By | 2004/01/07

WS-Eventing, from BEA, MS, and Tibco.

The good news is that finally, we’ve got a Web services spec that tackles the hard problem, interfaces. Yes, WS-Eventing is an application protocol (do we have to bind SOAP to it too?)

The bad news is that it continues the track record of abusing other application protocols (if indeed it’s objective is to be used with them, which I can only assume is the case); the more that goes in the SOAP envelope, the less suitable it is for use with them, as it just duplicates what they already provide. Once again we see WS-Addressing as the culprit; wsa:Action and wsa:To exist to replace their counterparts in any underlying application protocol. For example, for HTTP, the counterparts of wsa:Action and wsa:To are the request method and request URI, respectively.

A point of frustration for me is that the semantics of subscription itself are largely uniform. What I mean is, wouldn’t it be great if all Web services supported “subscribe”? So why not use HTTP like I’ve done, which is built for uniform semantics? Using a SOAP envelope with MONITOR and for the resultant notifications would be really sweet.

One pleasant surprise is the form that notifications take, as in this example. Notice the use of wsa:Action; the value is no longer a method, but is instead a type. Woot! That’s the first time I’ve seen the “action” semantic used properly in any Web services work. Presumably this is due to notification semantics being entirely geared towards simply getting some data to some other application; basically, POST. Of course, technically, I don’t believe any “action” is required in this case, as there’s no intent on behalf of the notification sender beyond simple data submission; the intent is determined and assigned by the recipient of the notification. But that’s still progress!

Another upside is the use of fine grained URIs for identifying the endpoints, e.g. “”, rather than something like “”.

Overall, very disappointing from a protocol and pub/sub POV, but the progress on resource identification, uniform semantics (even if it’s accidental 8-), and intent declaration is quite encouraging. Perhaps the next attempt will be done as an HTTP extension with a SOAP binding to MONITOR (the existing binding to POST would suffice for notifications).

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