Delegation Terminology / Linked XRD

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

I think there is a lot of confusion when it comes to delegation. XRD does not delegate anything. It simply describes resource attributes which for some application may mean delegation. But it is important to note that XRD only links to other resources.

A link inside an XRD is semantically equivalent to a link header or element associated with the same resource. Applications may want to limit how such links are consumed for performance and simplicity but it does not change the semantic meaning of all these links.

Also, XRD does not link to anything - it only describes link. It is the resource that links to other resources. XRD really can't do anything other than describe. All other verbs belong to the Subject (according to the XRD authority).

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If a resource has 2 link headers:

Link: <http://example.com/meta/1>; rel="describedby"; type="application/xrd+xml"
Link: <http://example.com/author/a>; rel="author"; type="text/html"

And the content of http://example.com/meta/1 is:

<XRD>
	<Link>
		<Rel>copyright</Rel>
		<URI>http://example.com/copyright</URI>
	</Link>
	<Link>
		<Rel>describedby</Rel>
		<URI>http://example.com/meta2</URI>
	</Link>
</XRD>

And the content of http://example.com/meta2 is:

<XRD>
	<Link>
		<Rel>http://example.org/rel/example</Rel>
		<URI>http://example.com/example</URI>
	</Link>
</XRD>

It is the same as if these 5 links were all in the resource header, resource body as Link elements, or in a single XRD associated with the resource.

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I need to catch up on the Linked XRD proposal but it seems like the idea is to explicitly say that when one XRD describes a link to another XRD with relation type describedby, that clients are required/recommended to go and fetch that other XRD as well. It might also suggest that there is an order of precedence between the links found in the first XRD to those found in the second. Do I have this right?

I don't think we can say much more (if any) than that if an XRD describes a link with relation type 'describedby' and media type 'application/xrd+xml', that information found there should be treated as a logical OR with the current XRD.

EHL