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Subject: Re: [wsbpel] Issue 6 - Rough draft of proposal for vote
Hi Danny, It was not a matter of complexity, rather there was an important concern about adoption; we made a careful attempt to limit the amount of new or "advanced" features that were uncommon in workflow practice. "Immediate evaluation" was one of those, but we also decided to mention it explicitly in the spec because we consider it a very powerful extension which the language should support at some point. Compared with some of the function BPEL has since introduced since then "immediate evaluation" looks like small change. Paco Danny van der Rijn To: Axel Martens/Watson/IBM@IBMUS <dannyv@tibco.com cc: "'wsbpel@lists.oasis-open.org'" <wsbpel@lists.oasis-open.org> > Subject: Re: [wsbpel] Issue 6 - Rough draft of proposal for vote 09/01/2004 04:18 PM while i like the simplicity, i am concerned that in WSFL 1.0, the "immediate" value was not allowed: "V1.0 remark: deferred is the only valid value for the when attribute in Version 1.0." can you comment on the reasons that it was not included in 1.0? i imagine that there were complexities introduced that were not desirable for a 1.0 spec. i wonder if those concerns are also relevant for BPEL. Axel Martens wrote: Dear all: I'd like to participate in your discussion with a proposal that comes with minimal changes to the current BPEL specification but is able to handle the mentioned use cases, and even more. First, I'd like to introduce myself. I am a post-doc researcher at IBM Watson, working in the group of Paco Curbera. In my PhD thesis I have dealt with Web service based business processes and Petri nets. Hence, I know the discussion on the mismatch between BPEL and the workflow pattern à la van der Aalst quite well. I completely agree on searching for a general solution to the request of issue 6 rather than fixing distinguished cases. The problem (or restriction) of BPEL is that you can't enable or disable an activity easily w.r.t. to the complex context, like you can do modeling with Petri nets or related methods. The deferred evaluation of the join condition (all incoming links has to be signaled first) is one of the mayor reasons for it. Hence, our group proposes to allow an immediate evaluation of the join condition, too. This feature was already covered by IBM's Flow Definition Language (FDL) and Web Service Flow Language (WSFL). The next paragraph explains the proposed change to syntax and semantics. Afterwards I show three examples to demonstrate the expressive power of this changes. Syntax & semantics: The changes to syntax are minimal. Each activity gets a new standard attribute "joinEvaluation" - of course, this attribute has only influence on activities that are target of at least two links. The attribute can have the values "deferred" (the default value) and "immediate". As a kind of syntactical sugar, each flow or the whole process might redefine the default value for all its child activities through the additional attribute "defaultJoinEvaluation". But, we do not insist on this change. In case, the attribute was set to "deferred", the join behavior stays untouched. If the attribute was set to "immediate", the join condition is evaluated each time one of the incoming links has changed its value. Some examples: "true" and "yet undefined" yields "yet undefined" "false" and "yet undefined" yields "false" "true" or "yet undefined" yields "true" "false" or "yet undefined" yields "yet undefined" This evaluation may yield "yet undefined", than nothing happens. If the join condition could be evaluated to "true" or "false", the activity is executed or skipped, and further changes to the link values are ignored. Because links are not allowed to cross the borders of a while loop and each time a while loop is entered, its internal link value should be cleared, there can't be any confusion. Examples: This changes together with the already existing concepts of scope and fault handling allow an strait forward mapping of the missing pattern. One-out-of-Many First, we take a look on a selection of one result out of three, where the not chosen alternatives are allowed to finish normally. The following figure shows the example of contacting three airlines and booking the first available (for reasons of clarity, the bpel-code is not fully specified). (Embedded image moved to file: pic03178.gif) Because of the immediate evaluation of the join condition, the activity "InvokeBooking" is started directly after the termination of one preceding activity. Whereas the flow is exited only if all its child activities are terminated. In case, the customer should be informed as soon as possible, the reply activity should be moved into the flow in sequence to "InvokeBooking". N-out-of-M The second example shows two new aspects. First, the join requires at least two of the three referees to have finished the review. This is done by a modified join condition. Second, after two results are available, still running activities of the flow are explicitly terminated. This is modeled by help of a scope around the flow with an empty fault handler that catches the "forcedTemination" fault. These mechanisms already exit. Hence, no further changes to the language itself are necessary. Of course, a user should be supported, e.g. by generating such