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JSON [RFC4627] has proven to be a highly useful object serialization and messaging format. In an attempt to harmonize the representation of Linked Data in JSON, this specification outlines a common JSON representation format for expressing directed graphs; mixing both Linked Data and non-Linked Data in a single document.
This document is merely a public working draft of a potential specification. It has no official standing of any kind and does not represent the support or consensus of any standards organisation.
This document is an experimental work in progress.
JSON, as specified in [RFC4627], is a simple language for representing data on the Web. Linked Data is a technique for creating a graph of interlinked data across different documents or Web sites. Data entities are described using IRIs, which are typically dereferencable and thus may be used to find more information about an entity, creating a "Web of Knowledge". JSON-LD is intended to be a simple publishing method for expressing not only Linked Data in JSON, but also for adding semantics to existing JSON.
JSON-LD is designed as a lightweight syntax that can be used to express Linked Data. It is primarily intended to be a way to use Linked Data in Javascript and other Web-based programming environments. It is also useful when building interoperable Web services and when storing Linked Data in JSON-based document storage engines. It is practical and designed to be as simple as possible, utilizing the large number of JSON parsers and libraries available today.
The syntax does not necessarily require applications to change their JSON, but allows to easily add meaning by simply adding or referencing a context. The syntax is designed to not disturb already deployed systems running on JSON, but provide a smooth upgrade path from JSON to JSON-LD with added semantics. Finally, the format is intended to be easy to parse, efficient to generate, and to require a very small memory footprint in order to operate.
This document is a detailed specification for a serialization of Linked Data in JSON. The document is primarily intended for the following audiences:
This specification does not describe the processing algorithms and programming interfaces, for those see [JSON-LD-API].
To understand the basics in this specification you must first be familiar with JSON, which is detailed in [RFC4627]. To understand the API and how it is intended to operate in a programming environment, it is useful to have working knowledge of the JavaScript programming language [ECMA-262] and WebIDL [WEBIDL].
JSON [RFC4627] defines several terms which are used throughout this document:
JSON-LD specifies a number of syntax tokens and keywords that are using in all algorithms described in this section:
@context@id@language@type@value:There are a number of ways that one may participate in the development of this specification:
The following section outlines the design goals and rationale behind the JSON-LD markup language.
A number of design considerations were explored during the creation of this markup language:
An Internationalized Resource Identifier (IRI), as described in [RFC3987], is a mechanism for representing unique identifiers on the web. In Linked Data, an IRI is commonly used for expressing a subject, a property or an object.
JSON-LD defines a mechanism to map JSON terms, i.e., keys and values, to IRIs. This does not mean that JSON-LD requires every key or value to be an IRI, but rather ensures that keys and values can be mapped to IRIs if the developer desires to transform their data into Linked Data. There are a few techniques that can ensure that developers will generate good Linked Data for the Web. JSON-LD formalizes those techniques.
We will be using the following JSON markup as the example for the rest of this section:
{
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"avatar": "http://twitter.com/account/profile_image/manusporny"
}
In JSON-LD, a context is used to map terms, i.e., keys with associated values in an JSON document, to IRIs. A term is a short word that may be expanded to an IRI. A term must have the lexical form of NCName (see [XML-NAMES]), or be an empty string.
The Web uses IRIs for unambiguous identification. The
idea is that these terms mean 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 Web Vocabulary terms. For example, the term name may
map directly to the IRI http://xmlns.com/foaf/0.1/name. This allows JSON-LD documents to be constructed
using the common JSON practice of simple name/value pairs while ensuring that the data is useful outside of the
page, API or database in which it resides. The value of a term mapping must be a simple string with the lexical form of an absolute IRI.
These Linked Data terms are typically collected in a context document that would look something like this:
{
"@context": {
"name": "http://xmlns.com/foaf/0.1/name",
"homepage": "http://xmlns.com/foaf/0.1/homepage",
"avatar": "http://xmlns.com/foaf/0.1/avatar"
}
}
This context document can then be used in an JSON-LD document by adding a single line. The JSON markup as shown in the previous section could be changed as follows to link to the context document:
{
"@context": "http://example.org/json-ld-contexts/person",
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"avatar": "http://twitter.com/account/profile_image/manusporny"
}
The additions above transform the previous JSON document into a JSON document
with added semantics because the @context specifies how the
name, homepage, and avatar
terms map to IRIs.
Mapping those keys to IRIs gives the data global context. If two
developers use the same IRI to describe a property, they are more than likely
expressing the same concept. This allows both developers to re-use each others
data without having to agree to how their data will inter-operate on a
site-by-site basis. Contexts may also contain datatype information
for certain terms as well as other processing instructions for
the JSON-LD processor.
Contexts may be specified in-line. This ensures that JSON-LD documents can be processed when a JSON-LD processor does not have access to the Web.
