JSON 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 an experimental work in progress.

Introduction

JSON, as specified in [[!RFC4627]], is a simple language for representing data on the Web. Linked Data is a technique for creating a network of inter-connected data across different Web documents and Web sites. A thing in this data network is typically identified using an IRI (Internationalized Resource Identifier), which is typically dereference-able, and thus may be used to find more information about the thing. The IRI allows a software program to start at one thing and follow links to other things in order to learn more about all of the things described on the Web.

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 inter-operable 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 one 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. Finally, the format is intended to be easy to parse, efficient to generate, and only requires a very small memory footprint in order to operate.

How to Read this Document

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 programming interfaces for the JSON-LD Syntax. The specification that describes the programming interfaces for JSON-LD documents is the JSON-LD Application Programming Interface [[JSON-LD-API]].

To understand the basics in this specification you must first be familiar with JSON, which is detailed in [[!RFC4627]].

JSON [[RFC4627]] defines several terms which are used throughout this document:

JSON object
An object structure is represented as a pair of curly brackets surrounding zero or more name-value pairs. A name is a string. A single colon comes after each name, separating the name from the value. A single comma separates a value from a following name. The names within an object SHOULD be unique.
array
In JSON, an array is an ordered collection of values. An array is represented as square brackets surrounding zero or more values that are separated by commas. While JSON-LD uses the same array representation as JSON, the collection is unordered by default. While order is preserved in regular JSON arrays, it is not in regular JSON-LD arrays unless specific markup is provided (see Sets and Lists).
string
A string is a sequence of zero or more Unicode (UTF-8) characters, wrapped in double quotes, using backslash escapes (if necessary). A character is represented as a single character string.
number
A number is is similar to that used in most programming languages, except that the octal and hexadecimal formats are not used and that leading zeros are not allowed.
true and false
Values that are used to express one of two possible boolean states.
null
Unless otherwise specified, a JSON-LD processor MUST act as if a key-value pair in the body of a JSON-LD document was never declared when the value equals null. If @value, @list, or @set is set to null in expanded form, then the entire JSON object is ignored. If @context is set to null, the active context is reset and when used within a context, it removes any definition associated with the key, unless otherwise specified.
subject definition
A JSON object used to represent a subject and one or more properties of that subject. A JSON object is a subject definition if it does not contain they keys @value, @list or @set and it has one or more keys other than @id.
subject reference
A JSON object used to reference a subject having only the @id key.

Syntax Tokens and Keywords

JSON-LD specifies a number of syntax tokens and keywords that are a core part of the language:

@context
Used to define the short-hand names that are used throughout a JSON-LD document. These short-hand names are called terms and help developers to express specific identifiers in a compact manner. The @context keyword is described in detail in the section titled The Context.
@graph
Used to explicitly express a linked data graph.
@id
Used to uniquely identify things that are being described in the document. This keyword is described in the section titled Identifying the Subject.
@value
Used to specify the data that is associated with a particular property in the graph. This keyword is described in the sections titled String Internationalization and Typed Values.
@language
Used to specify the native language for a particular value or the default language of a JSON-LD document. This keyword is described in the section titled String Internationalization.
@type
Used to set the data type of a subject or typed value. This keyword is described in the section titled Typed Values.
@container
Used to set the container of a particular value. This keyword is described in the section titled Sets and Lists.
@list
Used to express an ordered set of data. This keyword is described in the section titled Sets and Lists.
@set
Used to express an unordered set of data. This keyword is described in the section titled Sets and Lists.
:
The separator for JSON keys and values that use compact IRIs.

For the avoidance of doubt, all keys, keywords, and values in JSON-LD are case-sensitive.

Contributing

There are a number of ways that one may participate in the development of this specification:

Design Goals and Rationale

A number of design goals were established before the creation of this markup language:

Simplicity
No extra processors or software libraries should be necessary to use JSON-LD in its most basic form. The language will provide developers with a very easy learning curve. Developers need only know JSON and two keywords (@context and @id) to use the basic functionality in JSON-LD.
Compatibility
The JSON-LD markup must be 100% compatible with JSON. This ensures that all of the standard JSON libraries work seamlessly with JSON-LD documents.
Expressiveness
The syntax must be able to express directed graphs, which have been proven to be able to express almost every real world data model.
Terseness
The JSON-LD syntax must be very terse and human readable, requiring as little effort as possible from the developer.
Zero Edits, most of the time
JSON-LD must provide a mechanism that allows developers to specify context in a way that is out-of-band. This allows organizations that have already deployed large JSON-based infrastructure to add meaning to their JSON documents in a way that is not disruptive to their day-to-day operations and is transparent to their current customers. At times, mapping JSON to a graph representation can become difficult. In these instances, rather than having JSON-LD support an esoteric use case, we chose not to support the use case and support a simplified syntax instead. So, while Zero Edits is a goal, it is not always possible without adding great complexity to the language.
One-pass Processing
JSON-LD supports one-pass processing, which results in a very small memory footprint when processing documents. For example, to expand a JSON-LD document from a compacted form, only one pass is required over the data.

