5. Basic Concepts
This section is non-normative.
JSON [RFC4627] is a lightweight, language-independent data-interchange format.
It is easy to parse and easy to generate. However, it is difficult to integrate JSON
from different sources as the data has just local meaning. Furthermore, JSON has no
built-in support for hyperlinks - a fundamental building block on the Web. Let's look
at an example that we will be using for the rest of this section:
Example 1: Sample JSON document
{
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"image": "http://manu.sporny.org/images/manu.png"
}It's obvious to humans that the data is about a person whose name is "Manu Sporny"
and that the homepage property contains the URL of that person's homepage.
A machine doesn't have such an intuitive understanding and sometimes,
even for humans, it is difficult to resolve ambiguities in such representations. This problem
can be solved by using unambiguous identifiers to denote the different concepts instead of
tokens such as "name", "homepage", etc.
Linked Data, and the Web in general, uses IRIs
(Internationalized Resource Identifiers as described in [RFC3987]) for unambiguous
identification. The idea is to assign IRIs to something that may
be of use to other developers and that it is useful to give them an unambiguous identifier.
That is, it is useful for terms to expand to IRIs
so that developers don't accidentally step on each other's terms. Furthermore, developers and
machines are able to use this IRI (by using a web browser, for instance) to go to
the term and get a definition of what the term means. This process is known as IRI
dereferencing.
Leveraging the well-known schema.org vocabulary,
the example above could be unambiguously expressed as follows:
Example 2: Sample JSON-LD document using full IRIs instead of terms
{
"http://schema.org/name": "Manu Sporny",
"http://schema.org/url": { "@id": "http://manu.sporny.org/" },
"http://schema.org/image": { "@id": "http://manu.sporny.org/images/manu.png" }
}In the example above, every property is unambiguously identified by an IRI and all values
representing IRIs are explicitly marked as such by the
@id keyword. While this is a valid JSON-LD
document that is very specific about its data, the document is also overly verbose and difficult
to work with for human developers. To address this issue, JSON-LD introduces the notion
of a context as described in the next section.
5.1 The Context
This section is non-normative.
Simply speaking, a context is used to map terms to
IRIs. Terms are case sensitive
and any valid string that is not a reserved JSON-LD keyword
can be used as a term.
For the sample document in the previous section, a context would
look something like this:
Example 3: Context for the sample document in the previous section
{
"@context":
{
"name": "http://schema.org/name",
"image": {
"@id": "http://schema.org/image",
"@type": "@id"
},
"homepage": {
"@id": "http://schema.org/url",
"@type": "@id"
}
}
}As the context above shows, the value of a term definition can
either be a simple string, mapping the term to an IRI,
or a JSON object.
When a JSON object is
associated with a term, it is called an expanded term definition.
The example above specifies that the values of image and
homepage terms are IRIs.
They also allow terms to be used for index maps
and to specify whether array values are to be
interpreted as sets or lists.
Expanded term definitions may
be defined using absolute or
compact IRIs as keys, which is
mainly used to associate type or language information with an
absolute or compact IRI.
Contexts can either be directly embedded
into the document or be referenced. Assuming the context document in the previous
example can be retrieved at http://json-ld.org/contexts/person.jsonld,
it can be referenced by adding a single line and allows a JSON-LD document to
be expressed much more concisely as shown in the example below:
Example 4: Referencing a JSON-LD context
{
"@context": "http://json-ld.org/contexts/person.jsonld",
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"image": "http://manu.sporny.org/images/manu.png"
}The referenced context not only specifies how the terms map to
IRIs in the Schema.org vocabulary but also specifies that
the values of the homepage and image property
can be interpreted as an IRI ("@type": "@id",
see section 5.2 IRIs for more details). This information allows developers
to re-use each other's data without having to agree to how their data will interoperate
on a site-by-site basis. External JSON-LD context documents may contain extra
information located outside of the @context key, such as
documentation about the terms declared in the
document. Information contained outside of the @context value
is ignored when the document is used as an external JSON-LD context document.
JSON documents can be interpreted as JSON-LD without having to be modified by
referencing a context via an HTTP Link Header
as described in section 6.8 Interpreting JSON as JSON-LD. It is also
possible to apply a custom context using the JSON-LD API [JSON-LD-API].
In JSON-LD documents
contexts may also be specified in-line.
This has the advantage that documents can be processed even in the
absence of a connection to the Web. Ultimately, this is a modeling decision
and different use cases may require different handling.
Example 5: In-line context definition
{
"@context":
{
"name": "http://schema.org/name",
"image": {
"@id": "http://schema.org/image",
"@type": "@id"
},
"homepage": {
"@id": "http://schema.org/url",
"@type": "@id"
}
},
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"image": "http://manu.sporny.org/images/manu.png"
}5.2 IRIs
This section is non-normative.
IRIs (Internationalized Resource Identifiers
[RFC3987]) are fundamental to Linked Data as that is how most
nodes and properties
are identified. In JSON-LD, IRIs may be represented as an
absolute IRI or a relative IRI. An
absolute IRI is defined in [RFC3987] as containing a
scheme along with path and optional query and
fragment segments. A relative IRI is an IRI
that is relative to some other absolute IRI.
In JSON-LD all relative IRIs are resolved
relative to the base IRI associated with the document.
A string is interpreted as an IRI when it is the
value of an @id member:
Example 6: Values of @id are interpreted as IRI
{
...
"homepage": { "@id": "http://example.com/" }
...
