|FHIR Infrastructure Work Group||Maturity Level: 0||Ballot Status: Draft|
FHIR resources can be represented as an RDF graph serialised in the Turtle format , or as JSON-LD . The RDF format is defined to assist the process of bridging between operational data exchange and formal knowledge processing systems. While the RDF form offers a fully functional representation of FHIR resources, it has different operational characteristics to the JSON and XML representations, and would be implemented for different reasons. Systems focused on operational exchange of data would not generally use choose to use RDF.
This page describes:
A FHIR resource is represented by a series of RDF triples. The Turtle representation for a resource is described using this format:
Turtle Template [ a fhir:Observation; fhir:nodeRole fhir:treeRoot; # from Resource: id; meta; implicitRules; and language # from DomainResource: text; contained; extension; and modifierExtension fhir:Obervation.identifier [ Identifier ]; # 0..* Unique Id for this particular observation fhir:Obervation.status [ fhir:value "<code>" ]; # R! registered | preliminary | final | amended + fhir:Obervation.code [ CodeableConcept ]; # 1..1 R! Type of observation (code / type) fhir:Obervation.subject [ fhir:reference [ Patient|Group|Device|Location ] ]; # 0..1 Who and/or what this is about fhir:Obervation.encounter [ fhir:reference [ Encounter ] ]; # 0..1 Healthcare event during which this observation is made # effective[x]: 0..1 Clinically relevant time/time-period for observation. One of these 2: fhir:Obervation.effectiveDateTime [ fhir:value "<dateTime>" ]; fhir:Obervation.effectivePeriod [ Period ]; ]
Using this format:
text/turtle. Other MIME types:
application/json-ldfor the json-ld variant, and
text/shexfor the SHEX (RDF schemas)
Each resource is represented as a set of RDF triples represented using the Turtle syntax. When a resource has a persistent identity (e.g. it can be found at a particular URL - usually a FHIR RESTful server), then that URL is it's identity. Resources with no persistent identity (e.g. bundles from search results) have the the identity of the root document - "<>" in turtle syntax.
Some resources can contain other resources. Given that the relationships can appear in any order, it
cannot be assumed that the first encountered element represents the resource of interest that is being
represented by the set of turtle statements. The focal resource - where to start when parsing - is the
resource with the relationship
fhir:treeRoot. if there is
more than one node labelled as a 'treeRoot' in a set of turtle statements, it cannot be determined how
to parse them as a single resource.
Elements that can repeat are represented with a relationship
where [n] is a zero-based integer offset (ie.g the first element has an index of 0). Lists are never sparse; it is an error if there are missing items in the repeat sequence.
Note: this measn that the rdf:list structure is not used.
Primitive elements - elements with a primitive type - are represented as regular nodes so that the elements extensions can be represented. The actual value of the primitive type is represented using the fhir:value predicate:
The value has two parts: a literal string that contains the value, and, if applicable, one of the following schema types:
The choice is made based on the types as specified for the primitive type. Note that the correct schema type for a date/dateTime must be determined by inspecting the value of the date for precision.
The fhir:value property can never be empty. Either the relationship is absent, or it is present with at least one character of content. XHTML is represented as an escaped xs:string.
A Reference element is representing using the same rules as above:
fhir:Observation.subjectReference [ fhir:Reference.reference [ fhir:value "Patient/example" ]; fhir:Reference.display [ fhir:value "Example Patient" ]; ];
This allows faithful round tripping of the resource between the Turtle format and the JSON and XML formats. However it's very useful for an RDF processor if the RDF graph links to the target of the reference directly. This can be represented using the fhir:link property:
fhir:Observation.subjectReference [ fhir:link <http://hl7.org/fhir/Patient/example>; fhir:Reference.reference [ fhir:value "Patient/example" ] ];
The correct value for the fhir:link relationship must be determined by resolving the rules for resolving references for the various reference types to a literal URL that refers to the correct content in the local RDF context.
The fhir:link relationship can be added automatically as part of generating the resource representation, or it can be injected by a post-processor that knows how to convert the raw references into RDF-suitable references.
The same logic applies to the Coding data type. These are represented directly in turtle by serialising their properties as specified above:
fhir:Observation.code [ fhir:CodeableConcept.coding [ fhir:index 0; fhir:Coding.system [ fhir:value "http://loinc.org" ]; fhir:Coding.code [ fhir:value "29463-7" ]; fhir:Coding.display [ fhir:value "Body Weight" ] ]; fhir:CodeableConcept.coding [ fhir:index 1; fhir:Coding.system [ fhir:value "http://snomed.info/sct" ]; fhir:Coding.code [ fhir:value "27113001" ]; fhir:Coding.display [ fhir:value "Body weight" ] ] ];
For reasoners using the RDF graph, it's very useful to make the implicit concept references in these Codings explicit using a rdf:type assertion ("a" in turtle):
fhir:Observation.code [ fhir:CodeableConcept.coding [ fhir:index 0; a loinc:29463-7; fhir:Coding.system [ fhir:value "http://loinc.org" ]; fhir:Coding.code [ fhir:value "29463-7" ]; fhir:Coding.display [ fhir:value "Body Weight" ] ]; fhir:CodeableConcept.coding [ fhir:index 2; a sct:27113001; fhir:Coding.system [ fhir:value "http://snomed.info/sct" ]; fhir:Coding.code [ fhir:value "27113001" ]; fhir:Coding.display [ fhir:value "Body weight" ] ] ];
These rdf:type assertions can be made by any agent that knows how to convert from the code system to the correct ontological representation on the RDF context. Note that a few code systems have standard ontological representations, but many don't. Again, these assertions can be made by the serialiser, or injected by a post-processor.
FHIR uses ShEx for representing the turtle schema. See fhir.shex for definitions.
As a by-product of the RDF format, a JSON-LD format is defined to represent exactly the same information as the Turtle format, but using the JSON-LD syntax. Some specific notes about the JSON-LD format:
STU Note: The definition of the JSON-LD format means that the FHIR specification defines two different JSON formats, each with a different purpose. It seems unlikely that normal implementers will prefer the JSON-LD format over the other JSON format, since that is more dense, but there are other implementers who would prefer the the JSON-LD format. On the other hand, defining two different JSON formats seems like a recipe for confusion. Feedback from implementers is welcome.
In addition to the basic representation of FHIR resources in turtle format, a Turtle representation of the FHIR infrastructure and definitions is also published, for the following purposes: