Independent Submission U. Carion

Independent Submission U. Carion
Request for Comments: 8927 Segment
Category: Experimental November 2020
ISSN: 2070-1721

JSON Type Definition

Abstract

This document proposes a format, called JSON Type Definition (JTD),
for describing the shape of JavaScript Object Notation (JSON)
messages. Its main goals are to enable code generation from schemas
as well as portable validation with standardized error indicators.
To this end, JTD is intentionally limited to be no more expressive
than the type systems of mainstream programming languages. This
intentional limitation, as well as the decision to make JTD schemas
be JSON documents, makes tooling atop of JTD easier to build.

This document does not have IETF consensus and is presented here to
facilitate experimentation with the concept of JTD.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.

This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8927.

Copyright Notice

Copyright (c) 2020 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.

Table of Contents

1. Introduction
1.1. Terminology
1.2. Scope of Experiment
2. Syntax
2.1. Root vs. Non-root Schemas
2.2. Forms
2.2.1. Empty
2.2.2. Ref
2.2.3. Type
2.2.4. Enum
2.2.5. Elements
2.2.6. Properties
2.2.7. Values
2.2.8. Discriminator
2.3. Extending JTD's Syntax
3. Semantics
3.1. Allowing Additional Properties
3.2. Errors
3.3. Forms
3.3.1. Empty
3.3.2. Ref
3.3.3. Type
3.3.4. Enum
3.3.5. Elements
3.3.6. Properties
3.3.7. Values
3.3.8. Discriminator
4. IANA Considerations
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Rationale for Omitted Features
A.1. Support for 64-Bit Numbers
A.2. Support for Non-root Definitions
Appendix B. Comparison with CDDL
Appendix C. Example
Acknowledgments
Author's Address

1. Introduction

This document describes a schema language for JSON [RFC8259] called
JSON Type Definition (JTD).

There exist many options for describing JSON data. JTD's niche is to
focus on enabling code generation from schemas; to this end, JTD's
expressiveness is intentionally limited to be no more powerful than
what can be expressed in the type systems of mainstream programming
languages.

The goals of JTD are to:

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

* Be able to describe common JSON data types and structures (that
is, the data types and structures necessary to support most JSON
documents and that are widely understood in an interoperable way
by JSON implementations).

* Provide a single format that is readable and editable by both
humans and machines and that can be embedded within other JSON
documents. This makes JTD a convenient format for tooling to
accept as input or produce as output.

* Enable code generation from JTD schemas. JTD schemas are meant to
be easy to convert into data structures idiomatic to mainstream
programming languages.

* Provide a standardized format for error indicators when data does
not conform with a schema.

JTD is intentionally designed as a rather minimal schema language.
Thus, although JTD can describe some categories of JSON, it is not
able to describe its own structure; this document uses Concise Data
Definition Language (CDDL) [RFC8610] to describe JTD's syntax. By
keeping the expressiveness of the schema language minimal, JTD makes
code generation and standardized error indicators easier to
implement.

Examples in this document use constructs from the C++ programming
language. These examples are provided to aid the reader in
understanding the principles of JTD but are not limiting in any way.

JTD's feature set is designed to represent common patterns in JSON-
using applications, while still having a clear correspondence to
programming languages in widespread use. Thus, JTD supports:

* Signed and unsigned 8-, 16-, and 32-bit integers. A tool that
converts JTD schemas into code can use "int8_t", "uint8_t",
"int16_t", etc., or their equivalents in the target language, to
represent these JTD types.

* A distinction between "float32" and "float64". Code generators
can use "float" and "double", or their equivalents, for these JTD
types.

* A "properties" form of JSON objects, corresponding to some sort of
struct or record. The "properties" form of JSON objects is akin
to a C++ "struct".

* A "values" form of JSON objects, corresponding to some sort of
dictionary or associative array. The "values" form of JSON
objects is akin to a C++ "std::map".

* A "discriminator" form of JSON objects, corresponding to a
discriminated (or "tagged") union. The "discriminator" form of
JSON objects is akin to a C++ "std::variant".

The principle of common patterns in JSON is why JTD does not support
64-bit integers, as these are usually transmitted over JSON in non-
interoperable (i.e., ignoring the recommendations in Section 2.2 of
[RFC7493]) or mutually inconsistent ways. Appendix A.1 further
elaborates on why JTD does not support 64-bit integers.

The principle of clear correspondence to common programming languages
is why JTD does not support, for example, a data type for integers up
to 2**53-1.

It is expected that for many use cases, a schema language of JTD's
expressiveness is sufficient. Where a more expressive language is
required, alternatives exist in CDDL and others.

This document does not have IETF consensus and is presented here to
facilitate experimentation with the concept of JTD. The purpose of
the experiment is to gain experience with JTD and to possibly revise
this work accordingly. If JTD is determined to be a valuable and
popular approach, it may be taken to the IETF for further discussion
and revision.

