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Initial commit. This commit contains the third major design of a C++ library for JSON Schema validation. It is definitely not what I would consider production-ready, but I do think that the overall design of the library is robust.
2013-10-29 21:51:11 +01:00
Internet Engineering Task Force G. Luff, Ed.
Internet-Draft
Intended status: Informational K. Zyp
Expires: August 5, 2013 SitePen (USA)
G. Court
February 1, 2013
JSON Hyper-Schema: Hypertext definitions for JSON Schema
draft-luff-json-hyper-schema-00
Abstract
JSON Schema is a JSON based format for defining the structure of JSON
data. This document specifies hyperlink- and hypermedia-related
keywords of JSON Schema.
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 http://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 August 5, 2013.
Copyright Notice
Copyright (c) 2013 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
(http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Design Considerations . . . . . . . . . . . . . . . . . . 5
4. Schema keywords . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. links . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.1. Multiple links per URI . . . . . . . . . . . . . . . . 6
4.2. fragmentResolution . . . . . . . . . . . . . . . . . . . . 8
4.2.1. json-pointer fragment resolution . . . . . . . . . . . 8
4.3. media . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.1. Properties of "media" . . . . . . . . . . . . . . . . 9
4.3.2. Example . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. readOnly . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5. pathStart . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Link Description Object . . . . . . . . . . . . . . . . . . . 10
5.1. href . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1.1. URI Templating . . . . . . . . . . . . . . . . . . . . 11
5.2. rel . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2.1. Fragment resolution with "root" links . . . . . . . . 16
5.2.2. Security Considerations for "self" links . . . . . . . 17
5.3. title . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.4. targetSchema . . . . . . . . . . . . . . . . . . . . . . . 18
5.4.1. Security Considerations for "targetSchema" . . . . . . 19
5.5. mediaType . . . . . . . . . . . . . . . . . . . . . . . . 20
5.5.1. Security concerns for "mediaType" . . . . . . . . . . 21
5.6. Submission Link Properties . . . . . . . . . . . . . . . . 22
5.6.1. method . . . . . . . . . . . . . . . . . . . . . . . . 22
5.6.2. encType . . . . . . . . . . . . . . . . . . . . . . . 22
5.6.3. schema . . . . . . . . . . . . . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
6.1. Registry of Link Relations . . . . . . . . . . . . . . . . 23
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.1. Normative References . . . . . . . . . . . . . . . . . . . 24
7.2. Informative References . . . . . . . . . . . . . . . . . . 24
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
JSON Schema is a JSON based format for defining the structure of JSON
data. This document specifies hyperlink- and hypermedia-related
keywords of JSON Schema.
The term JSON Hyper-Schema is used to refer to a JSON Schema that
uses these keywords.
This specification will use the terminology defined by the JSON
Schema core specification [json-schema-core]. It is advised that
readers have a copy of this specification.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The terms "schema", "instance", "property" and "item" are to be
interpreted as defined in the JSON Schema core specification
[json-schema-core].
3. Overview
This document describes how JSON Schema can be used to define
hyperlinks on instance data. It also defines how to provide
additional information required to interpret JSON data as rich
multimedia documents.
Just as with the core JSON schema keywords, all the keywords
described in the "Schema Keywords" section are optional.
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Here is an example JSON Schema defining hyperlinks, and providing a
multimedia interpretation for the "imgData" property:
{
"title": "Written Article",
"type": "object",
"properties": {
"id": {
"title": "Article Identifier",
"type": "number"
},
"title": {
"title": "Article Title",
"type": "string"
},
"authorId": {
"type": "integer"
},
"imgData": {
"title": "Article Illustration (small)",
"type": "string",
"media": {
"binaryEncoding": "base64",
"type": "image/png"
}
}
},
"required" : ["id", "title", "authorId"],
"links": [
{
"rel": "full",
"href": "{id}"
},
{
"rel": "author",
"href": "/user?id={authorId}"
}
]
}
This example schema defines the properties of the instance. For the
"imgData" property, it specifies that that it should be base64-
decoded and the resulting binary data treated as a PNG image. It
also defines link relations for the instance, with URIs incorporating
values from the instance.
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An example of a JSON instance described by the above schema might be:
{
"id": 15,
"title": "Example data",
"authorId": 105,
"imgData": "iVBORw...kJggg=="
}
The base-64 data has been abbreviated for readability.