}Values that are interpreted as IRIs, can also be
expressed as relative IRIs. For example,
assuming that the following document is located at
http://example.com/about/, the relative IRI
../ would expand to http://example.com/ (for more
information on where relative IRIs can be
used, please refer to appendix B. JSON-LD Grammar).
Example 7: IRIs can be relative
{
...
"homepage": { "@id": "../" }
...
}Absolute IRIs can be expressed directly
in the key position like so:
Example 8: IRI as a key
{
...
"http://schema.org/name": "Manu Sporny",
...
}In the example above, the key http://schema.org/name
is interpreted as an absolute IRI because it contains a colon
(:) and it is neither a compact IRI nor a
blank node identifier.
Term-to-IRI expansion occurs if the key matches a term defined
within the active context:
Example 9: Term expansion from context definition
{
"@context":
{
"name": "http://schema.org/name"
},
"name": "Manu Sporny",
"status": "trollin'"
}JSON keys that do not expand to an IRI, such as status
in the example above, are not Linked Data and thus ignored when processed.
If type coercion rules are specified in the @context for
a particular term or property IRI, an IRI is generated:
Example 10: Type coercion
{
"@context":
{
...
"homepage":
{
"@id": "http://schema.org/homepage",
"@type": "@id"
}
...
}
...
"homepage": "http://manu.sporny.org/",
...
}In the example above, even though the value http://manu.sporny.org/
is expressed as a JSON string, the type coercion
rules will transform the value into an IRI when generating the
graph. See section 6.5 Type Coercion for more
details about this feature.
In summary, IRIs can be expressed in a variety of
different ways in JSON-LD:
- JSON object keys that have a term mapping in
the active context expand to an IRI
(only applies outside of the context definition).
- An IRI is generated for the string value specified using
@id or @type.
- An IRI is generated for the string value of any key for which there
are coercion rules that contain a
@type key that is
set to a value of @id or @vocab.
6. Advanced Concepts
JSON-LD has a number of features that provide functionality above and beyond
the core functionality described above. The following section describes this
advanced functionality in more detail.
6.4 Typed Values
This section is non-normative.
A value with an associated type, also known as a
typed value, is indicated by associating a value with
an IRI which indicates the value's type. Typed values may be
expressed in JSON-LD in three ways:
- By utilizing the
@type keyword when defining
a term within a @context section.
- By utilizing a value object.
- By using a native JSON type such as number, true, or false.
The first example uses the @type keyword to associate a
type with a particular term in the @context:
Example 21: Expanded term definition with type coercion
{
"@context":
{
"modified":
{
"@id": "http://purl.org/dc/terms/modified",
"@type": "http://www.w3.org/2001/XMLSchema#dateTime"
}
},
...
"@id": "http://example.com/docs/1",
"modified": "2010-05-29T14:17:39+02:00",
...
}The modified key's value above is automatically type coerced to a
dateTime value because of the information specified in the
@context. A JSON-LD processor will interpret the example above
as follows:
| Subject |
Property |
Value |
Value Type |
|---|
| http://example.com/docs/1 |
http://purl.org/dc/terms/modified |
2010-05-29T14:17:39+02:00 |
http://www.w3.org/2001/XMLSchema#dateTime |
The second example uses the expanded form of setting the type information
in the body of a JSON-LD document:
Example 22: Expanded value with type
{
"@context":
{
"modified":
{
"@id": "http://purl.org/dc/terms/modified"
}
},
...
"modified":
{
"@value": "2010-05-29T14:17:39+02:00",
"@type": "http://www.w3.org/2001/XMLSchema#dateTime"
}
...
}Both examples above would generate the value
2010-05-29T14:17:39+02:00 with the type
http://www.w3.org/2001/XMLSchema#dateTime. Note that it is
also possible to use a term or a compact IRI to
express the value of a type.
The @type keyword is also used to associate a type
with a node. The concept of a node type and
a value type are different.
Generally speaking, a node type specifies the type of thing
that is being described, like a person, place, event, or web page. A
value type specifies the data type of a particular value, such
as an integer, a floating point number, or a date.
Example 23: Example demonstrating the context-sensitivity for @type
{
...
"@id": "http://example.org/posts#TripToWestVirginia",
"@type": "http://schema.org/BlogPosting", <- This is a node type
"modified":
{
"@value": "2010-05-29T14:17:39+02:00",
"@type": "http://www.w3.org/2001/XMLSchema#dateTime" <- This is a value type
}
...
}The first use of @type associates a node type
(http://schema.org/BlogPosting) with the node,
which is expressed using the @id keyword.
The second use of @type associates a value type
(http://www.w3.org/2001/XMLSchema#dateTime) with the
value expressed using the @value keyword. As a
general rule, when @value and @type are used in
the same JSON object, the @type
keyword is expressing a value type.
Otherwise, the @type keyword is expressing a
node type. The example above expresses the following data:
| Subject |
Property |
Value |
Value Type |
|---|
| http://example.org/posts#TripToWestVirginia |
http://www.w3.org/1999/02/22-rdf-syntax-ns#type |
http://schema.org/BlogPosting |
- |
| http://example.org/posts#TripToWestVirginia |
http://purl.org/dc/terms/modified |
2010-05-29T14:17:39+02:00 |
http://www.w3.org/2001/XMLSchema#dateTime |
6.5 Type Coercion
This section is non-normative.
JSON-LD supports the coercion of values to particular data types.
