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Subject: Re: [oslc-core] Dialog discovery: triple and Link header for discovery of dialogs won't be on same resources

I'm trying to understand the point of ResourceShape  valueType and representation properties.

oslc:valueType description is: "A URI that indicates the value type, for example XML Schema or RDF URIs for literal value types, and OSLC-specified for others.  If this property is omitted, then the value type is unconstrained."  That is defining the type of the property, not its representation in a resource, and covers what would be captured in the RDFS/OWL range property of the property.

The description for property oslc:representation is "Should be http://open-services.net/ns/core#Reference, http://open-services.net/ns/core#Inlineor http://open-services.net/ns/core#Either". This is addressing whether the representation of the referenced object must or may be in the same resource representation as the subject of that property. This is probably included in order to limit the number of GETs required to do discovery. Its a resource representation optimization that has no semantic meaning.

Later on in the ResourceShape vocabulary, there is a comment: "<!-- ********** Property: oslc:valueType allowed values ********** -->" which is followed by the enumeration types for Resource, LocalResource and AnyResource. This seems completely orthogonal to the definition of oslc:valueType and its common use to specify expected type of the value of a property, and appears to overlap with the values for oslc:representation.

In particular, oslc:valueType of LocalResource could only have oslc:representation oslc:Inline, and all instances of oslc:representation Inline in the OSLC2 specification have oslc:valueType LocalResource.

What seems to have happened is that oslc:valueType got somewhat overloaded. If the valueType is a resource, then it can have a representation. No other value type can have a representation. A resource shape might specify multiple valueTypes for a property, one that is the property's type (i.e., the object of its range property), the other is a tag indicating how the property value (that is a resource) should be represented in an HTTP resource - inlined (blank node or relative URI) or as a (potentially) external GETtable resource in its own right.

But this overlaps with oslc:representation which says the same thing. So I think valueTypes of Resource, LocalResource or AnyResource are redundant and unnecessary (but can't be removed). The value of a property should simply have a  type, and if its type is a non-primitive resource, then that value should have a representation that MUST or MAY be in the same resource as the subject URI.

Given the description above, we should be able to decouple resources from their particular representation. That is, a ServiceProvider resource representation would expect its Service instances to be inlined in that representation. But this shouldn't mean the Service can't be an LDPC in its own right, and certainly a GET on a Service URI would return a resource representation in which the Service is inlined!.

So I don't think there's a problem here. We should:

1. deprecate the use of oslc:Resource, LocalResource and AnyResource since they are redundant with oslc:representation. Any value of oslc:representation applies to any resource, so that doesn't need to be stated. oslc:representation for a LocalResource has to be Inline. Clients can't make any assumptions about the representation for AnyResource. So again, all values of oslc:representation apply.

2. add oslc:representation back into the OSLC3 specifications (it was removed and doesn't appear in any of the shape .ttl files or generated tables)

3. Treat oslc:representation as a means of specifying what should be included in resource representations that reference properties, but does not constrain where servers actually manage those resources. That is, the value of a property with oslc:representation oslc:Inline could be a blank node, relative (or hash) URI, or a URI to a resource that that can also be the URI of a GET request, even though it would be accessed inline in any other referencing resource representation.

As a result, get on a ServiceProvider would include the publisher and services inline, but each Service could also be the URI of an LDPC in an OPTIONS, HEAD, or GET method that provides Link headers for discovery.

Jim Amsden, Senior Technical Staff Member
OSLC and Linked Lifecycle Data

From:        Martin P Pain <martinpain@uk.ibm.com>
To:        OASIS <oslc-core@lists.oasis-open.org>
Date:        09/02/2015 10:14 AM
Subject:        [oslc-core] Dialog discovery: triple and Link header for discovery of dialogs won't be on same resources
Sent by:        <oslc-core@lists.oasis-open.org>

We have three mechanisms of discovering dialogs:
Link headers & Prefer headers:
and oslc:selectionDialog/oslc:creationDislog triples in oslc:Service resources:

I had previously expected that these headers and these triples would be on the same resources, but they cannot be.

The triples are on oslc:Service resources, but
OSLC v2 requires that these resources be "Local Resources", which it defines to mean blank nodes (although I need to raise a separate issue to clarify whether the intention was to allow for hash URIs or not). "Local Resources" cannot have headers of their own, so oslc:Service resources cannot have Link header. (If they have hash URIs, technically they can have their own Link headers, but I don't suggest we go down that route.)

The Link and Prefer headers will be on LDPs themselves. (Although currently I don't think we have a good way of finding those LDPCs).

Is everyone else ok with the fact that these headers and triples will be on different resources?
I just wanted to make sure we're clear what the situation is and are ok with it.

(This might make more sense in the context of my previous email and the wiki page it links to, where I'm thinking about how a server with one or more LDP containers makes those containers and their capabilities discoverable using OSLC).

Martin Pain
Software Developer - Green Hat
Rational Test Virtualization Server, Rational Test Control Panel

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