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Network Working Group                                          U. Carion
Internet-Draft                                           August 05, 2019
Intended status: Informational
Expires: February 6, 2020


                          JSON Schema Language
                 draft-ucarion-json-schema-language-00

Abstract

   JSON Schema Language (JSL) is a portable method for describing the
   format of JavaScript Object Notation (JSON) data and the errors
   associated with ill-formed data.  JSL is designed to enable code
   generation from schemas.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 6, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Semantics . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.1.  Strict instance semantics . . . . . . . . . . . . . . . .   9
     3.2.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Forms . . . . . . . . . . . . . . . . . . . . . . . . . .  10
       3.3.1.  Empty . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.2.  Ref . . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.3.  Type  . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.4.  Enum  . . . . . . . . . . . . . . . . . . . . . . . .  15
       3.3.5.  Elements  . . . . . . . . . . . . . . . . . . . . . .  15
       3.3.6.  Properties  . . . . . . . . . . . . . . . . . . . . .  16
       3.3.7.  Values  . . . . . . . . . . . . . . . . . . . . . . .  18
       3.3.8.  Discriminator . . . . . . . . . . . . . . . . . . . .  19
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .  23
   Appendix A.  Comparison with CDDL . . . . . . . . . . . . . . . .  23
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  26
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   This document describes a schema language for JSON [RFC8259] called
   JSON Schema Language (JSL).

   The goals of JSL are to:

   o  Provide an unambiguous description of the overall structure of a
      JSON document.

   o  Be able to describe common JSON datatypes and structures.

   o  Provide a single format that is readable and editable by both
      humans and machines, and which can be embedded within other JSON
      documents.

   o  Enable code generation from schemas.

   o  Provide a standardized format for errors when data does not
      conform with a schema.

   JSL is intentionally designed as a rather minimal schema language.
   For example, JSL is homoiconic (it both describes, and is written in,
   JSON) yet is incapable of describing in detail its own structure.  By



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   keeping the expressiveness of the schema language minimal, JSL makes
   code generation and standardized errors easier to implement.

   It is expected that for many use-cases, a schema language of JSL's
   expressiveness is sufficient.  Where a more expressive language is
   required, alternatives exist in CDDL ([RFC8610], Concise Data
   Definition Language) and others.

   This document has the following structure:

   The syntax of JSL is defined in Section 2.  Section 3 describes the
   semantics of JSL; this includes determining whether some data
   satisfies a schema and what errors should be produced when the data
   is unsatisfactory.  Appendix A presents various JSL schemas and their
   CDDL equivalents.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

   The term "JSON Pointer", when it appears in this document, is to be
   understood as it is defined in [RFC6901].

   The terms "object", "member", "array", "number", "name", and "string"
   in this document are to be interpreted as described in [RFC8259].

   The term "instance", when it appears in this document, refers to a
   JSON value being validated against a JSL schema.

2.  Syntax

   This section describes when a JSON document is a correct JSL schema.

   JSL schemas may recursively contain other schemas.  In this document,
   a "root schema" is one which is not contained within another schema,
   i.e. it is "top level".

   A correct JSL schema MUST match the "schema" CDDL rule described in
   this section.  A JSL schema is a JSON object taking on an appropriate
   form.  It may optionally contain definitions (a mapping from names to
   schemas) and additional data.




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   schema = {
     form,
     ? definitions: { * tstr => schema }
   }

                   Figure 1: CDDL Definition of a Schema

   This is not a correct JSL schema, as its "definitions" object
   contains a number, which is not a schema:

   { "definitions": { "foo": 3 }}

   JSL schemas can take on one of eight forms.  These forms are defined
   so as to be mutually exclusive; a schema cannot satisfy multiple
   forms at once.

   form = empty /
     ref /
     type /
     enum /
     elements /
     properties /
     values /
     discriminator

               Figure 2: CDDL Definition of the Schema Forms

   The first form, "empty", is trivial.  It is meant for matching any
   instance:

   empty = {}

                Figure 3: CDDL Definition of the Empty Form

   Thus, this is a correct schema:

   {}

   The second form, "ref", is for when a schema is meant to be defined
   in terms of something in "definitions":

   ref = { ref: tstr }

                 Figure 4: CDDL Definition of the Ref Form

   For a schema to be correct, the "ref" value must refer to one of the
   definitions found at the root level of the schema it appears in.
   More formally, for a schema _S_ of the "ref" form:



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   o  Let _B_ be the root schema containing the schema, or the schema
      itself if it is a root schema.

   o  Let _R_ be the value of the member of _S_ with the name "ref".