Type coercion allows someone deploying JSON-LD to coerce the incoming or
outgoing values to the proper data type based on a mapping of data type IRIs to
terms. Using type coercion, value representation is preserved without requiring
the data type to be specified with each piece of data.
Type coercion is specified within an expanded term definition
using the @type key. The value of this key expands to an IRI.
Alternatively, the keywords @id or @vocab may be used
as value to indicate that within the body of a JSON-LD document, a string value of a
term coerced to @id or @vocab is to be interpreted as an
IRI. The difference between @id and @vocab is how values are expanded
to absolute IRIs. @vocab first tries to expand the value
by interpreting it as term. If no matching term is found in the
active context, it tries to expand it as compact IRI or absolute IRI
if there's a colon in the value; otherwise, it will expand the value using the
active context's vocabulary mapping, if present, or by interpreting it
as relative IRI. Values coerced to @id in contrast are expanded as
compact IRI or absolute IRI if a colon is present; otherwise, they are interpreted
as relative IRI.
Terms or compact IRIs used as the value of a
@type key may be defined within the same context. This means that one may specify a
term like xsd and then use xsd:integer within the same
context definition.
The example below demonstrates how a JSON-LD author can coerce values to
typed values and IRIs.
Example 24: Expanded term definition with types
{
"@context":
{
"xsd": "http://www.w3.org/2001/XMLSchema#",
"name": "http://xmlns.com/foaf/0.1/name",
"age":
{
"@id": "http://xmlns.com/foaf/0.1/age",
"@type": "xsd:integer"
},
"homepage":
{
"@id": "http://xmlns.com/foaf/0.1/homepage",
"@type": "@id"
}
},
"@id": "http://example.com/people#john",
"name": "John Smith",
"age": "41",
"homepage":
[
"http://personal.example.org/",
"http://work.example.com/jsmith/"
]
}The example shown above would generate the following data.
| Subject |
Property |
Value |
Value Type |
|---|
| http://example.com/people#john |
http://xmlns.com/foaf/0.1/name |
John Smith |
|
| http://example.com/people#john |
http://xmlns.com/foaf/0.1/age |
41 |
http://www.w3.org/2001/XMLSchema#integer |
| http://example.com/people#john |
http://xmlns.com/foaf/0.1/homepage |
http://personal.example.org/ |
IRI |
| http://work.example.com/jsmith/ |
IRI |
Terms may also be defined using absolute IRIs
or compact IRIs. This allows coercion rules
to be applied to keys which are not represented as a simple term.
For example:
Example 25: Term definitions using compact and absolute IRIs
{
"@context":
{
"foaf": "http://xmlns.com/foaf/0.1/",
"foaf:age":
{
"@id": "http://xmlns.com/foaf/0.1/age",
"@type": "xsd:integer"
},
"http://xmlns.com/foaf/0.1/homepage":
{
"@type": "@id"
}
},
"foaf:name": "John Smith",
"foaf:age": "41",
"http://xmlns.com/foaf/0.1/homepage":
[
"http://personal.example.org/",
"http://work.example.com/jsmith/"
]
}In this case the @id definition in the term definition is optional.
If it does exist, the compact IRI or IRI representing
the term will always be expanded to IRI defined by the @id
key—regardless of whether a prefix is defined or not.
Type coercion is always performed using the unexpanded value of the key. In the
example above, that means that type coercion is done looking for foaf:age
in the active context and not for the corresponding, expanded
IRI http://xmlns.com/foaf/0.1/age.
Note
Keys in the context are treated as terms for the purpose of
expansion and value coercion. At times, this may result in multiple representations for the same expanded IRI.
For example, one could specify that dog and cat both expanded to http://example.com/vocab#animal.
Doing this could be useful for establishing different type coercion or language specification rules. It also allows a compact IRI (or even an
absolute IRI) to be defined as something else entirely. For example, one could specify that
the term http://example.org/zoo should expand to
http://example.org/river, but this usage is discouraged because it would lead to a
great deal of confusion among developers attempting to understand the JSON-LD document.
6.7 Advanced Context Usage
This section is non-normative.
Section 5.1 The Context introduced the basics of what makes
JSON-LD work. This section expands on the basic principles of the
context and demonstrates how more advanced use cases can
be achieved using JSON-LD.
In general, contexts may be used at any time a
JSON object is defined. The only time that one cannot
express a context is inside a context definition itself. For example, a
JSON-LD document may use more than one context at different
points in a document:
Example 27: Using multiple contexts
[
{
"@context": "http://example.org/contexts/person.jsonld",
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"depiction": "http://twitter.com/account/profile_image/manusporny"
},
{
"@context": "http://example.org/contexts/place.jsonld",
"name": "The Empire State Building",
"description": "The Empire State Building is a 102-story landmark in New York City.",
"geo": {
"latitude": "40.75",
"longitude": "73.98"
}
}
]Duplicate context terms are overridden using a
most-recently-defined-wins mechanism.
Example 28: Scoped contexts within node objects
{
"@context":
{
"name": "http://example.com/person#name,
"details": "http://example.com/person#details"
}",
"name": "Markus Lanthaler",
...
"details":
{
"@context":
{
"name": "http://example.com/organization#name"
},
"name": "Graz University of Technology"
}
}In the example above, the name term is overridden
in the more deeply nested details structure. Note that this is
rarely a good authoring practice and is typically used when working with
legacy applications that depend on a specific structure of the
JSON object. If a term is redefined within a
context, all previous rules associated with the previous definition are
removed. If a term is redefined to null,
the term is effectively removed from the list of
terms defined in the active context.
