RE: interposition requirements

[Bill Pope <bpope@bowstreet.com>]

 

I believe what was discussed at the face-to-face was disallowing or restricting the ability of a sub-participant to register with the root coordinator.  Is that the point that you're addressing?   The other side of the coin, dis-allowing registration of a sub-participant with a sub-coordinator isn't really possible.  A root participant may initiate back end transactions, either BTP or other, without the root coordinator being aware.

 

One of the issues raised was the trust issue.  It is difficult to establish trust from a sub-participant back to the root coordinator when the only context information is a transaction identifier.

 

One of the arguments favoring registration of a sub-participant with the root coordinator was performance.  Would the proponents of this please correct me if I get this wrong.  The position was that flattening the coordination space provided more control of the participants.  Any time delays were minimized because messages were sent to all participants, including sub-participants at the same time rather than requiring a sub-coordinator to act as an application proxy and forward any commit or cancel requests.

 

My opinion is that this breaks down completely for orchestrated transactions.  Even for a transaction set where all of the root participants are atomic, individual sub-participant failures may be recoverable if the coordinator knows the context/semantic/business function in which the sub-participant is called.  For sub-participants this is not knowable at the root coordinator.  Even if the root coordinator knows the kind of thing that is to be called e.g., parcel delivery service, it will not know the business policies at the sub-participant site that define what thing to call in different situations.  This information could (and should IMO of application design) reside in the sub-coordinator.

 

=bill

-----Original Message-----
From: Mark Little [mailto:mcl@arjuna.com]
Sent: Monday, April 30, 2001 7:28 AM
To: bt models
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.

 

Mark.

 

----------------------------------------------
Dr. Mark Little (mark@arjuna.com)
Transactions Architect, HP Arjuna Labs
Phone +44 191 2064538
Fax   +44 191 2064203