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Re: Interpreting JSON as JSON-LD / Content-Type header spec. requirements

From: Gregg Kellogg <gregg@greggkellogg.net>
Date: Thu, 26 Oct 2017 13:31:39 -0400
Message-Id: <E2F81A55-9704-45BD-84DA-7E9576199766@greggkellogg.net>
Cc: Martynas Jusevičius <martynas@atomgraph.com>, public-linked-json@w3.org
To: Christopher Johnson <chjohnson39@gmail.com>
> On Oct 26, 2017, at 1:04 PM, Christopher Johnson <chjohnson39@gmail.com> wrote:
> 
> Hi list,
> 
> Following up on this:
> In a discussion regarding JSON-LD processing documents in relation to an implementation of LDP, a few specification points have been addressed:
> 
> 1. Since JSON-LD processing documents cannot be deserialized to RDF (and are therefore not JSON-LD documents), they MUST be served with a Content-Type application/json.

Can you clarify this? If the JSON-LD processing document is JSON-LD, it would include an @context, which would allow it to be interpreted as RDF, no? application/json can serve a JSON document that can be interpreted as JSON-LD by linking to a context, or it can return a JSON-LD document containing @context (or both). Am I missing something?

No clear what a “processing document” is in relation to LDP.

> 2. As application/json Content-Type, JSON-LD processing documents can only be persisted in LDP as rdf:type http://www.w3.org/ns/ldp#NonRDFSource <http://www.w3.org/ns/ldp#NonRDFSource>  .

This wouldn’t seem to restrict a client’s ability to process it as JSON-LD, though, would it?

