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Subject: Re: [wsbpel] Issue 6.2 - Proposal For Vote
Just want to add few brief comments: (A) During the last F2F, I thought Satish only suggested to limit early completion to <completionCondition> in <flow>, maybe in parallel <forEach>. I guess I got the wrong impression. Restricting early completion to parallel <forEach> only is a way too restrictive usage from viewpoint. And, restricting to <forEach> only make the issue 6 discussion totally dependent on Issue 147. And, if Issue 147 does not get passed, the discussion of Issue 6 becomes meaningless. (B) I tend to think it is not that common to have early completion logic to apply to a flow where control links are used. Because, when early completion condition is used, different branches within a flow are usually relatively independent tasks to achieve the same goal (i.e. completion condition). (Of course, I would be happy to hear convincing counter examples from others). (C) Ok. Let's say we want to cover the edge case of control-links + early-completion. Assume we accept Satish's supplementary interpretation of activity termination. Then, we shall be able to allow a flow with early-completion condition with control links without enforcing scope-based restriction. Then, it goes back to the same old question, as Satish rephrased it: "should we protect the naïve process designer from unintended pitfalls by enforcing a scope boundary for branches that may be prematurely terminated by early completion?" Do we trust our users that they can write a flow with control links + early completion and still get the compensation handler right? That is a conscious choice that we need to make. As of now, I am neutral to this question. (D) BTW, if we pass Issue 147 parallel for-each, I tend to think a scope-based restriction should be applied again to child activity in a parallel-each for to hold the iterator value properly for compensation ... (Of course, that should be the discussion of 147 ... and of course ... Yaron will yell at me again after he read this part of email ... ) Regards, Alex Yiu Satish Thatte wrote: One concern with making all children scopes is that this is kinda useless when your control flow is governed mostly by links, in the WSFL style. This also creates additional concerns relating to compensation order and the constraints imposed by the resolution of issue 10 on link structure crossing scope boundaries. This is one reason why I had suggested limiting the early completion proposal to the proposed parallel foreach construct, which, being a specialized form of flow, can have more peculiar semantics, although I am sure Yaron will not thank me for this suggestion :-) Satish -----Original Message----- From: Yaron Y. Goland [mailto:ygoland@bea.com] Sent: Tuesday, April 05, 2005 3:14 PM To: Alex Yiu Cc: wsbpel@lists.oasis-open.org; Trickovic, Ivana Subject: Re: [wsbpel] Issue 6.2 - Proposal For Vote Requiring that all the children of the flows be scopes is just bizarre. If such contortions are needed to make this proposal work then I would humbly suggest that this proposal is not ready for approval. Yaron Alex Yiu wrote:Hi all, Ivana and I were still discussing an open technical issue about this6.2proposal. That is: whether branches of <flow> should be restricted toascope-based only when <completionCondition> / earlyCompletion is used. However, since there is a Apr 1st deadline for proposal submission, itmay bebetter to submit the proposal on what was drafted so far withoutwaiting toresolve this open issue. And, let other people comment on this openissue.Here is my detailed viewpoints and analysis we need a scoped-onlyrestrictionwhen early completion is used: ======================================================== Scope-based-only branches restriction for early completion is needed,because:* early completion mechanics does not change fundamentallyregardlesswhether we are using <complete> activity or<completionCondition>* we need a scope to encapsulate activities of branch as _"unitsof work"_to minimize the need of defining <completionHandler> * a partial termination of a scope is NOT well-defined in BPEL,whiletermination of the whole scope is already well-defined in BPELspec.*_How termination mechanism works in BPEL: _* Here is my understanding of how the termination mechanism in BPELworks: When ascope is sent with a termination signal, the targeted scope will: (1)cascadethe termination signal inner child scopes (2) stop all the inneractivitieswithin the targeted scope (3) execute the termination handler. Then,done.As of now, the termination signal MUST alway go through a scope tootherNON-scope activity. And, the termination Handler of the correspondingscope MUSTgot activated. Hence, termination of the whole scope is well defined. However, apartialtermination of a scope is NOT well-defined. That is we have notdefined:* what should happen if some branches got terminated, while somebranchesare completed. * what should happen if the termination signal to sent toNON-scopeactivities without going through a <scope>. *_Example #1_* Consider the following, a flow with two sequence as branches, which isNOTscope-based. <scope name="A"> <flow> <completionCondition> ... </completionCondition> <sequence name="B1"> ... <scope name="B1SS1"> ... </scope> <receive name="B1R2" ... /> ... </sequence> <sequence name="B2"> ... <scope name="B2SS1"> ... </scope> <receive name="B2R2" ... /> ... </sequence> </flow> </scope> Say, sequence "B2" finishes. The completionCondition is fulfilled.Sequence"B1" is being terminated when it is waiting a message at the receive"B1R2".Now, we got the following questions _unanswered, if we don't want toinvent abrand variation of termination mechanism_: * Will the terminationHandler of scope "A" be activated? * Should the compensation handler of scope "B1SS1" be invoked?Which"handler" logic is responsible to determine that? Ok. Let's change it into scope-based branches. <scope name="A"> <flow> <completionCondition> ... </completionCondition> <scope name="B1"> <terminationHandler> ... </terminationHandler> <sequence> ... <scope name="B1SS1"> ... </scope> <receive name="B1R2" ... /> ... </sequence> </scope> <scope name="B2"> <terminationHandler> ... </terminationHandler> <sequence> ... <scope name="B2SS1"> ... </scope> <receive name="B2R2" ... /> ... </sequence> </scope> </flow> </scope> The above questions got _answered clearly, with reusing the currentterminationmechanism easily_: * Will the terminationHandler of scope "A" be activated? o No. * Should the compensation handler of scope "B1SS1" be invoked?Which"handler" logic