structures automatically on demand, but this is a task of tooling rather than language design. More complex relations Finally, the join condition can specify precisely which combination of link values is needed to proceed, instead of just the number. The last figure shows an example employing this feature: For booking a trip, it is necessary to find an available hotel and at least on kind of transportation. This example mixes all concepts. In case, no hotel or no transportation is available, an information for the customer is generated ("assign") and the flow is terminated ("throw"). Otherwise, the trip is booked ("InvokeBooking") and still running activities are allowed to terminate on their own before the customer is informed. Summary A minimal change to BPEL's syntax and an easy to implement change to its operational semantics enrich the expressive power enormously, such that more useful modeling pattern could be mapped into this language. To increase the convenience of usage, tools may provide predefined macros or generate necessary structures automatically on demand. Regards, Axel. ---------------------------------------- Axel Martens Post Doc Researcher Component Systems Group IBM TJ Watson Research Center Hawthorne, NY (USA) Phone: (914) 784-7480 E-mail: amarten@us.ibm.com "Trickovic, Ivana" <ivana.trickovic@s ap.com> To "'alex.yiu@oracle.com'" 09/01/2004 10:32 <alex.yiu@oracle.com> AM cc "'satisht@microsoft.com'" <satisht@microsoft.com>, "'edwink@collaxa.com'" <edwink@collaxa.com>, "'wsbpel@lists.oasis-open.org'" <wsbpel@lists.oasis-open.org> Subject RE: [wsbpel] Issue 6 - Rough draft of proposal for vote Alex, Regarding the proposed semantics: Would you elaborate how the fault (bpws:earlyCompletionFault) will be signaled to other incomplete flow activities? Those activities are enclosed within the flow activity. If I understand the fault handling model correctly there is no possibility to communicate faults to nested activities/scopes except canceling the enclosing activity (in this case, the flow activity). Regards, Ivana -----Original Message----- From: Alex Yiu [mailto:alex.yiu@oracle.com] Sent: Mittwoch, 1. September 2004 04:28 To: Satish Thatte Cc: edwink@collaxa.com; wsbpel@lists.oasis-open.org; Alex Yiu Subject: Re: [wsbpel] Issue 6 - Rough draft of proposal for vote Hi, Yaron and others, I agree with the concern of Edwin and Satish. We should define a general mechansim to express early completion condition. Then, define the "orJoin" attribute or other syntax as a macro/special syntax short cut. If you guys don't mind, I can supplement some alternative ideas to this issue. I am in the thinking-out-loud mode again. That means, I need to verify whether this idea makes sense to other people at Oracle. [ But, as Ron said, this thinking-out-loud mode has worked quite well for me in the past. ;-) ] We should allow a <completeCondition> subelement attached to parallel-flow-starter activity (e.g. <flow> or future potential parallel <forEach>). Syntax of completeCondition: It has two forms: (1) <completeCondition>a_boolean_expr</completeCondition> (2) <completeCondition minFlow="a_number" /> ... or ... <completeCondition countMinFlow="yes">a_number_expr</completeCondition> The first form is the most generic form of completion condition. The second form provides a convienent shorthand to achieves the N out of M semantics. The suggested orJoin="yes" is equivalent to: <flow> <completeCondition minFlow="1" /> ... </flow> Semantics of completeCondition: The complete condition will be evaluated at the completion of each flow. If the condition is true, a standard fault bpws:earlyCompletionFault will be signaled to other incomplete flow activities. Each incomplete flow activity can potentially be compensated if each flow is a scope activity. (Potentially, we can add static analysis to enforce that when a completionCondition is used in <flow>, <scope> must be used as activities within the flow.) The completeCondition itself is executed in the same scope context of the <flow> activity. [ And, the completeCondition should be executed serially, to avoid racing condition among flows (?)] I hope this rough draft of ideas serves as a good base of discussion for this issue. Thanks! Regards, Alex Yiu Satish Thatte wrote: I echo Edwin's second question. Yaron, your proposal clearly doesn't address the intent of 6. It seems too narrow to just address this corner case of completion of one among many (corner case from my perspective). Satish From: Edwin Khodabakchian [mailto:edwink@collaxa.com] Sent: Friday, August 27, 2004 5:38 PM To: wsbpel@lists.oasis-open.org Subject: RE: [wsbpel] Issue 6 - Rough draft of proposal for vote Hi Yaron, 2 questions: 1) are we sure that termination is the desired behavior for the non completed activity? 2) what happens if the join rule is: "the first search result which is non empty or includes at least 3 entries with more than 85% accuracy"? The point here is that a lot of join patterns might have logic in them. I know that in the past we talked about having something like <break> for flow. (This is something that a lot of our customers are asking us and might address the same us case is a more flexible way specially if the <break> can be configure to tell the engine to compensate or terminate). Best, Edwin
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