This document has the following structure. Section 2 defines the
syntax of JTD. Section 3 describes the semantics of JTD; this
includes determining whether some data satisfies a schema and what
error indicators should be produced when the data is unsatisfactory.
Appendix A discusses why certain features are omitted from JTD.
Appendix B presents various JTD 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.

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 JTD schema. This value can be
an entire JSON document, or it can be a value embedded within a JSON
document.

1.2. Scope of Experiment

JTD is an experiment. Participation in this experiment consists of
using JTD to validate or document interchanged JSON messages or
building tooling atop of JTD. Feedback on the results of this
experiment may be emailed to the author. Participants in this
experiment are anticipated to mostly be nodes that provide or consume
JSON-based APIs.

Nodes know if they are participating in the experiment if they are
validating JSON messages against a JTD schema or if they are relying
on another node to do so. Nodes are also participating in the
experiment if they are running code generated from a JTD schema.

The risk of this experiment "escaping" takes the form of a JTD-
supporting node expecting another node, which lacks such support, to
validate messages against some JTD schema. In such a case, the
outcome will likely be that the nodes fail to interchange information
correctly.

This experiment will be deemed successful when JTD has been
implemented by multiple independent parties and these parties
successfully use JTD to facilitate information interchange within
their internal systems or between systems operated by independent
parties.

If this experiment is deemed successful, and JTD is determined to be
a valuable and popular approach, it may be taken to the IETF for
further discussion and revision. One possible outcome of this
discussion and revision could be that a working group produces a
Standards Track specification of JTD.

Some implementations of JTD, as well as code generators and other
tooling related to JTD, are available at <https://github.com/
jsontypedef>.

2. Syntax

This section describes when a JSON document is a correct JTD schema.
Because Concise Data Definition Language (CDDL) is well suited to the
task of defining complex JSON formats, such as JTD schemas, this
section uses CDDL to describe the format of JTD schemas.

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

A JTD schema is a JSON object taking on an appropriate form. JTD
schemas may contain "additional data", discussed in Section 2.3.
Root JTD schemas may optionally contain definitions (a mapping from
names to schemas).

A correct root JTD schema MUST match the "root-schema" CDDL rule
described in this section. A correct non-root JTD schema MUST match
the "schema" CDDL rule described in this section.

; root-schema is identical to schema, but additionally allows for
; definitions.
;
; definitions are prohibited from appearing on non-root schemas.
root-schema = {
? definitions: { * tstr => { schema}},
schema,
}
; schema is the main CDDL rule defining a JTD schema.
;
; All JTD schemas are JSON objects taking on one of eight forms
; listed here.
schema = (
ref //
type //
enum //
elements //
properties //
values //
discriminator //
empty //
)
; shared is a CDDL rule containing properties that all eight schema
; forms share.
shared = (
? metadata: { * tstr => any },
? nullable: bool,
)
; empty describes the "empty" schema form.
empty = shared
; ref describes the "ref" schema form.
;
; There are additional constraints on this form that cannot be
; expressed in CDDL. Section 2.2.2 describes these additional
; constraints in detail.
ref = ( ref: tstr, shared )
; type describes the "type" schema form.
type = (
type: "boolean"
/ "float32"
/ "float64"
/ "int8"
/ "uint8"
/ "int16"
/ "uint16"
/ "int32"
/ "uint32"
/ "string"
/ "timestamp",
shared,
)
; enum describes the "enum" schema form.
;
; There are additional constraints on this form that cannot be
; expressed in CDDL. Section 2.2.4 describes these additional
; constraints in detail.
enum = ( enum: [+ tstr], shared )
; elements describes the "elements" schema form.
elements = ( elements: { schema }, shared )
; properties describes the "properties" schema form.
;
; This CDDL rule is defined so that a schema of the "properties" form
; may omit a member named "properties" or a member named
; "optionalProperties", but not both.
;
; There are additional constraints on this form that cannot be
; expressed in CDDL. Section 2.2.6 describes these additional
; constraints in detail.
properties = (with-properties // with-optional-properties)
with-properties = (
properties: { * tstr => { schema }},
? optionalProperties: { * tstr => { schema }},
? additionalProperties: bool,
shared,
)
with-optional-properties = (
? properties: { * tstr => { schema }},
optionalProperties: { * tstr => { schema }},
? additionalProperties: bool,
shared,
)
; values describes the "values" schema form.
values = ( values: { schema }, shared )
; discriminator describes the "discriminator" schema form.
;
; There are additional constraints on this form that cannot be
; expressed in CDDL. Section 2.2.8 describes these additional
; constraints in detail.
discriminator = (
discriminator: tstr,
; Note well: this rule is defined in terms of the "properties"
; CDDL rule, not the "schema" CDDL rule.
mapping: { * tstr => { properties } }
shared,
)

Figure 1: CDDL Definition of a Schema

The remainder of this section will describe constraints on JTD
schemas that cannot be expressed in CDDL. It will also provide
examples of valid and invalid JTD schemas.

