In recent years, [[YAML]] has emerged as a more concise format to represent information that had previously been serialized as JSON, including Linked Data. This document defines how to serialize linked data in YAML. Moreover, it registers the `application/ld+yaml` media type.
This document has been developed by the JSON-LD Community Group.
[[JSON-LD11]] is a JSON-based format to serialize Linked Data. In recent years, [[YAML]] has emerged as a more concise format to represent information that had previously been serialized as [[JSON]], including API specifications, data schemas, and Linked Data.
This document defines YAML-LD as a set of conventions on top of YAML which specify how to serialize Linked Data [[LINKED-DATA]] as [[YAML]] based on JSON-LD syntax, semantics, and APIs.
Since YAML is more expressive than JSON, both in the available data types and in the document structure (see [[I-D.ietf-httpapi-yaml-mediatypes]]), this document identifies constraints on YAML such that any YAML-LD document can be represented in JSON-LD.
To take better advantage of the broader expressivity of YAML, this document also defines a means of extending the JSON-LD internal representation to allow a more complete expression of native data types within YAML-LD, and allows use of the complete [[[JSON-LD11-API]]] [[JSON-LD11-API]] Application Programming Interface to manipulate extended YAML-LD documents.
To understand the basics of this specification, one must be familiar with the following:
This document is intended primarily for two main audiences, comprised of software developers and IT and non-IT professionals, as described below.
Among related technologies, JSON-LD familiarity would be required to build most YAML-LD capable applications, while RDF familiarity is only required when it is desired to convert YAML-LD to RDF graphs, or vice versa.
IT and non-IT professionals who want to read and/or produce Linked Data documents in YAML-LD format. Such documents can be —
For these users, familiarity with JSON-LD is not required, but understanding of Linked Data principles might be beneficial. The YAML-LD Primer is meant as a gentle introduction to the essential information necessary to use YAML-LD in practice.
This document uses the following terms as defined in external specifications and defines terms specific to JSON-LD.
A YAML-LD stream is a YAML stream of YAML-LD documents.
For interoperability considerations on YAML streams, see the relevant section in YAML Media Type.
A YAML-LD document is any YAML document from which a conversion to [[JSON]] produces a valid JSON-LD document which can be interpreted as [[LINKED-DATA]].
The term media type is imported from [[RFC6838]].
The term JSON is imported from [[JSON]]
The term JSON document represents a serialization of a resource conforming to the [[JSON]] grammar.
The terms JSON-LD document, and value object are imported from [[JSON-LD11]].
The terms internal representation, and documentLoader are imported from [[JSON-LD11-API]].
The terms array, boolean, map, map entry, null, and string are imported from [[INFRA]].
The term number is imported from [[ECMASCRIPT]].
The terms YAML, YAML representation graph, YAML stream, YAML directive, TAG directive, YAML document, YAML sequence (either block sequence or flow sequence), YAML mapping (either block mapping or flow mapping), node, scalar, node anchor, node tags, and alias node, are imported from [[YAML]].
The term content negotiation is imported from [[RFC9110]].
The terms RDF literal, language-tagged string, datatype IRI, and language tag are imported from [[RDF11-CONCEPTS]].
The terms fragment and fragment identifier in this document are to be interpreted as in [[URI]].
The term Linked Data is imported from [[LINKED-DATA]].
This specification makes use of the following namespace prefixes:
| Prefix | IRI |
|---|---|
| ex | http://example.org/ |
| i18n | https://www.w3.org/ns/i18n# |
| rdf | http://www.w3.org/1999/02/22-rdf-syntax-ns# |
| xsd | http://www.w3.org/2001/XMLSchema# |
These are used within this document as part of a compact IRI
as a shorthand for the resulting IRI, such as dcterms:title
used to represent http://purl.org/dc/terms/title.
A YAML-LD document complies with this specification if it follows the normative statements from this specification and can be interpreted as [[JSON-LD11]] after transformation into the internal representation. For convenience, normative statements for documents are often phrased as statements on the properties of the document.
A YAML-LD document complies with the YAML-LD JSON profile of this specification if it follows the normative statements from this specification and can be transformed into a JSON-LD representation, then back to a conforming YAML-LD document, without loss of semantic information.
