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

358 SHE Framework
Buffered message flow
This type models typically asynchronous communication. The sender can always send
without bothering about synchronisation with the receiver. This type of flow makes
objects temporal independent. This type of flow is useful in distributed systems and in
general for the communication with subsystems that require temporal independence.
11.4.5.7 Design of Composites
In Sections 4.6 and 4.8 we described concepts for modelling various sorts of composites
and their properties. These concepts can be used for the recognition of particular
structures that emerge in MFDs. We can derive rules for the construction of various
sorts of composites. Some examples:
Message flows that connect an aggregate with its environment must be connected
to the aggregate object of the aggregate.
Dependence requires connection by a message flow that represents a request for a
service. The direction of the request is from the dependent object to the receiver.
If an object owns another object, only the owner sends requests to the owned object.
A whole dominated part can only give a reply after a request. If the part and the
whole are not concurrent, a message flow with reply is appropriate.
Besides the visible structure of flows there is a less eye catching structure which is that
of dynamic links. The identifiers of groups of collaborating objects form a name space.
Such a space determines the ability of weakly distributed communication, and must be
designed carefully. For instance distribution boundaries can indicate the desirability
of separate name spaces. The use of identifiers is shown on the flows. Identifiers are
implemented by parameters in messages. These parameters are textually annotated on
message flows in MFDs. The annotation of these texts is a specification of the sort of
communication.
Modules of distribution can be designed to communicate autistically. Autistic communication
is modelled by messages that do not use parameters for passing object
identifiers.
In general we recommend to use weakly distributed communication as much as possible
because this does not restrict the possibilities for structure transformations. When
the model becomes stable and no further transformations are expected it is the right
time to implement autistic communication in a model. This is achieved by removing
the parameters that carry identifiers from the messages that flow between strongly
distributed modules.
Besides distribution there may be other reasons to restrict the space where particular
identifiers can be used. We introduced the concept of visibility to reason about this. The
concept of visibility is directly related to the use of identifiers for message passing.