fields.</t>
                            <t>Split the additionalItems/items into two sections</t>
                            <t>Reworked properties/patternProperties/additionalProperties definition</t>
                            <t>Added "examples" keyword</t>
                            <t>Added "contains" keyword</t>
                            <t>Allow empty "required" and "dependencies" arrays</t>
                            <t>Fixed "type" reference to primitive types</t>
                            <t>Added "const" keyword</t>
                            <t>Added "propertyNames" keyword</t>
                        </list>
                    </t>
                    <t hangText="draft-wright-json-schema-validation-00">
                        <list style="symbols">
                            <t>Added additional security considerations</t>
                            <t>Removed reference to "latest version" meta-schema, use numbered version instead</t>
                            <t>Rephrased many keyword definitions for brevity</t>
                            <t>Added "uriref" format that also allows relative URI references</t>
                        </list>
                    </t>
                    <t hangText="draft-fge-json-schema-validation-00">
                        <list style="symbols">
                            <t>Initial draft.</t>
                            <t>Salvaged from draft v3.</t>
                            <t>Redefine the "required" keyword.</t>
                            <t>Remove "extends", "disallow"</t>
                            <t>Add "anyOf", "allOf", "oneOf", "not", "definitions", "minProperties",
                            "maxProperties".</t>
                            <t>"dependencies" member values can no longer be single strings; at
                            least one element is required in a property dependency array.</t>
                            <t>Rename "divisibleBy" to "multipleOf".</t>
                            <t>"type" arrays can no longer have schemas; remove "any" as a possible
                            value.</t>
                            <t>Rework the "format" section; make support optional.</t>
                            <t>"format": remove attributes "phone", "style", "color"; rename
                            "ip-address" to "ipv4"; add references for all attributes.</t>
                            <t>Provide algorithms to calculate schema(s) for array/object
                            instances.</t>
                            <t>Add interoperability considerations.</t>
                        </list>
                    </t>
                </list>
            </t>
        </section>
    </back>
</rfc>