A YAML-LD document complies with the YAML-LD extended profile of this specification if it follows the normative statements from this specification and can be transformed into the JSON-LD extended internal representation, then back to a conforming YAML-LD document, without loss of semantic information.
To ease writing and collaborating on [[JSON-LD11]] documents, it is becoming common practice to serialize them as [[YAML]]. This requires a registered media type, not only to enable content negotiation of linked data documents in YAML, but also to define the expected behavior of applications that process these documents, including fragment identifiers and interoperability considerations.
This is because YAML is more flexible than [[JSON]]:
The first goal of this specification is to allow a JSON-LD document to be processed and serialized into YAML, and then back into JSON-LD, without losing any semantic information.
This is always possible, because a YAML representation graph can always represent a tree, because JSON data types are a subset of YAML's, and because JSON encoding is UTF-8.
The subset of YAML-LD which supports serialisation of JSON-LD documents is defined as the YAML-LD JSON profile of YAML-LD.
Example: The JSON-LD document below
Can be serialized as YAML as follows. Note that entries starting with `@` need to be enclosed in quotes (as shown in this example), because `@` is a reserved character in YAML.
This document is based on YAML 1.2.2, but YAML-LD is not tied to a specific version of YAML. Implementers concerned about features related to a specific YAML version can specify it in documents using the `%YAML` directive (see ).
As [[YAML]] has well-defined representation requirements, all YAML-LD streams MUST form a well-formed stream and use alias node defined by a previous node with a corresponding anchor; otherwise, a loading-document-failed error has been detected and processing is aborted.
A YAML-LD document MUST be encoded in UTF-8, to ensure interoperability with [[JSON]]; otherwise, an invalid-encoding error has been detected and processing is aborted.
Since anchor names are a serialization detail, such anchors MUST NOT be used to convey relevant information, MAY be altered when processing the document, and MAY be dropped when interpreting the document as JSON-LD.
Not sure how to test that anchors are not used to convey information. As the Internal Representation has does not have a way of expressing anchors, also not sure how to test for this.
A YAML-LD document MAY contain anchored nodes and alias nodes, but its representation graph MUST NOT contain cycles; otherwise, a loading-document-failed error has been detected and processing is aborted. When interpreting the document as JSON-LD, alias nodes MUST be resolved by value to their target nodes.
When processing using the YAML-LD JSON profile, documents MUST NOT contain alias nodes; otherwise, a profile-error error has been detected and processing is aborted. The YAML-LD extended profile allows full use of anchor names and alias nodes subject to the requirements described above in this section.
The YAML-LD document in the following example contains alias nodes for the `{"@id": "countries:ITA"}` object:
While the representation graph (and eventually the in-memory representation of the data structure, e.g., a Python dictionary or a Java hashmap) will still contain references between nodes, the JSON-LD serialization will not, as shown below:
Every YAML-LD file is a YAML-LD stream and might contain multiple YAML-LD documents, as shown in the example below.
Each of the individual YAML documents in the stream is converted into a separate JSON-LD document and processed separately.
The current text does not support this, and only supports a single YAML document. This is inconsistent with the processing description in .
YAML-LD processing is defined by converting YAML to the internal representation and using [[[JSON-LD11-API]]] to process on that representation, after which the representation is converted back to YAML. As information specific to a given YAML document structure is lost in this transformation, much of the specifics of that original representation are therefore lost in that conversion, limiting the ability to fully round-trip a YAML-LD document back to an equivalent representation. Consequently, round-tripping in this context is limited to preservation of the semantic representation of a document, rather than a specific syntactic representation.
If the {{JsonLdOptions/extendedYAML}} API flag is `true`, the processing result will be in the extended internal representation.
The conversion process represented here is compatible with the description of "Composing the Representation Graph" from the 3.1.2 Load section of [[YAML]]. The steps described below for converting to the internal representation operate upon that .
When operating using the YAML-LD JSON profile, it is intended that the common feature provided by most YAML libraries of transforming YAML directly to JSON satisfies the requirements for parsing a YAML-LD file.
As a developer, I want to be able to convert JSON-LD documents to YAML-LD by simply serializing the document using any standard YAML library, So that the resulting YAML is valid YAML-LD, resolving to the same graph as the original JSON-LD.
A YAML stream is composed of zero or more YAML documents.
YAML streams may correspond more directly to [[[RFC7464]]], which are not presently part of the JSON-LD representation model. The description here more closely aligns with how JSON-LD interprets HTML Scripts.
