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Subject: Issue - 107 - Opacity and the meaning of nothingness in abstract processes
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An example of implicitly hiding data is the way that abstract processes deal with some of the variables on message operations. If one doesn't want to specify where a message is recorded on a receive then one may simply leave off the variable attribute.
An example of explicitly hiding data would be introducing something like an <opaque> activity. For example, imagine that one is defining an abstract process and the start activities of the abstract process are to be omitted since they are not relevant to the functionality the abstract process is describing. In that case one could start a process with an <opaque> activity. By putting in <opaque> one is explicitly communicating to the reader of the abstract process 'There is something here but its definition is not relevant to the goal of the abstract process.'
The advantage of explicit versus implicit hiding is in error detection.
In an implicit system where one simply leaves off variables or omits activities there is no way to tell the difference between an error, e.g. the person intended to include the variable or activity but forget and true omission. This is a well known problem with treating the absence of information as information.
In an explicit system where one would have <opaque> activities and special reserved bpel:opaque place holders for attributes there is no possibility for confusion. If an opaque value is specified then one has an unambiguous declaration from the designer of the abstract process as to their intention.
The issue before the group is - should we consistently use an implicit
or explicit omission mechanism for specifying when information has been
intentionally omitted in an abstract process?
Changes: 18 Mar 2004 - new issue
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