2.1. Root vs. Non-root Schemas

The "root-schema" rule in Figure 1 permits a member named
"definitions", but the "schema" rule does not permit for such a
member. This means that only root (i.e., "top-level") JTD schemas
can have a "definitions" object, and subschemas may not.

Thus,

{ "definitions": {} }

is a correct JTD schema, but

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

is not, because subschemas (such as the object at "/definitions/foo")
must not have a member named "definitions".

2.2. Forms

JTD schemas (i.e., JSON objects satisfying the "schema" CDDL rule in
Figure 1) must take on one of eight forms. These forms are defined
so as to be mutually exclusive; a schema cannot satisfy multiple
forms at once.

2.2.1. Empty

The "empty" form is defined by the "empty" CDDL rule in Figure 1.
The semantics of the "empty" form are described in Section 3.3.1.

Despite the name "empty", schemas of the "empty" form are not
necessarily empty JSON objects. Like schemas of any of the eight
forms, schemas of the "empty" form may contain members named
"nullable" (whose value must be "true" or "false") or "metadata"
(whose value must be an object) or both.

Thus,

{}

and

{ "nullable": true }

and

{ "nullable": true, "metadata": { "foo": "bar" }}

are correct JTD schemas of the "empty" form, but

{ "nullable": "foo" }

is not, because the value of the member named "nullable" must be
"true" or "false".

2.2.2. Ref

The "ref" form is defined by the "ref" CDDL rule in Figure 1. The
semantics of the "ref" form are described in Section 3.3.2.

For a schema of the "ref" form to be correct, the value of the member
named "ref" 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:

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

* 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_.

Thus,

{
"definitions": {
"coordinates": {
"properties": {
"lat": { "type": "float32" },
"lng": { "type": "float32" }
}
}
},
"properties": {
"user_location": { "ref": "coordinates" },
"server_location": { "ref": "coordinates" }
}
}

is a correct JTD schema and demonstrates the point of the "ref" form:
to avoid redefining the same thing twice. However,

{ "ref": "foo" }

is not a correct JTD schema, as there are no top-level "definitions",
and so the "ref" form cannot be correct. Similarly,

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

is not a correct JTD schema, as there is no member named "bar" in the
top-level "definitions".

2.2.3. Type

The "type" form is defined by the "type" CDDL rule in Figure 1. The
semantics of the "type" form are described in Section 3.3.3.

As an example of a correct JTD schema of the "type" form,

{ "type": "uint8" }

is a correct JTD schema, whereas

{ "type": true }

and

{ "type": "foo" }

are not correct schemas, as neither "true" nor the JSON string "foo"
are in the list of permitted values of the "type" member described in
the "type" CDDL rule in Figure 1.

2.2.4. Enum

The "enum" form is defined by the "enum" CDDL rule in Figure 1. The
semantics of the "enum" form are described in Section 3.3.4.

For a schema of the "enum" form to be correct, the value of the
member named "enum" must be a nonempty array of strings, and that
array must not contain duplicate values. More formally, for a schema
_S_ of the "enum" form:

* Let _E_ be the value of the member of _S_ with name "enum".

If the schema is correct, then there MUST NOT exist any pair of
elements of _E_ that encode equal string values, where string
equality is defined as in Section 8.3 of [RFC8259].

Thus,

{ "enum": [] }

is not a correct JTD schema, as the value of the member named "enum"
must be nonempty, and

{ "enum": ["a\\b", "a\u005Cb"] }

is not a correct JTD schema, as

"a\\b"

and

"a\u005Cb"

encode strings that are equal by the definition of string equality
given in Section 8.3 of [RFC8259]. By contrast,

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

is an example of a correct JTD schema of the "enum" form.

2.2.5. Elements

The "elements" form is defined by the "elements" CDDL rule in
Figure 1. The semantics of the "elements" form are described in
Section 3.3.5.

As an example of a correct JTD schema of the "elements" form,

{ "elements": { "type": "uint8" }}

is a correct JTD schema, whereas

{ "elements": true }

and

{ "elements": { "type": "foo" } }

are not correct schemas, as neither

true

nor

{ "type": "foo" }

are correct JTD schemas, and the value of the member named "elements"
must be a correct JTD schema.

2.2.6. Properties

The "properties" form is defined by the "properties" CDDL rule in
Figure 1. The semantics of the "properties" form are described in
Section 3.3.6.

For a schema of the "properties" form to be correct, properties must
either be required (i.e., in "properties") or optional (i.e., in
"optionalProperties"), but not both.

More formally, 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; that
is, no member of _P_ may have a name equal to the name of any member
of _O_, under the definition of string equality given in Section 8.3
of [RFC8259].

