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Fig. 10. Basic Communication Pattern
sense the specification of the interaction among the agents is a choreography of
the capabilities of the agents that are involved in the interaction.
When it comes to executing the protocol at run time one has at least two
options to choose from. In the first option the interaction protocol would be di-
rectly interpreted by a protocol interpreter that is included in the agents' bodies.
In the second option the protocol is transformed into local behaviors for each
of the participating agents which can be directly executed in some execution
environment. The first option is more flexible but the second option is easier to
implement. For this reason we use the second option. This has also the advantage
that the local behavior can be produced with a model-to-model transformation
at the PIM4Agents level which means that the resulting behaviors can be trans-
formed into all different execution environment for which a transformation of
PIM4Agents models is available. We therefore do not interpret the protocol
model directly but transform the protocol model at design time into capabili-
ties that provide the respective communication behavior that is required by the
contract to which the protocol belongs. To achieve this a separate capability is
generated from the protocol model for each of the given actors. We can iden-
tify a basic pattern which allows to already design a large number of different
protocols (at least regarding those which are explicitly represented by models
at design time). This basic pattern is displayed in Fig. 10. It always starts with
one actor sending a message to another actor and then waiting for all answers
to this message.
Behind all actors of a protocol any number of agent instances might be hiding
except for the actor that contains the start message flow. Only one individual
agent is allowed to play the role that is connected to this actor. Only the start
message flow actually sends multiple messages to all agent instances hiding be-
hind the actor that receives this message. All subsequent messages are exchanged
in a bilateral manner. However, this means that the start message flow spans
off a set of parallel interaction threads. For some protocols it is necessary to
synchronize (see Fig. 11) these interaction threads. For example in the Contract
Net Protocol ([5] see Fig. 9) the manager has to wait till all bidders have replied
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