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Subject: RE: [wsbpel] Issue 6.2 - Proposal For Vote
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 this 6.2 > proposal. That is: whether branches of <flow> should be restricted to a > scope-based only when <completionCondition> / earlyCompletion is used. > > However, since there is a Apr 1st deadline for proposal submission, it may be > better to submit the proposal on what was drafted so far without waiting to > resolve this open issue. And, let other people comment on this open issue. > > Here is my detailed viewpoints and analysis we need a scoped-only restriction > when early completion is used: > > ======================================================== > > Scope-based-only branches restriction for early completion is needed, because: > > * early completion mechanics does not change fundamentally regardless > whether we are using <complete> activity or <completionCondition> > * we need a scope to encapsulate activities of branch as _"units of work"_ > to minimize the need of defining <completionHandler> > * a partial termination of a scope is NOT well-defined in BPEL, while > termination of the whole scope is already well-defined in BPEL spec. > > > *_How termination mechanism works in BPEL: _* > Here is my understanding of how the termination mechanism in BPEL works: When a > scope is sent with a termination signal, the targeted scope will: (1) cascade > the termination signal inner child scopes (2) stop all the inner activities > within the targeted scope (3) execute the termination handler. Then, done. > > As of now, the termination signal MUST alway go through a scope to other > NON-scope activity. And, the termination Handler of the corresponding scope MUST > got activated. > > Hence, termination of the whole scope is well defined. However, a partial > termination of a scope is NOT well-defined. That is we have not defined: > > * what should happen if some branches got terminated, while some branches > are completed. > * what should happen if the termination signal to sent to NON-scope > activities without going through a <scope>. > > > *_Example #1_* > > Consider the following, a flow with two sequence as branches, which is NOT > scope-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 to invent a > brand 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 current termination > mechanism 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 to invoke the > compensation handler of scope "B1SS1". By default, the > terminationHandler of scope "B1" will invoke that compensation handler. > > > Hence, we minimize the need of "completionHandler". I hope I have refreshed your > memory on why we come up with this scope-based branches. (Regardless we use > <complete> activity or not). > > > *_Example #2_* > > One important to remember: <flow> only controls the parallelism of execution. It > does not interfere the compensation and termination structure of a process. > > Without scope-based-branch only restriction, the net effect of a <flow> is just > a parallellized execution of activities inside the flow in an undeterministic > order. So, in one extreme case of this undeterministic order, one inner branch > got completed first and then the other inner branch got executed. Hence, one > extreme case of <flow> is equivalent to the following nested sequence pattern in > the context of compensation and termination. (I replace <flow> in exmple #1 with > an outter <sequence> ... You can also think of one control-link is used from > sequence "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 early completion, > we need to do this "partial" termination magic. We don't have a well-defined way > to terminate sequence "EX2B2" without affecting sequence "EX2B1". Similarly, we > don't have a well-defined way to terminate sequence "EX2B2" without affecting > "EX2B2SS1" > > Now think again, if we have scope-based-only branches, all activities are > separated in units-of-work. Now those branches have independent compensation and > termination structure now. > > Last not least, if we map this early completion idea to transaction world, it > does not seem like a good idea to allow partial transaction-work rollback > (mapped to termination & compensation) without any units-of-work boundary marker > or 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 when all >>its (directly) nested activities have completed, either successfully or >>unsuccessfully. 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 for all >>its 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 of execution >>of 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 activity completes. >>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. You may a link to this group and all your TCs in OASIS >>at: >>https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php >> >> >> > --------------------------------------------------------------------- To unsubscribe from this mail list, you must leave the OASIS TC that generates this mail. You may a link to this group and all your TCs in OASIS at: https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
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