Thus,

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

is not a correct JTD schema, as "confusing" appears in both
"properties" and "optionalProperties". By contrast,

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

is a correct JTD schema of the "properties" form, describing a
paginated list of users and demonstrating the recursive nature of the
syntax of JTD schemas.

2.2.7. Values

The "values" form is defined by the "values" CDDL rule in Figure 1.
The semantics of the "values" form are described in Section 3.3.7.

As an example of a correct JTD schema of the "values" form,

{ "values": { "type": "uint8" }}

is a correct JTD schema, whereas

{ "values": true }

and

{ "values": { "type": "foo" } }

are not correct schemas, as neither

true

nor

{ "type": "foo" }

are correct JTD schemas, and the value of the member named "values"
must be a correct JTD schema.

2.2.8. Discriminator

The "discriminator" form is defined by the "discriminator" CDDL rule
in Figure 1. The semantics of the "discriminator" form are described
in Section 3.3.8. Understanding the semantics of the "discriminator"
form will likely aid the reader in understanding why this section
provides constraints on the "discriminator" form beyond those in
Figure 1.

To prevent ambiguous or unsatisfiable constraints on the
"discriminator" property of a tagged union, an additional constraint
on schemas of the "discriminator" form exists. For schemas of the
"discriminator" form:

* Let _D_ be the member of the schema with the name "discriminator".

* Let _M_ be the member of the schema 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 _S_:

* If _S_ has a member _N_ whose name equals "nullable", _N_'s value
MUST NOT be the JSON primitive value "true".

* 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 _D_'s value.

Thus,

{
"discriminator": "event_type",
"mapping": {
"can_the_object_be_null_or_not?": {
"nullable": true,
"properties": { "foo": { "type": "string" } }}
}
}
}

is an incorrect schema, as a member of "mapping" has a member named
"nullable" whose value is "true". This would suggest that the
instance may be null. Yet, the top-level schema lacks such a
"nullable" set to "true", which would suggest that the instance in
fact cannot be null. If this were a correct JTD schema, it would be
unclear which piece of information takes precedence.

JTD handles such possible ambiguity by disallowing, at the syntactic
level, the possibility of contradictory specifications of whether an
instance described by a schema of the "discriminator" form may be
null. The schemas in a discriminator "mapping" cannot have
"nullable" set to "true"; only the discriminator itself can use
"nullable" in this way.

It also follows that

{
"discriminator": "event_type",
"mapping": {
"is_event_type_a_string_or_a_float32?": {
"properties": { "event_type": { "type": "float32" }}
}
}
}

and

{
"discriminator": "event_type",
"mapping": {
"is_event_type_a_string_or_an_optional_float32?": {
"optionalProperties": { "event_type": { "type": "float32" }}
}
}
}

are incorrect schemas, as "event_type" is both the value of
"discriminator" and a member name in one of the "mapping" member
"properties" or "optionalProperties". This is ambiguous, because
ordinarily the "discriminator" keyword would indicate that
"event_type" is expected to be a string, but another part of the
schema specifies that "event_type" is expected to be a number.

JTD handles such possible ambiguity by disallowing, at the syntactic
level, the possibility of contradictory specifications of
discriminator "tags". Discriminator "tags" cannot be redefined in
other parts of the schema.

By contrast,

{
"discriminator": "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" }
}
}
}
}

is a correct schema, describing a pattern of data common in JSON-
based messaging systems. Section 3.3.8 provides examples of what
this schema accepts and rejects.

2.3. Extending JTD's Syntax

This document does not describe any extension mechanisms for JTD
schema validation, which is described in Section 3. However, schemas
are defined to optionally contain a "metadata" keyword, whose value
is an arbitrary JSON object. Call the members of this object
"metadata members".

Users MAY add metadata members to JTD schemas to convey information
that is not pertinent to validation. For example, such metadata
members could provide hints to code generators or trigger some
special behavior for a library that generates user interfaces from
schemas.

Users SHOULD NOT expect metadata members to be understood by other
parties. As a result, if consistent validation with other parties is
a requirement, users MUST NOT use metadata members to affect how
schema validation, as described in Section 3, works.

Users MAY expect metadata members to be understood by other parties
and MAY use metadata members to affect how schema validation works,
if these other parties are somehow known to support these metadata
members. For example, two parties may agree, out of band, that they
will support an extended JTD with a custom metadata member that
affects validation.

3. Semantics

This section describes when an instance is valid against a correct
JTD schema and the error indicators to produce when an instance is
invalid.

3.1. Allowing Additional Properties

Users will have different desired behavior with respect to
"unspecified" members in an instance. For example, consider the JTD
schema in Figure 2:

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

Figure 2: An Illustrative JTD Schema

Some users may expect that

{"a": "foo", "b": "bar"}

satisfies the schema in Figure 2. Others may disagree, as "b" is not
one of the properties described in the schema. In this document,
allowing such "unspecified" members, like "b" in this example,
happens when evaluation is in "allow additional properties" mode.