Basic Concepts

JSON-LD is designed to ensure that Linked Data concepts can be marked up in a way that is simple to understand and create by Web authors. 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.

Linking Data

The following definition for Linked Data is the one that will be used for this specification.

  1. Linked Data is a set of documents, each containing a representation of a linked data graph.
  2. A linked data graph is an unordered labeled directed graph, where nodes are subjects or objects, and edges are properties.
  3. A subject is any node in a linked data graph with at least one outgoing edge.
  4. A subject SHOULD be labeled with an IRI (an Internationalized Resource Identifier as described in [[!RFC3987]]).
  5. An object is a node in a linked data graph with at least one incoming edge.
  6. An object MAY be labeled with an IRI.
  7. A node MAY be a subject and object at the same time.
  8. A property is an edge of the linked data graph.
  9. A property SHOULD be labeled with an IRI.
  10. An IRI that is a label in a linked data graph SHOULD be dereferencable to a Linked Data document describing the labeled subject, object or property.
  11. A value is an object with a label that is not an IRI

An illustration of a linked data graph would probably help here.

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.

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:

The Context

In JSON-LD, a context is used to map terms, i.e., properties with associated values in an JSON document, to IRIs. A term is a short word that expands to an IRI. Terms MAY be defined as any valid JSON string other than a JSON-LD keyword. To avoid forward-compatibility issues, terms starting with an @ character SHOULD NOT be used as they might be used as keywords in future versions of JSON-LD. Furthermore, the use of empty terms ("") is discouraged as not all programming languages are able to handle empty property names.

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 vocabulary terms. Furthermore, developers, and machines, are able to use this IRI (by plugging it directly into a web browser, for instance) to go to the term and get a definition of what the term means. This mechanism is analogousto the way 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. 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 either; 1) a simple string with the lexical form of an absolute IRI or 2) compact IRI, or 3) an JSON object containing an @id, @type, @language, or @container keyword (all other keywords are ignored by a JSON-LD processor).

These Linked Data terms are typically collected in a context document that would look something like this:

Assuming that this context document can be retrieved at http://json-ld.org/contexts/person, it can be referenced from a JSON-LD document by adding a single line. The JSON markup shown in the previous section could be changed as follows:

The additions above transform the previous JSON document into a JSON document with added semantics because the @context specifies how the name, homepage, and depiction 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 interoperate on a site-by-site basis. Contexts may also contain type 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.

Contexts MAY be used at any time a JSON object is defined. A JSON object MAY specify multiple contexts, using an array, which is processed in order. This is useful when an author would like to use an existing context and add application-specific terms to the existing context. Duplicate context terms MUST be overridden using a last-defined-overrides mechanism.

If a term is re-defined within a context, all previous rules associated with the previous definition are removed. A term defined in a previous context MUST be removed, if it is re-defined to null.

The set of contexts defined within a specific JSON object are referred to as local contexts. Setting the context to null effectively sets the local context to its initial state. 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:

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.

From JSON to JSON-LD

If a set of terms such as, name, homepage, and depiction, 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:

Doing this allows JSON to be unambiguously machine-readable without requiring developers to drastically change their workflow.

The example above does not use the @id keyword to set the subject of the node being described above. This type of node is called an unlabeled node and is considered to be a weaker form of Linked Data. It is advised that all nodes described in JSON-LD are given unique identifiers via the @id keyword unless the data is not intended to be linked to from other data sets.

A JSON object used to define property values is called a subject definition. Subject definitions do not require a @id, in which case they are considered to be an unlabeled node.

IRIs

Expressing IRIs are fundamental to Linked Data as that is how most subjects, all properties and many objects are identified. IRIs can be expressed in a variety of different ways in JSON-LD.

  1. Except within a context definition, terms in the key position in a JSON object that have a mapping to an absolute IRI or another term in the active context are expanded to an IRI by JSON-LD processors.
  2. An IRI is generated for the string value specified using @id or @type.
  3. An IRI is generated for the string value of any key for which there are coercion rules in effect that identify the value as an @id.

IRIs may be represented as an absolute IRI, a relative IRI, a term, or a compact IRI.

