Pádraic Brady has posted a two part look at a service discovery process as it pertains to things like OpenID via Yadis.

From part one:

This first entry is an examination of Yadis, a Specification which is relied upon by OpenID. Of course Yadis is a topic of its own, since it is also used by Light-Weight Identity (LID), sxip Identity, and mIDm. You can bet more will join in once Yadis becomes more firmly established.

In this first part, he talks about what Yadis is, how it can help with service discovery, and how it would follow a "breadcrumb trail" to get to the information it needs (like getting the actual Yadi slocation from the initial OpenID request).

From part two:

After all this fuss and running about, it's easy to miss the point of Yadis - getting that final Yadis XRD document that describes the Services associated with a given ID.

Part two looks more at working with the Yadis Resource Descriptor information to grap the Type information out of it for the actual locations you'll needed. He also links to some more information on the Yadis specification in PDF form.