Evaluation of a schema does not allow additional properties by
default, but this can be overridden by having the schema include a
member named "additionalProperties", where that member has a value of
"true".

More formally, evaluation of a schema _S_ is in "allow additional
properties" mode if there exists a member of _S_ whose name equals
"additionalProperties" and whose value is a boolean "true".
Otherwise, evaluation of _S_ is not in "allow additional properties"
mode.

See Section 3.3.6 for how allowing unknown properties affects schema
evaluation, but briefly, the schema

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

rejects

{ "a": "foo", "b": "bar" }

However, the schema

{
"additionalProperties": true,
"properties": { "a": { "type": "string" }}
}

accepts

{ "a": "foo", "b": "bar" }

Note that "additionalProperties" does not get "inherited" by
subschemas. For example, the JTD schema

{
"additionalProperties": true,
"properties": {
"a": {
"properties": {
"b": { "type": "string" }
}
}
}
}

accepts

{ "a": { "b": "c" }, "foo": "bar" }

but rejects

{ "a": { "b": "c", "foo": "bar" }}

because the "additionalProperties" at the root level does not affect
the behavior of subschemas.

Note from Figure 1 that only schemas of the "properties" form may
have a member named "additionalProperties".

3.2. Errors

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

The standard error indicator 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:

* 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.

* 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 JTD schema forms, the
rules dictating whether an instance is accepted, as well as the error
indicators to produce when an instance is invalid.

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

3.3.1. Empty

The "empty" form is meant to describe instances whose values are
unknown, unpredictable, or otherwise unconstrained by the schema.
The syntax of the "empty" form is described in Section 2.2.1.

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

3.3.2. Ref

The "ref" form is for when a schema is 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. The syntax of the "ref" form is described in
Section 2.2.2.

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

* If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value
"null", then the schema accepts the instance.

Otherwise:

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

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

- Let _D_ be the member of _B_ with the name "definitions". Per
Section 2, we know _D_ exists.

- Let _S_ be the value of the member of _D_ whose name equals
_R_. Per Section 2.2.2, we know _S_ exists and is a schema.

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

For example, the schema

{
"definitions": { "a": { "type": "float32" }},
"ref": "a"
}

accepts

123

but rejects

null

with the error indicator

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

The schema

{
"definitions": { "a": { "type": "float32" }},
"ref": "a",
"nullable": true
}

accepts

null

because the schema has a "nullable" member whose value is "true".

Note that "nullable" being "false" has no effect in any of the forms
described in this document. For example, the schema

{
"definitions": { "a": { "nullable": false, "type": "float32" }},
"ref": "a",
"nullable": true
}

accepts

null

In other words, it is not the case that putting a "false" value for
"nullable" will ever override a "nullable" member in schemas of the
"ref" form; it is correct, though ineffectual, to have a value of
"false" for the "nullable" member in a schema.

3.3.3. Type

The "type" form is meant to describe instances whose value is a
boolean, number, string, or timestamp [RFC3339]. The syntax of the
"type" form is described in Section 2.2.3.

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

* If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value
"null", then the schema accepts the instance.

Otherwise:

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:

+============+=========================================+
| If _"T"_ | then the instance is accepted if it is |
| equals ... | ... |
+============+=========================================+
| boolean | equal to "true" or "false" |
+------------+-----------------------------------------+
| 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 that follows the standard |
| | format described in [RFC3339], as |
| | refined by Section 3.3 of [RFC4287] |
+------------+-----------------------------------------+

Table 1: Accepted Values for Type

"float32" and "float64" are distinguished from each other 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. Tools that generate code from JTD schemas
will likely produce different code for "float32" than for
"float64".

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:

+========+===========================+===========================+
| _"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

and

10.0

and

1.0e1

encode values with zero fractional part, whereas

10.5

encodes a number with a non-zero fractional part. Thus, the schema

{"type": "int8"}

accepts

10

and

10.0

and

1.0e1

but rejects

10.5

as well as

false

because "false" is not a number at all.

If the instance is not accepted, then the error indicator 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, the schema

{"type": "boolean"}

accepts

false

but rejects

127

The schema

{"type": "float32"}

accepts

10.5

and

127

but rejects

false

The schema

{"type": "string"}

accepts

"1985-04-12T23:20:50.52Z"

and

"foo"

but rejects

false

The schema

{"type": "timestamp"}

accepts

"1985-04-12T23:20:50.52Z"

but rejects

"foo"

and

false

The schema

{"type": "boolean", "nullable": true}

accepts

null

and

false

but rejects

127

In all of the examples of rejected instances given in this section,
the error indicator to produce is:

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

3.3.4. Enum

The "enum" form is meant to describe instances whose value must be
one of a given set of string values. The syntax of the "enum" form
is described in Section 2.2.4.

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

* If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value
"null", then the schema accepts the instance.