{
"@context": {
"name": "http://xmlns.com/foaf/0.1/name",
"homepage": "http://xmlns.com/foaf/0.1/homepage",
"avatar": "http://xmlns.com/foaf/0.1/avatar"
},
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"avatar": "http://twitter.com/account/profile_image/manusporny"
}
Contexts may be used at any time a JSON object is defined, and a JSON object may specify multiple contexts, to be processed in order, for example to include standard prefix definitions along with an author-specific prefix definition.
The set of contexts defined within a specific JSON Object is termed a local context. The active context refers to the accumulation of local contexts that are in scope at a specific point within the document. The following example specifies an external context and then layers a local context on top of the external context:
{
"@context": [
"http://example.org/json-ld-contexts/person",
{
"pic": "http://xmlns.com/foaf/0.1/avatar"
}
],
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"pic": "http://twitter.com/account/profile_image/manusporny"
}
JSON-LD strives to ensure that developers don't have to change the JSON that is going into and being returned from their Web APIs. This means that developers can also specify a context for JSON data in an out-of-band fashion. This is described later in this document.
JSON-LD uses a special type of machine-readable document called a Web Vocabulary to define terms that are then used to describe concepts and "things" in the world. Typically, these Web Vocabulary documents have prefixes associated with them and contain a number of term declarations. Prefixes are helpful when a developer wants to mix multiple vocabularies together in a context, but does not want to go to the trouble of defining every single term in every single vocabulary. Some Web Vocabularies may have dozens of terms defined. If a developer wants to use 3-4 different vocabularies, the number of terms that would have to be declared in a single context could become quite large. To reduce the number of different terms that must be defined, JSON-LD also allows prefixes to be used to compact IRIs.
For example, the IRI http://xmlns.com/foaf/0.1/
specifies a Web Vocabulary which may be represented using the
foaf prefix. The foaf Web Vocabulary
contains a term called name. If you join the
foaf prefix with the name suffix,
you can build a compact IRI that will expand out into an absolute IRI for the
http://xmlns.com/foaf/0.1/name vocabulary term.
That is, the compact IRI, or short-form, is foaf:name and the
expanded-form is http://xmlns.com/foaf/0.1/name. This vocabulary
term is used to specify a person's name.
Developers, and machines, are able to use this IRI (plugging it directly into a web browser, for instance) to go to the term and get a definition of what the term means. Much like we can use WordNet today to see the definition of words in the English language. Developers and machines need the same sort of definition of terms. IRIs provide a way to ensure that these terms are unambiguous.
The context provides a collection of vocabulary terms and prefixes that can be used to expand JSON keys and values into IRIs.
To ensure the best possible performance, it is a best practice to put the context definition at the top of the JSON-LD document. If it isn't listed first, processors have to save each key-value pair until the context is processed. This creates a memory and complexity burden for one-pass processors.
If a set of terms such as, name, homepage, and avatar, are defined in a context, and that context is used to resolve the names in JSON objects, machines are able to automatically expand the terms to something meaningful and unambiguous, like this:
{
"http://xmlns.com/foaf/0.1/name": "Manu Sporny",
"http://xmlns.com/foaf/0.1/homepage": "http://manu.sporny.org"
"http://rdfs.org/sioc/ns#avatar": "http://twitter.com/account/profile_image/manusporny"
}
Doing this allows JSON to be unambiguously machine-readable without requiring developers to drastically change their workflow.
Please note that this JSON-LD document doesn't define the subject and will thus result in an unlabeled or blank node.
JSON-LD is designed to ensure that Linked Data concepts can be marked up in a way that is simple to understand and author by Web developers. In many cases, regular JSON markup can become Linked Data with the simple addition of a context. As more JSON-LD features are used, more semantics are added to the JSON markup.
Expressing IRIs are fundamental to Linked Data as that is how most subjects and many object are named. IRIs can be expressed in a variety of different ways in JSON-LD.
@id or @type.@id.IRIs may be represented as an absolute IRI, a term, or a prefix:suffix construct.
IRIs can be expressed directly in the key position like so:
{
...
"http://xmlns.com/foaf/0.1/name": "Manu Sporny",
...
}
In the example above, the key
http://xmlns.com/foaf/0.1/name is interpreted as an IRI, as
opposed to being interpreted as a string.
Term expansion occurs for IRIs if the value matches a term defined within the active context:
{
"@context": { "name": "http://xmlns.com/foaf/0.1/name" },
...
"name": "Manu Sporny",
...
}
Prefixes are expanded when the form of the value is prefix:suffix, and the
prefix matches a term defined within the active context:
{
"@context": { "foaf": "http://xmlns.com/foaf/0.1/" },
...
"foaf:name": "Manu Sporny",
...
