Re: [wsbpel] Issue - 107 - Extension points and opacity

[Yuzo Fujishima <fujishima@bc.jp.nec.com>]

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
> 
> 
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> 
> 
>