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

6.3 Communication 169
statement after the message send statement of the message. There may be no statements
in between, because that would imply parallelism instead of waiting. If there is more
than one possible answer the behaviour after the send statement must be an or statement
that contains all possible receive statements (the or statement is explained in Subsection
6.6.6). This statement can specify a (non-deterministic) choice.
Again we emphasise the importance of choosing expressive names. Message with reply
flows often represent a request to a server. In this case the message name must have the
flavour of a request. The possible answers must also be denoted in the flow text. We
must abbreviate them in case they are too long. We can for instance skip their parameters.
The guideline is to place a slash symbol between the message and its possible replies.
Example:
giveStatus/notMounted,mounted
Summary
The message with reply flow symbol is useful for modelling of:
synchronous message passing with reply and waiting;
a client-server model with waiting for reply;
a double rendezvous in one symbol, provided that waiting is required.
The POOSL description needs two statement pairs, and the waiting must be specified
explicitly. The statement pair that specifies the passing of the reply may be constructed
for multiple answers by using an or statement.
6.3.6.3 The Continuous Flow Symbol
A continuous flow symbol represents a time-continuous flow. A continuous flow has a
value at every instant, within a time interval that the source of the flow is actively
producing that flow. The symbol has a double arrowhead, see Figure 6.6. The symbol is
designed to visualise that there is a message (value) immediately after each acceptance
of a previous message (continuously).
A continuous flow is useful to model physical devices such as sensors and actors. By
using the method of Ward and Mellor (Structured Development for Real-Time Systems
[WM85]) we experienced that continuous flows are a useful concept. Other methods,
such as Hatley and Pirbhai [HP88] do not offer this concept. Their notion of continuous
signals is differently defined as ’signals that may take on one of an arbitrarily large number
of ordered numeric values.’ The notion of time-continuous should not be confused with
this notion of a continuous function, where the range of values is continuous. A timecontinuous
flow may transport values from either a discrete set of values or from a
continuous range of values. Sources such as sensors can give for instance a voltage as
a time-continuous output. Such an output can be a time-continuous flow. A name of a
flow must adequately represent the quantity that is measured for instance temperature
(See Figure 6.6).