}
Terms are case sensitive, and must be matched using a case-sensitive comparison.
foaf:name above will automatically expand out to the IRI
http://xmlns.com/foaf/0.1/name. See Prefixes for more details.
An IRI is generated when a value is associated with a key using
the @id keyword:
{
...
"homepage": { "@id": "http://manu.sporny.org" }
...
}
Specifying a JSON Object with an @id key is
used to represent an IRI, but it also is the mechanism by which a subject is defined.
This is an example of chaining in JSON-LD, an issue covered further in Chaining.
If type coercion rules are specified in the @context for
a particular term or property IRI, an IRI is generated:
{
"@context": {
...
"homepage": {
"@id": "http://xmlns.com/foaf/0.1/homepage", "@type": "@id"
}
...
}
...
"homepage": "http://manu.sporny.org/",
...
}
Even though the value http://manu.sporny.org/ is a string,
the type coercion rules will transform the value into an IRI when processed
by a JSON-LD Processor
To be able to externally reference nodes, it is important that each node has an unambiguous identifier. IRIs are a fundamental concept of Linked Data, and nodes should have a de-referencable identifier used to name and locate them. For nodes to be truely linked, de-referencing the identifier should result in a representation of that node. Associating an IRI with a node tells an application that the returned document contains a description of the node requested.
JSON-LD documents may also contain descriptions of other nodes, so it is necessary to be able to uniquely identify each node which may be externally referenced.
A subject
of an object in JSON is declared using the @id key. The subject is the
first piece of information needed by the JSON-LD processor in order to
create the (subject, property, object) tuple, also known as a triple.
{
...
"@id": "http://example.org/people#joebob",
...
}
The example above would set the subject to the IRI
http://example.org/people#joebob.
To ensure the best possible performance, it is a best practice to
put the @id key before other key-value pairs in an object. If
it isn't listed first, processors have to save each key-value pair until
@id is processed before they can create valid triples. This
creates a memory and complexity burden for one-pass processors.
The type of a particular subject can be specified using the
@type key. Specifying the type in this way will generate a
triple of the form (subject, type, type-iri).
To be Linked Data, types must be uniquely identified by an IRI.
{
...
"@id": "http://example.org/people#joebob",
"@type": "http://xmlns.com/foaf/0.1/Person",
...
}
Regular text strings, also referred to as plain literals, are easily expressed using regular JSON strings.
{
...
"name": "Mark Birbeck",
...
}
JSON-LD makes an assumption that strings with associated language encoding information are not very common when used in JavaScript and Web Services. Thus, it takes a little more effort to express strings with associated language information.
{
...
"name":
{
"@value": "花澄",
"@language": "ja"
}
...
}
The example above would generate a plain literal for
花澄 and associate the ja language code with the triple
that is generated. Languages must be expressed in [BCP47] format.
A value with an associated datatype, also known as a typed literal, is indicated by associating a literal with an IRI which indicates the literal's datatype. Typed literals may be expressed in JSON-LD in three ways:
@type keyword when defining a term within a
@context section.The first example uses the @type keyword to express a typed literal:
{
"@context": {
"xsd": "http://www.w3.org/2001/XMLSchema#",
"modified": {
"@id": "http://purl.org/dc/terms/modified",
"@type": "xsd:dateTime"
}
}
...
"modified": "2010-05-29T14:17:39+02:00",
...
}
The second example uses the expanded form for specifying objects:
{
...
"modified": {
"@value": "2010-05-29T14:17:39+02:00",
"@type": "xsd:dateTime"
}
...
}
Both examples above would generate an object with the literal value of
2010-05-29T14:17:39+02:00 and the datatype of
http://www.w3.org/2001/XMLSchema#dateTime.
The third example uses a built-in native JSON type, a number, to express a datatype:
{
...
"@id": "http://example.org/people#joebob",
"age": 31
...
}
The example above is really just a shorthand for the following:
{
...
"@id": "http://example.org/people#joebob",
"age": {
"@value": "31",
"@type": "http://www.w3.org/2001/XMLSchema#integer"
}
...
}
The @type keyword is also used to associate a type with an object. Although the same keyword
is used in both places, the concept of object type and literal datatype are, in fact, different. This is similar
to object-oriented programming languages where both scalar and structured types use the same class inheritance
mechanism, even though scalar types and structured types are inherently different.
A JSON-LD author can express multiple triples in a compact way by using arrays. If a subject has multiple values for the same property, the author may express each property as an array.
In JSON-LD, multiple objects on a property are not ordered. This is because typically graphs are not inherently ordered data structures. To see more on creating ordered collections in JSON-LD, see Lists.
{
...
"@id": "http://example.org/people#joebob",
"nick": [ "joe", "bob", "jaybee" ],
...
