XRD: Single resource, resource set?

[Eran Hammer-Lahav <eran@hueniverse.com>]

- The XRD describes resources. So it seems logical to allow it to list the
resources it describes. Previously, I suggested using the URI element to do
so at the XRD level. However, this is incorrect because the URI element at
the Link (Service) level implies redundancy but at the XRD level, there is
no implied relationship between the multiple resources the XRD describes. As
long as the resources described share the same descriptor, they can all be
listed and have nothing else in common.

This brings up the following:

* Should a single XRD be allowed to describe a group of resources that are
not interchangeable?

* Should we use a different element name, such as <About> instead? I fear
<URI> at the XRD level will be confusing (it confused me).

* Do we need a way to indicate an 'equivalent' relationship between the
resources described by the XRD and other resources? We can always do it
using a <Link> relationship, but today XRDS has a few ___ID elements that
provide such specialized mechanism.

I had a lot of trouble dealing with CanonicalID, mostly in light of the idea
of a single XRD describing multiple resources. So I took it out to try and
see what it looks like. I came up with something like this:

<XRD>
    <Expires>2009-01-01T08:30:00Z</Expires>
    <About>http://example.com/resource/1</About>
    <Type>http://example.com/type/profile_photo</Type>

    <Link>
        <URI>http://example.com/resource/2</URI>
        <Rel>http://example.com/rel/profile</Rel>
    </Link>
</XRD>

Now, if we want to say that the resource the XRD describes has aliases, we
can do it in two ways:

<XRD>
    <Expires>2009-01-01T08:30:00Z</Expires>
    <About>http://example.com/resource/1</About>
    <Type>http://example.com/type/profile_photo</Type>
    <Equiv>http://example.com/resource/1_copy</Equiv>

    <Link>
        <URI>http://example.com/resource/2</URI>
        <Rel>http://example.com/rel/profile</Rel>
    </Link>
</XRD>

And

<XRD>
    <Expires>2009-01-01T08:30:00Z</Expires>
    <About>http://example.com/resource/1</About>
    <Type>http://example.com/type/profile_photo</Type>

    <Link>
        <URI>http://example.com/resource/1_copy</URI>
        <Rel>equiv</Rel>
    </Link>
    <Link>
        <URI>http://example.com/resource/2</URI>
        <Rel>http://example.com/rel/profile</Rel>
    </Link>
</XRD>

So it is not clear we need the special element, unless it supports a very
common use case for which the Link method is too heavy. I don't have such a
use case.

EHL