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Subject: Re: [wsbpel] Issue - 107 - Extension points and opacity
I actually see abstract processes being used in two different contexts. #1 - To define the externally visible contract for a protocol. That is, system A wishes to work with system B. System B publishes an abstract process. System A writes its code to work with System B's abstract process and so is able to interoperate with System B's executable process. #2 - To provide templates for those wishing to provide implementations of standardized services. For example, the International Association of Widget makers provides a standard for a widget request service. As part of that standard they provide an abstract process definition that specifies how someone implementing the widget request service is to behave. So when a widget maker decides to implement the widget request service they will validate their executable code against the abstract process definition. In other words, they must make sure that their executable process does the same thing the abstract process definition specifies. This is the inverse of the previous example. To your point about being able to put in an assign but not a receive, I would offer the following as a counter example. Imagine that a widget maker implementing the widget request service adds in a receive to their implementation of the widget request service that waits for up-to-date information from the widget factories as to the current number of available widgets. This receive not be specified in the abstract process definition since it is not relevant to that definition how the widget request service knows how many widgets are available. So in this case it is completely reasonable for the executable process to add in a receive that was never specified in the abstract process definition. Finally, as to your assertion that opaque is not necessary it is very difficult to respond to an assertion. In my original e-mail below I provide two examples where the use of either nothing or empty causes ambiguities and then show how opaque resolves those ambiguities. If you could illustrate that the ambiguities I describe don't actually exist then it would seem likely that you would have made the case that opaque is unnecessary. Thanks, Yaron Yuzo Fujishima wrote: > > > Hi, > > I believe that an abstract process defines the behavior of a process that > is observable from outside. In other words, an abstract process is a > contract to outside parties. In addition, I assume that an abstract > process must be unambiguous as a contract although it does not restrict > implementation details. > > In my opinion, the executable process corresponding to an abstract process > can be implemented in any way the implementor like as far as the contract > is satisfied. At the same time, any implementation that does not satisfy > the contract should be deemed incompliant. > > Therefore, I think putting <assign> in the place of <opaque> should be > acceptable (at least in most cases, I believe) while putting <receive> is > in many cases illegal because it does modify the observable behavior. > > That being said, my conclusion is that I don't see the necessity of > introducing <opaque>. One can derive an executable process from an abstract > process definition by inserting any activity anywhere as far as the > contract > is satisfied. > > Yuzo Fujishima > NEC Corporation > > Yaron Y. Goland wrote: > > > One of the big concerns that came up on today's discussion of issue 107 > > was how the opacity proposal altered the ability to extend an abstract > > process. > > > > For example, imagine the following abstract process that uses opaque: > > > > process > > opaque > > receive > > reply > > > > If someone wanted to build an executable process based on this abstract > > process would it be 'legal' to write something like the following and > > still have it be considered valid against the abstract process? > > > > process > > ... > > receive > > assign > > reply > > > > In other words, could the executable process programmer insert the > > 'assign' activity between the receive and reply activity and still have > > their executable process be considered valid against the abstract > > process definition? > > > > In my proposal the answer is - yes. In my opinion one is free to extend > > an abstract process at absolutely any point one wants. The purpose of > > validating an executable process against an abstract process is to make > > sure that the executable process does 'at least' what the abstract > > process does but it is free to do much more. > > > > The purpose of opaque is not to restrict where one can extend an > > abstract process when creating an executable process. The purpose of > > opaque is to specify where the abstract process is intentionally leaving > > out data in situations where the absence of information could be > ambiguous. > > > > For example, let's say that an abstract process has the following code > > snippet: > > > > process > > scope > > eventHandlers > > onEvent ... > > ... > > > > In this case the onEvent handler in the abstract process has no content. > > > > So now the reader of the abstract process is left with a quandary. Is > > the onEvent handler empty because the definition of the event handler is > > left up to the implementer or is the event handler empty because the > > programmer of the abstract process screwed up and forgot to put in the > > definition for the event handler? > > > > One way around this ambiguity is to do the following: > > > > process > > scope > > eventHandlers > > onEvent ... > > empty > > ... > > > > By inserting empty it is clear to the reader that the onEvent handler > > was intended to not have any definition. The only problem is that empty > > has its own very specific meaning - do nothing. So now there is another > > ambiguity. Is the abstract process author telling the executable process > > author 'Write your own code for this event handler' or are they telling > > the executable process author 'You are required to catch the messages > > identified by this event handler but then you should do nothing with > > those messages'? > > > > This is where opaque comes in. > > process > > scope > > eventHandlers > > onEvent ... > > opaque > > ... > > > > Now there is no ambiguity. The abstract author is explicitly telling the > > executable author 'You must catch the messages identified by this event > > handler but how you handle them is up to you.' > > > > It is only necessary, however, to use opaque if its absence would lead > > to ambiguity. So, going back to the example, that started this mail, > > there is no need to specify > > > > process > > opaque > > receive > > opaque > > reply > > > > In order to tell the executable programmer "You MAY insert activities > > between the receive and reply". > > > > It's fine to just specify: > > > > process > > opaque > > receive > > reply > > > > The right to insert additional activities between the receive and reply > > is always implicit and unambiguous so opaque is not needed. > > > > I hope this clarifies things, > > > > Thanks, > > > > Yaron > > > > > > To unsubscribe from this mailing list (and be removed from the roster of > > the OASIS TC), go to > > > http://www.oasis-open.org/apps/org/workgroup/wsbpel/members/leave_workgroup.php. > > > > > > > > > >
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