is responsible to determine that? o It is up to the terminationHandler to decide whether toinvoke thecompensation handler of scope "B1SS1". By default, the terminationHandler of scope "B1" will invoke thatcompensation handler.Hence, we minimize the need of "completionHandler". I hope I haverefreshed yourmemory on why we come up with this scope-based branches. (Regardlesswe use<complete> activity or not). *_Example #2_* One important to remember: <flow> only controls the parallelism ofexecution. Itdoes not interfere the compensation and termination structure of aprocess.Without scope-based-branch only restriction, the net effect of a<flow> is justa parallellized execution of activities inside the flow in anundeterministicorder. So, in one extreme case of this undeterministic order, oneinner branchgot completed first and then the other inner branch got executed.Hence, oneextreme case of <flow> is equivalent to the following nested sequencepattern inthe context of compensation and termination. (I replace <flow> inexmple #1 withan outter <sequence> ... You can also think of one control-link isused fromsequence "EX2B1" to "EX2B2"). -------------------------------- <scope name="EX2A"> <sequence> <sequence name="EX2B1"> ... <scope name="EX2B1SS1"> ... </scope> <receive name="EX2B1R2" ... /> ... </sequence> <sequence name="EX2B2"> ... <scope name="EX2B2SS1"> ... </scope> <receive name="EX2B2R2" ... /> ... </sequence> </sequence> </scope> -------------------------------- Say, the execution is waiting at receive "EX2B2R2". Now due to earlycompletion,we need to do this "partial" termination magic. We don't have awell-defined wayto terminate sequence "EX2B2" without affecting sequence "EX2B1".Similarly, wedon't have a well-defined way to terminate sequence "EX2B2" withoutaffecting"EX2B2SS1" Now think again, if we have scope-based-only branches, all activitiesareseparated in units-of-work. Now those branches have independentcompensation andtermination structure now. Last not least, if we map this early completion idea to transactionworld, itdoes not seem like a good idea to allow partial transaction-workrollback(mapped to termination & compensation) without any units-of-workboundary markeror checkpoints (mapped to <scope>). ======================================================== Thanks!!! Thank you so much for reading this long email! Regards, Alex Yiu Trickovic, Ivana wrote:Motivation =========== The current semantics of the flow activity is that it completes whenallits (directly) nested activities have completed, either successfullyorunsuccessfully. However, there are scenarios where it is necessary to have ability to complete the flow activity before all its nested activities complete in order to speed up the process execution. For example, a process waits in parallel for 3 reviews of a paper. If 2 positive reviews are received the process may continue with the execution without waiting for the last, third response. The completion condition of may have the following flavors: * Wait for N out of M nested activities to complete * Wait until after boolean condition C evaluates to true The completion condition is interesting for a flow activity enclosing identical nested activities and for the parallel for-each activity (still under discussion). Proposal ========= Syntax: <flow standard-attributes> standard-elements <links>? <link name="ncname">+ </links> <completionCondition>? activity+ </flow> <completionCondition> <branches expressionLanguage="URI"? countCompletedBranchesOnly="yes|no"?> an-integer-expression </branches>? <booleanExpression expressionLanguage="URI"?> a-boolean-expression </booleanExpression>? </completionCondition> Semantics: (1) The completionCondition element is an optional element of the flow activity. Default behavior of the flow activity is that it waits forallits nested activities to complete. (2) There are two kinds of completion condition: A> <booleanExpression>: A boolean condition operating upon process variables. It is evaluated at the end of execution of each nested activity. B> <branches>: An integer value expression which is used to define condition of flavor N out of M. It is evaluated at the end ofexecutionof each nested activity. This condition has "at least N out of M" semantics. (The exact N out of M condition semantics involve resolving racing condition among nested activities.) (3) Both conditions (<branches> and <booleanExpression>) may be specified at the same time. They will be evaluated at the end of execution of each nested activity. If at least one condition evaluates to true the <flow> activity completes successfully terminating all remaining running nested activities. If both conditions are specified, the <branches> will be evaluated first. If the boolean condition is specified the evaluation of the condition is done in a serialized fashion with respect to the nested activities directly enclosed in the flow activity. (4) If the integer value evaluated from the <branches> expression is larger than the number of nested activities in the <flow>, then bpws:invalidBranchCondition fault MUST be thrown. Note that the number of branches may be known only during runtime in some cases. Static analysis should be encouraged to detect this erroneous situation at design time when possible. (E.g. when the branches expression is a constant.) (5) <branches> expression has an optional attribute "countCompletedBranchesOnly". Its default value is "no". If countCompletedBranchesOnly is "no", it means the BPEL processor will count branches which have completed (either successfully or unsuccessfully). If countCompletedBranchesOnly is "yes", it means the BPEL processor will count branches which have completed successfully only. (6) If flow activity specifies a completionCondition element the completion condition is evaluated each time a nested activitycompletes.If the completion condition evaluates to true the flow activity completes successfully. All still running nested activities will be terminated. (7) Standard BPEL termination semantics applies to running nested activities when the completion condition is met. The termination of running nested activities follows the termination semantics defined in the specification (see section 13.4.4 Semantics of Activity Termination). (8) If all nested activities of the flow activity have been completed but the completion condition evaluates to false the "bpws:completionConditionFailure" MUST be thrown by the flow activity. (end) ---------------- Ivana --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. 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