High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 160

Since a lease can be automatically extended
by publications without a respective acknowledge-
ment message from the subscriber, there is a risk
that the publisher side runs into a kind of infinite
loop. In other words, after publishing the initial
node value, the publisher may renew the lease
and the toPublish flag again and again, while the
subscriber disappeared in the meantime. To cope
with that, an exceptional termination condition
for publications was integrated. The publisher
counts the number of automatic lease renewals
and removes the toPublish flag, when the value
of the counter equals the given lease factor. Hence,
that exceptional condition forces the subscriber
to renew the subscription again if it still requires
information about the respective node value.
Consequently, each subscription resets the coun-
ter at the publisher.
It is obvious that this communication scheme
is well suited for low power applications such as
environmental or structural health monitoring.
Besides this high efficiency in power consumption,
another main goal of this chapter is robustness
of event detection, which consequently requires
a stable and reliable communication scheme.
Therefore the introduced approach can operate in a
reliable mode as well. The reliable mode combines
the advantages of the introduced publish/subscribe
scheme with the reliability benefits provided by
ACK-based communication. The reliable mode
introduces retransmissions on the application
level. Usually retransmissions in case of message
loss can be assumed to be part of the Medium Ac-
cess Control (MAC) layer. If retransmissions are
not provided by the MAC protocol or if the link
reliability of the underlying network is unknown,
the publish/subscribe scheme can provide a similar
feature on the application level.
The reliable mode is activated by setting
the reliableMode attribute in the <EVENT>
element of the event specification. The reliable
mode applies the introduced lease-based publish/
subscribe scheme, too, but enforces to explicitly
acknowledge every transmitted data packet. If

the ACK-message fails while the reliable mode
is enabled, the already renewed lease is removed
immediately. A publication in response to an
initial subscription is implicitly used as an ACK-
message. This introduces a little overhead indeed,
but still outperforms usual ACK-based variants.
Figure 12 illustrates the performance of the reli-
able mode when applied to the same scenario as
shown before in Figure 11. Similarly, it compares
the behavior of the ACK-based variant (a) to the
cases of no changes (b) and changes (c) of EDT
node values in the publish/subscribe approach.
Even here, the reliable mode still saves 50 percent
of the required messages.
Finally, the reliable mode can be applied to a
specific event specification and hence, allows
customizing the used communication scheme for
each configured EDT. In contrast to other ap-
proaches where the communication protocol is
identical for all configured tasks, the EDT engine
may execute both modes simultaneously depend-
ing on the configured EDTs. To summarize, it is
a simple fact that the lease-based publish/subscribe
provides a considerable benefit with respect to
the number of required transmissions, even if the
applied lease interval is rather short. This theory
has been proven by simulations, which are pre-
sented in the next sections.

Side Effects of Collaboration

In spite of presented benefits of collaborative event
detection, there exist some side effects that must
be considered separately. Collaboration obviously
includes distribution of partial evaluation results to
subscribing nodes to enable these to evaluate their
respective EDT. Certainly, that may essentially
affect the final evaluation results at these sensor
nodes. More precisely, the final EDT evaluation
result at subscribing nodes may highly or directly
depend on received (published) values. Hence,
several subscriber nodes may always generate
the same final evaluation result as their publisher.
This is not of concern for usual detection, since