multiple capabilities of the same kind

[Doug Tody]

On Fri, 15 Feb 2008, Ray Plante wrote:

> Hi Doug,
> 
> On Fri, 15 Feb 2008, Doug Tody wrote:
> > How do we handle a service which supports two versions of the same
> > interface - are these separate capability elements as well?  I suspect
> > so, in which case having multiple capabilities of the same basic type
> > is probably unavoidable.
> 
> There is an explicit mechanism for this.  In the case in which the two
> versions both use the same capability type and the capability metadata are the
> same, then one can list multiple <interface> elements, each with a different
> "version" attribute.
> 
> It may be that version 2.0 of a protocol may define a different or extended
> set of capability metedata.  In this case, the new version would define a new
> capability type.  Thus, the two versions would be distinguished by 2 different
> capability elements.

Ok - either will work with a service discovery query as I described.
In either case if we query for a specific version of a service, we get
back the correct capability element, and we don't care (for discovery
purposes) if the same capability element is used for both versions.
The only caveat in the multiple-version-per-capability case is that
the client should include an explicit VERSION parameter when it calls
the service, or it may get the wrong version.  But this will always be
wise when we get to the situation where we have multiple versions.

> The ambiguous case that result in the same difficulty that Paul referred to is
> if the two versions can be described with the same capability type but the
> values of the capability metadata are different.  At the moment, the best way
> to handle this case is to register these as separate resources.

Guess I still don't understand why this is so.  These are two
separate capability elements, hence if the query is really looking
for capability elements (which are what is now a "service") it
should work.  Maybe the issue is that we need to query the registry
for capability elements rather than resources, now that we can have
multiple capabilities per resource?

That is, the result of a service discovery query should be one or
more resource-capability combinations.  If so, it does not matter if
some of these reference a common resource.

	- Doug