   If the schema is correct, then _B_ must have a member _D_ with the
   name "definitions", and _D_ must contain a member whose name equals
   _R_.

   Here is a correct example of "ref" being used to avoid re-defining
   the same thing twice:

   {
     "definitions": {
       "coordinates": {
         "properties": {
           "lat": { "type": "number" },
           "lng": { "type": "number" }
         }
       }
     },
     "properties": {
       "user_location": { "ref": "coordinates" },
       "server_location": { "ref": "coordinates" }
     }
   }

   However, this schema is incorrect, as it refers to a definition that
   doesn't exist:

   {
     "definitions": { "foo": { "type": "number" }},
     "ref": "bar"
   }

   This schema is incorrect as well, as it refers to a definition that
   doesn't exist at the root level.  The non-root definition is
   immaterial:

   {
     "definitions": { "foo": { "type": "number" }},
     "elements": {
       "definitions": { "bar": { "type": "number" }},
       "ref": "bar"
     }
   }





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   The third form, "type", constrains instances to have a particular
   primitive type.  The precise meaning of each of the primitive types
   is described in Section 3.

   type = { type: "boolean" / num-type / "string" / "timestamp" }
   num-type = "number" / "float32" / "float64" /
     "int8" / "uint8" / "int16" / "uint16" / "int32" / "uint32"

                Figure 5: CDDL Definition of the Type Form

   For example, this schema constrains instances to be strings that are
   correct [RFC3339] timestamps:

   { "type": "timestamp" }

   The fourth form, "enum", describes instances whose value must be one
   of a finite, predetermined set of values:

   enum = { enum: [+ tstr] }

                Figure 6: CDDL Definition of the Enum Form

   The values within "[+ tstr]" MUST NOT contain duplicates.  Thus, the
   following is a correct schema:

   { "enum": ["IN_PROGRESS", "DONE", "CANCELED"] }

   But this is not a correct schema, as "B" is duplicated:

   { "enum": ["A", "B", "B"] }

   The fifth form, "elements", describes instances that must be arrays.
   A further sub-schema describes the elements of the array.

   elements = { elements: schema }

              Figure 7: CDDL Definition of the Elements Form

   Here is a schema describing an array of [RFC3339] timestamps:

   { "elements": { "type": "timestamp" }}

   The sixth form, "properties", describes JSON objects being used as a
   "struct".  A schema of this form specifies the names of required and
   optional properties, as well as the schemas each of those properties
   must satisfy:





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   ; One of properties or optionalProperties may be omitted,
   ; but not both.
   properties = with-properties / with-optional-properties

   with-properties = {
     properties: * tstr => schema,
     ? optionalProperties * tstr => schema
   }

   with-optional-properties = {
     ? properties: * tstr => schema,
     optionalProperties: * tstr => schema
   }

             Figure 8: CDDL Definition of the Properties Form

   If a schema has both a member named "properties" (with value _P_) and
   another member named "optionalProperties" (with value _O_), then _O_
   and _P_ MUST NOT have any member names in common.  This is to prevent
   ambiguity as to whether a property is optional or required.

