Object interactions

Object interactions in OOPN are initiated by transitions. Transitions in high-level Petri nets may have guards and actions:

• Transition guards are evaluated during transition firability testing. There must be no side effect. It only allows atomic decision if the transition can or cannot be executed.

• On the other hand, transition actions are evaluated during transition execution. There is also possibible to think about non-atomic execution of transition action (start and end of action) and a possibility of a side effect (such as a message sending in OOPN).

In OOPN, each expression of inscription language specifies a message sending. It means that both guard and action contain message sendings. If the receiver of the message is a primitive object (i.e. not described by an OOPN class), then the semantics is the same as in the case of the well-known high-level Petri nets. Let us now suppose a non-primitive object as a receiver. Since there is a difference in semantics of a guard and an action we can distinguish two kinds of object interactions. Both these interactions are specified as message sending:

• Message sending in transition guard means atomic synchronous interaction, which is realized by synchronous ports invocation (will be discussed later).

• Message sending in transition action means a client-server (request-reply) interaction which is realized by invocation of method nets.

Invocation of method nets

• Method nets are high-level Petri nets with message patterns attached to them and with parameter-places and a return-place.

• A method net can access places belonging to corresponding object net.

• Method nets are dynamically instantiated by message passing specified by transition actions. Of course, polymorphism takes place here, just like in Smalltalk.

• Events connected to a method net invocation are: send (F - fork, creation of a new process - instance of method net) and receive (J - join, terminating of the method net instance).

Invocation of synchronous ports

• Synchronous ports are virtual transitions in object net with message patterns attached to them.

• They are intended for synchronous interaction of objects. The synchronous interactions (invocation of synchronous ports) are specified in transition guards as message sendings (polymorphism aplies here).

• A (sender) transition is firable only if the receiver of the message in its guard agree with it (the involved synchronous port is firable).

• The invoked synchronous port (all invoked s. ports) are executed simultaneously with the sender transition.

• The semantics of the synchronous interaction can be described as a transition which is a fusion of the sender transition and the synchronous port (respecting polymorphism and parameters binding). It behaves as the fusion transition.