Otherwise:

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 error indicator 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"

and

"DONE"

and

"CANCELED"

but rejects all of

0

and

1

and

2

and

"UNKNOWN"

and

null

with the error indicator

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

The schema

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

accepts

"PENDING"

and

null

but rejects

1

and

"UNKNOWN"

with the error indicator

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

3.3.5. Elements

The "elements" form is meant to describe instances that must be
arrays. A further subschema describes the elements of the array.
The syntax of the "elements" form is described in Section 2.2.5.

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

* If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value
"null", then the schema accepts the instance.

Otherwise:

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 error indicator
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 error
indicators for this case are the union of all the errors
arising from evaluating _S_ against elements of the
instance.

For example, the schema

{
"elements": {
"type": "float32"
}
}

accepts

[]

and

[1, 2, 3]

but rejects

null

with the error indicator

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

and rejects

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

with the error indicators

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

The schema

{
"elements": {
"type": "float32"
},
"nullable": true
}

accepts

null

and

[]

and

[1, 2, 3]

but rejects

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

with the error indicators

[
{ "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". The syntax of the "properties" form is described in
Section 2.2.6.

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

* If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value
"null", then the schema accepts the instance.

Otherwise:

- The instance must be an object.

Otherwise, the schema rejects the instance. The error
indicator 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".

- 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". Per Section 2.2.6, we know _P_ is an
object. For every member name in _P_, a member of the same
name in the instance must exist.

Otherwise, the schema rejects the instance. The error
indicator 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.

- 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).

For every member _I_ of the instance, find a member with the
same name as _I_'s in _P_ or _O_. Per Section 2.2.6, we know 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:

o If no such member in _P_ or _O_ exists and validation is not
in "allow additional properties" mode (see Section 3.1),
then the schema rejects the instance.

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

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

The error indicators for this case are the union of the
error indicators from evaluating _S_ against _I_'s value.

If an instance is an object, it may have multiple errors arising
from the second and third bullet in the list above. In this case,
the error indicators are the union of the errors.

For example, the schema

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

accepts

{ "a": "foo", "b": "bar" }

and

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

and

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

and

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

but rejects

null

with the error indicator

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

and rejects

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

with the error indicators

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

If instead the schema had "additionalProperties: true" but was
otherwise the same:

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

and the instance remained the same:

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

then the error indicators from evaluating the instance against the
schema would be:

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

These are the same errors as before, except the final error
(associated with the additional member named "e" in the instance)
is no longer present. This is because "additionalProperties:
true" enables "allow additional properties" mode on the schema.

Finally, the schema

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

accepts

null

but rejects

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

with the error indicators

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

3.3.7. Values

The "values" form is meant to describe instances that are JSON
objects being used as an associative array. The syntax of the
"values" form is described in Section 2.2.7.

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

* If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value
"null", then the schema accepts the instance.

Otherwise:

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 error indicator
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 error
indicators for this case are the union of all the error
indicators arising from evaluating _S_ against member values
of the instance.

For example, the schema

{
"values": {
"type": "float32"
}
}

accepts

{}

and

{"a": 1, "b": 2}

but rejects

null

with the error indicator

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

and rejects

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

with the error indicators

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

The schema

{
"nullable": true,
"values": {
"type": "float32"
}
}

accepts

null

but rejects

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

with the error indicators

[
{ "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. The syntax of the "discriminator" form is described in
Section 2.2.8.

When a schema is of the "discriminator" form, it validates that:

* the instance is an object,

* the instance has a particular "tag" property,

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

* 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 B 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:

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

* Let _M_ be the schema member with the name "mapping".

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

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

If the schema has a member named "nullable" whose value is the
boolean "true", and the instance is the JSON primitive value "null",
then the schema accepts the instance. Otherwise, the instance is
accepted if and only if all of the following are true:

* The instance is an object.

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

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

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

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

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

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

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

* If the instance is a JSON object, _I_ exists, and _S_ exists, then
the instance must satisfy _S_'s value. Per Section 2, we know
_S_'s value is a schema of 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 error indicators for this case shall be error
indicators from evaluating _S_'s value against the instance, with
the "discriminator tag exemption" applied to _I_.

The list items above are defined in a mutually exclusive way. For
any given instance and schema, exactly one of the list items above
will apply.

For example, the schema

{
"discriminator": "version",
"mapping": {
"v1": {
"properties": {
"a": { "type": "float32" }
}
},
"v2": {
"properties": {
"a": { "type": "string" }
}
}
}
}

rejects

null

with the error indicator

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

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

Also rejected is

{}

with the error indicator

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

(This is the case of _I_ not existing.)

Also rejected is

{ "version": 1 }

with the error indicator

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

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

Also rejected is

{ "version": "v3" }

with the error indicator

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

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

Also rejected is

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

with the error indicator

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

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

Finally, the schema accepts

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

This instance is accepted even though "version" is not mentioned by
"/mapping/v2/properties"; the "discriminator tag exemption" ensures
that "version" is not treated as an additional property when
evaluating the instance against _S_'s value.

