Subject: Issue - R26 - Default Compensation Order Conflict
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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".
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:
- scope "S2" has a direct peer-scope dependency on scope "S1"
- the scope-controlled set of activities of scope "S1" contains default termination handlers
- 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".
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