}
The markup shown above would generate the following triples:
<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>
"jaybee" .
Multiple literals may also be expressed using the expanded form for objects:
{
"@id": "http://example.org/articles/8",
"dc:title":
[
{"@value": "Das Kapital", "@language": "de"},
{"@value": "Capital", "@language": "en"}
]
}
The markup shown above would generate the following triples:
<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 .
Because graphs do not describe ordering for links between nodes, in contrast to plain JSON, multi-valued properties in JSON-LD do not provide an ordering of the listed objects. For example, consider the following simple document:
{
...
"@id": "http://example.org/people#joebob",
"nick": [ "joe", "bob", "jaybee" ],
...
}
This results in three triples being generated, each relating the subject to an individual object, with no inherent order.
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:
{
...
"@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 through
alternate representations as described in [JSON-LD-API]. If every use of a given multi-valued property is a list, this
may be abbreviated by adding an @type coercion:
{
"@context": {
...
"nick": {
"@id": "http://xmlns.com/foaf/0.1/nick",
"@list": true
}
},
...
"@id": "http://example.org/people#joebob",
"nick": [ "joe", "bob", "jaybee" ],
...
}
List coercion is specified within an expanded term definition using the @list key.
The value of this key, if present, must be true.
This indicates that array values of keys coerced as @list are to be serialized
as a List.
JSON-LD has a number of features that provide functionality above and beyond the core functionality described above. The following sections outline the features that are specific to JSON-LD.
Authors may choose to declare JSON-LD contexts in external documents to promote re-use of contexts as well as reduce the size of JSON-LD documents.
In order to use an external context, an author may specify an IRI
to a valid JSON-LD document. The referenced document must have a
top-level JSON Object. The value of any @context key
within that object is substituted for the IRI within the referencing document
to have the same effect as if the value were specified inline within the
referencing document.
The following example demonstrates the use of an external context:
{
"@context": "http://example.org/json-ld-contexts/person",
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"avatar": "http://twitter.com/account/profile_image/manusporny"
}
Authors may also import multiple contexts or a combination of external and local contexts by specifying a list of contexts:
{
"@context":
[
"http://example.org/json-ld-contexts/person",
{ "foaf": "http://xmlns.com/foaf/0.1/" },
"http://example.org/json-ld-contexts/event"
]
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"avatar": "http://twitter.com/account/profile_image/manusporny"
"celebrates":
{
"@type": "Event",
"description": "International Talk Like a Pirate Day",
"date": "R/2011-09-19"
}
}
Each context in a list will be evaluated in-order. Duplicate mappings within the contexts must be overwritten on a last-defined-overrides basis. The context list must contain either de-referenceable IRIs or JSON Objects that conform to the context syntax as described in this document.
An author may nest contexts within JSON objects, with the more deeply nested contexts overriding the values in previously defined contexts:
{
"@context":
{
"name": "http://example.com/person#name",
"details": "http://example.com/person#details"
},
"name": "Markus",
...
"details":
{
"@context": { "name": "http://example.com/organization#name" },
"name": "Acme, Ltd."
}
}
In the example above, the name prefix is overridden in the
more deeply nested details structure. Note that this is
rarely a good authoring practice and is typically used when the
JSON object has legacy applications using the structure of the object.
External JSON-LD context documents may contain extra information located
outside of the @context key, such as
documentation about the prefixes declared in the document.
When importing a @context value from an external JSON-LD context
document, any extra information contained outside of the
@context value must be discarded. It is
also recommended that a human-readable document encoded in HTML+RDFa
[HTML-RDFA] or other Linked Data compatible format is served as well to
explain the correct usage of the JSON-LD context document.
Within a context definition, terms may be defined using an expanded notation to allow for additional information associated with the term to be specified (see Type Coercion and Lists).
Instead of using a string representation of an IRI, the IRI is specified using an object having
an @id key. The value of this key must be an absolute IRI.
{
"@context": {
"name": {"@id": "http://xmlns.com/foaf/0.1/name"},
"homepage": {"@id": "http://xmlns.com/foaf/0.1/homepage"},
"avatar": {"@id": "http://xmlns.com/foaf/0.1/avatar"}
},
"name": "Manu Sporny",
"homepage": "http://manu.sporny.org/",
"avatar": "http://twitter.com/account/profile_image/manusporny"
}
There is an open issue (#43) on allowing non-terms in the key position to allow coercion to be specified for CURIEs or absolute IRIs.
JSON-LD allows a default value to use as the language for plain literals. It is commonly the case that documents are written using a single language. As described in String Internationalization, a language-tagged literal may be specified as follows:
{
...
"name":
{
"@value": "花澄",
"@language": "ja"
}
...
}
It is also possible to apply a particular language code to all
plain literals by setting the @language key in the
@context:
{
"@context:"
{
"@language": "ja"
},
...