By contrast, consider the same schema but with "nullable" being
"true". The schema

{
"nullable": true,
"discriminator": "version",
"mapping": {
"v1": {
"properties": {
"a": { "type": "float32" }
}
},
"v2": {
"properties": {
"a": { "type": "string" }
}
}
}
}

accepts

null

To further illustrate the "discriminator" form with examples, recall
the JTD schema in Section 2.2.8, reproduced here:

{
"discriminator": "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" }
}
}
}
}

This schema accepts

{ "event_type": "account_deleted", "account_id": "abc-123" }

and

{
"event_type": "account_payment_plan_changed",
"account_id": "abc-123",
"payment_plan": "PAID"
}

and

{
"event_type": "account_payment_plan_changed",
"account_id": "abc-123",
"payment_plan": "PAID",
"upgraded_by": "users/mkhwarizmi"
}

but rejects

{}

with the error indicator

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

and rejects

{ "event_type": "some_other_event_type" }

with the error indicator

[
{
"instancePath": "/event_type",
"schemaPath": "/mapping"
}
]

and rejects

{ "event_type": "account_deleted" }

with the error indicator

[{
"instancePath": "",
"schemaPath": "/mapping/account_deleted/properties/account_id"
}]

and rejects

{
"event_type": "account_payment_plan_changed",
"account_id": "abc-123",
"payment_plan": "PAID",
"xxx": "asdf"
}

with the error indicator

[{
"instancePath": "/xxx",
"schemaPath": "/mapping/account_payment_plan_changed"
}]

4. IANA Considerations

This document has no IANA actions.

5. Security Considerations

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

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

6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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>.

[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.

[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>.

6.2. Informative References

[JSON-SCHEMA]
Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON
Schema: A Media Type for Describing JSON Documents", Work
in Progress, Internet-Draft, draft-handrews-json-schema-
02, 17 September 2019, <https://tools.ietf.org/html/draft-
handrews-json-schema-02>.

[OPENAPI] OpenAPI Initiative, "OpenAPI Specification", February
2020, <https://spec.openapis.org/oas/v3.0.3>.

[RFC7071] Borenstein, N. and M. Kucherawy, "A Media Type for
Reputation Interchange", RFC 7071, DOI 10.17487/RFC7071,
November 2013, <https://www.rfc-editor.org/info/rfc7071>.

[RFC7493] Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
DOI 10.17487/RFC7493, March 2015,
<https://www.rfc-editor.org/info/rfc7493>.

Appendix A. Rationale for Omitted Features

This appendix is not normative.

This section describes possible features that are intentionally left
out of JSON Type Definition and justifies why these features are
omitted.

A.1. Support for 64-Bit Numbers

This document does not allow "int64" or "uint64" as values for the
JTD "type" keyword (see Sections 2.2.3 and 3.3.3). Such hypothetical
"int64" or "uint64" types would behave like "int32" or "uint32"
(respectively) but with the range of values associated with 64-bit
instead of 32-bit integers. That is:

* "int64" would accept numbers between -(2**63) and (2**63)-1

* "uint64" would accept numbers between 0 and (2**64)-1

Users of "int64" and "uint64" would likely expect that the full range
of signed or unsigned 64-bit integers could interoperably be
transmitted as JSON without loss of precision. But this assumption
is likely to be incorrect, for the reasons given in Section 2.2 of
[RFC7493].

"int64" and "uint64" likely would have led users to falsely assume
that the full range of 64-bit integers can be interoperably processed
as JSON without loss of precision. To avoid leading users astray,
JTD omits "int64" and "uint64".

A.2. Support for Non-root Definitions

This document disallows the "definitions" keyword from appearing
outside of root schemas (see Figure 1). Conceivably, this document
could have instead allowed "definitions" to appear on any schema,
even non-root ones. Under this alternative design, "ref"s would
resolve to a definition in the "nearest" (i.e., most nested) schema
that both contained the "ref" and had a suitably named "definitions"
member.

For instance, under this alternative approach, one could define
schemas like the one in Figure 3.

{
"properties": {
"foo": {
"definitions": {
"user": { "properties": { "user_id": {"type": "string" }}}
},
"ref": "user"
},
"bar": {
"definitions": {
"user": { "properties": { "user_id": {"type": "string" }}}
},
"ref": "user"
},
"baz": {
"definitions": {
"user": { "properties": { "userId": {"type": "string" }}}
},
"ref": "user"
}
}
}

Figure 3: A Hypothetical Schema Had This Document Permitted Non-root
Definitions. This Is Not a Correct JTD Schema.

If schemas like that in Figure 3 were permitted, code generation from
JTD schemas would be more difficult, and the generated code would be
less useful.