Any error reported in a recursive processing step MUST result in the failure of this processing step.
From the YAML grammar, a YAML document MAY be preceded by a Document Prefix and/or a set of directives followed by a YAML bare document, which is composed of a single node.
Any error reported in a recursive processing step MUST result in the failure of this processing step.
Both block sequences and flow sequences are directly aligned with an array in the internal representation.
Any error reported in a recursive processing step MUST result in the failure of this processing step.
Both block mappings and flow mappings are directly aligned with a map in the internal representation.
Any error reported in a recursive processing step MUST result in the failure of this processing step.
Implementations may retain the representation as an YAML Integer, or YAML Floating Point, but a JSON-LD processor must treat them uniformly as a number, although the specific type of number value SHOULD be retained for round-tripping.
The conversion result is the value of the entry in the |named nodes| map having the node entry. If none exist, the document is invalid, and processing MUST end in failure.
If an alias node is encountered when processing the YAML representation graph and the {{JsonLdOptions/extendedYAML}} flag is `false`, the YAML-LD JSON profile has been selected. A profile-error error has been detected and processing MUST be aborted.
If a cycle is detected, a processing error MUST be returned, and processing aborted.
The conversion process from the internal representation involves turning that representation back into a YAML representation graph and relies on the description of "Serializing the Representation Graph" from the 3.1.1 Dump section of [[YAML]] for the final serialization.
As the internal representation is rooted by either an array or a map, the process of transforming the internal representation to YAML begins by preparing an empty representation graph which will be rooted with either a YAML mapping or YAML sequence.
Although outside of the scope of this specification, processors MAY use YAML directives, including TAG directives, and Document markers, as appropriate for best results. Specifically, if the {{JsonLdOptions/extendedYAML}} API flag is `true`, the document SHOULD use the `%YAML` directive with version set to at least `1.2`. To improve readability and reduce document size, the document MAY use a `%TAG` directive appropriate for RDF literals contained within the representation.
The use of `%TAG` directives in YAML-LD is similar to the use of the `PREFIX` directive in [[?Turtle]] or the general use of terms as prefixes to create Compact IRIs in [[JSON-LD11]]: they not change the meaning of the encoded scalars.
Although allowed within the YAML Grammar, some current YAML parsers do not allow the use of `"#"` within a tag URI. Substituting the `"%23"` escape is a workaround for this problem, that will hopefully become unnecessary as implementations are updated.
A concrete proposal in that direction would be to use a tag at the top-level of any "idiomatic" YAML-LD document, applying to the whole object/array that makes the document.
It might also include a version to identify the specification that it relates to, allowing for version announcement that could be used for future-proofing.
The following block is one example:
!yaml-ld
$context: http://schema.org/
$type: Person
name: Pierre-Antoine Champin
See for an example of serializing the extended internal representation.
This algorithm describes the steps to convert each element from the internal representation into corresponding YAML nodes by recursively processing each element |n|.
This section identifies two application profiles for operating with YAML-LD:
Application profiles allow publishers to use YAML-LD either for maximum interoperability, or for maximum expressivity. The YAML-LD JSON profile provides for complete round-tripping between YAML-LD documents and JSON-LD documents. The YAML-LD extended profile allows for fuller use of YAML features to enhance the ability to represent a larger number of native datatypes and reduce document redundancy.
Application profiles can be set using the {{JsonLdProcessor}} API interface, as well as an HTTP request profile (see ).
The YAML-LD JSON profile is based on the YAML Core Schema, which interprets only a limited set of node tags. YAML scalars with node tags outside of the defined range SHOULD be avoided and MUST be converted to the closest scalar type from the YAML Core Schema, if found. See for specifics.
Keys used in a YAML mapping MUST be strings.
Although YAML-LD documents MAY include node anchors, documents MUST NOT use alias nodes.
A YAML stream MUST include only a single YAML document, as the JSON-LD internal representation only supports a single document model.
The YAML-LD extended profile extends the YAML Core Schema, allowing node tags to specify RDF literals by using a JSON-LD extended internal representation capable of directly representing RDF literals.
As with the YAML-LD JSON profile, YAML-LD documents in the YAML-LD extended profile MUST NOT use encodings other than UTF-8.
As with the YAML-LD JSON profile, keys used in a YAML mapping MUST be strings.