"name": "花澄"
"occupation": "科学者"
...
}
The example above would generate a plain literal for
花澄 and 科学者 and associate the ja language
code with each literal.
It is possible to override the default language by using the expanded form of a literal:
{
"@context:"
{
"@language": "ja"
},
...
"name": "花澄"
"occupation":
{
"@value": "Scientist",
"@language": "en"
}
...
}
It is also possible to override the default language and specify a plain
literal by omitting the @language tag when expressing the
expanded literal value:
{
"@context:"
{
"@language": "ja"
},
...
"name": "花澄"
"occupation":
{
"@value": "Ninja"
}
...
}
Vocabulary terms in Linked Data documents may draw from a number of different Web vocabularies. At times, declaring every single term that a document uses can require the developer to declare tens, if not hundreds of potential vocabulary terms that may be used across an application. This is a concern for at least three reasons; the first is the cognitive load on the developer, the second is the serialized size of the context, the third is future-proofing application contexts. In order to address these issues, the concept of a prefix mechanism is introduced.
A prefix is a compact way of expressing a base
IRI to a Web Vocabulary.
Generally, these prefixes are used by concatenating the prefix and
a suffix separated by a colon (:).
The prefix is a term taken from the active context
and is a short string identifying a particular IRI in a JSON-LD document.
For example, the prefix foaf may be used as a short
hand for the Friend-of-a-Friend Web Vocabulary, which is identified using
the IRI http://xmlns.com/foaf/0.1/. A developer may append any of
the FOAF Vocabulary terms to the end of the prefix to specify a short-hand
version of the absolute IRI for the vocabulary term. For example,
foaf:name would be expanded out to the IRI
http://xmlns.com/foaf/0.1/name. Instead of having to remember
and type out the entire IRI, the developer can instead use the prefix in
their JSON-LD markup.
To generate an IRI out of a prefix:suffix construct,
the value is first split into a prefix and suffix at the first
occurrence of a colon (:). If the active context contains a term
mapping for prefix, an IRI is generated by prepending the mapped
prefix to the (possibly empty) suffix using textual concatenation.
If no prefix mapping is defined, the value is used directly as an IRI. If the
prefix is an underscore (_), the IRI remains unchanged.
The ability to use prefixes reduces the need for developers
to declare every vocabulary term that they intend to use in
the JSON-LD context. This reduces document serialization size because
every vocabulary term need not be declared in the context.
Prefix also
reduce the cognitive load on the developer. It is far easier to
remember foaf:name than it is to remember
http://xmlns.com/foaf/0.1/name. The use of prefixes also
ensures that a context document does not have to be updated in lock-step
with an externally defined Web Vocabulary. Without prefixes, a developer
would need to keep their application context terms in lock-step with an
externally defined Web Vocabulary. Rather, by just declaring the
Web Vocabulary prefix, one can use new terms as they're declared
without having to update the application's JSON-LD context.
Consider the following example:
{
"@context": {
"dc": "http://purl.org/dc/elements/1.1/",
"ex": "http://example.org/vocab#"
},
"@id": "http://example.org/library",
"@type": "ex:Library",
"ex:contains": {
"@id": "http://example.org/library/the-republic",
"@type": "ex:Book",
"dc:creator": "Plato",
"dc:title": "The Republic",
"ex:contains": {
"@id": "http://example.org/library/the-republic#introduction",
"@type": "ex:Chapter",
"dc:description": "An introductory chapter on The Republic.",
"dc:title": "The Introduction"
}
}
}
In this example, two different vocabularies are referred to using
prefixes. Those prefixes are then used as type and property values using
the prefix:suffix notation.
Prefixes, also known as CURIEs, are defined more formally in RDFa Core 1.1, Section 6 "CURIE Syntax Definition" [RDFA-CORE]. JSON-LD does not support the square-bracketed CURIE syntax as the mechanism is not required to disambiguate IRIs in a JSON-LD document like it is in HTML documents.
To be consistent with JSON-LD in general, anywhere an IRI is expected, normal IRI expansion rules
apply (see IRIs). Within a context definition, this can mean that terms defined
within a given 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 literals:
{
"@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:
{
"@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"}
},
...
}
The only exception is that terms must not be used in a circular manner, meaning that the definition of term-1 may not depend on the definition of term-2 if term-2 also depends on term-1. For example, the following context definition is illegal:
{
"@context":
{
"term1": {"@id": "term2:foo"},
"term2": {"@id": "term1:bar"}
},
...
}
Since JSON is capable of expressing typed information such as doubles, integers, and boolean values. As demonstrated below, JSON-LD utilizes that information to create typed literals:
{
...
// The following two values are automatically converted to a type of xsd:double
// and both values are equivalent to each other.