   Thus, this is not a correct schema, as "confusing" appears in both
   "properties" and "optionalProperties":

   {
     "properties": { "confusing": {} },
     "optionalProperties": { "confusing": {} }
   }

   Here is a correct schema, describing a paginated list of users:

   {
     "properties": {
       "users": {
         "elements": {
           "properties": {
             "id": { "type": "string" },
             "name": { "type": "string" },
             "create_time": { "type": "timestamp" }
           },
           "optionalProperties": {
             "delete_time": { "type": "timestamp" }
           }
         }
       },
       "next_page_token": { "type": "string" }
     }
   }



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   The seventh form, "values", describes JSON objects being used as an
   associative array.  A schema of this form specifies the form all
   member values must satisfy, but places no constraints on the member
   names:

   values = { values: * tstr => schema }

               Figure 9: CDDL Definition of the Values Form

   Thus, this is a correct schema, describing a mapping from strings to
   numbers:

   { "values": { "type": "number" }}

   Finally, the eighth form, "discriminator", describes JSON objects
   being used as a discriminated union.  A schema of this form specifies
   the "tag" (or "discriminator") of the union, as well as a mapping
   from tag values to the appropriate schema to use.

   ; Note well: the values of mapping are of the properties form.
   discriminator = { tag: tstr, mapping: * tstr => properties }

           Figure 10: CDDL Definition of the Discriminator Form

   To prevent ambiguous or unsatisfiable contstraints on the "tag" of a
   discriminator, an additional constraint on schemas of the
   discriminator form exists.  For schemas of the discriminator form:

   o  Let _D_ be the schema member with the name "discriminator".

   o  Let _T_ be the member of _D_ with the name "tag".

   o  Let _M_ be the member of _D_ with the name "mapping".

   If the schema is correct, then all member values _S_ of _M_ will be
   schemas of the "properties" form.  For each member _P_ of _S_ whose
   name equals "properties" or "optionalProperties", _P_'s value, which
   must be an object, MUST NOT contain any members whose name equals
   _T_'s value.

   Thus, this is an incorrect schema, as "event_type" is both the value
   of "tag" and a member name in one of the "mapping" member
   "properties":








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   {
     "tag": "event_type",
     "mapping": {
       "is_event_type_a_string_or_a_number?": {
         "properties": { "event_type": { "type": "number" }}
       }
     }
   }

   However, this is a correct schema, describing a pattern of data
   common in JSON-based messaging systems:

   {
     "tag": "event_type",
     "mapping": {
       "account_deleted": {
         "properties": {
           "account_id": { "type": "string" }
         }
       },
       "account_payment_plan_changed": {
         "properties": {
           "account_id": { "type": "string" },
           "payment_plan": { "enum": ["FREE", "PAID"] }
         },
         "optionalProperties": {
           "upgraded_by": { "type": "string" }
         }
       }
     }
   }

3.  Semantics

   This section describes when an instance is valid against a correct
   JSL schema, and the standardized errors to produce when an instance
   is invalid.

3.1.  Strict instance semantics

   Users will have different desired behavior with respect to
   "unspcecified" members in an instance.  For example:

   { "properties": { "a": { "type": "string" }}}

   Some users may expect that {"a": "foo", "b": "bar"} satisfies the
   above schema.  Others may disagree.  JSL addresses this point of




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   contention by leaving it to implementations whether to accept such
   "unspecified" members, or whether to reject them.

   Rejecting "unspecified" members is called "strict instance
   semantics".  Whether to use strict instance semantics is not
   specified within a schema - it is considered out-of-band information.

   Implementations MAY allow users to choose whether to use strict
   instance semantics.  Implementations SHOULD document whether they use
   strict instance semantics by default.

   See Section 3.3.6 for how strict instance semantics affects schema
   evaluation.

3.2.  Errors

   To facilitate consistent validation error handling, this document
   specifies a standard error format.  Implementations SHOULD support
   producing errors in this standard form.

   The standard error format is a JSON array.  The order of the elements
   of this array is not specified.  The elements of this array are JSON
   objects with two members:

   o  A member with the name "instancePath", whose value is a JSON
      string encoding a JSON Pointer.  This JSON Pointer will point to
      the part of the instance that was rejected.

   o  A member with the name "schemaPath", whose value is a JSON string
      encoding a JSON Pointer.  This JSON Pointer will point to the part
      of the schema that rejected the instance.

   The values for "instancePath" and "schemaPath" depend on the form of
   the schema, and are described in detail in Section 3.3.

