Specification of Reactive Hardware/Software Systems - Electronic ...

6.3 Communication 157
between process objects connected by static channels is different from the communication
between rather dynamic data objects. This section describes various aspects and
problems of object communication.
6.3.2 Communication Concepts for Reactive Systems
The desire to be able to design a wide variety of complex distributed reactive systems at
various levels of abstraction requires that we offer the primitives to model various sorts
of communication. The following sorts of pair-wise communication have been identified.
Synchronous communication with reply:
Simultaneous activity of a sending and a receiving object, with the purpose
of information transport. This information may be substantial in the form
of data or the simple fact that the communication took place without data
transport. There are three forms of synchronous communication:
1. Synchronous communication with reply and waiting:
This form is needed to model sequential behaviour. The activities of the
sending object are suspended, until the required operation is performed
by the receiving object, and the result has been returned.
2. Synchronous communication with reply, waiting and time-out:
Is similar to the previous form. The difference is that the sending object
stops waiting after a predetermined period of time. The sender must
decide what to do with results that come back too late.
3. Synchronous communication with reply but without waiting:
The sender may continue performing its activities after the first synchronous
communication that transfers the request. A second synchronous
communication is necessary to transport a possible result to
the sender. This form is needed to model fully concurrent behaviour.
Synchronous communication without reply:
Simultaneous activity of a sending and a receiving object, with the purpose
of transport of information. This information may be substantial in the form
of data or the simple fact that the communication took place. The sending
object does not expect a result so it can continue its activities immediately.
This form enables concurrent behaviour.
Asynchronous communication without reply:
Sending of a message may happen, independent of the readiness of the receiver.
The sending object continues performing its own activities without
waiting for confirmation. The message can only be handled by the receiving
process when it is ready to receive it. This form of communication requires
buffering of messages to overcome the period between message sending and
readiness of the receiver. A lack of readiness can also be handled by discarding
a message.