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:]
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 ()
Transactions Architect,
HP Arjuna Labs
Phone +44 191 2064538
Fax +44 191
2064203
