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A condition C=(A, R, B) represents a single condition that a message object m must fulfill in order to be delivered. B is a comparison function which can be viewed as a binary predicate: B(o1: obj, o2: obj): bool. The two arguments are (1) a predefined result R and (2) the result of the invocation chain represented by the accessor A. A condition is thus evaluated against a message object m, and evaluates positively iff m satisfies that condition: C(o: obj): bool, and C(m) = B(R, A(m)).

Figure 6 outlines the different evaluation stages of a condition. A similar scheme can be found for object queries in object-oriented data management systems, e.g., TIGUKAT [SO95].[*]

Figure 6: Applying a Condition to a Message Object

Patrick Eugster