YAML-LD docucments MAY use alias nodes, as long as dereferencing these aliases does not result in a loop.
As with the YAML-LD JSON profile, a YAML stream MUST include only a single YAML document, as the JSON-LD extended internal representation only supports a single document model.
Consier something like `!id` as a local tag to denote IRIs.
This specification defines the JSON-LD extended internal representation , an extension of the JSON-LD internal representation.
In addition to maps, arrays, and strings, the internal representation allows native representation of numbers, boolean values, and nulls. The extended internal representation allows for native representation of RDF literals, both with a datatype IRI, and language-tagged strings.
When transforming from the extended internal representation to the internal representation — for example when serializing to JSON or to the YAML-LD JSON profile — implementations MUST transform RDF literals to the closest native representation of the internal representation:
An alternative would be to transform such literals to JSON-LD value objects, and we may want to provide a means of transforming between the internal representation and extended internal representation using value objects, but this treatment is consistent with [[YAML]] Core Schema Tag Resolution.
This specification extends the [[[JSON-LD11-API]]] [[JSON-LD11-API]] Application Programming Interface and the [[[JSON-LD11-FRAMING]]] [[JSON-LD11-FRAMING]] Application Programming Interface to manage the serialization and deserialization of [[YAML]] and to enable an option for setting the YAML-LD extended profile.
The JSON-LD Processor interface is the high-level programming structure that developers use to access the JSON-LD transformation methods. The updates below is an experimental extension of the {{JsonLdProcessor}} interface defined in the JSON-LD 1.1 API [[JSON-LD11-API]] to serialize output as YAML rather than JSON.
Otherwise, if both the {{JsonLdOptions/useNativeTypes}} and {{JsonLdOptions/extendedYAML}} flags are set and the datatype IRI of |value| is not `xsd:string`:
- If |value| is a language-tagged string set |converted value| to a new RDF literal composed of the lexical form of |value| and datatype IRI composed of `https://www.w3.org/ns/i18n#` followed by the language tag of |value|.
- Otherwise, et |converted value| to |value|.
- Otherwise, if |value| is an RDF literal, |value| is left unmodified. This will only be the case when processing a value from an extended internal representation.
The {{JsonLdOptions}} type is used to pass various options to the {{JsonLdProcessor}} methods.
partial dictionary JsonLdOptions {
boolean extendedYAML = false;
};
In addition to those options defined in the JSON-LD 1.1 API [[JSON-LD11-API]] and JSON-LD 1.1 Framing [[JSON-LD11-FRAMING]], this specification defines these additional options:
This section describes an update to the built-in {{LoadDocumentCallback}} to load YAML streams and documents into the internal representation, or into the extended internal representation if the {{JsonLdOptions/extendedYAML}} API flag is `true`.
The {{LoadDocumentCallback}} algorithm in [[JSON-LD11-API]] is updated as follows:
These updates are intended to be compatible with other updates to the {{LoadDocumentCallback}}, such as Process HTML as defined in [[JSON-LD11-API]].
The YamlLdErrorCode represents the collection of valid YAML-LD error codes, which extends the {{JsonLdErrorCode}} definitions.
enum YamlLdErrorCode {
"invalid-encoding",
"mapping-key-error",
"profile-error"
};
See Security considerations in JSON-LD 1.1. Also, see the YAML media type registration.
For general interoperability considerations on the serialization of JSON documents in [[YAML]], see YAML and the Interoperability consideration of application/yaml [[I-D.ietf-httpapi-yaml-mediatypes]].
The YAML-LD format and the media type registration are not restricted to a specific version of YAML, but implementers that want to use YAML-LD with YAML versions other than 1.2.2 need to be aware that the considerations and analysis provided here, including interoperability and security considerations, are based on the YAML 1.2.2 specification.
This section has been submitted to the Internet Engineering Steering Group (IESG) for review, approval, and registration with IANA.
This section describes the information required to register the above media type according to [[RFC6838]]
profileA non-empty list of space-separated URIs identifying specific
constraints or conventions that apply to a YAML-LD document according to [[RFC6906]].
A profile does not change the semantics of the resource representation
when processed without profile knowledge, so that clients both with
and without knowledge of a profiled resource can safely use the same
representation. The profile parameter MAY be used by
clients to express their preferences in the content negotiation process.