3.1. Design Considerations
The purpose of this document is to define keywords for the JSON
Schema that allow JSON data to be understood as hyper-text.
JSON data on its own requires special knowledge from the client about
the format in order to be interpretable as hyper-text. This document
proposes a way to describe the hyper-text and hyper-media
interpretation of such JSON formats, without defining reserved
keywords or otherwise restricting the structure of the JSON data.
4. Schema keywords
4.1. links
The "links" property of schemas is used to associate Link Description
Objects with instances. The value of this property MUST be an array,
and the items in the array must be Link Description Objects, as
defined below.
An example schema using the "links" keyword could be:
{
"title": "Schema defining links",
"links": [
{
"rel": "full",
"href": "{id}"
},
{
"rel": "parent",
"href": "{parent}"
}
]
}
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4.1.1. Multiple links per URI
A single URI might have more than one role with relation to an
instance. This is not a problem - the same URI can be used in more
than one Link Description Object.
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For example, this schema describes a format for blog posts, accessed
via HTTP. The links describe how to access the comments for the
post, how to search the comments, and how to submit new comments, all
with the same URI:
{
"title": "News post",
...
"links": [
{
"rel": "comments",
"href": "/{id}/comments"
},
{
"rel": "search",
"href": "/{id}/comments",
"schema": {
"type": "object",
"properties": {
"searchTerm": {
"type": "string"
},
"itemsPerPage": {
"type": "integer",
"minimum": 10,
"multipleOf": 10,
"default": 20
}
},
"required": ["searchTerm"]
}
},
{
"title": "Post a comment",
"rel": "create",
"href": "/{id}/comments",
"method": "POST",
"schema": {
"type": "object",
"properties": {
"message": {
"type": "string"
}
},
"required": ["message"]
}
}
]
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}
If the client follows the first link, the URI might be expanded to
"/15/comments". For the second link, the method is "GET" (the
default for HTTP) so a client following this link would add the
parameters to the URL to produce something like: "/15/
comments?searchTerm=JSON&itemsPerPage=50". The third link defines a
possible interaction where a client would POST to a URI (such as
"/15/comments"), where the post-data was a JSON representation of the
new comment, for example:
{
"message": "This is an example comment"
}
4.2. fragmentResolution
When addressing a JSON document, the fragment part of the URI may be
used to refer to a particular instance within the document.
This keyword indicates the method to use for finding the appropriate
instance within a document, given the fragment part. The default
fragment resolution protocol is "json-pointer", which is defined
below. Other fragment resolution protocols MAY be used, but are not
defined in this document.
If the instance is described by a schema providing the a link with
"root" relation, or such a link is provided in using the HTTP Link
header [RFC5988], then the target of the "root" link should be
considered the document root for the purposes of all fragment
resolution methods that use the document structure (such as "json-
pointer"). The only exception to this is the resolution of "root"
links themselves.
4.2.1. json-pointer fragment resolution
The "json-pointer" fragment resolution protocol uses a JSON Pointer
[json-pointer] to resolve fragment identifiers in URIs within
instance representations.
4.3. media
The "media" property indicates that this instance contains non-JSON
data encoded in a JSON string. It describes the type of content and
how it is encoded.
The value of this property MUST be an object, and SHOULD be ignored
for any instance that is not a string.
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4.3.1. Properties of "media"
The value of the "media" keyword MAY contain any of the following
properties:
4.3.1.1. binaryEncoding
If the instance value is a string, this property defines that the
string SHOULD be interpreted as binary data and decoded using the
encoding named by this property. RFC 2045, Sec 6.1 [RFC2045] lists
the possible values for this property.
4.3.1.2. type
The value of this property must be a media type, as defined by RFC
2046 [RFC2046]. This property defines the media type of instances
which this schema defines.
If the "binaryEncoding" property is not set, but the instance value
is a string, then the value of this property SHOULD specify a text
document type, and the character set SHOULD be the character set into
which the JSON string value was decoded (for which the default is
Unicode).
4.3.2. Example
Here is an example schema, illustrating the use of "media":
{
"type": "string",
"media": {
"binaryEncoding": "base64",
"type": "image/png"
}
}
Instances described by this schema should be strings, and their
values should be interpretable as base64-encoded PNG images.
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Another example:
{
"type": "string",
"media": {
"mediaType": "text/html"
}
}
Instances described by this schema should be strings containing HTML,
using whatever character set the JSON string was decoded into
(default is Unicode).
