2 Why We Need Model-Based Testing

272 Reactive Systems
16.5.3 Passive testing
An important special case of adaptive on-the-fly testing occurs when all the actions
are observable. In that case the role of the strategy reduces to checking conformance
without ever needing to select tester actions. Passive testing can be used, for example,
to validate existing logs of events of a concurrent system. In that case each log
represents a sequence of events produced by a single agent. Multiple per-agent
event logs can be merged into a single log, for example by using the multiplexing
technique discussed below.
16.6 Partially ordered runs
This section discusses the problem of producing a sequence of events of a reactive
system, such that the sequence respects an underlying partial order of those events.
If the IUT is sequential, such as a single-threaded program, events can be enqueued
in the order they are observed by the stepper. This is, for example, the case
with the bag example above. If the IUT is concurrent, such as a multithreaded program
or a distributed system, the problem is that the chronological order in which
the events are observed by the stepper and enqueued into the observation queue
may differ from the chronological order in which they actually happened, due to
buffering and communication delays. This may render the testing process incorrect.
Dependencies between agents, where agents are either threads or processes,
usually impose a partial order on the events. For example, send events happen before
corresponding receive events in communicating systems. In multithreaded programs,
a partial order of events is defined by access to shared resources. An example of
a shared resource is a lock.
Suppose that each shared resource R is associated with a usage count ucR that
is initially 0. View R as a unary function symbol, and assume that each access (e.g.
read or write) to R generates a resource usage event R(ucR) and causes the usage
count ucR to be incremented by one.
Now suppose that there are multiple agents and each agent A generates a sequence
of events (eA 1 ,eA 2 ,...). Say that an event eA i happens immediately before an event
, if one of the following two conditions holds:
e B j
1. A and B are the same agent and j = i + 1.
2. A and B are distinct agents, e A i is a resource usage event R(k), and e B j is a
resource usage event R(k + 1) for some shared resource R.
Figure 16.7 illustrates events of a concurrent system with two agents A1 and
A2. The sequence of events of an agent is shown as labeled dots along the corresponding
horizontal line. An arrow from event x to event y indicates that x happens
immediately before y.
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