"measure:cups": 5.3,
"measure:cups": 5.3e0,
// The following value is automatically converted to a type of xsd:double as well
"space:astronomicUnits": 6.5e73,
// The following value should never be converted to a language-native type
"measure:stones": { "@value": "4.8", "@type": "xsd:decimal" },
// This value is automatically converted to having a type of xsd:integer
"chem:protons": 12,
// This value is automatically converted to having a type of xsd:boolean
"sensor:active": true,
...
}
When dealing with a number of modern programming languages,
including JavaScript ECMA-262, there is no distinction between
xsd:decimal and xsd:double values. That is,
the number 5.3 and the number
5.3e0 are treated as if they were the same. When converting from
JSON-LD to a language-native format and back, datatype information is lost in a
number of these languages. Thus, one could say that 5.3 is a
xsd:decimal and 5.3e0 is an
xsd:double in JSON-LD, but when both values are
converted to a language-native format the datatype difference between the two
is lost because the machine-level representation will almost always be a
double.
Implementers should be aware of this potential round-tripping issue between
xsd:decimal and xsd:double. Specifically
objects with a datatype of xsd:decimal must not be converted
to a language native type.
JSON-LD supports the coercion of values to particular data types. Type coercion allows someone deploying JSON-LD to coerce the incoming or outgoing types to the proper data type based on a mapping of data type IRIs to property types. Using type coercion, value representation is preserved without requiring the data type to be specified with each usage.
Type coercion is specified within an expanded term definition
using the @type key. The values of this key represent datatype IRIs and must take the form of
term, prefix:suffix, absolute IRI or the keyword @id. Specifying
@id indicates that within the body of a JSON-LD document, string values of keys coerced as
@id are to be interpreted as IRIs, or labeled nodes and are subject to IRI expansion.
Terms or prefixes used as the value of a
@type key may be defined within the same context.
The example below demonstrates how a JSON-LD author can coerce values to plain literals, typed literals, IRIs and lists.
{
"@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"}
},
"name": "John Smith",
"age": "41",
"homepage": "http://example.org/home/"
}
The example above would generate the following turtle:
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> . [ foaf:name "John Smith"; foaf:age "41"^^xsd:integer; foaf:homepage <http://example.org/home> ] .
Object chaining is a JSON-LD feature that allows an author to use the definition of JSON-LD objects as property values. This is a commonly used mechanism for creating a parent-child relationship between two subjects.
The example shows an two subjects related by a property from the first subject:
{
...
"name": "Manu Sporny",
"knows": {
"@type": "Person",
"name": "Gregg Kellogg",
}
...
}
An object definition, like the one used above, may be used as a JSON value at any point in JSON-LD.
At times, it becomes necessary to be able to express information without
being able to specify the subject. Typically, this type of node is called
an unlabeled node or a blank node. In JSON-LD, unlabeled node identifiers are
automatically created if a subject is not specified using the
@id keyword. However, authors may provide identifiers for
unlabeled nodes by using the special _ (underscore)
prefix. This allows to reference the node locally within the
document but not in an external document.
{
...
"@id": "_:foo",
...
}
The example above would set the subject to _:foo, which can
then be used later on in the JSON-LD markup to refer back to the
unlabeled node. This practice, however, is usually frowned upon when
generating Linked Data. If a developer finds that they refer to the unlabeled
node more than once, they should consider naming the node using a resolve-able
IRI.
JSON-LD allows all of the syntax keywords, except for @context,
to be aliased. This feature allows more legacy JSON content to be supported
by JSON-LD. It also allows developers to design domain-specific implementations
using only the JSON-LD context.
{
"@context":
{
"url": "@id",
"a": "@type",
"name": "http://schema.org/name"
},
"url": "http://example.com/about#gregg",
"a": "http://schema.org/Person",
"name": "Gregg Kellogg"
}
In the example above, the @id and @type
keywords have been given the aliases url and
a, respectively.
JSON-LD is a specification for representing Linked Data in JSON. A common
way of working with Linked Data is through RDF, the Resource Description Framework.
RDF can be expressed using JSON-LD by associating JSON-LD concepts such as @id
and @type with the equivalent IRIs in RDF. Further information about
RDF may be found in [RDF-PRIMER].
The JSON-LD markup examples below demonstrate how JSON-LD can be used to express semantic data marked up in other languages such as RDF/XML, Turtle, RDFa, Microformats, and Microdata. These sections are merely provided as proof that JSON-LD is very flexible in what it can express across different Linked Data approaches. Details of transforming JSON-LD into RDF are defined in [JSON-LD-API].
The following are examples of representing RDF as expressed in [TURTLE] into JSON-LD.
