A State-Based Programming Model for Wireless Sensor Networks

5.5. Hierarchical Composition 91
state B, lifetime varB
state B’, lifetime varB’
inB() inB’() outB()
f
outB’()
real time
0 1
2 discrete time
Figure 5.7: Possible mapping of real-time to discrete time for parallel machines.
of OSM operates on sets of concurrent events rather than on individual events.
All elements of a set are handled in the same discrete state-machine step. The
enqueue operation takes a set of concurrent events as a parameter and appends
this set as a whole to the end of the queue. The dequeue operation removes the
set of concurrent events from the queue that has been inserted earliest. Whenever
the system is ready to make a transition, it uses the dequeue operation to
determine the one or more events to trigger transitions.
All events emitted programmatically in exit and entry actions, respectively,
are enqueued as separate sets of events, even though they are emitted during
the same state machine step. That is, each state-machine step consumes one set
of events, and may generate up to two new sets: one for events emitted in exit
actions and another set for events emitted in entry actions. Clearly, the arrival
rate of event sets must not be larger than the actual service rate, which is the
responsibility of the programmer.
5.4.2 Progress in Parallel Machines
The execution semantics of a set of parallel state machines can now be described
as follows. At the beginning of each state-machine step, the earliest set of concurrent
events is dequeued unless the queue is empty. Each of the parallel state
machines considers the set of dequeued (concurrent) events separately to derive
a set of possible transitions. Individual events may also trigger transitions
in multiple state machines. Similarly, if multiple concurrent events are available,
multiple transitions could fire in a single state machine. To resolve such
ambiguous cases, priorities must be assigned to transitions. Only the transition
with the highest priority is triggered in each state machine. Actions are executed
as described in Sect. 5.3. The dequeued set of concurrent events is then dropped.
5.5 Hierarchical Composition
Another important abstraction supported by OSM are state hierarchies, where
a single state (i.e., a superstate) is further refined by embedding another state,
state machine (as depicted in Fig. 5.8 (a)) or multiple, concurrent state machines
(as depicted in Fig. 5.8 (b)). Just as uncomposed states, superstates can have
state variables, can be source or target of a transition, and can have entry and