4.4. readOnly
If it has a value of boolean true, this keyword indicates that the
instance property SHOULD NOT be changed, and attempts by a user agent
to modify the value of this property are expected to be rejected by a
server.
The value of this keyword MUST be a boolean. The default value is
false.
4.5. pathStart
This property is a URI that defines what the instance's URI MUST
start with in order to validate. The value of the "pathStart"
property MUST be resolved relative to the closest URI Resolution
Scope (as defined in the JSON Schema core specification
[json-schema-core]), using the rules from RFC 3986, Sec 5 [RFC3986].
When multiple schemas have been referenced for an instance, the user
agent can determine if this schema is applicable for a particular
instance by determining if the URI of the instance begins with the
the value of the "pathStart" property. If the URI of the instance
does not start with this URI, or if another schema specifies a
starting URI that is longer and also matches the instance, this
schema SHOULD NOT be considered to describe the instance. Any schema
that does not have a pathStart property SHOULD be considered
applicable to all the instances for which it is referenced.
5. Link Description Object
A Link Description Object (LDO) is used to describe a single link
relation. In the context of a schema, it defines the link relations
of the instances of the schema, and can be parameterized by the
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instance values. A Link Description Object (LDO) must be an object.
The link description format can be used without JSON Schema, and use
of this format can be declared by referencing the normative link
description schema as the schema for the data structure that uses the
links. The URI of the normative link description schema is:
http://json-schema.org/links (latest version) or
http://json-schema.org/draft-04/links (draft-04 version).
"Form"-like functionality can be defined by use of the "schema"
keyword, which supplies a schema describing the data to supply to the
server.
5.1. href
The value of the "href" link description property is a template used
to determine the target URI of the related resource. The value of
the instance property SHOULD be resolved as a URI-Reference per RFC
3986 [RFC3986] and MAY be a relative reference to a URI. The base
URI to be used for relative URI resolution SHOULD be the URI used to
retrieve the instance object (not the schema).
The base URI to be used for relative URI resolution SHOULD is defined
as follows:
if the data has a link defined, with a relation of "self", then
the "href" value of that link is used, unless the relation of the
link being resolved is also "self"
otherwise, the URI should be resolved against the link with
relation "self" belonging to the closest parent node in the JSON
document, if it exists
otherwise, the URI used to fetch the document should be used.
This property is not optional.
5.1.1. URI Templating
The value of "href" is to be used as a URI Template, as defined in
RFC 6570 [RFC6570]. However, some special considerations apply:
5.1.1.1. Pre-processing
The URI Template specification [RFC6570] restricts the set of
characters available for variable names. Property names in JSON,
however, can be any UTF-8 string.
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To allow the use of any JSON property name in the template, before
using the value of "href" as a URI Template, the following pre-
processing rules MUST be applied, in order:
5.1.1.1.1. Bracket escaping
The purpose of this step is to allow the use of brackets to percent-
encode variable names inside curly brackets. Variable names to be
escaped are enclosed within rounded brackets, with the close-rounded-
bracket character ")" being escaped as a pair of close-rounded-
brackets "))". Since the empty string is not a valid variable name
in RFC 6570, an empty pair of brackets is replaced with "%65mpty".
The rules are as follows:
Find the largest possible sections of the text such that:
do not contain an odd number of close-rounded-bracket characters
")" in sequence in that section of the text
are surrounded by a pair of rounded brackets: ( ), where
the surrounding rounded brackets are themselves contained within a
pair of curly brackets: { }
Each of these sections of the text (including the surrounding rounded
brackets) MUST be replaced, according to the following rules:
If the brackets contained no text (the empty string), then they
are replaced with "%65mpty" (which is "empty" with a percent-
encoded "e")
Otherwise, the enclosing brackets are removed, and the inner text
used after the following modifications
all pairs of close-brackets "))" are replaced with a single
close bracket
after that, the text is replaced with its percent-encoded
equivalent, such that the result is a valid RFC 6570 variable
name (note that this requires encoding characters such as "*"
and "!")
5.1.1.1.2. Replacing $
After the above substitutions, if the character "$" (dollar sign)
appears within a pair of curly brackets, then it MUST be replaced
with the text "%73elf" (which is "self" with a percent-encoded "s").