If the profile parameter is given, a server SHOULD return a document that
honors the profiles in the list which it recognizes,
and MUST ignore the profiles in the list which it does not recognize.
It is RECOMMENDED that profile URIs are dereferenceable and provide
useful documentation at that URI. For more information and background
please refer to [[RFC6906]].
This specification allows the use of the `profile` parameters listed in and additionally defines the following:
http://www.w3.org/ns/json-ld#extended
When used as a media type parameter [[RFC4288]]
in an HTTP Accept header field [[RFC9110]],
the value of the profile parameter MUST be enclosed in quotes (") if it contains
special characters such as whitespace, which is required when multiple profile URIs are combined.
When processing the "profile" media type parameter, it is important to note that its value contains one or more URIs and not IRIs. In some cases it might therefore be necessary to convert between IRIs and URIs as specified in section 3 Relationship between IRIs and URIs of [[RFC3987]].
Fragment identifiers used with application/ld+yaml are treated as in RDF syntaxes, as per RDF 1.1 Concepts and Abstract Syntax [[RDF11-CONCEPTS]] and do not follow the process defined for application/yaml.
FIXME
REMOVE THIS SECTION BEFORE PUBLICATION.
Transforming the above entry into a JSON-LD document
results in:
Processing this entry in Python, I get the following
structure that preserve the references to
mutable objects (e.g., the `temperature` dict)
but not to scalars (e.g., the `value` keyword).
Since all these implementations pre-date this
specification, some more interoperable choices include the following:
* forbidding cycles in YAML-LD documents
* considering all references in YAML-LD as static,
i.e., a shorthand way to repeat specific patterns
Here, we propose to YAML-LD users a bit of advice which, although optional, might suggest one or two useful thoughts.
Follow JSON-LD best practices …in order to achieve a greater level of reusability, performance, and human friendliness among YAML-LD aware systems. The [[json-ld-bp]] document is as relevant to YAML-LD as it is to [[JSON-LD11]].
Do not force users to author contexts Instead, provide pre-built contexts that the user can reference by URL for a majority of common use cases.
YAML-LD is intended to simplify the authoring of Linked Data for a wide range of domain experts; its target audience is not comprised solely of IT professionals. [[YAML]] is chosen as a medium to minimize syntactic noise, and to keep the authored documents concise and clear. [[JSON-LD11]] (and hence YAML-LD) Context comprises a special language of its own. A requirement to author such a context would make the domain expert's job much harder, which we, as system architects and developers, should try to avoid.
Use a default context
If most, or all, of a user's documents are based on one particular context, try to make it the default in order to rescue the user from copy-pasting the same technical incantation from one document to another.For instance, according to [[JSON-LD11-API]], the `expand()` method of a JSON-LD processor accepts an `expandContext` argument which can be used to provide a default system context.
Alias JSON-LD keywords If possible, map JSON-LD keywords containing the `@` character to keywords that do not contain it.
The `@` character is reserved in YAML, and thus requires quoting (or escaping), as in the following example:
The need to quote these keywords has to be learnt, and introduces one more little irregularity to the document author's life. Further, on most keyboard layouts, typing quotes will require `Shift`, which reduces typing speed, albeit slightly.
In order to avoid this, the context might introduce custom mappings for the necessary keywords. For instance, [[schema-org]] context redefines `@id` as just `id` — which seems to be much more convenient to type, and no more difficult to remember.
Use YAML-LD Convenience Context
YAML-LD users may use a JSON-LD context provided as part of this specification, henceforth known as the convenience context, which defines a standardized mapping of every `@`-keyword to a `$`-keyword, except `@context`.
The convenience context contains an alias to every JSON-LD keyword which the JSON-LD 1.1
specification permits aliasing — which means all the keywords except @context. The reserved `@` character is
replaced by `$`, which is not reserved and therefore does not require quoting. Consider
reformatted using the convenience context:
The applicability of this context depends on the domain and is left to the architect's best judgement.
This document is meant as a gentle introduction to YAML-LD for software developers and IT professionals as well as professionals in domains other than computer science.
Comments
Comments in YAML-LD documents are treated as white space. This behavior is consistent with other Linked Data serializations such as [[?TURTLE]]. See Interoperability considerations of [[I-D.ietf-httpapi-yaml-mediatypes]] for more details.