I finally got to take a look at the WS-MetadataExchange specification while hanging out in Don's office last week. The spec is fairly straightforward, it defines a mechanism for one to request the WSDL, Policy or XML Schema of a target namespace (i.e. a URI) from an XML Web Service endpoint. Basically one can ask what services an endpoint supports and what the messages the end point accepts should look like.
Both Don and Omri have suggested that WS-MetadataExchange can solve a problem I had with the SOAP-based version of the ATOM API. The problem is how an ATOM client is supposed to know what services an ATOM end point supports. Here are three descriptions of ATOM-enabled sites that I might want to interact with as an RSS Bandit user.
A weblog that supports user comments posted anonymously and provides the ability to search the weblog archives. The user comments must use a subset of HTML. For example, Sam Ruby's weblog.
A weblog that doesn't have comments enabled but does provide the ability to search the weblog archives. For example, Mark Pilgrim's weblog
A weblog that only supports comments that have been authenicated with TypeKey and doesn't support search. Again user comments must use a subset of HTML. Any Movable Type blog that supports TypeKey is an example.
All three would require a smart client to give the user visual hints and clues as to how they can interact with the site. At the very minimum a search box that is grayed out when the target weblog doesn't support search.
So far the only mechanism I've seen proposed for solving this problem in the case of the ATOM API is the link element used for locating service endpoints. This allows you to get the URI of service end points like where to post comments or where to send search results if they exist but do not answer finer grained questions. Questions such as “What subset of HTML can I use in comments?” or “Do I need to be authenticated before I post comments” are currently not answered by any of the draft ATOM specs.
So far WS-MetadataExchange or something like it look like the best way to support such scenarios for SOAP-enabled ATOM end points in a way that is consistent with the Global XML Web Services architecture. I would be interested in seeing an ATOM-specific solution evolve as well since some of this issues hurt usability of weblogs. I've lost count of the amount of times I've posted a comment or seen someone post a comment only to complain about the fact that the weblog doesn't support HTML or mangled some text. Having a way to inquire about this in a standard way would definitely improve the user experience.