An absolute IRI is defined in [[!RFC3987]] containing a scheme along with path and optional query and fragment segments. A relative IRI is an IRI that is relative some other absolute IRI. In JSON-LD all relative IRIs are resolved relative to the base IRI associated with the document (typically, the directory that contains the document or the document itself).

IRIs can be expressed directly in the key position like so:

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 because it contains a colon (:) delimiting a valid IRI scheme.

Term expansion occurs for IRIs if the value matches a term defined within the active context:

Terms are case sensitive, and MUST be matched using a case-sensitive comparison.

JSON keys that do not expand to an absolute IRI are ignored, or removed in some cases, by the [[JSON-LD-API]]. However, JSON keys that do not include a mapping in the context are still considered valid expressions in JSON-LD documents - the keys just don't have any machine-readable, semantic meaning.

Prefixes are expanded when the form of the value is a compact IRI represented as a prefix:suffix combination, and the prefix matches a term defined within the active context:

foaf:name above will automatically expand out to the IRI http://xmlns.com/foaf/0.1/name. See Compact IRIs for more details.

An IRI is generated when a JSON object is used in the value position that contains an @id keyword:

Specifying a JSON object with an @id key is used to identify that object using an IRI. This facility MAY also be used to link to another JSON object using a mechanism called embedding, which is covered in the section titled Embedding.

If type coercion rules are specified in the @context for a particular term or property IRI, an IRI is generated:

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 processed by a JSON-LD Processor.

Identifying the Subject

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 truly 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.

The example above would set the subject to the IRI http://example.org/people#joebob.

A JSON object used to define property values is called a subject definition. Subject definitions do not require a @id, in which case they are considered to be an unlabeled node.

To ensure the best possible performance, it is a best practice to put the @id keyword 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 start generating triples. Not specifying the @id keyword first creates a memory and complexity burden for one-pass processors.

Specifying the Type

The type of a particular subject can be specified using the @type keyword. Specifying the type in this way will generate a triple of the form (subject, type, type-IRI). To be considered Linked Data, types MUST be uniquely identified by an IRI.

String Internationalization

In different scenarios it is important to annotate a string with its language. In JSON-LD this is possible in a variety of ways. Firstly, it is possible to define a default language for a JSON-LD document by setting the @language key in the @context or in a term definition:

The example above would associate the ja language code with the two strings 花澄 and 科学者. Languages MUST be expressed in [[!BCP47]] format.

It is possible to override the default language by using the expanded form of a value:

It is also possible to override the default language or specify a plain value by omitting the @language tag or setting it to null when expressing the expanded value:

Please note that language associations MUST only be applied to plain literal strings. That is, typed values or values that are subject to type coercion won't be language tagged.

To clear the default language for a subtree, @language can be set to null in a local context as follows:

JSON-LD allows to associate language information with terms. See Expanded Term Definition for more details.

Sets and Lists

A JSON-LD author can express multiple values in a compact way by using arrays. But, because graphs do not describe ordering for links between nodes, in contrast to plain JSON, arrays in JSON-LD do not provide an ordering of the listed objects by default. For example, consider the following simple document:

The markup shown above would result in three triples being generated, each relating the subject to an individual object, with no inherent order:

Multiple values may also be expressed using the expanded object form:

The markup shown above would generate the following triples, again 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:

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:

List of lists are not allowed in this version of JSON-LD. If a list of lists is detected, a JSON-LD processor will throw an exception.

Similarly to @list, there exists the keyword @set to describe unordered sets. While its use in the body of a JSON-LD document represents just syntactic sugar that MUST be optimized away when processing the document, it is very 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 a compacted document. This makes post-processing of the data easier as the data is in a deterministic form.

The use of @container in the body of a JSON-LD document, i.e., outside @context is ignored by JSON-LD processors.

Advanced Concepts

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.

Typed Values

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:

  1. By utilizing the @type keyword when defining a term within a @context section.
  2. By utilizing the expanded form for specifying objects.
  3. By using a native JSON type.

The first example uses the @type keyword to express a typed value:

The second example uses the expanded form for specifying objects:

Both examples above would generate an object with the value of 2010-05-29T14:17:39+02:00 and the type of http://www.w3.org/2001/XMLSchema#dateTime.

The third example uses a built-in native JSON type, a number, to express a type:

The example above is really just a shorthand for the following:

The @type keyword is also used to associate a type with a subject. Although the same keyword is used in both places, the concept of an object type and a value type are 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.

Compact IRIs

Terms in Linked Data documents may draw from a number of different 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 are used across an application. This is a concern for at least three reasons: the first is the cognitive load on the developer of remembering all of the terms, the second is the serialized size of the context if it is specified inline, the third is future-proofing embedded application contexts that may not be easy to change after they are deployed. In order to address these issues, the concept of a compact IRI is introduced.