Multiple contexts may be combined using an array, which is processed
in order. The set of contexts defined within a specific JSON object are
referred to as local contexts. The
active context refers to the accumulation of
local contexts that are in scope at a
specific point within the document. Setting a local context
to null effectively resets the active context
to an empty context. The following example specifies an external context
and then layers an embedded context on top of the external context:
Example 29: Combining external and local contexts
{
"@context": [
"http://json-ld.org/contexts/person.jsonld",
{
"pic": "http://xmlns.com/foaf/0.1/depiction"
}
],
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"pic": "http://twitter.com/account/profile_image/manusporny"
}Note
When possible, the context definition should be put
at the top of a JSON-LD document. This makes the document easier to read and
might make streaming parsers more efficient. Documents that do not have the
context at the top are still conformant JSON-LD.
Note
To avoid forward-compatibility issues, terms
starting with an @ character are to be avoided as they
might be used as keywords in future versions
of JSON-LD. Terms starting with an @ character that are not
JSON-LD 1.0 keywords are treated as any other term, i.e.,
they are ignored unless mapped to an IRI. Furthermore, the use of
empty terms ("") is not allowed as
not all programming languages are able to handle empty JSON keys.
6.8 Interpreting JSON as JSON-LD
Ordinary JSON documents can be interpreted as JSON-LD by referencing a JSON-LD
context document in an HTTP Link Header. Doing so allows JSON to
be unambiguously machine-readable without requiring developers to drastically
change their documents and provides an upgrade path for existing infrastructure
without breaking existing clients that rely on the application/json
media type.
In order to use an external context with an ordinary JSON document, an author
MUST specify an IRI to a valid JSON-LD document in
an HTTP Link Header [RFC5988] using the http://www.w3.org/ns/json-ld#context
link relation. The referenced document MUST have a top-level JSON object.
The @context subtree within that object is added to the top-level
JSON object of the referencing document. If an array
is at the top-level of the referencing document and its items are
JSON objects, the @context
subtree is added to all array items. All extra information located outside
of the @context subtree in the referenced document MUST be
discarded. Effectively this means that the active context is
initialized with the referenced external context.
The following example demonstrates the use of an external context with an
ordinary JSON document:
Example 30: Referencing a JSON-LD context from a JSON document via an HTTP Link Header
GET /ordinary-json-document.json HTTP/1.1
Host: example.com
Accept: application/ld+json,application/json,*/*;q=0.1
====================================
HTTP/1.1 200 OK
...
Content-Type: application/json
Link: <http://json-ld.org/contexts/person.jsonld>; rel="http://www.w3.org/ns/json-ld#context"; type="application/ld+json"
{
"name": "Markus Lanthaler",
"homepage": "http://www.markus-lanthaler.com/",
"image": "http://twitter.com/account/profile_image/markuslanthaler"
}Please note that JSON-LD documents
served with the application/ld+json
media type MUST have all context information, including references to external
contexts, within the body of the document. Contexts linked via a
http://www.w3.org/ns/json-ld#context HTTP Link Header MUST be
ignored for such documents.
6.9 String Internationalization
This section is non-normative.
At times, it is important to annotate a string
with its language. In JSON-LD this is possible in a variety of ways.
First, it is possible to define a default language for a JSON-LD document
by setting the @language key in the context:
Example 31: Setting the default language of a JSON-LD document
{
"@context":
{
...
"@language": "ja"
},
"name": "花澄",
"occupation": "科学者"
}The example above would associate the ja language
code with the two strings 花澄 and 科学者.
Languages codes are defined in [BCP47]. The default language applies to all
string values that are not type coerced.
To clear the default language for a subtree, @language can
be set to null in a local context as follows:
Example 32: Clearing default language
{
"@context": {
...
"@language": "ja"
},
"name": "花澄",
"details": {
"@context": {
"@language": null
},
"occupation": "Ninja"
}
}Second, it is possible to associate a language with a specific term
using an expanded term definition:
Example 33: Expanded term definition with language
{
"@context": {
...
"ex": "http://example.com/vocab/",
"@language": "ja",
"name": { "@id": "ex:name", "@language": null },
"occupation": { "@id": "ex:occupation" },
"occupation_en": { "@id": "ex:occupation", "@language": "en" },
"occupation_cs": { "@id": "ex:occupation", "@language": "cs" }
},
"name": "Yagyū Muneyoshi",
"occupation": "忍者",
"occupation_en": "Ninja",
"occupation_cs": "Nindža",
...
}The example above would associate 忍者 with the specified default
language code ja, Ninja with the language code
en, and Nindža with the language code cs.
The value of name, Yagyū Muneyoshi wouldn't be
associated with any language code since @language was reset to
null in the expanded term definition.
Just as in the example above, systems often need to express the value of a
property in multiple languages. Typically, such systems also try to ensure that
developers have a programmatically easy way to navigate the data structures for
the language-specific data. In this case, language maps
may be utilized.
Example 34: Language map expressing a property in three languages
{
"@context":
{
...
"occupation": { "@id": "ex:occupation", "@container": "@language" }
},
"name": "Yagyū Muneyoshi",
"occupation":
{
"ja": "忍者",
"en": "Ninja",
"cs": "Nindža"
}
...