Gregg

> 3. A (recommended) HTTP header for clients who need to identify a processing document to a specification compliant LDP implementation MAY be Accept: application/ld+json; profile="http://example.org/context.json <http://www.google.com/url?q=http%3A%2F%2Fexample.org%2Fcontext.json&sa=D&sntz=1&usg=AFQjCNGVZtpiz4NkamqiB3uatpw9ZmXeyw>"
> 
> Christopher Johnson
> Scientific Associate
> Universitätsbibliothek Leipzig
> 
> On 23 October 2017 at 00:59, Gregg Kellogg <gregg@greggkellogg.net <mailto:gregg@greggkellogg.net>> wrote:
>> On Oct 22, 2017, at 11:58 AM, Martynas Jusevičius <martynas@atomgraph.com <mailto:martynas@atomgraph.com>> wrote:
>> 
>> Why should LDP be concerned with any specific serialization?
> 
> I think the concern is likely captured in issue #491. A context cache isn’t really something the spec needs to handle explicitly, as it already provides the means to specify an alternative document loader, with which a client could implement a context cache.
> 
> Gregg
> 
>> On Sun, 22 Oct 2017 at 20.44, Gregg Kellogg <gregg@greggkellogg.net <mailto:gregg@greggkellogg.net>> wrote:
>>> On Oct 21, 2017, at 8:49 PM, Christopher Johnson <chjohnson39@gmail.com <mailto:chjohnson39@gmail.com>> wrote:
>>> 
>>> Hi,
>>> 
>>> Thanks for the reply.  jsonld-java does not execute the remote-doc tests, yet.  Interesting thing about the 0009 test is that the Link references a non-HTTP document.  Processing Link headers is not that easy, but I would assume that the test client should also try to dereference the Link Header using an HTTP client.  I will check how jsonld.js does this.
>> 
>> It is HTTP, but should be HTTPS, I’ll update the .htaccess file that sets this. However, it should redirect and work anyway.
>> 
>>> The rational is buried in the GitHub issue tracker, but as I recall, application/ld+json is intended to represent a JSON-LD document that can be fully interpreted based on its content, rather than rely on out-of-band information. This allows the document to be used outside of it’s HTTP context (URI base issues aside).
>>> 
>>> ​By "URI base issues", I assume that you mean when @context is referenced with an IRI in a document.  Perhaps, this form and also the JSON-LD profiles that do not include context (i.e. http://www.w3.org/ns/json-ld#expanded <http://www.w3.org/ns/json-ld#expanded>) should be mentioned in the spec as exclusions to the #context constraint if these are intended to also be served as application/ld+json.  
>> 
>> By “URI base issues”, I mean the resolution of non-absolute document-relative IRIs; but, this is a common issue among any format that uses relative IRIs where the document is used outside of a scheme that can be used to determine the base IRI (URL). Implementations may provide a means of setting this through options. If the `@context` uses a relative IRI, it will have the same issue.
>> 
>> However, the spec mostly is concerned with JSON(-LD) documents served over HTTP(S); certainly when the Link header is involved.
>> 
>>> The concern that I would like to address relates primarily with the "HTTP context" dependency that you indicate.  If a client depends on an processed document format (e.g. framed or compacted) and the @context or frame is only served by HTTP, then any client-side JSON-LD processing (required when a document is served in a non-expected format like RDF or JSON-LD expanded) requires some hypermedia mechanism whereby these dependencies can be dereferenced and validated before deserialization.  
>> 
>> You can also specify the context through the API (both for expansion and compaction). Same with the frame.
>> 
>> If dependencies are provided outside the document envelope (Link header or API option), you can certainly do this. If inside the document, then you need to deserialize to find the references (consider an @context embedded within some object).
>> 
>>> An undefined point is how an LDP server can persist @context in RDF for a particular document graph or document collection.  This seems to be related to a the client expectation of a particular document format that can only be provided by an remote reference (z.B. the "HTTP context").  A client could issue some form of negotiated "Accept-Processing-Document" request, and the server could respond with an "processing document" IRI, (but only if it had persisted it) …
>> 
>> There isn’t a way to represent a context in RDF, per-se, other than when serialized as JSON-LD which includes a top-level @context.
>> 
>> These seem like reasonable concerns, however, but perhaps they should be considered in LDP. At most, I could see the API including some form of @context lookup hash that would allow an implementation to look for context references that are in this hash and use them rather than perform a dereference.
>> 
>>> Furthermore, it seems then that an "HTTP context" is a actually a dependency for client-side processing of the document , and could be serviced similarly to other compile time package dependency managers (e.g. Maven Central, npm, etc.).  This frees the producer from having to provide custom headers for every possible document processing relation.  When deserializing from JSON-LD, the producer could persist a context hash with the graph that pointed to a package, and let the client do the rest.  For LDP implementations, where the data can be serialized dynamically from RDF, the client does have the potential to declare profile preferences, though I am not sure why it would if it were doing the processing.  The protocol regarding negotiation of JSON-LD processing options is undefined at the moment, though this is yet another "feature" that could be considered for server-side negotiated processing.
>> 
