CIF 3

Event declaration placementΒΆ

Consider the following CIF specification (the producer/consumer example from the lesson on synchronizing events):

automaton producer:
  event produce, provide;

  location producing:
    initial;
    edge produce goto idle;

  location idle:
    edge provide goto producing;
end

automaton consumer:
  event consume;

  location idle:
    initial;
    edge producer.provide goto consuming;

  location consuming:
    edge consume goto idle;
end

The specification could also be specified as follows:

automaton producer:
  event produce, provide, consume; // Declaration of event 'consume' moved.

  location producing:
    initial;
    edge produce goto idle;

  location idle:
    edge provide goto producing;
end

automaton consumer:
  location idle:
    initial;
    edge producer.provide goto consuming;

  location consuming:
    edge producer.consume goto idle; // Event 'consume' from 'producer'.
end

The consume event is now declared in the producer automaton rather than the consumer automaton, but the locations and edges have not changed. This modified specification exhibits the same behavior as the original.

It should be clear that while events can be declared in various places, it is best to declare them where they belong. That is, the consume event is only used by the consumer automaton, and is thus best declared in that automaton. Similarly, the produce event is only used by the producer automaton.

The provide event however is used by both automata. In such cases the event is usually declared where it is initiated. In the example above, the producer provides the product to the consumer, and not the other way around. Therefore, the provide event is declared in the producer automaton, rather than in the consumer automaton.

However, the modeler is free to choose the best place to declare the event. If no choice can be made between the automata, the event can also be declared outside the automata, as follows:

event provide; // Event 'provide' now declared outside the automata.

automaton producer:
  event produce;

  location producing:
    initial;
    edge produce goto idle;

  location idle:
    edge provide goto producing;
end

automaton consumer:
  event consume;

  location idle:
    initial;
    edge provide goto consuming; // Can directly refer to 'provide' event.

  location consuming:
    edge consume goto idle;
end

This specification also has the same behavior. Only the placement of the event declarations has changed.

The place where an event is declared is of no influence to the implicit (default) alphabets of the automata. The implicit alphabet of an automaton is determined solely based on the events that occur on the edges of that automaton.