}The example above expresses exactly the same information as the previous
example but consolidates all values in a single property. To access the
value in a specific language in a programming language supporting dot-notation
accessors for object properties, a developer may use the
property.language pattern. For example, to access the occupation
in English, a developer would use the following code snippet:
obj.occupation.en.
Third, it is possible to override the default language by using a
value object:
Example 35: Overriding default language using an expanded value
{
"@context": {
...
"@language": "ja"
},
"name": "花澄",
"occupation": {
"@value": "Scientist",
"@language": "en"
}
}This makes it possible to specify a plain string by omitting the
@language tag or setting it to null when expressing
it using a value object:
Example 36: Removing language information using an expanded value
{
"@context": {
...
"@language": "ja"
},
"name": {
"@value": "Frank"
},
"occupation": {
"@value": "Ninja",
"@language": "en"
},
"speciality": "手裏剣"
}6.10 IRI Expansion within a Context
This section is non-normative.
In general, normal IRI expansion rules apply
anywhere an IRI is expected (see section 5.2 IRIs). Within
a context definition, this can mean that terms defined
within the context may also be used within that context as long as
there are no circular dependencies. For example, it is common to use
the xsd namespace when defining typed values:
Example 37: IRI expansion within a context
{
"@context":
{
"xsd": "http://www.w3.org/2001/XMLSchema#",
"name": "http://xmlns.com/foaf/0.1/name",
"age":
{
"@id": "http://xmlns.com/foaf/0.1/age",
"@type": "xsd:integer"
},
"homepage":
{
"@id": "http://xmlns.com/foaf/0.1/homepage",
"@type": "@id"
}
},
...
}In this example, the xsd term is defined
and used as a prefix for the @type coercion
of the age property.
Terms may also be used when defining the IRI of another
term:
Example 38: Using a term to define the IRI of another term within a context
{
"@context":
{
"foaf": "http://xmlns.com/foaf/0.1/",
"xsd": "http://www.w3.org/2001/XMLSchema#",
"name": "foaf:name",
"age":
{
"@id": "foaf:age",
"@type": "xsd:integer"
},
"homepage":
{
"@id": "foaf:homepage",
"@type": "@id"
}
},
...
}Compact IRIs
and IRIs may be used on the left-hand side of a
term definition.
Example 39: Using a compact IRI as a term
{
"@context":
{
"foaf": "http://xmlns.com/foaf/0.1/",
"xsd": "http://www.w3.org/2001/XMLSchema#",
"name": "foaf:name",
"foaf:age":
{
"@type": "xsd:integer"
},
"foaf:homepage":
{
"@type": "@id"
}
},
...
}
In this example, the compact IRI form is used in two different
ways.
In the first approach, foaf:age declares both the
IRI for the term (using short-form) as well as the
@type associated with the term. In the second
approach, only the @type associated with the term is
specified. The full IRI for
foaf:homepage is determined by looking up the foaf
prefix in the
context.
Absolute IRIs may also be used in the key position in a context:
Example 40: Associating context definitions with absolute IRIs
{
"@context":
{
"foaf": "http://xmlns.com/foaf/0.1/",
"xsd": "http://www.w3.org/2001/XMLSchema#",
"name": "foaf:name",
"foaf:age":
{
"@id": "foaf:age",
"@type": "xsd:integer"
},
"http://xmlns.com/foaf/0.1/homepage":
{
"@type": "@id"
}
},
...
}In order for the absolute IRI to match above, the absolute IRI
needs to be used in the JSON-LD document. Also note that foaf:homepage
will not use the { "@type": "@id" } declaration because
foaf:homepage is not the same as http://xmlns.com/foaf/0.1/homepage.
That is, terms are looked up in a context using
direct string comparison before the prefix lookup mechanism is applied.
Note
While it is possible to define a compact IRI, or
an absolute IRI to expand to some other unrelated IRI
(for example, foaf:name expanding to
http://example.org/unrelated#species), such usage is strongly
discouraged.
The only exception for using terms in the context is that
circular definitions are not allowed. That is,
a definition of term1 cannot depend on the
definition of term2 if term2 also depends on
term1. For example, the following context definition
is illegal:
Example 41: Illegal circular definition of terms within a context
{
"@context":
{
"term1": "term2:foo",
"term2": "term1:bar"
},
...
}6.11 Sets and Lists
This section is non-normative.
A JSON-LD author can express multiple values in a compact way by using
arrays. Since graphs do not describe ordering for links
between nodes, arrays in JSON-LD do not provide an ordering of the
contained elements by default. This is exactly the opposite from regular JSON
arrays, which are ordered by default. For example, consider the following
simple document:
Example 42: Multiple values with no inherent order
{
...
"@id": "http://example.org/people#joebob",
"nick": [ "joe", "bob", "JB" ],
...
}The example shown above would result in the following data being generated,
each relating the node to an individual value, with no inherent order:
| Subject |
Property |
Value |
|---|
| http://example.org/people#joebob |
http://xmlns.com/foaf/0.1/nick |
joe |
| http://example.org/people#joebob |
http://xmlns.com/foaf/0.1/nick |
bob |
| http://example.org/people#joebob |
http://xmlns.com/foaf/0.1/nick |
JB |
Multiple values may also be expressed using the expanded form:
Example 43: Using an expanded form to set multiple values
{
"@id": "http://example.org/articles/8",
"dc:title":
[
{
"@value": "Das Kapital",
"@language": "de"
},
{
"@value": "Capital",
"@language": "en"
}
]
}The example shown above would generate the following data, again with
no inherent order:
| Subject |
Property |
Value |
Language |
|---|
| http://example.org/articles/8 |
http://purl.org/dc/terms/title |
Das Kapital |
de |
| http://example.org/articles/8 |
http://purl.org/dc/terms/title |
Capital |
en |
As the notion of ordered collections is rather important in data
modeling, it is useful to have specific language support. In JSON-LD,
a list may be represented using the @list keyword as follows:
Example 44: An ordered collection of values in JSON-LD
{
...
