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Fig. 8. The Interaction Aspect of PIM4Agents
Regarding the metamodel definitions the Interaction aspect (see Figure 8)
is one of the most complex in the PIM4Agents metamodel. However, the most
important concepts to understand are: Protocol, Actor, MessageFlow, and Mes-
sageScope. The protocol is actually the core concept that defines an interac-
tion. The reason to not directly replace the concept of interaction with the
concept protocol is that protocols are intuitively understood as message based
interactions and we want to be open to allow in future work other forms of
interaction, too.
A protocol is in the first place composed of a set of actors. For each actor a set
of message flows defines the different states in which the interaction could end up.
Message scopes connect two message flows of two different actors. An attribute
tells whether the message flow has a fork operator (i.e. it sends a message)
or a join operator which means that the message flow receives a message. The
message scope refers to the concrete message that is sent from the sender (actor)
to the receiver (actor). In each actor exactly one message flow is marked as initial
message flow which means that the protocol execution will start for this actor
in the state that is defined by this message flow. However, in the whole protocol
there is only one actor for which the initial message flow has a forkOperator
(i.e. the initial message flow sends a message). We call this message flow start
message flow of the protocol.
 
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