Architecture Modeling - SPES 2020

Sequences of Events
It is typical to specify the order of the occurrences of different events. To do so, there are two
language constructs:
and
Event and then Event
N times Event
Architecture Modeling
Sequences with “and then”
The “and then” construct postulates a sequence of events. The following example demonstrates the
use of the pattern.
Requirement: When a crash was detected, first the airbag shall inflate and then the doors
shall be unlocked.
whenever CrashDetected occurs InflateAirbag and then UnlockDoors
occurs.
This pattern does not say anything about timing, it just specifies the order. To express timing
behaviour it is possible to use intervals with the events. E.g. if the door shall be unlocked in
at least 50 ms after the airbag inflated the pattern would look like this:
whenever CrashDetected occurs ReCheckSensors and then InflateAirbag
and then UnlockDoors during [0ms,50ms] and then EngineOff during
[0ms,300ms] occurs.
The “during” with timing entities differs semantically from the filter that can be used to constrain
events. If “during” with timing entities is used in an “and then” statement the interval is demanding, i.e.
that the event has to occur in the specified interval. The timer starts with the occurrence of the
preceding event. If the event does not occur in the specified interval the “and then” statement
continues its execution from the latest previous event in the chain, that was not constraint by timing
properties. The clock for the interval with the timing constraints is restarted by this event. This means
for the above example that a failing of the EngineOff events 300ms after the unlocking of the doors,
the observer would wait for a new InflateAirbag message, since this event has no timing constraints to
the preceding event.
In contrast to timing intervals the use of events in the intervals is semantically identical to the filter
described in section 0.
This backtracking behaviour can be further specified by the use of brackets.
whenever a occurs b and then (c and then d and then e during
[0ms,7ms]) occurs.
If e does not occur in the specified interval the observer would wait for a new c instead of a
new d.
114/ 156