3.3.  Forms

   This section describes, for each of the eight JSL schema forms, the
   rules dictating whether an instance is accepted, as well as the
   standardized errors to produce when an instance is invalid.

   The forms a correct schema may take on are formally described in
   Section 2.








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3.3.1.  Empty

   The empty form is meant to describe instances whose values are
   unknown, unpredictable, or otherwise unconstrained by the schema.

   If a schema is of the empty form, then it accepts all instances.  A
   schema of the empty form will never produce any errors.

3.3.2.  Ref

   The ref form is for when a schema is meant to be defined in terms of
   something in the "definitions" of the root schema.  The ref form
   enables schemas to be less repetitive, and also enables describing
   recursive structures.

   If a schema is of the ref form, then:

   o  Let _B_ be the root schema containing the schema, or the schema
      itself if it is a root schema.

   o  Let _D_ be the member of _B_ with the name "definitions".  By
      Section 2, _D_ exists.

   o  Let _R_ be the value of the schema member with the name "ref".

   o  Let _S_ be the value of the member of _D_ whose name equals _R_.
      By Section 2, _S_ exists, and is a schema.

   The schema accepts the instance if and only if _S_ accepts the
   instance.  Otherwise, the standard errors to return in this case are
   the union of the errors from evaluating _S_ against the instance.

   For example, the schema:

   {
     "definitions": { "a": { "type": "number" }},
     "ref": "a"
   }

   Accepts 123 but not false.  The standard errors to produce when
   evaluting false against this schema are:

   [{ "instancePath": "", "schemaPath": "/definitions/a/type" }]

   Note that the ref form is defined to only look up definitions at the
   root level.  Thus, with the schema:





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   {
     "definitions": { "a": { "type": "number" }},
     "elements": {
       "definitions": { "a": { "type": "boolean" }},
       "ref": "foo"
     }
   }

   The instance 123 is accepted, and false is rejected.  The standard
   errors to produce when evaluating false against this schema are:

   [{ "instancePath": "", "schemaPath": "/definitions/a/type" }]

   Though non-root definitions are not syntactically disallowed in
   correct schemas, they are entirely immaterial to evaluating
   references.

3.3.3.  Type

   The type form is meant to describe instances whose value is a
   boolean, number, string, or timestamp ([RFC3339]).

   If a schema is of the type form, then let _T_ be the value of the
   member with the name "type".  The following table describes whether
   the instance is accepted, as a function of _T_'s value:


























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   +-------------------+----------------------------------------------+
   | If _T_ equals ... | then the instance is accepted if it is ...   |
   +-------------------+----------------------------------------------+
   | boolean           | equal to "true" or "false"                   |
   |                   |                                              |
   | number            | a JSON number                                |
   |                   |                                              |
   | float32           | a JSON number                                |
   |                   |                                              |
   | float64           | a JSON number                                |
   |                   |                                              |
   | int8              | See Table 2                                  |
   |                   |                                              |
   | uint8             | See Table 2                                  |
   |                   |                                              |
   | int16             | See Table 2                                  |
   |                   |                                              |
   | uint16            | See Table 2                                  |
   |                   |                                              |
   | int32             | See Table 2                                  |
   |                   |                                              |
   | uint32            | See Table 2                                  |
   |                   |                                              |
   | string            | a JSON string                                |
   |                   |                                              |
   | timestamp         | a JSON string encoding a [RFC3339] timestamp |
   +-------------------+----------------------------------------------+

                     Table 1: Accepted Values for Type

   "float32" and "float64" are distinguished from "number" in their
   intent.  "float32" indicates data intended to be processed as an IEEE
   754 single-precision float, whereas "float64" indicates data intended
   to be processed as an IEEE 754 double-precision float. "number"
   indicates no specific intent.  Tools which generate code from JSL
   schemas will likely produce different code for "float32", "float64",
   and "number".