"@id": "http://example.org/people#joebob",
"foaf:nick":
{
"@list": [ "joe", "bob", "jaybee" ]
},
...
}This describes the use of this array as being ordered,
and order is maintained when processing a document. If every use of a given multi-valued
property is a list, this may be abbreviated by setting @container
to @list in the context:
Example 45: Specifying that a collection is ordered in the context
{
"@context":
{
...
"nick":
{
"@id": "http://xmlns.com/foaf/0.1/nick",
"@container": "@list"
}
},
...
"@id": "http://example.org/people#joebob",
"nick": [ "joe", "bob", "jaybee" ],
...
}Note
List of lists are not allowed in this version of JSON-LD.
This decision was made due to the extreme amount of added complexity when
processing lists of lists.
While @list is used to describe ordered lists,
the @set keyword is used to describe unordered sets.
The use of @set in the body of a JSON-LD document
is optimized away when processing the document, as it is just syntactic
sugar. However, @set is helpful when used within the context
of a document.
Values of terms associated with a @set or @list container
are always represented in the form of an array,
even if there is just a single value that would otherwise be optimized to
a non-array form in compact form (see
section 6.18 Compacted Document Form). This makes post-processing of
JSON-LD documents easier as the data is always in array form, even if the
array only contains a single value.
6.13 Named Graphs
This section is non-normative.
At times, it is necessary to make statements about a graph
itself, rather than just a single node. This can be done by
grouping a set of nodes using the @graph
keyword. A developer may also name data expressed using the
@graph keyword by pairing it with an
@id keyword as shown in the following example:
Example 49: Identifying and making statements about a graph
{
"@context": {
"generatedAt": {
"@id": "http://www.w3.org/ns/prov#generatedAtTime",
"@type": "http://www.w3.org/2001/XMLSchema#date"
},
"Person": "http://xmlns.com/foaf/0.1/Person",
"name": "http://xmlns.com/foaf/0.1/name",
"knows": "http://xmlns.com/foaf/0.1/knows"
},
"@id": "http://example.org/graphs/73",
"generatedAt": "2012-04-09",
"@graph":
[
{
"@id": "http://manu.sporny.org/i/public",
"@type": "Person",
"name": "Manu Sporny",
"knows": "http://greggkellogg.net/foaf#me"
},
{
"@id": "http://greggkellogg.net/foaf#me",
"@type": "Person",
"name": "Gregg Kellogg",
"knows": "http://manu.sporny.org/i/public"
}
]
}The example above expresses a named graph that is identified
by the IRI http://example.org/graphs/73. That
graph is composed of the statements about Manu and Gregg. Metadata about
the graph itself is expressed via the generatedAt property,
which specifies when the graph was generated. An alternative view of the
information above is represented in table form below:
| Graph |
Subject |
Property |
Value |
Value Type |
|---|
| |
http://example.org/graphs/73 |
http://www.w3.org/ns/prov#generatedAtTime |
2012-04-09 |
http://www.w3.org/2001/XMLSchema#date |
| http://example.org/graphs/73 |
http://manu.sporny.org/i/public |
http://www.w3.org/2001/XMLSchema#type |
http://xmlns.com/foaf/0.1/Person |
|
| http://example.org/graphs/73 |
http://manu.sporny.org/i/public |
http://xmlns.com/foaf/0.1/name |
Manu Sporny |
|
| http://example.org/graphs/73 |
http://manu.sporny.org/i/public |
http://xmlns.com/foaf/0.1/knows |
http://greggkellogg.net/foaf#me |
|
| http://example.org/graphs/73 |
http://greggkellogg.net/foaf#me |
http://www.w3.org/2001/XMLSchema#type |
http://xmlns.com/foaf/0.1/Person |
|
| http://example.org/graphs/73 |
http://greggkellogg.net/foaf#me |
http://xmlns.com/foaf/0.1/name |
Gregg Kellogg |
|
| http://example.org/graphs/73 |
http://greggkellogg.net/foaf#me |
http://xmlns.com/foaf/0.1/knows |
http://manu.sporny.org/i/public |
|
When a JSON-LD document's top-level structure is an
object that contains no other
properties than @graph and
optionally @context (properties that are not mapped to an
IRI or a keyword are ignored),
@graph is considered to express the otherwise implicit
default graph. This mechanism can be useful when a number
of nodes exist at the document's top level that
share the same context, which is, e.g., the case when a
document is flattened. The
@graph keyword collects such nodes in an array
and allows the use of a shared context.
