Re: [wsbpel] Issue - R26 - Default Compensation Order Conflict

[Danny van der Rijn <dannyv@tibco.com>]

Yeah, my relaxation was to change to Consider scopes A and B such that B has a *Peer Scope Dependency or a control dependency* on A, but that still doesn't fix this issue.  I told you that I hadn't understood the issue completely!  While this doesn't fix this issue, it's still probably worth examining.

Where Dieter says there's a contradiction, I think that the spec doesn't actually contradict itself.  The behavior would be as Dieter describes.  The question in my mind is Do we have a bug here?

Danny

Rania Khalaf wrote:

Hi,

I was writing this mail during our discussion on the call today, so decided to just wrap it up and send it as a recap and follow-on questions.

 First just a quick note that  A and B are not peer scopes per the def (their parents, s1 and s2 are) so the wording below is off but Danny was  suggesting today  mod'ing it a bit.

If s1 and s2 have completed, then there is no problem because the peer scope stuff takes care of it.

The problem is if S1 was still running but S2 has completed.

Then the spec dictates that first you do termination of S1 , then once that completes, you do compensation of S0. Problem is that S1's termination compensates A. This will always happen ( per today's def of termination handlers ) before B if S1 was still running and S2 had completed.

Now looking at proposals discussed to tackle it:

Dieter's proposal would make sure that one never has the case where S1 is still running but S2 has completed so we will avoid this wierd behavior at runtime. 

I'm not sure I follow how the idea Danny is showing (coz he said he will relax the wording) addresses the clash between the compensation routines of the  termination handler (s1) and the scope that kicked of the termination (s0). I mean, I don't see how we wouldn't have to change the termination/fault handling behavior to enforce the proposal. Or are you suggesting following it up with a static validation check of some sort ?

Alex is in support of Danny's direction, but i am confused about how that would work so let's follow up on e-mail.

regards,
Rania


Danny van der Rijn wrote:

I'm not sure I understand your case completely, but if I do, my understanding is that by the definition in 12.5.1, there exists a Peer Scope Dependency from S1 to S2.  I think the intention of "Rule 1" in that section was to say that this should cause compensation of S2 before S1, but, in fact, it does not.

My alternate suggestion would be to move the 3 definitions before "Rule 1" and to Modify the following the first sentence in the following paragraph

"Consider scopes A and B such that B has a control dependency on A. Assuming both A and B completed successfully and both must be compensated as part of default compensation behavior, the compensation handler of B MUST run to completion before the compensation handler of A is started. "

to say

"Consider scopes A and B such that B has a *Peer Scope Dependency* on A. Assuming both A and B completed successfully and both must be compensated as part of default compensation behavior, the compensation handler of B MUST run to completion before the compensation handler of A is started."



ws-bpel issues list editor wrote:

This issue has been added to the wsbpel issue list with a status of "received". The status will be changed to "open" if a motion to open the issue is proposed and that motion is approved by the TC. A motion could also be proposed to close it without further consideration. Otherwise it will remain as "received".

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Issue - R26 - Default Compensation Order Conflict

Status: received
Date added: 30 Oct 2006
Date submitted: 27 October 2006
Submitter: Dieter Koenig
Description: Consider the following hierarchy of scopes. In this example, none of the scopes is isolated, all scopes only have default fault handlers and default termination handlers, and scopes "A" and "B" have custom compensation handlers.

scope name="S0"
   flow
      scope name="S1"
         scope name="A"
            source link="fromAtoB"
         ...
      scope name="S2"
         scope name="B"
            target link="fromAtoB"
         ...
      activity name="E"
  

Assume a point in time where scopes "A" and "B" and "S2" have completed successfully, and scope "S1" and activity "E" are still running.

Further assume that activity "E" now throws a fault which is caught by the default fault handler of scope "S0".

1. All running activities inside of scope "S0" are terminated, and the default termination handler of scope "S1" compensates scope "A".
2. The default fault handler of scope "S0" then compensates scope "S2", which in turn compensates scope "B".

The observed compensation order "A then B" is caused by the sequence "first termination, then fault handling". OTOH, the required compensation order "B then A" implied by the control dependency of "B" on "A".

This is a contradiction.

Note that this situation cannot occur when the scope "S1" is an isolated scope. In this case, the link "fromAtoB" cannot leave the scope "S1" before it completes. In this case, scope "S1" always completes before scope "S2", and compensation of "A" cannot be caused by a termination handler before scope "B" is compensated.

Moreover, this conflict is irrelevant if scope "S1" OR scope "S2" only contain default compensation handlers, because in this case, no custom compensation logic can be executed in the wrong order.
Submitter's proposal: In order to avoid conflicts between the termination-handling / fault-handling sequence and the default compensation order, different strategies can be considered, for example, enforcing the absence of such conflicts during static analysis. A less restrictive approach is proposed here, which only affects the runtime behavior.

At the end of section 12.5.2, add the following paragraph:

"The default compensation order is initiated by default termination handlers and default fault handlers, and recursively carried forward by compensation handlers. The termination phase always precedes the fault handling phase (see section 12.6. Termination Handlers). This sequence must not create a conflict with the default compensation order of scopes that are in a control dependency relationship. If all of the following is true:

1. scope "S2" has a direct peer-scope dependency on scope "S1"
2. the scope-controlled set of activities of scope "S1" contains default termination handlers
3. the scope-controlled set of activities of scope "S1" AND scope "S2" both contain at least one custom compensation handler then a WS-BPEL processor MUST execute these scopes as if "S2" had a control dependency on scope "S1".

Changes: 30 Oct 2006 - new issue

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