   If _T_ starts with "int" or "uint", then the instance is accepted if
   and only if it is a JSON number encoding a value with zero fractional
   part.  Depending on the value of _T_, this encoded number must
   additionally fall within a particular range:









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    +--------+---------------------------+---------------------------+
    | _T_    | Minimum Value (Inclusive) | Maximum Value (Inclusive) |
    +--------+---------------------------+---------------------------+
    | int8   | -128                      | 127                       |
    |        |                           |                           |
    | uint8  | 0                         | 255                       |
    |        |                           |                           |
    | int16  | -32,768                   | 32,767                    |
    |        |                           |                           |
    | uint16 | 0                         | 65,535                    |
    |        |                           |                           |
    | int32  | -2,147,483,648            | 2,147,483,647             |
    |        |                           |                           |
    | uint32 | 0                         | 4,294,967,295             |
    +--------+---------------------------+---------------------------+

                     Table 2: Ranges for Integer Types

   Note that 10, 10.0, and 1.0e1 encode values with zero fractional
   part. 10.5 encodes a number with a non-zero fractional part.  Thus
   {"type": "int8"} accepts 10, 10.0, and 1.0e1, but not 10.5.

   If the instance is not accepted, then the standard error for this
   case shall have an "instancePath" pointing to the instance, and a
   "schemaPath" pointing to the schema member with the name "type".

   For example:

   o  The schema {"type": "boolean"} accepts false, but rejects 127.

   o  The schema {"type": "number"} accepts 10.5, 127 and 128, but
      rejects false.

   o  The schema {"type": "int8"} accepts 127, but rejects 10.5, 128 and
      false.

   o  The schema {"type": "string"} accepts "1985-04-12T23:20:50.52Z"
      and "foo", but rejects 127.

   o  The schema {"type": "timestamp"} accepts
      "1985-04-12T23:20:50.52Z", but rejects "foo" and 127.

   In all of the rejected examples just given, the standard error to
   produce is:

   [{ "instancePath": "", "schemaPath": "/type" }]





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3.3.4.  Enum

   The enum form is meant to describe instances whose value must be one
   of a finite, predetermined set of string values.

   If a schema is of the enum form, then let _E_ be the value of the
   schema member with the name "enum".  The instance is accepted if and
   only if it is equal to one of the elements of _E_.

   If the instance is not accepted, then the standard error for this
   case shall have an "instancePath" pointing to the instance, and a
   "schemaPath" pointing to the schema member with the name "enum".

   For example, the schema:

   { "enum": ["PENDING", "DONE", "CANCELED"] }

   Accepts "PENDING", "DONE", and "CANCELED", but it rejects both 123
   and "UNKNOWN" with the standard errors:

   [{ "instancePath": "", "schemaPath": "/enum" }]

3.3.5.  Elements

   The elements form is meant to describe instances that must be arrays.
   A further sub-schema describes the elements of the array.

   If a schema is of the elements form, then let _S_ be the value of the
   schema member with the name "elements".  The instance is accepted if
   and only if all of the following are true:

   o  The instance is an array.  Otherwise, the standard error for this
      case shall have an "instancePath" pointing to the instance, and a
      "schemaPath" pointing to the schema member with the name
      "elements".

   o  If the instance is an array, then every element of the instance
      must be accepted by _S_. Otherwise, the standard errors for this
      case are the union of all the errors arising from evaluating _S_
      against elements of the instance.

   For example, if we have the schema:

   {
     "elements": {
       "type": "number"
     }
   }



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   Then the instances [] and [1, 2, 3] are accepted.  If instead we
   evaluate false against that schema, the standard errors are:

   [{ "instancePath": "", "schemaPath": "/elements" }]

   Finally, if we evaluate the instance:

   [1, 2, "foo", 3, "bar"]

   The standard errors are:

   [
     { "instancePath": "/2", "schemaPath": "/elements/type" },
     { "instancePath": "/4", "schemaPath": "/elements/type" }
   ]

3.3.6.  Properties

   The properties form is meant to describe JSON objects being used as a
   "struct".

   If a schema is of the properties form, then the instance is accepted
   if and only if all of the following are true:

   o  The instance is an object.