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The purpose of this stage is to allow the use of the instance value
itself (instead of its object properties or array items) in the URI
Template, by the special value "%73elf".
5.1.1.1.3. Choice of special-case values
The special-case values of "%73elf" and "%65mpty" were chosen because
they are unlikely to be accidentally generated by either a human or
automated escaping.
5.1.1.1.4. Examples
For example, here are some possible values for "href", followed by
the results after pre-processing:
Input Output
-----------------------------------------
"no change" "no change"
"(no change)" "(no change)"
"{(escape space)}" "{escape%20space}"
"{(escape+plus)}" "{escape%2Bplus}"
"{(escape*asterisk)}" "{escape%2Aasterisk}"
"{(escape(bracket)}" "{escape%28bracket}"
"{(escape))bracket)}" "{escape%29bracket}"
"{(a))b)}" "{a%29b}
"{(a (b)))}" "{a%20%28b%29}
"{()}" "{%65mpty}
"{+$*}" "{+%73elf*}
"{+($)*}" "{+%24*}
Note that in the final example, because the "+" was outside the
brackets, it remained unescaped, whereas in the fourth example the
"+" was escaped.
5.1.1.2. Values for substitution
After pre-processing, the URI Template is filled out using data from
the instance. To allow the use of any object property (including the
empty string), array index, or the instance value itself, the
following rules are defined:
For a given variable name in the URI Template, the value to use is
determined as follows:
If the variable name is "%73elf", then the instance value itself
MUST be used.
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If the variable name is "%65mpty", then the instances's empty-
string ("") property MUST be used (if it exists).
If the instance is an array, and the variable name is a
representation of a non-negative integer, then the value at the
corresponding array index MUST be used (if it exists).
Otherwise, the variable name should be percent-decoded, and the
corresponding object property MUST be used (if it exists).
5.1.1.2.1. Converting to strings
When any value referenced by the URI template is null, a boolean or a
number, then it should first be converted into a string as follows:
null values SHOULD be replaced by the text "null"
boolean values SHOULD be replaced by their lower-case equivalents:
"true" or "false"
numbers SHOULD be replaced with their original JSON
representation.
In some software environments the original JSON representation of a
number will not be available (there is no way to tell the difference
between 1.0 and 1), so any reasonable representation should be used.
Schema and API authors should bear this in mind, and use other types
(such as string or boolean) if the exact representation is important.
5.1.1.3. Missing values
Sometimes, the appropriate values will not be available. For
example, the template might specify the use of object properties, but
the instance is an array or a string.
If any of the values required for the template are not present in the
JSON instance, then substitute values MAY be provided from another
source (such as default values). Otherwise, the link definition
SHOULD be considered not to apply to the instance.
5.2. rel
The value of the "rel" property indicates the name of the relation to
the target resource. This property is not optional.
The relation to the target SHOULD be interpreted as specifically from
the instance object that the schema (or sub-schema) applies to, not
just the top level resource that contains the object within its
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hierarchy. A link relation from the top level resource to a target
MUST be indicated with the schema describing the top level JSON
representation.
Relationship definitions SHOULD NOT be media type dependent, and
users are encouraged to utilize existing accepted relation
definitions, including those in existing relation registries (see RFC
4287 [RFC4287]). However, we define these relations here for clarity
of normative interpretation within the context of JSON Schema defined
relations:
self If the relation value is "self", when this property is
encountered in the instance object, the object represents a
resource and the instance object is treated as a full
representation of the target resource identified by the specified
URI.
full This indicates that the target of the link is the full
representation for the instance object. The instance that
contains this link may not be the full representation.
describedBy This indicates the target of the link is a schema
describing the instance object. This MAY be used to specifically
denote the schemas of objects within a JSON object hierarchy,
facilitating polymorphic type data structures.
root This relation indicates that the target of the link SHOULD be
treated as the root or the body of the representation for the
purposes of user agent interaction or fragment resolution. All
other data in the document can be regarded as meta-data for the
document. The URI of this link MUST refer to a location within
the instance document, otherwise the link MUST be ignored.
The following relations are applicable for schemas (the schema as the
"from" resource in the relation) if they require no parameterization
with data from the instance:
instances This indicates the target resource that represents a
collection of instances of a schema.
create This indicates a target to use for creating new instances of
a schema. This link definition SHOULD be a submission link with a
non-safe method (like POST).