Code generation would be more difficult because it would force code
generators to implement a name-mangling scheme for types generated
from definitions. This additional difficulty is not immense, but it
adds complexity to an otherwise relatively trivial task.

Generated code would be less useful because generated, mangled struct
names are less pithy than human-defined struct names. For instance,
the "user" definitions in Figure 3 might have been generated into
types named "PropertiesFooUser", "PropertiesBarUser", and
"PropertiesBazUser"; obtuse names like these are less useful to
human-written code than names like "User".

Furthermore, even though "PropertiesFooUser" and "PropertiesBarUser"
would be essentially identical, they would not be interchangeable in
many statically typed programming languages. A code generator could
attempt to circumvent this by deduplicating identical definitions,
but then the user might be confused as to why the subtly distinct
"PropertiesBazUser", defined from a schema allowing a property named
"userId" (not "user_id"), was not deduplicated.

Because there seem to be implementation and usability challenges
associated with non-root definitions, and because it would be easier
to later amend JTD to permit for non-root definitions than to later
amend JTD to prohibit them, this document does not permit non-root
definitions in JTD schemas.

Appendix B. Comparison with CDDL

This appendix is not normative.

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

The JTD schema

{}

accepts the same instances as the CDDL rule

root = any

The JTD schema

{
"definitions": {
"a": { "elements": { "ref": "b" }},
"b": { "type": "float32" }
},
"elements": {
"ref": "a"
}
}

accepts the same instances as the CDDL rule

root = [* a]
a = [* b]
b = number

The JTD schema

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

accepts the same instances as the CDDL rule

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

The JTD schema

{"type": "boolean"}

accepts the same instances as the CDDL rule

root = bool

The JTD schemas:

{"type": "float32"}

and

{"type": "float64"}

both accept the same instances as the CDDL rule

root = number

The JTD schema

{"type": "string"}

accepts the same instances as the CDDL rule

root = tstr

The JTD schema

{"type": "timestamp"}

accepts the same instances as the CDDL rule

root = tdate

The JTD schema

{ "elements": { "type": "float32" }}

accepts the same instances as the CDDL rule

root = [* number]

The JTD schema

{
"properties": {
"a": { "type": "boolean" },
"b": { "type": "float32" }
},
"optionalProperties": {
"c": { "type": "string" },
"d": { "type": "timestamp" }
}
}

accepts the same instances as the CDDL rule

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

The JTD schema

{ "values": { "type": "float32" }}

accepts the same instances as the CDDL rule

root = { * tstr => number }

Finally, the JTD schema

{
"discriminator": "a",
"mapping": {
"foo": {
"properties": {
"b": { "type": "float32" }
}
},
"bar": {
"properties": {
"b": { "type": "string" }
}
}
}
}

accepts the same instances as the CDDL rule

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

Appendix C. Example

This appendix is not normative.

As a demonstration of JTD, in Figure 4 is a JTD schema closely
equivalent to the plain-English definition "reputation-object"
described in Section 6.2.2 of [RFC7071]:

{
"properties": {
"application": { "type": "string" },
"reputons": {
"elements": {
"additionalProperties": true,
"properties": {
"rater": { "type": "string" },
"assertion": { "type": "string" },
"rated": { "type": "string" },
"rating": { "type": "float32" },
},
"optionalProperties": {
"confidence": { "type": "float32" },
"normal-rating": { "type": "float32" },
"sample-size": { "type": "float64" },
"generated": { "type": "float64" },
"expires": { "type": "float64" }
}
}
}
}
}

Figure 4: A JTD Schema Describing "reputation-object" from
Section 6.2.2 of [RFC7071]

This schema does not enforce the requirement that "sample-size",
"generated", and "expires" be unbounded positive integers. It does
not express the limitation that "rating", "confidence", and "normal-
rating" should not have more than three decimal places of precision.

The example in Figure 4 can be compared against the equivalent
example in Appendix H of [RFC8610].

Acknowledgments

Carsten Bormann provided lots of useful guidance and feedback on
JTD's design and the structure of this document.

Evgeny Poberezkin suggested the addition of "nullable" and thoroughly
vetted this document for mistakes and opportunities for
simplification.

Tim Bray suggested the current "ref" model and the addition of
"enum". Anders Rundgren suggested extending "type" to have more
support for numerical types. James Manger suggested additional
clarifying examples of how integer types work. Adrian Farrel
suggested many improvements to help make this document clearer.

Members of the IETF JSON mailing list -- in particular, Pete Cordell,
Phillip Hallam-Baker, Nico Williams, John Cowan, Rob Sayre, and Erik
Wilde -- provided lots of useful feedback.

OpenAPI's "discriminator" object [OPENAPI] inspired the
"discriminator" form. [JSON-SCHEMA] influenced various parts of
JTD's early design.

Author's Address

Ulysse Carion
Segment.io, Inc
100 California Street
San Francisco, CA 94111
United States of America

Email: [email protected]