The JSON-LD context has direct equivalents for the Turtle @prefix declaration:
@prefix foaf: <http://xmlns.com/foaf/0.1/> . <http://manu.sporny.org/#me> a foaf:Person; foaf:name "Manu Sporny"; foaf:homepage <http://manu.sporny.org/> .
{
"@context": {
"foaf": "http://xmlns.com/foaf/0.1/"
},
"@id": "http://manu.sporny.org/#me",
"@type": "foaf:Person",
"foaf:name": "Manu Sporny",
"foaf:homepage": { "@id": "http://manu.sporny.org/" }
}
JSON-LD has no equivalent for the Turtle @base declaration. Authors could, of course,
use a prefix definition to resolve relative IRIs. For example, an empty prefix could be used
to get a similar effect to @base:
{
"@context": {
"": "http://manu.sporny.org/",
"foaf": "http://xmlns.com/foaf/0.1/"
},
"@id": ":#me",
"@type": "foaf:Person",
"foaf:name": "Manu Sporny",
"foaf:homepage": { "@id": ":" }
}
Both Turtle and JSON-LD allow chaining of objects, although Turtle only allows chaining of objects which use nlank node identifiers.
@base <http://manu.sporny.org/> . @prefix foaf: <http://xmlns.com/foaf/0.1/> . <#me> a foaf:Person; foaf:name "Manu Sporny"; foaf:knows [ a foaf:Person; foaf:name "Gregg Kellogg" ] .
{
"@context": {
"": "http://manu.sporny.org/",
"foaf": "http://xmlns.com/foaf/0.1/"
},
"@id": ":#me",
"@type": "foaf:Person",
"foaf:name": "Manu Sporny",
"foaf:knows": {
"@type": "foaf:Person",
"foaf:name": "Gregg Kellogg"
}
}
Both JSON-LD and Turtle can represent sequential lists of values.
@prefix foaf: <http://xmlns.com/foaf/0.1/> . <http://example.org/people#joebob> a foaf:Person; foaf:name "Joe Bob"; foaf:nick ( "joe" "bob" "jaybee") .
{
"@context": {
"foaf": "http://xmlns.com/foaf/0.1/"
},
"@id": "http://example.org/people#joebob",
"@type": "foaf:Person",
"foaf:name": "Joe Bob",
"foaf:nick": { "@list": [ "joe", "bob", "jaybe" ] }
}
The following example describes three people with their respective names and homepages.
<div prefix="foaf: http://xmlns.com/foaf/0.1/"> <ul> <li typeof="foaf:Person"> <a rel="foaf:homepage" href="http://example.com/bob/" property="foaf:name" >Bob</a> </li> <li typeof="foaf:Person"> <a rel="foaf:homepage" href="http://example.com/eve/" property="foaf:name" >Eve</a> </li> <li typeof="foaf:Person"> <a rel="foaf:homepage" href="http://example.com/manu/" property="foaf:name" >Manu</a> </li> </ul> </div>
An example JSON-LD implementation is described below, however, there are other ways to mark-up this information such that the context is not repeated.
{
"@context": { "foaf": "http://xmlns.com/foaf/0.1/" },
"@id": [
{
"@id": "_:bnode1",
"@type": "foaf:Person",
"foaf:homepage": "http://example.com/bob/",
"foaf:name": "Bob"
},
{
"@id": "_:bnode2",
"@type": "foaf:Person",
"foaf:homepage": "http://example.com/eve/",
"foaf:name": "Eve"
},
{
"@id": "_:bnode3",
"@type": "foaf:Person",
"foaf:homepage": "http://example.com/manu/",
"foaf:name": "Manu"
}
]
}
The following example uses a simple Microformats hCard example to express how the Microformat is represented in JSON-LD.
<div class="vcard"> <a class="url fn" href="http://tantek.com/">Tantek Çelik</a> </div>
The representation of the hCard expresses the Microformat terms in the
context and uses them directly for the url and fn
properties. Also note that the Microformat to JSON-LD processor has
generated the proper URL type for http://tantek.com.
{
"@context":
{
"vcard": "http://microformats.org/profile/hcard#vcard",
"url": {"@id": "http://microformats.org/profile/hcard#url", "@type": "@id"},
"fn": "http://microformats.org/profile/hcard#fn"
},
"@id": "_:bnode1",
"@type": "vcard",
"url": "http://tantek.com/",
"fn": "Tantek Çelik"
}
The microdata example below expresses book information as a microdata Work item.
