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Subject: interposition requirements

I've been talking to Jim Webber and Savas P. who attended last weeks face-to-face. From their notes, and from little I could hear on the teleconference, it would appear that there was some discussion about sub-participant registration. In this email I'll term the web services that a client interacts with directly the root-participants, and any web service that those services talk to the sub-participants.
Now, a root participant receives a context it must be BTP-aware (and I think we agreed on this at the meeting). Therefore, at some point in its life it will register with the coordinator as a root-participant that can be driven during the termination protocol. If during the course of an application invocation a root participant needs to contact another BTP-aware service, the question is, where does the sub-participant register?
In a traditional TP environment, if all participants had to register with the root coordinator then performance would typically be in the drain, especially if one node has lots of participants. Therefore, the typical way to get round this is to use interposition, whereby a proxy for the transaction manager (a sub-coordinator) is registered with the root, and local participants register with the sub-coordinator. The advantage to performance is obvious. However, interposition is not mandated, and a participant that wants to register back with the root coordinator is (usually) free to do so.
So, do we want something like this in BTP? From what I've heard, the answer was no. I disagree with this for several reasons:
(i) performance: see above.
(ii) trust: the root coordinator may only trust requests (e.g., registration) from the root services.
(iii) connectivity: there may be no direct route to the root coordinator for the sub-participants, e.g., an intranet with only a few external ip addresses, but which has lots of internal web services.
What I would suggest is that we allow interposition, such that if a sub-participant receives a context, it is up to the service that sent it whether or not it wants to modify the context to point to a sub-coordinator and not directly back to the root coordinator. The sub-coordinator piece to this puzzle is not web service specific, so we're not putting a burden on the application developer; it's something that can be developed once for all time.
Dr. Mark Little (mark@arjuna.com)
Transactions Architect, HP Arjuna Labs
Phone +44 191 2064538
Fax   +44 191 2064203

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