A compact IRI is a way of expressing an IRI using a prefix and suffix separated by a colon (:) which is similar to the CURIE Syntax in [[RDFA-CORE]]. 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 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.

Terms are interpreted as compact IRIs if they contain at least one colon and the first colon is not followed by two slashes (//, as in http://example.com). To generate the full IRI, 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 interpreted as an absolute IRI. If the prefix is an underscore (_), the IRI remains unchanged. This effectively means that every term containing a colon will be interpreted by a JSON-LD processor as an IRI.

Consider the following example:

In this example, two different vocabularies are referred to using prefixes. Those prefixes are then used as type and property values using the compact IRI prefix:suffix notation.

It's also possible to use compact IRIs within the context as shown in the following example:

External Contexts

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 MUST 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:

Authors may also import multiple contexts or a combination of external and local contexts by specifying a list of contexts:

Each context in a list will be evaluated in-order. Duplicate mappings among 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:

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 is served as well to explain the correct usage of the JSON-LD context document.

Referencing Contexts from JSON Documents

Ordinary JSON documents can be transformed into JSON-LD documents by referencing to an external JSON-LD context in an HTTP Link Header. Doing this allows JSON to be unambiguously machine-readable without requiring developers to drastically change their workflow 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 describedby link relation. The referenced document MUST have a top-level JSON object. The @context subtree within that object is added to the top-level object of the referencing document. If an array is at the top-level of the referencing document and its items are 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.

The following example demonstrates the use of an external context with an ordinary JSON document:

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.

Expanded Term Definition

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 also Type Coercion and Sets and Lists).

Instead of using a string representation of an IRI, the IRI MAY be specified using an object having an @id key. The value of the @id key MUST be either a term, a compact IRI, or an absolute IRI.

This allows additional information to be associated with the term. This MAY be used for Type Coercion, Sets and Lists), or to associate language information with a term as shown in the following example:

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.

Expanded terms MAY also be defined using compact IRIs or absolute IRIs as keys. If the definition does not include an @id key, the expanded IRI is determined by performing expansion of the key within the current active context. This mechanism is mainly used to associate type or language information with a compact IRI or an absolute IRI.

Although it is possible to define a compact IRI or IRI to expand to some other IRI, such usage is strongly discouraged.

Type Coercion

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 type IRIs and MUST take the form of term, compact IRI, 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.

Terms or compact IRIs 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 typed values, IRIs and lists.

The example above would generate the following Turtle:

Terms may also be defined using absolute IRIs or compact IRIs. This allows coercion rules to by applied to keys which are not represented as a simple term. For example:

In this case, the @id definition is optional, but if it does exist, the compact IRI or IRI is treated as a term so that the actual definition of a prefix becomes unnecessary.

Keys in the context are treated as terms for the purpose of expansion and value coercion. This allows multiple representations for the same expanded IRI, which may be useful for establishing different type coercion rules. It also allows a compact IRI (or even an absolute IRI) to be defined as something else entirely, but this usage is discouraged.

Type coercion is performed using the unexpanded value of the key, which MUST exactly match a definition in the active context.

IRI Expansion Within a Context

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 values:

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:

Not only terms, but also compact IRIs and IRIs MAY be used on the left-hand side of a definition.

In this example, the compact IRI form is used in two different ways. The first way, as shown with foaf:age declares both the IRI for the term (using short-form) as well as the @type associated with the term. The second way, only declares the @type associated with the term. In the second case, the JSON-LD processor will still derive the full IRI by looking up the foaf prefix in the context for foaf:homepage.

Absolute IRIs MAY also be used on the left-hand side of a context:

In order for the absolute IRI to match above, the absolute IRI MUST also 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, a JSON-LD processor will use direct string comparison when looking up terms in a context before it applies the prefix lookup mechanism.

The only exception for using terms in the context is that they MUST NOT be used in a circular manner. That is, a definition of term-1 MUST NOT depend on the definition of term-2 if term-2 also depends on term-1. For example, the following context definition is illegal:

Embedding

Object embedding 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 two subjects related by a property from the first subject:

An object definition, like the one used above, MAY be used as a JSON value at any point in JSON-LD.

