- From: Jim McCusker <mccusj@rpi.edu>
- Date: Thu, 26 Feb 2015 22:41:51 +0000
- To: David Booth <david@dbooth.org>, public-linked-json@w3.org, Manu Sporny <msporny@digitalbazaar.com>, Markus Lanthaler <markus.lanthaler@gmx.net>
- Cc: Pat Hayes <phayes@ihmc.us>
- Message-ID: <CAAtgn=TYab9gtWO4KjCh-C7BxpC7zfRM35YwJPkqXkdjp_FnUw@mail.gmail.com>
Blank node IDs are only significant in RDF if they are objects of more than
one statement. The two graphs you show above are semantically equivalent,
so you should be fine omitting the @id. I would check to see if there are
any edge cases where blank nodes might be referred to twice, but based on
the original serialization in FRIR, that seems unlikely (especially since
JSON is a tree).
Jim
On Thu Feb 26 2015 at 3:35:20 PM David Booth <david@dbooth.org> wrote:
> On 02/25/2015 10:11 AM, Manu Sporny wrote:
> > So, count us in - send the questions to the mailing list and it looks
> > like you have multiple community members that would be willing to help
> out.
>
> Thanks Manu (and Markus and Jim and any others)! Okay, my first
> question regards blank nodes.
>
> Here is an except of a FHIR JSON data:
>
> {
> "dob": "1972-11-30",
> "_dob": {
> "id": "314159",
> "extension": [{
> "url" : "http://example.org/fhir/extensions#text",
> "valueString" : "Easter 1970"
> }]
> }
>
> To turn this into JSON-LD, I've created an @context:
>
> {
> "@context":
> {
> "@vocab": "http://example/fhir/vocab#",
> "fhir": "http://example/fhir#",
> "xsd": "http://www.w3.org/2001/XMLSchema#",
> "dob":
> {
> "@id": "fhir:dob",
> "@type": "xsd:date"
> },
> "_dob":
> {
> "@id": "fhir:_dob",
> "@type": "@id"
> }
> }
> }
>
> and I've linked it from the FHIR JSON document (thus involving a single,
> constant, one-line change to the existing format). Here is the
> resulting JSON-LD, with three # comments added for reference later:
>
> { # _:b0
> "@context": "http://dbooth.org/2015/fhir/json-ld/dob-context.jsonld",
> "dob": "1972-11-30",
> "_dob": { # _:b1
> "id": "314159",
> "extension": [{ # _:b2
> "url" : "http://example.org/fhir/extensions#text",
> "valueString" : "Easter 1970"
> }]
> }
>
> As you can see, none of the JSON objects above has been given an @id, so
> when this is interpreted as RDF, blank nodes (_:b0, _:b1, _:b2) are
> generated to represent those unidentified objects. Here is the RDF
> interpretation in Turtle:
>
> @prefix fhir: <http://example/fhir#> .
> @prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
>
> _:b0 fhir:_dob _:b1 ;
> fhir:dob "1972-11-30"^^xsd:date .
>
> _:b1 <http://example/fhir/vocab#extension> _:b2 ;
> <http://example/fhir/vocab#id> "314159" .
>
> _:b2 <http://example/fhir/vocab#url>
> "http://example.org/fhir/extensions#text" ;
> <http://example/fhir/vocab#valueString>
> "Easter 1970" .
>
> One of the key requirements is for FHIR data to be round-trippable
> between the different data representations, such as XML and JSON. This
> means that if we interpret some given FHIR JSON-LD data as RDF, we need
> to be able to serialize from RDF back to the *same* JSON-LD. Here is
> the result after serializing the above Turtle back into JSON-LD (using
> jena riot), with line numbers added for reference:
>
> 1. {
> 2. "@context": {
> 3. "fhir": "http://example/fhir#",
> 4. "rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#",
> 5. "rdfs": "http://www.w3.org/2000/01/rdf-schema#",
> 6. "xsd": "http://www.w3.org/2001/XMLSchema#"
> 7. },
> 8. "@graph": [
> 9. {
> 10. "@id": "_:fc7725329340449efa72b6f7f5d7182eeb3",
> 11. "http://example/fhir/vocab#url":
> "http://example.org/fhir/extensions#text",
> 12. "http://example/fhir/vocab#valueString": "Easter 1970"
> 13. },
> 14. {
> 15. "@id": "_:fc7725329340449efa72b6f7f5d7182eeb2",
> 16. "http://example/fhir/vocab#extension": {
> 17. "@id": "_:fc7725329340449efa72b6f7f5d7182eeb3"
> 18. },
> 19. "http://example/fhir/vocab#id": "314159"
> 20. },
> 21. {
> 22. "@id": "_:fc7725329340449efa72b6f7f5d7182eeb1",
> 23. "fhir:_dob": {
> 24. "@id": "_:fc7725329340449efa72b6f7f5d7182eeb2"
> 25. },
> 26. "fhir:dob": {
> 27. "@type": "xsd:date",
> 28. "@value": "1972-11-30"
> 29. }
> 30. }
> 31. ]
> 32. }
>
> As you can see, there are several differences from the original JSON-LD,
> which is not a surprise. I already know that a generic RDF JSON-LD
> serializer will not suffice for this purpose -- a special purpose
> FHIR-aware JSON-LD serializer will be needed -- but that's okay, because
> FHIR already requires a special-purpose serializer for its existing JSON
> format anyway. Some of the differences could be readily handled by a
> special-purpose FHIR JSON-LD serializer (such as the outer @graph
> wrapper and the embedded @context), but not all.
>
> In this message I want to focus specifically on the blank nodes, so for
> the moment I'll ignore other differences. Blank node labels are
> arbitrary in RDF, so they might be serialized differently by different
> serializers or in different runs of the same serializer, so I had been
> thinking that, to enable predictable round tripping, it may be best to
> generate predictable URIs instead of using blank nodes. However, a
> downside of this is that AFAIK it would require the FHIR JSON-LD
> instance data -- not merely the @context -- to contain an explicit @id
> property on every FHIR JSON-LD object. Is this correct? If so, FHIR
> JSON users are likely to reject that option as too onerous, because most
> of them don't care about RDF. If not, how would URIs be specified in
> the @context for JSON-LD objects that have no explicit @ids?
>
> However, I am now thinking that it might be better to allow those
> unidentified JSON-LD objects to become blank nodes in the RDF
> interpretation, and instead have the FHIR-specific JSON-LD serializer
> suppress them (if they are blank nodes). So instead of serializing a
> bit of RDF as:
>
> {
> "@id": "_:fc7725329340449efa72b6f7f5d7182eeb3",
> "http://example/fhir/vocab#url":
> "http://example.org/fhir/extensions#text",
> "http://example/fhir/vocab#valueString": "Easter 1970"
> },
>
> it would instead omit the @id property and serialize it as:
>
> {
> "http://example/fhir/vocab#url":
> "http://example.org/fhir/extensions#text",
> "http://example/fhir/vocab#valueString": "Easter 1970"
> },
>
> Does this seem like a good idea? Any hidden problems? Are there other
> approaches that you think would be better?
>
> Thanks,
> David
>
Received on Thursday, 26 February 2015 22:42:24 UTC