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Subject: Re: [oslc-core] Dialog discovery: triple and Link header for discovery of dialogs won't be on same resources
- From: "Jim Amsden" <jamsden@us.ibm.com>
- To: OASIS <oslc-core@lists.oasis-open.org>
- Date: Fri, 25 Sep 2015 14:24:55 -0400
I agree with Arthur on the use of blank
nodes. But I think blank nodes have nothing to do with RDF or its meaning
- rather they are syntactical mechanism to deal with URIs in an RDF/XML
resource representation for which the creator is either not interested
in, does not know, or doesn't care about assigning a unique URI to an object.
That flies in the face of web best practices.
Jim Amsden, Senior Technical Staff Member
OSLC and Linked Lifecycle Data
919-525-6575
From:
Arthur Ryman <arthur.ryman@gmail.com>
To:
Martin P Pain <martinpain@uk.ibm.com>
Cc:
Jim Amsden/Raleigh/IBM@IBMUS,
OASIS <oslc-core@lists.oasis-open.org>
Date:
09/25/2015 12:09 PM
Subject:
Re: [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>
Martin,
My responses are inlined below:
- "I agree that we should re-introduce
olsc:representation into v3. " - I'm sure you will agree with this.
Agreed.- "I am pretty sure that valueType
is never the same as range" - I' confident that you will agree with
this.
Agreed, for two reasons.:
1) rdfs:range defines an inference rule which lets you
infer a type triple when the given property used as the predicate of a
triple. In contrast Shapes define constraints, so oslc:valueType defines
a check on the value of any RDF node that is the object of a triple whose
predicate is the given property. This distinction between inference rules
and constraints is the source of a lot of confusion in developers coming
from an OO background, which is basically all of us. The W3C chartered
the Data Shapes WG to define what most developers wanted, namely a way
to describe and check constraints in RDF data.
2) The effect that developers want from rdfs:range can
be achieved by using oslc:valueType with a value of oslc:Resource, oslc:LocalResource,
or oslc:AnyResource and oslc:range, i.e. if the value of a property is
non-literal then you can provide information about the value's rdf:type
using oslc:range.
oslc:valueType is primarily used to constrain the syntactic
type of an RDF node, i.e. IRI, blank node, or literal. In the case of literal,
we decided to be prescriptive and restrict the choice to a small number
of XSD types, which are enumerated in the spec.- "I also agree that, if we were
starting from scratch, then we should only have one valueType for RDF resources"
- I expect you would disagree with this. I think it would be useful to
have some scenarios/examples (covering at least GET and POST cases, probably
PUT as well) showing in what cases requiring or prohibiting blank nodes
would be useful (and when requiring or prohibiting inlining would be useful)
to show the value of having these three valueTypes for resources.
I
do disagree based on the way people use RDF in practice, i.e. as not being
very strongly typed. Multiple values for oslc:valueType are primarily useful
for literal types. The meaning of having multiple values is that the node
must match at least one of them. RDF is intrinsically permissive. So this
boils down to a policy decision for OSLC, i.e. to impose strong typing
or not. The tradeoff is ease-of-implementation (with one type) versus flexibility.
Perhaps this could be handled with a SHOULD requirement, i.e. state that
properties SHOULD use just one type of RDF node.- "Any references in the spec to
oslc:LocalResource would become oslc:AnyResource with an oslc:representation
of oslc:Inline." - even if we don't deprecate any of the resource
valueTypes, it feels like when talking about GETs (which the vast majority
of the shapes in the spec do.... I think.... at least that's how I read
them) there's no need to force something to be a blank node - as long as
it is inlined when it should be, as far as I can see that's what matters
to a client doing a GET. When POSTing I can understand that a server doesn't
want the client to force certain IRIs in the body, so requiring blank nodes
might make more sense. This suggests to me that perhaps we ought to have
separate shapes for POSTing and getting...not that we have the time to
make those sort of changes. This is very much getting into the scenarios
discussion that I suggest above - showing what the value & motivations
are for requiring or prohibiting blank nodes, and also looking at the difference
between GETs and POSTs.
- (Aside: you said in issue 34 that "
In RDF, IRI are opaque identifiers so you really should not talk about
fragments since it's not meaningful from an RDF viewpoint." - however,
when POSTing, especially to create a resource, it seems that there is a
distinction. It seems to me that it should be perfectly valid and acceptable
for a representation being POSTed in a creation request to define its own
relative fragment identifiers [e.g. <#me> - with no base IRI defined],
which will be resolved relative to the IRI the server mints for the new
resource [if the server allows relative graphs]. However if the representation
contains triples whose subject is another resource entirely, specifying
its absolute IRI on the same server, then that is ambiguous as to whether
it wants the server to create that resource or just store triples about
it in this resource's graph/representation - it depends what sort of model
the server is using, and I expect most implementations would want ot avoid
that sort of thing. I'm not sure I made myself clear here, but what I wastrying
to say is that in GETs I can see that IRIs are opaque, whether they contain
fragment identifiers or not, but that in POSTs there is a difference between
an IRI to another resource and relative fragment identifiers.)
Personally, I think OSLC should actively discourage the
use of blank nodes, because they are not used correctly. In RDF, the meaning
of a blank node is as a placeholder when the IRI of a resource is not known.
The interpretation of triples is in terms of existentially statements,
e.g. Somebody likes Martin. However, developers use blank nodes when they
want the RDF to be compact or they don't realize that they could use hash
IRIs to identify parts of a resource.
When you POST a resource, the server in effect copies
it into a new resource. The server is free to coin new IRIs, including
fragment IRIs. This behaviour should be described in the API spec for the
POST service.
Yes, there can be different Shapes for POST vs GET. The
Creation Factory should link to a shape used for POST. The Query Capability
or the resource itself should like to its GET shape.
-- Arthur
I withdraw my suggestion from that email
to deprecate oslc:Resource and oslc:LocalResource. However I'd like to
see examples (which hopefully already exist) of when requiring or prohibiting
blank nodes is useful, so we can be confident of what should be in the
shapes in the spec.
Thanks,Martin
Pain Software Developer - Green Hat Rational Test Virtualization Server, Rational Test Control Panel |
IBM United Kingdom Limited Registered in England and Wales with number
741598 Registered office: PO Box 41, North Harbour, Portsmouth, Hants.
PO6 3AU
From: Arthur
Ryman <arthur.ryman@gmail.com>
To: Jim
Amsden <jamsden@us.ibm.com>
Cc: OASIS
<oslc-core@lists.oasis-open.org>
Date: 24/09/2015
23:14
Subject: Re:
[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>
Jim,
Still catching up after vacation. Your comments contain some misinterpretations.
I've replied in other several places. Is this still an issue. fyi, I tried
to clarify the meaning of these vocabulary terms in the W3C submission.
Please refer to http://www.w3.org/Submission/2014/SUBM-shapes-20140211/#valueType.
-- Arthur
On Wed, Sep 2, 2015 at 5:12 PM, Jim Amsden <jamsden@us.ibm.com>
wrote:
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
919-525-6575
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: http://tools.oasis-open.org/version-control/browse/wsvn/oslc-core/jra-editing/specs/dialogs.html#discovery_link
and oslc:selectionDialog/oslc:creationDislog triples in oslc:Service resources:
http://tools.oasis-open.org/version-control/browse/wsvn/oslc-core/jra-editing/specs/discovery.html#dialogs
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 |
IBM United Kingdom Limited Registered in England and Wales with number
741598 Registered office: PO Box 41, North Harbour, Portsmouth, Hants.
PO6 3AU
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number
741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number
741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6
3AU
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