      Otherwise, the standard error for this case shall have an
      "instancePath" pointing to the instance, and a "schemaPath"
      pointing to the schema member with the name "properties" if such a
      schema member exists; if such a member doesn't exist, "schemaPath"
      shall point to the schema member with the name
      "optionalProperties".

   o  If the instance is an object and the schema has a member named
      "properties", then let _P_ be the value of the schema member named
      "properties". _P_, by Section 2, must be an object.  For every
      member name in _P_, a member of the same name in the instance must
      exist.

      Otherwise, the standard error for this case shall have an
      "instancePath" pointing to the instance, and a "schemaPath"
      pointing to the member of _P_ failing the requirement just
      described.

   o  If the instance is an object, then let _P_ be the value of the
      schema member named "properties" (if it exists), and _O_ be the
      value of the schema member named "optionalProperties" (if it
      exists).



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      For every member _I_ of the instance, find a member with the same
      name as _I_'s in _P_ or _O_. By Section 2, it is not possible for
      both _P_ and _O_ to have such a member.  If the "discriminator tag
      exemption" is in effect on _I_ (see Section 3.3.8), then ignore
      _I_. Otherwise:

      *  If no such member in _P_ or _O_ exists and validation is using
         strict instance semantics, then the instance is rejected.

         The standard error for this case has an "instancePath" pointing
         _I_, and a "schemaPath" pointing to the schema.

      *  If such a member in _P_ or _O_ does exist, then call this
         member _S_. If _S_ rejects _I_'s value, then the instance is
         rejected.

         The standard error for this case is the union of the errors
         from evaluating _S_ against _I_'s value.

   An instance may have multiple errors arising from the second and
   third bullet in the above.  In this case, the standard errors are the
   union of the errors.

   For example, if we have the schema:

   {
     "properties": {
       "a": { "type": "string" },
       "b": { "type": "string" }
     },
     "optionalProperties": {
       "c": { "type": "string" },
       "d": { "type": "string" }
     }
   }

   Then each of the following instances (one on each line) are accepted:

   { "a": "foo", "b": "bar" }
   { "a": "foo", "b": "bar", "c": "baz" }
   { "a": "foo", "b": "bar", "c": "baz", "d": "quux" }
   { "a": "foo", "b": "bar", "d": "quux" }

   If we evaluate the instance 123 against this schema, then the
   standard errors are:

   [{ "instancePath": "", "schemaPath": "/properties" }]




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   If instead we evalute the instance:

   { "b": 3, "c": 3, "e": 3 }

   The standard errors, using strict instance semantics, are:

   [
     { "instancePath": "",
       "schemaPath": "/properties/a" },
     { "instancePath": "/b",
       "schemaPath": "/properties/b/type" },
     { "instancePath": "/c",
       "schemaPath": "/optionalProperties/c/type" },
     { "instancePath": "/e",
       "schemaPath": "" }
   ]

   If we the same instance were evaluated, but without strict instance
   semantics, the final element of the above array of errors would not
   be present.

3.3.7.  Values

   The elements form is meant to describe instances that are JSON
   objects being used as an associative array.

   If a schema is of the values form, then let _S_ be the value of the
   schema member with the name "values".  The instance is accepted if
   and only if all of the following are true:

   o  The instance is an object.  Otherwise, the standard error for this
      case shall have an "instancePath" pointing to the instance, and a
      "schemaPath" pointing to the schema member with the name "values".

   o  If the instance is an object, then every member value of the
      instance must be accepted by _S_. Otherwise, the standard errors
      for this case are the union of all the errors arising from
      evaluating _S_ against member values of the instance.

