Section 17.3. Ordering Constraints

17.3. Ordering Constraints

When the port types that are wired together are associated with process models that constrain the order in which operations of the port types can be used, the validity of the wiring can be checked. In Figure 17-3, the wiring shown is valid. That is, the interaction between the two port types, A and B, is consistent. B sends out a message via operation o5, which is consumed by operation o1. After consumption of the message, A sends out a message via operation o2 at some time. This message is consumed by B via operation o6, which is fine because o6 is ready for use after o5 sends the message. Under certain conditions (y=0 in the example), B sends a message via operation o7. This is fine because after o2 has been used to send a message, o4 is always ready for use.

Figure 17-3. Process models as constraints and matchmaking.

Note that a plug-link from o7 to o3 (instead of o4) would be problematic, because o3 will not always be available (only in case x>42 in this example). That is, in some scenarios, o7 might send a message for consumption by o3, but A would not receive this message because it is in a state that makes it impossible to receive the message. Thus, plug-links allow checking the validity of an intended interaction.

Assume two port types are given, each of which is associated with a process model, but no wiring between them has been specified. An interesting question is whether or not these two port types are compatible. Is it possible to define plug-links in such a way that a valid interaction between the two port types can take place? An algorithm solving this problem could be used to find partners that exchange messages in a compatible way. This goes far beyond simply matching signatures of port types by adding order of interactions between partners to the scope of matchmaking.