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Subject: RE: [sca-assembly] ISSUE 59: Conversations with more than 2 participants


Mike,
 
We are on the edge of a slippery slope.  We are already on it by having support for conversations at all.  I'm suggesting that we take one more step down the slope, by formalizing something that is already possible with the SCA-J and SCA-C++ specs (sharing conversation IDs), but promoting it to being an assembly concept.
 
You seem to be suggesting that we just go ahead and slide all the way to the bottom of the slope.  The problem with doing this is that there are quite a few issues that would have to be resolved in order for this to be generalized into a more general "shared context" concept, and we have seen on today's call that there is a bit of reluctance to take this on.
 
Michael
 


From: Mike Edwards [mailto:mike_edwards@uk.ibm.com]
Sent: Tuesday, March 25, 2008 7:38 AM
To: OASIS Assembly
Subject: Re: [sca-assembly] NEW ISSUE: Conversations with more than 2 participants


Folks,

Is this really a part of a bigger picture?

Is what we are looking at here not so much a "shared conversation" as a "shared context", where some set of
components are sharing/using some set of data, that may include conversations, but could include other
information too?  (Dare I apply the term "session" to this "shared context" ??)

One thing that I note is that the idea of "propagatesConversation" is probably too simple a description of what
needs to occur:

- take the "diamond case"

  B1
 /  \
A    C
 \  /
  B2

- If B1 terminates the conversation with C by calling some method marked "endConversation" and then starts
another conversation with C, what happens at some future point when B2 calls C?  I would expect that the
original conversation with C has now terminated so that if B2 used the original conversation, an error would
occur.  But should not B2 actually simply use the new conversation that B1 started with C?  In other words,
the "conversation with C" is part of the "shared context" and both B1 and B2 use that shared context whenever
they communicate with C...

Viewing this as "shared context" may also deal with some more complex cases:

  D - B1
 /      \
A        C
 \      /
  B2---/

Let's assume that D isn't conversational, but that B1 and B2 are conversational.

It seems strange to me to envisage propagating a conversation through D.  But
thinking of it as shared context in which they all participate - all can access any
of the shared context that they need.

The question then is the lifecycle of the shared context - when is it created, when
is it destroyed.  Which components participate in it.

I note that this looks very similar to some of the transaction context that has been
debated in the Policy group.  It's just that now we're expanding its use to other
circumstances and other context data.


Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com



"Michael Rowley" <mrowley@bea.com>

20/03/2008 20:39

To
"OASIS Assembly" <sca-assembly@lists.oasis-open.org>
cc
Subject
[sca-assembly] NEW ISSUE: Conversations with more than 2 participants





 
RAISER: Michael Rowley
 
TARGET: SCA Assembly Specification WD03, section 8.3 “Conversational Interfaces”
 
DESCRIPTION:
 
Line 2386 says: “Conversations occur between one client and one target service.”
 
However, we have encountered situations where the user would like for a single conversation to occur among a larger group of participants, rather than considering every conversation to be limited to two parties.
 
In many circumstances, you can’t really tell.  If a call chain looks like this:
  A – B – C
 
Then it isn’t obvious how it would be different if you consider the A/B conversation to be the same conversation as the B/C conversation or different.  However, if you have a diamond calling pattern like this:
 
  B1
 /  \
A    C
 \  /
  B2
 
Then it is important.  If there are multiple conversations, then there will be multiple instances of C, one for its conversation with B1 and one for its conversation with B2.  If it is all considered to be one conversation, then there will only be one instance of C.  (It is easiest to think of this in terms of instances, but even if C were “stateless”, it would still be semantically sharing state for the conversations with B1 and B2).
 
In a common case, all of the components within a composite should share a single conversation.  However, the mechanism should also allow for conversation sharing at a smaller scope.
 
Note that the diamond pattern shown above is the way that the SCA Java specification treats the request scope.  The request scope is like a conversation that is shared among all of the local services involved in the handling of a single remotable request.  If the assembly specification introduced N-ary conversations, then the SCA-J specification would be able to eliminate the concept of the request scope in favor of this more general mechanism.
 
PROPOSAL:
 
Introduce a new “propagateConversation” annotation on components (or possibly an implementation intent).
 
If component B is so marked, then the conversation ID that is sent by its client (A) will also be passed to any downstream services that are also conversational.  If a component that does not offer a conversational service is marked with “propagateConversation” then it just means that all of the conversational references will share the same conversation ID (this is what would occur with “A” in the diamond pattern above).
 
If a composite is marked with propagatesConversation, then it is as if all of the components of the composite were similarly marked.
 
It is unclear what should happen in an A – B – C pattern if A and C are conversational, but B is not.  Should B be required to be able to propagate the transaction, even if it is not itself conversational?
 
 
 
 






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