This page presents straw man data models for objects that may not need Linked Data Platform Containers (LDPCs).
The defining characteristics of this class of objects:
- They do not contain bitstreams, but rather, metadata about bitstreams held elsewhere
- They may contain pointer(s) to the referenced bitstreams (or bitstream objects)
The related concept in LDP (Linked Data Platform) terms: Non-LDPC LDPRs
- LDPR: Linked Data Platform Resource
- LDPC: Linked Data Platform Container
To what extent can/should this class of objects be modeled as LDPRs not contained within a LDPC?
What are the benefits of such a model?
What are the drawbacks or limitations?
Example 1: Fedora container object with a child resource at a specified path redirecting to external content
Request 1: create container object
curl -i -X PUT "http://localhost:8080/rest/my_external_datastream_obj"
HTTP/1.1 201 Created
ETag: "f93bbf77fee0fa16da4872102a54240e7135eaf2"
Last-Modified: Thu, 12 Mar 2015 20:56:08 GMT
Content-Length: 53
Location: http://localhost:8080/rest/my_external_datastream_obj
Content-Type: text/plain
Server: Jetty(8.1.12.v20130726)
curl -H "Accept: text/turtle" "http://localhost:8080/rest/my_external_datastream_obj"
<http://localhost:8080/rest/my_external_datastream_obj> a ldp:RDFSource , ldp:Container , <http://www.jcp.org/jcr/nt/1.0folder> , <http://www.jcp.org/jcr/nt/1.0hierarchyNode> , <http://www.jcp.org/jcr/nt/1.0base> , <http://www.jcp.org/jcr/mix/1.0created> , fedora:Container , fedora:Resource , fedora:Resource , <http://www.jcp.org/jcr/mix/1.0created> , <http://www.jcp.org/jcr/mix/1.0lastModified> , <http://www.jcp.org/jcr/mix/1.0referenceable> ;
fedora:lastModifiedBy "bypassAdmin"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:uuid "fe7a8497-d826-490d-a786-ba601e039055"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:createdBy "bypassAdmin"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:primaryType "nt:folder"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:created "2015-03-12T20:56:08.468Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> ;
fedora:mixinTypes "fedora:Container"^^<http://www.w3.org/2001/XMLSchema#string> , "fedora:Resource"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:lastModified "2015-03-12T20:56:08.468Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> ;
fedora:writable "true"^^<http://www.w3.org/2001/XMLSchema#boolean> ;
fedora:hasParent <http://localhost:8080/rest/> ;
fedora:exportsAs <http://localhost:8080/rest/my_external_datastream_obj/fcr:export?format=jcr/xml> .
<http://localhost:8080/rest/my_external_datastream_obj/fcr:export?format=jcr/xml> dc:format <http://fedora.info/definitions/v4/repository#jcr/xml> .
<http://fedora.info/definitions/v4/repository#jcr/xml> rdfs:label "jcr/xml"^^<http://www.w3.org/2001/XMLSchema#string> .
Request 2: create child external content object
curl -i -X PUT -H"Content-Type: message/external-body; access-type=URL; URL=\"http://en.wikipedia.org/wiki/File:Harry_Whittier_Frees_-_What%27s_Delaying_My_Dinner.jpg\"" "http://localhost:8080/rest/my_external_datastream_obj/extds" HTTP/1.1 201 Created ETag: "cbc2d06dd76fe1b3f6379e5dc0b8212ab056443e" Last-Modified: Thu, 12 Mar 2015 21:08:33 GMT Link: <http://localhost:8080/rest/my_external_datastream_obj/extds/fcr:metadata>; rel="describedby" Content-Length: 59 Location: http://localhost:8080/rest/my_external_datastream_obj/extds Content-Type: text/plain Server: Jetty(8.1.12.v20130726) http://localhost:8080/rest/my_external_datastream_obj/extds curl -i "http://localhost:8080/rest/my_external_datastream_obj/extds" HTTP/1.1 307 Temporary Redirect ETag: "cbc2d06dd76fe1b3f6379e5dc0b8212ab056443e" Last-Modified: Thu, 12 Mar 2015 21:08:33 GMT Content-Type: message/external-body; access-type=URL; url="http://en.wikipedia.org/wiki/File:Harry_Whittier_Frees_-_What%27s_Delaying_My_Dinner.jpg" Content-Length: 0 Accept-Ranges: bytes Content-Disposition: attachment; filename=""; creation-date="Thu, 12 Mar 2015 21:08:33 GMT"; modification-date="Thu, 12 Mar 2015 21:08:33 GMT"; size=0 Link: <http://www.w3.org/ns/ldp#Resource>;rel="type" Link: <http://www.w3.org/ns/ldp#NonRDFSource>;rel="type" Link: <http://localhost:8080/rest/my_external_datastream_obj/extds/fcr:metadata>; rel="describedby" Allow: DELETE,HEAD,GET,PUT,OPTIONS Location: http://en.wikipedia.org/wiki/File:Harry_Whittier_Frees_-_What%27s_Delaying_My_Dinner.jpg Server: Jetty(8.1.12.v20130726)
The complete thing
curl -H "Accept: text/turtle" -H "Prefer: return=representation; include=\"http://fedora.info/definitions/v4/repository#EmbedResources\";" "http://localhost:8080/rest/my_external_datastream_obj"
<http://localhost:8080/rest/my_external_datastream_obj> a ldp:RDFSource , ldp:Container , <http://www.jcp.org/jcr/nt/1.0folder> , <http://www.jcp.org/jcr/nt/1.0hierarchyNode> , <http://www.jcp.org/jcr/nt/1.0base> , <http://www.jcp.org/jcr/mix/1.0created> , fedora:Container , fedora:Resource , fedora:Resource , <http://www.jcp.org/jcr/mix/1.0created> , <http://www.jcp.org/jcr/mix/1.0lastModified> , <http://www.jcp.org/jcr/mix/1.0referenceable> ;
fedora:lastModifiedBy "bypassAdmin"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:uuid "fe7a8497-d826-490d-a786-ba601e039055"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:createdBy "bypassAdmin"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:primaryType "nt:folder"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:created "2015-03-12T20:56:08.468Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> ;
fedora:mixinTypes "fedora:Container"^^<http://www.w3.org/2001/XMLSchema#string> , "fedora:Resource"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:lastModified "2015-03-12T21:08:33.787Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> ;
fedora:writable "true"^^<http://www.w3.org/2001/XMLSchema#boolean> ;
fedora:hasParent <http://localhost:8080/rest/> ;
ldp:contains <http://localhost:8080/rest/my_external_datastream_obj/extds> .
