Category Archives: Tech

After an email exchange with Anish about my previous blog entry I would like to clarify what I wrote on Sunday. I agree that EPRs can be used as identifiers. Just like phone numbers. For example, a specification could require that the Reference Properties be constituted of the element, in effect making the EPR an identity representation by virtue of containing one. And this is fine.

My point is not that this should not be done, it is that the WS-Addressing working group does not need to concern itself with, or even acknowledge this usage of EPRs. The Ws-Addressing working group should concentrate on producing an XML element to package the information necessary to invoke a Web service endpoint. This is what is asked of it. Considering whether and how this can be used for identity consideration is at best a waste of time and at worst a source of unneeded complexity and formalism. Keep it simple.

Note: the most alert readers will have noticed that the prefix of the namespace for the MUWS ResourceId element has changed between my entry on Sunday and this entry from “muws-xs” to “muws-xs-1”. Looks like a detail but this is a sign of an important and very cool improvement that was decided by the WSDM TC today. I am implementing this change in the MUWS spec right now (this blog entry is my little “pause” from the editing work, scary as it sounds). Stay tuned for more. WSDM 1.0 is coming soon to a printer (and a JVM) near you.

I haven’t yet decided whether to join the WS-Addressing working group at W3C and I must say that the deluge of emails going on over there doesn’t do anything to alleviate my main fear about joining the group, i.e. that it will suck a lot of time. Just as I am trying to free more time to work internally on architectural issues related to the MAE (Management for the Adpative Enterprise) effort rather than externally, in standards bodies. But I am monitoring the mailing list of the WS-Addressing working group as what happens to the EPR concept is important to many specifications I am involved with, such as WSDM, WS-Notification and the WSRF patchwork. And it’s always interesting to read what the smart people involved in this discussion have to say. It feels like the Web services communitty has been waiting for an opportunity to have this discussion for a long long time.

One thing that is clear to me after using EPRs for a while is that they are not meant to be identifiers, just references. Their only goal is to tell you how to talk to an endpoint. Francisco Curbera hits the nail on the head in this email message. Identifying endpoints and whatever it is that they represent is a different thing altogether and not in scope for WS-Addressing. It is in scope for management applications of Web services and this is exaclty what WSDM MUWS (Management Using Web Services) provides through the muws-xs:ResourceId property, along with (optionally) the “correletable properties” capability.

Other issues that the WS-Addressing group now has to deal with include usage of Reference Properties (and especially how they get mapped to SOAP headers) and what Reference Parameters are good for. Those got slipped into the version of the spec submitted to W3C and were not available to us when designing WSDM, WSN and WSRF. Not that we would necessarily have used them anyway. Another issue that hasn’t yet showed up on the WS-Addressing mailing list is whether it is appropriate for someone to modify EPRs it receives if there is some “out of band” understanding of how they are built. My guess is that the group will punt on that one (which is reasonable) but I am looking forward to the discussion and will raise this question if no group member does. I might write more on some of the topics in this last paragraph in later entries.