Example 50: Using @graph to explicitly express the default graph
{
"@context": ...,
"@graph":
[
{
"@id": "http://manu.sporny.org/i/public",
"@type": "foaf:Person",
"name": "Manu Sporny",
"knows": "http://greggkellogg.net/foaf#me"
},
{
"@id": "http://greggkellogg.net/foaf#me",
"@type": "foaf:Person",
"name": "Gregg Kellogg",
"knows": "http://manu.sporny.org/i/public"
}
]
}In this case, embedding doesn't work as each node object
references the other. This is equivalent to using multiple
node objects in array and defining
the @context within each node object:
Example 51: Context needs to be duplicated if @graph is not used
[
{
"@context": ...,
"@id": "http://manu.sporny.org/i/public",
"@type": "foaf:Person",
"name": "Manu Sporny",
"knows": "http://greggkellogg.net/foaf#me"
},
{
"@context": ...,
"@id": "http://greggkellogg.net/foaf#me",
"@type": "foaf:Person",
"name": "Gregg Kellogg",
"knows": "http://manu.sporny.org/i/public"
}
]B. JSON-LD Grammar
This appendix restates the syntactic conventions described in the
previous sections more formally.
A JSON-LD document MUST be a valid JSON document as described
in [RFC4627].
A JSON-LD document MUST be a single node object
or an array whose elements are each
node objects at the top level.
In contrast to JSON, in JSON-LD the keys in objects
MUST be unique.
Note
JSON-LD allows keywords to be aliased
(see section 6.15 Aliasing Keywords for details). Whenever a keyword is
discussed in this grammar, the statements also apply to an alias for
that keyword. For example, if the active context
defines the term id as an alias for @id,
that alias may be legitimately used as a substitution for @id.
Note that keyword aliases are not expanded during context
processing.
B.2 Node Objects
A node object represents zero or more properties of a
node in the graph serialized by the
JSON-LD document. A JSON object is a
node object if it exists outside of a JSON-LD
context and:
- it does not contain the
@value, @list,
or @set keywords, and
- it is not the top-most JSON object in the JSON-LD document
consisting of no other members than
@graph and
@context.
The properties of a node in
a graph may be spread among different
node objects within a document. When
that happens, the keys of the different
node objects need to be merged to create the
properties of the resulting node.
A node object MUST be a JSON object. All keys
which are not IRIs,
compact IRIs, terms
valid in the active context, or one of the following
keywords MUST be ignored when processed:
@context,@id,@graph,@type,@reverse, or@index
If the node object contains the @context
key, its value MUST be null, an absolute IRI,
a relative IRI, a context definition, or
an array composed of any of these.
If the node object contains the @id key,
its value MUST be an absolute IRI, a relative IRI,
or a compact IRI (including
blank node identifiers).
See section 5.3 Node Identifiers,
section 6.3 Compact IRIs, and
section 6.14 Identifying Blank Nodes for further discussion on
@id values.
If the node object contains the @graph
key, its value MUST be
a node object or
an array of zero or more node objects.
If the node object contains an @id keyword,
its value is used as the label of a named graph.
See section 6.13 Named Graphs for further discussion on
@graph values. As a special case, if a JSON object
contains no keys other than @graph and @context, and the
JSON object is the root of the JSON-LD document, the
JSON object is not treated as a node object; this
is used as a way of defining node
definitions that may not form a connected graph. This allows a
context to be defined which is shared by all of the constituent
node objects.
If the node object contains the @type
key, its value MUST be either an absolute IRI, a
relative IRI, a compact IRI
(including blank node identifiers),
a term defined in the active context expanding into an absolute IRI, or
an array of any of these.
See section 5.4 Specifying the Type for further discussion on
@type values.
If the node object contains the @reverse key,
its value MUST be a JSON object containing members representing reverse
properties. Each value of such a reverse property MUST be an absolute IRI,
a relative IRI, a compact IRI, a blank node identifier,
a node object or an array containing a combination of these.
If the node object contains the @index key,
its value MUST be a string. See
section 6.16 Data Indexing for further discussion
on @index values.
Keys in a node object that are not
keywords MAY expand to an absolute IRI
using the active context. The values associated with keys that expand
to an absolute IRI MUST be one of the following:
- string,
- number,
- true,
- false,
- null,
- node object,
- value object,
- list object,
- set object,
- an array of zero or more of the possibilities above,
- a language map, or
- an index map
B.3 Value Objects
A value object is used to explicitly associate a type or a
language with a value to create a typed value or a language-tagged
string.
A value object MUST be a JSON object containing the
@value key. It MAY also contain a @type,
a @language, an @index, or an @context key but MUST NOT contain
both a @type and a @language key at the same time.
A value object MUST NOT contain any other keys that expand to an
absolute IRI or keyword.
The value associated with the @value key MUST be either a
string, a number, true,
false or null.
The value associated with the @type key MUST be a
term, a compact IRI,
an absolute IRI, a relative IRI, or null.
The value associated with the @language key MUST have the
lexical form described in [BCP47], or be null.
The value associated with the @index key MUST be a
string.
See section 6.4 Typed Values and
section 6.9 String Internationalization
for more information on value objects.
B.4 Lists and Sets
A list represents an ordered set of values. A set
represents an unordered set of values. Unless otherwise specified,
arrays are unordered in JSON-LD. As such, the
@set keyword, when used in the body of a JSON-LD document,
represents just syntactic sugar which is optimized away when processing the document.
However, it is very helpful when used within the context of a document. Values
of terms associated with a @set or @list container
will always be represented in the form of an array when a document
is processed—even if there is just a single value that would otherwise be optimized to
a non-array form in compact document form.
This simplifies post-processing of the data as the data is always in a
deterministic form.
A list object MUST be a JSON object that contains no
keys that expand to an absolute IRI or keyword other
than @list, @context, and @index.
A set object MUST be a JSON object that contains no
keys that expand to an absolute IRI or keyword other
than @list, @context, and @index.