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For example, if a schema is defined:
{
"links": [{
"rel": "self",
"href": "{id}"
}, {
"rel": "up",
"href": "{upId}"
}, {
"rel": "children",
"href": "?upId={id}"
}]
}
And if a collection of instance resources were retrieved with JSON
representation:
GET /Resource/
[{
"id": "thing",
"upId": "parent"
}, {
"id": "thing2",
"upId": "parent"
}]
This would indicate that for the first item in the collection, its
own (self) URI would resolve to "/Resource/thing" and the first
item's "up" relation SHOULD be resolved to the resource at
"/Resource/parent". The "children" collection would be located at
"/Resource/?upId=thing".
Note that these relationship values are case-insensitive, consistent
with their use in HTML and the HTTP Link header [RFC5988].
5.2.1. Fragment resolution with "root" links
The presence of a link with relation "root" alters what the root of
the document is considered to be. For fragment resolution methods
(such as JSON Pointer fragments) that navigate through the document,
the target of the "root" link should be the starting point for such
methods.
The only exception is "root" links themselves. When calculating the
target of links with relation "root", existing "root" links MUST NOT
be taken into consideration.
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For example, say we have the following schema:
{
"links": [{
"rel": "root",
"href": "#/myRootData"
}]
}
And the following data, returned from the URI:
"http://example.com/data/12345":
{
"myRootData": {
"title": "Document title"
},
"metaData": {
...
}
}
To correctly resolve the URL "http://example.com/data/12345", we must
take the "root" link into account. Here are some example URIs, along
with the data they would resolve to:
URI Data
-----------------------------------------------------------------------
http://example.com/data/12345 {"title": "Document title"}
http://example.com/data/12345#/title "Document title"
5.2.2. Security Considerations for "self" links
When link relation of "self" is used to denote a full representation
of an object, the user agent SHOULD NOT consider the representation
to be the authoritative representation of the resource denoted by the
target URI if the target URI is not equivalent to or a sub-path of
the the URI used to request the resource representation which
contains the target URI with the "self" link.
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For example, if a hyper schema was defined:
{
"links": [{
"rel": "self",
"href": "{id}"
}]
}
And a resource was requested from somesite.com:
GET /foo/
With a response of:
Content-Type: application/json; profile=/schema-for-this-data
[{
"id": "bar",
"name": "This representation can be safely treated \
as authoritative "
}, {
"id": "/baz",
"name": "This representation should not be treated as \
authoritative the user agent should make request the resource\
from '/baz' to ensure it has the authoritative representation"
}, {
"id": "http://othersite.com/something",
"name": "This representation\
should also not be treated as authoritative and the target\
resource representation should be retrieved for the\
authoritative representation"
}]
5.3. title
This property defines a title for the link. The value must be a
string.
User agents MAY use this title when presenting the link to the user.
5.4. targetSchema
This property value is advisory only, and is a schema that defines
the expected structure of the JSON representation of the target of
the link, if the target of the link is returned using JSON
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representation.
5.4.1. Security Considerations for "targetSchema"
This property has similar security concerns to that of "mediaType".
Clients MUST NOT use the value of this property to aid in the
interpretation of the data received in response to following the
link, as this leaves "safe" data open to re-interpretation.
For example, suppose two programmers are having a discussion about
web security using a text-only message board. Here is some data from
that conversation, with a URI of:
http://forum.example.com/topics/152/comments/13
{
"topicId": 152,
"commentId": 13,
"from": {
"name": "Jane",
"id": 5
},
"to": {
"name": "Jason",
"id": 8
},
"message": "It's easy, you just add some HTML like
this: <script>doSomethingEvil()</script>"
}
The message string was split over two lines for readability.
A third party might then write provide the following Link Description
Object at another location:
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{
"rel": "evil-attack",
"href": "http://forum.example.com/topics/152/comments/13",
"targetSchema": {
"properties": {
"message": {
"description": "Re-interpret the message text as HTML",
"media": {
"type": "text/html"
}
}
}
}
}
If the client used this "targetSchema" value when interpreting the
above data, then it might display the contents of "message" as HTML.
At this point, the JavaScript embedded in the message might be
executed (in the context of the "forum.example.com" domain).
5.5. mediaType
The value of this property is advisory only, and represents the media
type RFC 2046 [RFC2046], that is expected to be returned when
fetching this resource. This property value MAY be a media range
instead, using the same pattern defined in RFC 2161, section 14.1 -
HTTP "Accept" header [RFC2616].