<dl itemscope
itemtype="http://purl.org/vocab/frbr/core#Work"
itemid="http://purl.oreilly.com/works/45U8QJGZSQKDH8N">
<dt>Title</dt>
<dd><cite itemprop="http://purl.org/dc/terms/title">Just a Geek</cite></dd>
<dt>By</dt>
<dd><span itemprop="http://purl.org/dc/terms/creator">Wil Wheaton</span></dd>
<dt>Format</dt>
<dd itemprop="http://purl.org/vocab/frbr/core#realization"
itemscope
itemtype="http://purl.org/vocab/frbr/core#Expression"
itemid="http://purl.oreilly.com/products/9780596007683.BOOK">
<link itemprop="http://purl.org/dc/terms/type" href="http://purl.oreilly.com/product-types/BOOK">
Print
</dd>
<dd itemprop="http://purl.org/vocab/frbr/core#realization"
itemscope
itemtype="http://purl.org/vocab/frbr/core#Expression"
itemid="http://purl.oreilly.com/products/9780596802189.EBOOK">
<link itemprop="http://purl.org/dc/terms/type" href="http://purl.oreilly.com/product-types/EBOOK">
Ebook
</dd>
</dl>
Note that the JSON-LD representation of the microdata information stays true to the desires of the microdata community to avoid contexts and instead refer to items by their full IRI.
[
{
"@id": "http://purl.oreilly.com/works/45U8QJGZSQKDH8N",
"@type": "http://purl.org/vocab/frbr/core#Work",
"http://purl.org/dc/terms/title": "Just a Geek",
"http://purl.org/dc/terms/creator": "Whil Wheaton",
"http://purl.org/vocab/frbr/core#realization":
[ "http://purl.oreilly.com/products/9780596007683.BOOK", "http://purl.oreilly.com/products/9780596802189.EBOOK" ]
},
{
"@id": "http://purl.oreilly.com/products/9780596007683.BOOK",
"@type": "http://purl.org/vocab/frbr/core#Expression",
"http://purl.org/dc/terms/type": "http://purl.oreilly.com/product-types/BOOK"
},
{
"@id": "http://purl.oreilly.com/products/9780596802189.EBOOK",
"@type": "http://purl.org/vocab/frbr/core#Expression",
"http://purl.org/dc/terms/type": "http://purl.oreilly.com/product-types/EBOOK"
}
]
The following definition for Linked Data is the one that will be used for this specification.
Note that the definition for Linked Data above is silent on the topic of unlabeled nodes. Unlabeled nodes are not considered Linked Data. However, this specification allows for the expression of unlabled nodes, as most graph-based data sets on the Web contain a number of associated nodes that are not named and thus are not directly de-referenceable.
Developers would also benefit by allowing other vocabularies to be used automatically with their JSON API. There are over 200 Web Vocabulary Documents that are available for use on the Web today. Some of these vocabularies are:
You can use these vocabularies in combination, like so:
{
"@type": "foaf:Person",
"foaf:name": "Manu Sporny",
"foaf:homepage": "http://manu.sporny.org/",
"sioc:avatar": "http://twitter.com/account/profile_image/manusporny"
}
Developers can also specify their own Vocabulary documents by modifying the
active context in-line using the @context keyword,
like so:
{
"@context": { "myvocab": "http://example.org/myvocab#" },
"@type": "foaf:Person",
"foaf:name": "Manu Sporny",
"foaf:homepage": "http://manu.sporny.org/",
"sioc:avatar": "http://twitter.com/account/profile_image/manusporny",
"myvocab:personality": "friendly"
}
The @context keyword is used to change how the JSON-LD
processor evaluates key-value pairs. In this case, it was used to
map one string ('myvocab') to another string, which is interpreted as
a IRI. In the example above, the myvocab string is replaced
with "http://example.org/myvocab#" when it
is detected. In the example above, "myvocab:personality" would
expand to "http://example.org/myvocab#personality".
This mechanism is a short-hand, called a Web Vocabulary prefix, and provides developers an unambiguous way to map any JSON value to RDF.
This section is included merely for standards community review and will be submitted to the Internet Engineering Steering Group if this specification becomes a W3C Recommendation.
formcompacted, expanded,
and normalized. If no form is specified in an HTTP
request header to a responding application, such as a Web server,
the application may choose any form. If no form is specified for a
receiving application, the form must not be assumed to take any
particular form.application/json MIME media type.eval()
function. It is recommended that a conforming parser does not attempt to
directly evaluate the JSON-LD serialization and instead purely parse the
input into a language-native data structure. The editors would like to thank Mark Birbeck, who provided a great deal of the initial push behind the JSON-LD work via his work on RDFj, Dave Longley, Dave Lehn and Mike Johnson who reviewed, provided feedback, and performed several implementations of the specification, and Ian Davis, who created RDF/JSON. Thanks also to Nathan Rixham, Bradley P. Allen, Kingsley Idehen, Glenn McDonald, Alexandre Passant, Danny Ayers, Ted Thibodeau Jr., Olivier Grisel, Niklas Lindström, Markus Lanthaler, and Richard Cyganiak for their input on the specification.