Please note that the @index key will be ignored when being processed.
In both cases, the value associated with the keys @list and @set
MUST be one of the following types:
See section 6.11 Sets and Lists for further discussion on sets and lists.
B.5 Language Maps
A language map is used to associate a language with a value in a
way that allows easy programmatic access. A language map may be
used as a term value within a node object if the term is defined
with @container set to @language. The keys of a
language map MUST be strings representing
[BCP47] language codes with and the values MUST be any of the following types:
See section 6.9 String Internationalization for further discussion
on language maps.
B.6 Index Maps
An index map allows keys that have no semantic meaning,
but should be preserved regardless, to be used in JSON-LD documents.
An index map may
be used as a term value within a node object if the
term is defined with @container set to @index.
The values of the members of an index map MUST be one
of the following types:
See section 6.16 Data Indexing for further information on this topic.
B.7 Context Definitions
A context definition defines a local context in a
node object.
A context definition MUST be a JSON object whose
keys MUST either be terms,
compact IRIs, absolute IRIs,
or the keywords @language, @base,
and @vocab.
If the context definition has a @language key,
its value MUST have the lexical form described in [BCP47] or be null.
If the context definition has a @base key,
its value MUST be an absolute IRI, a relative IRI,
or null.
If the context definition has a @vocab key,
its value MUST be a absolute IRI, a compact IRI,
a term, or null.
The value of keys that are not keywords MUST be either an
absolute IRI, a compact IRI, a term,
a blank node identifier, a keyword, null,
or an expanded term definition.
An expanded term definition is used to describe the mapping
between a term and its expanded identifier, as well as other
properties of the value associated with the term when it is
used as key in a node object.
An expanded term definition MUST be a JSON object
composed of zero or more keys from @id, @reverse,
@type, @language or @container. An
expanded term definition SHOULD NOT contain any other keys.
If an expanded term definition has an @reverse member,
it MUST NOT have an @id member at the same time. If an
@container member exists, its value MUST be null,
@set, or @index.
If the term being defined is not a compact IRI or
absolute IRI and the active context does not have an
@vocab mapping, the expanded term definition MUST
include the @id key.
If the expanded term definition contains the @id
keyword, its value MUST be null, an absolute IRI,
a blank node identifier, a compact IRI, a term,
or a keyword.
If the expanded term definition contains the @type
keyword, its value MUST be an absolute IRI, a
compact IRI, a term, null, or the one of the
keywords @id or @vocab.
If the expanded term definition contains the @language keyword,
its value MUST have the lexical form described in [BCP47] or be null.
If the expanded term definition contains the @container
keyword, its value MUST be either @list, @set,
@language, @index, or be null. If the value
is @language, when the term is used outside of the
@context, the associated value MUST be a language map.
If the value is @index, when the term is used outside of
the @context, the associated value MUST be an
index map.
Terms MUST NOT be used in a circular manner. That is,
the definition of a term cannot depend on the definition of another term if that other
term also depends on the first term.
See section 5.1 The Context for further discussion on contexts.
F. Acknowledgements
This section is non-normative.
The authors would like to extend a deep appreciation and the most sincere
thanks to Mark Birbeck, who contributed foundational concepts
to JSON-LD via his work on RDFj. JSON-LD uses a number of core concepts
introduced in RDFj, such as the context as a mechanism to provide an
environment for interpreting JSON data. Mark had also been very involved in
the work on RDFa as well. RDFj built upon that work. JSON-LD exists
because of the work and ideas he started nearly a decade ago in 2004.
A large amount of thanks goes out to the JSON-LD Community Group
participants who worked through many of the technical issues on the mailing
list and the weekly telecons - of special mention are François Daoust,
Stéphane Corlosquet, Lin Clark, and Zdenko 'Denny' Vrandečić.
The work of David I. Lehn and Mike Johnson are appreciated for
reviewing, and performing several early implementations
of the specification. Thanks also to Ian Davis for this work on RDF/JSON.
Thanks to the following individuals, in order of their first name, for
their input on the specification: Adrian Walker, Alexandre Passant,
Andy Seaborne, Ben Adida, Blaine Cook, Bradley Allen, Brian Peterson,
Bryan Thompson, Conal Tuohy, Dan Brickley, Danny Ayers, Daniel Leja,
Dave Reynolds, David Booth, David I. Lehn, David Wood, Dean Landolt,
Ed Summers, elf Pavlik,
Eric Prud'hommeaux, Erik Wilde, Fabian Christ, Jon A. Frost, Gavin Carothers,
Glenn McDonald, Guus Schreiber, Henri Bergius, Jose María Alvarez Rodríguez,
Ivan Herman, Jack Moffitt, Josh Mandel, KANZAKI Masahide, Kingsley Idehen,
Kuno Woudt, Larry Garfield, Mark Baker, Mark MacGillivray, Marko Rodriguez,
Marios Meimaris,
Melvin Carvalho, Nathan Rixham, Olivier Grisel, Paolo Ciccarese, Pat Hayes,
Patrick Logan, Paul Kuykendall, Pelle Braendgaard,
Peter Patel-Schneider, Peter Williams, Pierre-Antoine Champin,
Richard Cyganiak, Roy T. Fielding, Sandro Hawke, Srecko Joksimovic,
Stephane Fellah, Steve Harris, Ted Thibodeau Jr., Thomas Steiner, Tim Bray,
Tom Morris, Tristan King, Sergio Fernández, Werner Wilms, and William Waites.