>> If the server maintains such a cache, then presumably it could be included in a JSON-LD serialization when serialized. It’s similar to prefix management for Turtle, where the service may have a configured set of prefixes it uses when serializing resources.
>> 
>> There is also an open-issue to allow the client to request (#491) [1], which could also be used for specifying a context the service should use when serializing compacted/framed JSON-LD (presumably, subject to white/black listing on the server side).
>> 
>>> It seems a gray area as to where the JSON-LD processing should occur.  In writing this, I have convinced myself that a clean solution is to have the producer provide expanded or RDF with some externally defined reference to processing options and context dependencies and let the client do the processing into its desired format.  No need for negotiation of preferences!  However, "tailored JSON serialization" is generally what some clients expect, though this level of service certainly has a cost in complexity for the producer.
>> 
>> The JSON-LD spec was originally written from the perspective that a client may apply algorithms to the received JSON-LD to put it in the most convenient form for local processing, but I think that, in actuality, it’s more common for clients to work with the results the server sends directly, thus the need for additional request profile information (tying us further to HTTP(S)).
>> 
>> This is a complicated enough issue that we might consider creating an ad-hoc conference call to discuss in more detail, or perhaps piggy-back on some existing LDP call (if such exists).
>> 
>> Gregg
>> 
>> [1] https://github.com/json-ld/json-ld.org/issues/491 <https://github.com/json-ld/json-ld.org/issues/491>
>> 
>> 
>>> Christopher Johnson
>>> Scientific Associate
>>> Universitätsbibliothek Leipzig
>> 
>>> On 19 October 2017 at 19:03, Gregg Kellogg <gregg@greggkellogg.net <mailto:gregg@greggkellogg.net>> wrote:
>> 
>>>> On Oct 17, 2017, at 8:29 PM, Christopher Johnson <chjohnson39@gmail.com <mailto:chjohnson39@gmail.com>> wrote:
>>>> 
>>>> Hi list,
>>>> 
>>>> In am writing tests and possibly a​n async document loader implementation for jsonld-java that could check Content-Type and Link headers before fetching.  The current one does not.
>>> 
>>> There are tests for this in the JSON-LD test suite, specifically remote-doc-0009-0011.
>>> 
>>> > curl -I https://json-ld.org/test-suite/tests/remote-doc-0009-in.jsonld <https://json-ld.org/test-suite/tests/remote-doc-0009-in.jsonld>
>>> HTTP/1.1 200 OK
>>> Accept-Ranges: bytes
>>> Access-Control-Allow-Origin: *
>>> Content-Length: 77
>>> Content-Type: application/ld+json
>>> Date: Thu, 19 Oct 2017 16:54:08 GMT
>>> Etag: "20d78-4d-4e582e01c8079"
>>> Last-Modified: Tue, 03 Sep 2013 23:16:15 GMT
>>> Link: <remote-doc-0009-context.jsonld>; rel="http://www.w3.org/ns/json-ld#context <http://www.w3.org/ns/json-ld#context>"
>>> Server: Apache/2.2.22 (Ubuntu)
>>> Vary: Accept-Encoding
>>> 
>>>>  I would like ​to ​research and find actual examples where the JSON-LD specification about the Interpretation requirement has been observed.  Anyone know a site that serve​s​ Content-Type application/json with a Link header that provides a "http://www.w3.org/ns/json-ld#context <http://www.w3.org/ns/json-ld#context>" relation?
>>>> 
>>>> Also could some​one please explain why application/ld+json is forbidden to provide an #context relation with a Link Header?
>>> 
>>> The rational is buried in the GitHub issue tracker, but as I recall, application/ld+json is intended to represent a JSON-LD document that can be fully interpreted based on its content, rather than rely on out-of-band information. This allows the document to be used outside of it’s HTTP context (URI base issues aside).
>>> 
>>>> Is an #context relation (if served as application/ld+json) allowed to specify a  http://www.w3.org/ns/json-ld <http://www.w3.org/ns/json-ld> profile?
>>> 
>>> I think there’s a test for this, where it is specifically ignored (0009).
>>> 
>>>> Or could a non http://www.w3.org/ns/json-ld <http://www.w3.org/ns/json-ld> Content-Type profile be served as application/json and provide a secondary #context relation if it were itself an #context ?
>>> 
>>> Need a more specific example of this.
>>> 
>>>> Should the document loader check (and ​subsequently dereference the relations) if "second-step" remote documents provide Link and/or Content-Type profile?  Should there be a processing "order of precedence”?
>>> 
>>> The remote-doc test manifests  does have some redirect logic, but you’d need to look more specifically to see if it addresses what you’re concerned about; we can always add more tests. https://json-ld.org/test-suite/tests/remote-doc-manifest.jsonld <https://json-ld.org/test-suite/tests/remote-doc-manifest.jsonld>.
>>> 
>>>> Just trying to understand required hypermedia options and possible use cases.
>>> 
>>> Happy to help, also you can chat on Giiter or IRC.
>>> 
>>> Gregg
>> 
>>>> Thanks,
>>>> Christopher Johnson
>>>> Scientific Associate
>>>> Universitätsbibliothek Leipzig
>>>> 
>>>> [1] https://stackoverflow.com/questions/39551829/usage-for-profile-parameter-for-json-ld-requests <https://stackoverflow.com/questions/39551829/usage-for-profile-parameter-for-json-ld-requests>
>>>> [2] https://json-ld.org/spec/latest/json-ld/#interpreting-json-as-json-ld <https://json-ld.org/spec/latest/json-ld/#interpreting-json-as-json-ld>
>>>> [3] https://github.com/IIIF/api/issues/1066 <https://github.com/IIIF/api/issues/1066>
>>>> ​[4] https://github.com/ProfileNegotiation/I-D-Accept--Schema​ <https://github.com/ProfileNegotiation/I-D-Accept--Schema%E2%80%8B>
>>>> 
>>>> 
>>>> 
>>> 
>>> 
>> 
> 
> 


Received on Thursday, 26 October 2017 17:32:14 UTC

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