   For example, if we have the schema:

   {
     "values": {
       "type": "number"
     }
   }





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   Then the instances {} and {"a": 1, "b": 2} are accepted.  If instead
   we evaluate false against that schema, the standard errors are:

   [{ "instancePath": "", "schemaPath": "/values" }]

   Finally, if we evaluate the instance:

   { "a": 1, "b": 2, "c": "foo", "d": 3, "e": "bar" }

   The standard errors are:

   [
     { "instancePath": "/c", "schemaPath": "/values/type" },
     { "instancePath": "/e", "schemaPath": "/values/type" }
   ]

3.3.8.  Discriminator

   The discriminator form is meant to describe JSON objects being used
   in a fashion similar to a discriminated union construct in C-like
   languages.  When a schema is of the "discriminator" form, it
   validates:

   o  That the instance is an object,

   o  That the instance has a particular "tag" property,

   o  That this "tag" property's value is a string within a set of valid
      values, and

   o  That the instance satisfies another schema, where this other
      schema is chosen based on the value of the "tag" property.

   The behavior of the discriminator form is more complex than the other
   keywords.  Readers familiar with CDDL may find the final example in
   Appendix A helpful in understanding its behavior.  What follows in
   this section is a description of the discriminator form's behavior,
   as well as some examples.

   If a schema is of the "discriminator" form, then:

   o  Let _D_ be the schema member with the name "discriminator".

   o  Let _T_ be the member of _D_ with the name "tag".

   o  Let _M_ be the member of _D_ with the name "mapping".





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   o  Let _I_ be the instance member whose name equals _T_'s value. _I_
      may, for some rejected instances, not exist.

   o  Let _S_ be the member of _M_ whose name equals _I_'s value. _S_
      may, for some rejected instances, not exist.

   The instance is accepted if and only if:

   o  The instance is an object.

      Otherwise, the standard error for this case shall have an
      "instancePath" pointing to the instance, and a "schemaPath"
      pointing to _D_.

   o  If the instance is a JSON object, then _I_ must exist.

      Otherwise, the standard error for this case shall have an
      "instancePath" pointing to the instance, and a "schemaPath"
      pointing to _T_.

   o  If the instance is a JSON object and _I_ exists, _I_'s value must
      be a string.

      Otherwise, the standard error for this case shall have an
      "instancePath" pointing to _I_, and a "schemaPath" pointing to
      _T_.

   o  If the instance is a JSON object and _I_ exists and has a string
      value, then _S_ must exist.

      Otherwise, the standard error for this case shall have an
      "instancePath" pointing to _I_, and a "schemaPath" pointing to
      _M_.

   o  If the instance is a JSON object, _I_ exists, and _S_ exists, then
      the instance must satisfy _S_'s value.  By Section 2, _S_'s value
      must have the properties form.  Apply the "discriminator tag
      exemption" afforded in Section 3.3.6 to _I_ when evaluating
      whether the instance satisfies _S_'s value.

      Otherwise, the standard errors for this case shall be standard
      errors from evaluating _S_'s value against the instance, with the
      "discriminator tag exemption" applied to _I_.

   Each of the list items above are defined to be mutually exclusive.
   For the same instance and schema, only one of the list items above
   will apply.




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   To illustrate the discriminator form, if we have the schema:

   {
     "discriminator": {
       "tag": "version",
       "mapping": {
         "v1": {
           "properties": {
             "a": { "type": "number" }
           }
         },
         "v2": {
           "properties": {
             "a": { "type": "string" }
           }
         }
       }
     }
   }

   Then if we evaluate the instance:

   "example"

   Against this schema, the standard errors are:

   [{ "instancePath": "", "schemaPath": "/discriminator" }]

   (This is the case of the instance not being an object.)

   If we instead evaluate the instance:

   {}

   Then the standard errors are:

   [{ "instancePath": "", "schemaPath": "/discriminator/tag" }]

   (This is the case of _I_ not existing.)

   If we instead evaluate the instance:

   { "version": 1 }

   Then the standard errors are:

   [{ "instancePath": "/version", "schemaPath": "/discriminator/tag" }]




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   (This is the case of _I_ existing, but having a string value.)

   If we instead evaluate the instance:

   { "version": "v3" }

   Then the standard errors are:

   [
     { "instancePath": "/version",
       "schemaPath": "/discriminator/mapping" }
   ]

   (This is the case of _I_ existing and having a string value, but _S_
   not existing.)