This property is analogous to the "type" property of <a> elements in
HTML (advisory content type), or the "type" parameter in the HTTP
Link header [RFC5988]. User agents MAY use this information to
inform the interface they present to the user before the link is
followed, but this information MUST NOT use this information in the
interpretation of the resulting data. When deciding how to interpret
data obtained through following this link, the behaviour of user
agents MUST be identical regardless of the value of the this
property.
If this property's value is specified, and the link's target is to be
obtained using any protocol that supports the HTTP/1.1 "Accept"
header RFC 2616, section 14.1 [RFC2616], then user agents MAY use the
value of this property to aid in the assembly of that header when
making the request to the server.
If this property's value is not specified, then the value should be
taken to be "application/json".
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For example, if a schema is defined:
{
"links": [{
"rel": "self",
"href": "/{id}/json"
}, {
"rel": "alternate",
"href": "/{id}/html",
"mediaType": "text/html"
}, {
"rel": "alternate",
"href": "/{id}/rss",
"mediaType": "application/rss+xml"
}, {
"rel": "icon",
"href": "{id}/icon",
"mediaType": "image/*"
}]
}
A suitable instance described by this schema would have four links
defined. The link with a "rel" value of "self" would have an
expected MIME type of "application/json" (the default). The two
links with a "rel" value of "alternate" specify the locations of HTML
and RSS versions of the current item. The link with a "rel" value of
"icon" links to an image, but does not specify the exact format.
A visual user agent displaying the item from the above example might
present a button representing an RSS feed, which when pressed passes
the target URI (calculated "href" value) to an view more suited to
displaying it, such as a news feed aggregator tab.
Note that presenting the link in the above manner, or passing the URI
to a news feed aggregator view does not constitute interpretation of
the data, but an interpretation of the link. The interpretation of
the data itself is performed by the news feed aggregator, which
SHOULD reject any data that would not have also been interpreted as a
news feed, had it been displayed in the main view.
5.5.1. Security concerns for "mediaType"
The "mediaType" property in link definitions defines the expected
format of the link's target. However, this is advisory only, and
MUST NOT be considered authoritative.
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When choosing how to interpret data, the type information provided by
the server (or inferred from the filename, or any other usual method)
MUST be the only consideration, and the "mediaType" property of the
link MUST NOT be used. User agents MAY use this information to
determine how they represent the link or where to display it (for
example hover-text, opening in a new tab). If user agents decide to
pass the link to an external program, they SHOULD first verify that
the data is of a type that would normally be passed to that external
program.
This is to guard against re-interpretation of "safe" data, similar to
the precautions for "targetSchema".
5.6. Submission Link Properties
The following properties also apply to Link Description Objects, and
provide functionality analogous to HTML forms, in providing a means
for submitting extra (often user supplied) information to send to a
server.
5.6.1. method
This property defines which method can be used to access the target
resource. In an HTTP environment, this might be "GET" or "POST" (or
other HTTP methods).
Some link relation values imply a set of appropriate HTTP methods to
be used for the link. For example, a client might assume that a link
with a relation of "edit" can be used in conjuction with the "PUT"
HTTP method. If the client does not know which methods might be
appropriate, then this SHOULD default to "GET".
5.6.2. encType
If present, this property indicates a query media type format that
the server supports for querying or posting to the collection of
instances at the target resource. The query can be suffixed to the
target URI to query the collection with property-based constraints on
the resources that SHOULD be returned from the server or used to post
data to the resource (depending on the method).
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For example, with the following schema:
{
"links": [{
"encType": "application/x-www-form-urlencoded",
"method": "GET",
"href": "/Product/",
"properties": {
"name": {
"description": "name of the product"
}
}
}]
}
This indicates that the client can query the server for instances
that have a specific name.
For example:
/Product/?name=Slinky
If no encType or method is specified, only the single URI specified
by the href property is defined. If the method is POST,
"application/json" is the default media type.
5.6.3. schema
This property contains a schema which defines the acceptable
structure of the submitted request. For a GET request, this schema
would define the properties for the query string and for a POST
request, this would define the body.
Note that this is separate from the URI templating of "href" (which
uses data from the instance, not submitted by the user). It is also
separate from the "targetSchema" property, which provides a schema
for the data that the client should expect to be returned when they
follow the link.