Named Graphs

The @graph keyword is used to identify a set of JSON-LD object definitions that may not be directly related through a property, or where embedding is not appropriate. For example:

In this case, embedding doesn't work as each JSON-LD object references the other. Using the @graph keyword allows multiple resources to be defined within an array, and allows the use of a shared context. This is equivalent to using multiple JSON object definitions in array and defining the @context within each object:

The @graph keyword takes on additional meaning when it is used along with other properties, or is used within an embedded JSON-LD object. In this case, the set of subject definitions, or subject references contained within a @graph is given a name, based on the label of the JSON-LD object containing a @graph property, either an IRI, or an unlabeled node. This allows statements to be made about an entire linked data graph, rather than just a single JSON-LD object.

This example says that there is a linked data graph identified by http://example.org/linked-data-graph which is composed of the statements about Manu and Gregg and a reference to another IRI, which could make statements about Markus. Additionally, there is information about the graph itself, which indicates a time at which this information as asserted to be true.

Identifying Unlabeled Nodes

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.

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 de-referenceable IRI.

Aliasing Keywords

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.

In the example above, the @id and @type keywords have been given the aliases url and a, respectively.

Expanded Document Form

The JSON-LD API [[JSON-LD-API]] defines an method for expanding a JSON-LD document. Expansion is the process of taking a JSON-LD document and applying a context such that all IRIs, datatypes, and literal values are expanded so that the context is no longer necessary.

For example, assume the following JSON-LD input document:

Running the JSON-LD Expansion algorithm against the JSON-LD input document provided above would result in the following output:

Compact Document Form

The JSON-LD API [[JSON-LD-API]] defines a method for compacting a JSON-LD document. Compaction is the process of taking a JSON-LD document and applying a context such that the most compact form of the document is generated. JSON is typically expressed in a very compact, key-value format. That is, full IRIs are rarely used as keys. At times, a JSON-LD document may be received that is not in its most compact form. JSON-LD, via the API, provides a way to compact a JSON-LD document.

For example, assume the following JSON-LD input document:

Additionally, assume the following developer-supplied JSON-LD context:

Running the JSON-LD Compaction algorithm given the context supplied above against the JSON-LD input document provided above would result in the following output:

The compaction algorithm enables a developer to map any document into an application-specific compacted form by first expanding the document. While the context provided above mapped http://xmlns.com/foaf/0.1/name to name, it could have also mapped it to any arbitrary string provided by the developer.

Markup Examples

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 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]].

Turtle

The following are examples of representing RDF as expressed in [[TURTLE]] into JSON-LD.

Prefix definitions

The JSON-LD context has direct equivalents for the Turtle @prefix declaration:

JSON-LD has no equivalent for the Turtle @base declaration. Authors could, of course, use a prefix definition to resolve relative IRIs:

Embedding

Both Turtle and JSON-LD allow embedding of objects, although Turtle only allows embedding of objects which use unlabeled node identifiers.

Lists

Both JSON-LD and Turtle can represent sequential lists of values.

RDFa

The following example describes three people with their respective names and homepages.

An example JSON-LD implementation using a single context is described below.

Microformats

The following example uses a simple Microformats hCard example to express how the Microformat is represented in JSON-LD.

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/.

Microdata

The microdata example below expresses book information as a microdata Work item.

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.

IANA Considerations

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.

application/ld+json

Type name:
application
Subtype name:
ld+json
Required parameters:
None
Optional parameters:
form
Determines the serialization form for the JSON-LD document. The only valid value at the moment is expanded. If no form is specified in an HTTP request header to an HTTP server, the server MAY choose any form. If no form is specified in an HTTP response, the form MUST NOT be assumed to take any particular form.
Encoding considerations:
The same as the application/json MIME media type.
Security considerations:
Since JSON-LD is intended to be a pure data exchange format for directed graphs, the serialization SHOULD NOT be passed through a code execution mechanism such as JavaScript's 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.
Interoperability considerations:
Not Applicable
Published specification:
The JSON-LD specification.
Applications that use this media type:
Any programming environment that requires the exchange of directed graphs. Implementations of JSON-LD have been created for JavaScript, Python, Ruby, PHP and C++.
Additional information:
Magic number(s):
Not Applicable
File extension(s):
.jsonld
Macintosh file type code(s):
TEXT
Person & email address to contact for further information:
Manu Sporny <msporny@digitalbazaar.com>
Intended usage:
Common
Restrictions on usage:
None
Author(s):
Manu Sporny, Gregg Kellogg, Markus Lanthaler, Dave Longley
Change controller:
W3C

Fragment identifiers used with application/ld+json resources MAY identify a node in the linked data graph expressed in the resource. This idiom, which is also used in RDF [[RDF-CONCEPTS]], gives a simple way to "mint" new, document-local IRIs to label nodes and therefore contributes considerably to the expressive power of JSON-LD.

Acknowledgements

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.