High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 151

Figure 6. Binary event specification of the introduced fire detection example. It contains all information
necessary for configuring sensing devices according to the event specification. The numbers displayed
on top of the binary event specification represent the respective offset addresses in the byte stream.

consumption in contrast to other approaches by
omitting the distribution of sensed data at each
detection interval.
Configuration and maintenance of EDTs is
performed by the EDT-engine on each sensor
node. This includes implementation features for
generating and evaluating EDTs as well for par-
titioning complex events into less complex ones
based on the sensing facilities of individual sen-
sor nodes. It further introduces efficient means to
detect nodes for collaboration, which may provide
missing information to evaluate the complete
EDT. Finally, simulation results of a prototype
implementation applied to a failure scenario un-
derline the robustness and the cost-efficiency of
the presented approach.
On the sensor nodes, the EDT-engine, which
is depicted in Figure 7, configures the sensor
node with respect to each received binary event
specification. First, the EDT generator processes
the sensor data element. As a result, it generates
the representation of the phenomenon to be sensed
as an EDT. Depending on the sensing features
and resources provided by the node, the EDT
adaptation splits this EDT into local and remote
parts. Local parts can be evaluated by the node
itself, whereas remote parts have to be requested
from external sources, e.g., from neighboring
nodes. The EDT adaptation further configures
application-related constraints as parameters of
the EDT, i.e., collaboration regions, handlers,
sensing intervals etc.

The EDT processing unit integrates the final
EDT and maintains compliance with all param-
eters of the configured EDTs. The EDT process-
ing unit consists of the EDT evaluation, an EDT
scheduler and a Handler box. The EDT sched-
uler autonomously schedules all EDTs with respect
to their configured evaluation intervals. This
schedule is currently implemented by timers as-
signed to each EDT. On timer wakeup, the respec-
tive EDT is enqueued into a queue that holds all
EDTs pending for execution. That guarantees
evaluation of all EDTs, even if several of them
are triggered simultaneously or with short lags.
This queue is in principle a First In-First Out
(FIFO) queue, but enqueued EDTs are secondary
sorted with respect to their assigned priority. The
priorities can be low , normal or high . EDTs with
a high priority are ranked first, of course. Simi-
larly the EDTs with a low priority are added to
the end of the queue. Future implementations are
supposed to use available schedulers of the un-
derlying OS, such as an integrated Earliest Dead-
line First (EDF) scheduler, to provide a more
precise and fair scheduling.
On dispatching an EDT into the EDT evaluation
for execution, the sensing devices are triggered to
deliver actual sensor readings required to decide
about the existence of the described phenomenon.
In case of a positive evaluation result, the Handler
box is called to execute the respectively associ-
ated handler methods. The EDT evaluation also
manages collaboration with other sensor nodes if
necessary. Details about collaboration are given