10.4.3.3 Using inevitable and optional

In interfaces, two modeling events may be used as abstract triggers, i.e. inevitable and optional:

on inevitable: imperative-statement;
on optional:   imperative-statement;

Where inevitable implies that if no other triggers occur, this trigger is guaranteed to occur, and optional implies that the trigger may or may never occur.

Note that an inevitable event is not allways guaranteed to occur, it is only inevitable in the absence of other events.

An example of an interface using both inevitable and optional.

interface inevitable_optional
{
  in bool hello ();
  in void bye ();
  out void world ();
  out void cruel ();

  behavior
  {
    enum status {IDLE, WORLD, CRUEL};
    status state = status.IDLE;

    [state.IDLE]
    {
      on hello: {state = status.WORLD; reply (true);}
      on hello: {state = status.CRUEL; reply (false);}
    }
    [state.WORLD] on inevitable: {state = status.IDLE; world;}
    [state.CRUEL]
    {
      on optional: {state = status.WORLD; cruel;}
      on bye: state = status.IDLE;
    }
  }
}

In the interface above a reply value of true on hello informs the client sending the hello that the world can be waited on. However in case the reply value of hello is false and the client would sit there waiting for cruel to happen, they may sit there forever because cruel might never happen. This is what we refer to as a deadlock. To avoid this deadlock as a client, they must make sure that they can handle a cruel in case it does happen and that they have another way of making progress in case cruel never happens.

Conversely, the implementation of this interface may choose to perform the cruel always, never or intermittently after a hello followed by a false, but it must (being contractually required) always do a world after a hello followed by a true.