<http://localhost:8080/rest/my_external_datastream_obj/extds> a <http://www.jcp.org/jcr/nt/1.0resource> , <http://www.jcp.org/jcr/nt/1.0base> , <http://www.jcp.org/jcr/mix/1.0mimeType> , <http://www.jcp.org/jcr/mix/1.0lastModified> , fedora:Binary , fedora:Resource , <http://www.jcp.org/jcr/mix/1.0created> , <http://www.jcp.org/jcr/mix/1.0lastModified> , <http://www.jcp.org/jcr/mix/1.0referenceable> ;
fedora:lastModifiedBy "bypassAdmin"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:uuid "86aa4107-768d-4e2d-9064-78be072c0888"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:mimeType "message/external-body; access-type=URL; url=\"http://en.wikipedia.org/wiki/File:Harry_Whittier_Frees_-_What%27s_Delaying_My_Dinner.jpg\""^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:digest <urn:sha1:da39a3ee5e6b4b0d3255bfef95601890afd80709> ;
premis:hasSize "0"^^<http://www.w3.org/2001/XMLSchema#long> ;
fedora:createdBy "bypassAdmin"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:primaryType "nt:resource"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:created "2015-03-12T21:08:33.787Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> ;
fedora:mixinTypes "fedora:Binary"^^<http://www.w3.org/2001/XMLSchema#string> ;
fedora:lastModified "2015-03-12T21:08:33.787Z"^^<http://www.w3.org/2001/XMLSchema#dateTime> ;
premis:hasOriginalName ""^^<http://www.w3.org/2001/XMLSchema#string> .
<http://localhost:8080/rest/my_external_datastream_obj> fedora:exportsAs <http://localhost:8080/rest/my_external_datastream_obj/fcr:export?format=jcr/xml> .
<http://localhost:8080/rest/my_external_datastream_obj/fcr:export?format=jcr/xml> dc:format <http://fedora.info/definitions/v4/repository#jcr/xml> .
<http://fedora.info/definitions/v4/repository#jcr/xml> rdfs:label "jcr/xml"^^<http://www.w3.org/2001/XMLSchema#string> .
Example 2: LDP-RS as datastreams
This strawman proposal addresses the following issues:
- Allow Fedora to create LDP-RS (RDF resources) that are NOT containers (LDPC), as allowed per the LDP spec. These will henceforth be called LDP-RS-NCs ("LDP RDF Source, Non-containers") for the sake of this discussion
- Allow the LDP-RS-NCs created by Fedora to contain arbitrary RDF
- No requirements on allowable subjects, predicates, or objects in the triples
- No constraints on constructs such as blank nodes
- Allow the LDP-RS-NCs created to by Fedora to be 'pure' in that they are logically unmodified from client requests
- No server managed triples are added by Fedora
- Allow the LDP-RS interaction model as defined by LDP on these resources
- Retrieve triples in specified representation (xml, turtle, ntriples, etc)
- Logical PUT operation (i.e. logical contents of graph are stored, not necessarily the serialization bytes)
- SPARQL Update semantics (PATCH)
- Interpretation of relative URIs as per the LDP spec (for example, allowing null relative URIs "<>" to refer to the to-be-created resource URI)
Proposal
LDP-RS-NCs in Fedora shall be datastreams, differing only from binary datastreams (exposed as LDP-NRs in LDP) in the following significant ways:
- Fedora's HTTP APIs support LDP-RS operations on these resources such as the logical PUT and PATCH, resolution of relative URIs in the content, etc.
- Fedora may describe these resources using slightly different metadata in their corresponding 'fcr:metadata' resources. For example, LDP-RS-NCs may have a different mixin type than LDP-NRs
- Basic validation as well-formed RDF or SPARQL/update statements is a prerequisite for accepting content.
- The binary representation of an LDP-RS-NC as persisted in Fedora as datastream content may potentially be opaque (a decision would need to be made on this)
- The serialization format may be specified in advance. For example, Fedora may chose to serialize as turtle, always
- Fedora may perhaps chose to store the exact binary content supplied by the client via PUT, and record metadata indicating which RDF serialization had been persisted.
- Likewise, the content may be entirely opaque, leaving it an implementation decision that can change at any time.
An LDP-RS-NC shall be created when the client specifies an LDPR interaction model for newly created resources as per section 5.2.3.4 of the LDP spec.