   If the instance evaluated were:

   { "version": "v2", "a": 3 }

   Then the standard errors are:

   [
     {
       "instancePath": "/a",
       "schemaPath": "/discriminator/mapping/v2/properties/a/type"
     }
   ]

   (This is the case of _I_ and _S_ existing, but the instance not
   satisfying _S_'s value.)

   Finally, if instead the instance were:

   { "version": "v2", "a": "foo" }

   Then the instance satisfies the schema.  No standard errors are
   returned.  This would be the case even if evaluation were using
   strict instance semantics, as the "discriminator tag exemption" would
   ensure that "version" is not treated as an unexpected property when
   evaluating the instance against _S_'s value.

4.  IANA Considerations

   No IANA considerations.






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5.  Security Considerations

   Implementations of JSON Schema Language will necessarily be
   manipulating JSON data.  Therefore, the security considerations of
   [RFC8259] are all relevant here.

   Implementations which evaluate user-inputted schemas SHOULD implement
   mechanisms to detect, and abort, circular references which might
   cause a naive implementation to go into an infinite loop.  Without
   such mechanisms, implementations may be vulnerable to denial-of-
   service attacks.

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,
              <https://www.rfc-editor.org/info/rfc6901>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

Appendix A.  Comparison with CDDL

   This appendix is informative.





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   To aid the reader familiar with CDDL, this section illustrates how
   JSL works by presenting JSL schemas and CDDL schemas which accept and
   reject the same instances.

   The JSL schema {} accepts the same instances as the CDDL rule:

   root = any

   The JSL schema:

   {
     "definitions": {
       "a": { "elements": { "ref": "b" }},
       "b": { "type": "number" }
     },
     "elements": {
       "ref": "a"
     }
   }

   Corresponds to the CDDL schema:

   root = [* a]

   a = [* b]
   b = number

   The JSL schema:

   { "enum": ["PENDING", "DONE", "CANCELED"]}

   Accepts the same instances as the CDDL rule:

   root = "PENDING" / "DONE" / "CANCELED"

   The JSL schema {"type": "boolean"} corresponds to the CDDL rule:

   root = bool

   The JSL schema {"type": "number"} corresponds to the CDDL rule:

   root = number

   The JSL schema {"type": "string"} corresponds to the CDDL rule:

   root = tstr

   The JSL schema {"type": "timestamp"} corresponds to the CDDL rule:



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   root = tdate

   The JSL schema:

   { "elements": { "type": "number" }}

   Corresponds to the CDDL rule:

   root = [* number]

   The JSL schema:

   {
     "properties": {
       "a": { "type": "boolean" },
       "b": { "type": "number" }
     },
     "optionalProperties": {
       "c": { "type": "string" },
       "d": { "type": "timestamp" }
     }
   }

   Corresponds to the CDDL rule:

   root = { a: bool, b: number, ? c: tstr, ? d: tdate }

   The JSL schema:

   { "values": { "type": "number" }}

   Corresponds to the CDDL rule:

   root = { * tstr => number }

   Finally, the JSL schema:















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   {
     "discriminator": {
       "tag": "a",
       "mapping": {
         "foo": {
           "properties": {
             "b": { "type": "number" }
           }
         },
         "bar": {
           "properties": {
             "b": { "type": "string" }
           }
         }
       }
     }
   }

   Corresponds to the CDDL rule:

   root = { a: "foo", b: number } / { a: "bar", b: tstr }

Acknowledgments

   Thanks to Gary Court, Francis Galiegue, Kris Zyp, Geraint Luff, Jason
   Desrosiers, Daniel Perrett, Erik Wilde, Ben Hutton, Evgeny
   Poberezkin, Brad Bowman, Gowry Sankar, Donald Pipowitch, Dave Finlay,
   Denis Laxalde, Henry Andrews, and Austin Wright for their work on the
   initial drafts of JSON Schema, which inspired JSON Schema Language.

   Thanks to Tim Bray, Carsten Bormann, and James Manger for their help
   on JSON Schema Language.

Author's Address

   Ulysse Carion

   Email: ulyssecarion@gmail.com













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