6. IANA Considerations
6.1. Registry of Link Relations
This registry is maintained by IANA per RFC 4287 [RFC4287] and this
specification adds four values: "full", "create", "instances",
"root". New assignments are subject to IESG Approval, as outlined in
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RFC 5226 [RFC5226]. Requests should be made by email to IANA, which
will then forward the request to the IESG, requesting approval.
7. References
7.1. Normative References
[RFC2045] Freed, N. and N. Borenstein,
"Multipurpose Internet Mail Extensions
(MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs
to Indicate Requirement Levels", BCP 14,
RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L.
Masinter, "Uniform Resource Identifier
(URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed.,
"The Atom Syndication Format", RFC 4287,
December 2005.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M.,
Nottingham, M., and D. Orchard, "URI
Template", RFC 6570, March 2012.
[json-pointer] Bryan, P., Zyp, K., and M. Nottingham,
"JSON Pointer", August 2012, <http://
tools.ietf.org/html/
draft-ietf-appsawg-json-pointer-03>.
[json-schema-core] Galiegue, F., Zyp, K., and G. Court,
"JSON Schema: core definitions and
terminology", 2013, <http://
tools.ietf.org/html/
draft-zyp-json-schema-04>.
7.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J.,
Frystyk, H., Masinter, L., Leach, P., and
T. Berners-Lee, "Hypertext Transfer
Protocol -- HTTP/1.1", RFC 2616,
June 1999.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines
for Writing an IANA Considerations
Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC2046] Freed, N. and N. Borenstein,
"Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types", RFC 2046,
November 1996.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988,
October 2010.
[W3C.REC-html401-19991224] Hors, A., Raggett, D., and I. Jacobs,
"HTML 4.01 Specification", World Wide Web
Consortium Recommendation REC-html401-
19991224, December 1999, <http://
www.w3.org/TR/1999/REC-html401-19991224>.
Appendix A. Change Log
draft-04
* Resolution of link URIs ("href") is now affected by rel="self"
links on the instance
* Define "title" for LDOs
* Use URI Templates for the "href" property
* Split hyper-schema definition out from main schema.
* Capitalised the T in "encType", and the O in "readOnly"
* Moved "mediaType" and "contentEncoding" to the new "media"
property (renamed "type" and "binaryEncoding")
* Added "mediaType" property to LDOs
* Replaced "slash-delimited" fragment resolution with "json-
pointer".
* Added "template" LDO attribute.
* Improved wording of sections.
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draft-03
* Added example and verbiage to "extends" attribute.
* Defined slash-delimited to use a leading slash.
* Made "root" a relation instead of an attribute.
* Removed address values, and MIME media type from format to
reduce confusion (mediaType already exists, so it can be used
for MIME types).
* Added more explanation of nullability.
* Removed "alternate" attribute.
* Upper cased many normative usages of must, may, and should.
* Replaced the link submission "properties" attribute to "schema"
attribute.
* Replaced "optional" attribute with "required" attribute.
* Replaced "maximumCanEqual" attribute with "exclusiveMaximum"
attribute.
* Replaced "minimumCanEqual" attribute with "exclusiveMinimum"
attribute.
* Replaced "requires" attribute with "dependencies" attribute.
* Moved "contentEncoding" attribute to hyper schema.
* Added "additionalItems" attribute.
* Added "id" attribute.
* Switched self-referencing variable substitution from "-this" to
"@" to align with reserved characters in URI template.
* Added "patternProperties" attribute.
* Schema URIs are now namespace versioned.
* Added "$ref" and "$schema" attributes.
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draft-02
* Replaced "maxDecimal" attribute with "divisibleBy" attribute.
* Added slash-delimited fragment resolution protocol and made it
the default.
* Added language about using links outside of schemas by
referencing its normative URI.
* Added "uniqueItems" attribute.
* Added "targetSchema" attribute to link description object.
draft-01
* Fixed category and updates from template.
draft-00
* Initial draft.
Authors' Addresses
Geraint Luff (editor)
Cambridge
UK
EMail: luffgd@gmail.com
Kris Zyp
SitePen (USA)
530 Lytton Avenue
Palo Alto, CA 94301
USA
Phone: +1 650 968 8787
EMail: kris@sitepen.com
Gary Court
Calgary, AB
Canada
EMail: gary.court@gmail.com
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