High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 159

subscriber renew the lease period automatically
upon notification of a state change. When data
is published during a valid lease, the expiration
date of the toPublish flag is reset to the full lease
period. Similarly, the subscribing node renews
the expiration date of a current subscription when
receiving a respective publication.
Figure 11 displays sequence charts of both
lease extension cases as well as the ACK-based
scheme for comparison. In the ACK-based vari-
ant, displayed in (a), the subscriber requests
information at each detection interval, which is
accordingly responded by the publishing node.

In contrast to that, (b) and (c) illustrate the lease
allocation in case of no change (b) and change (c)
in EDT node values while applying a lease factor
of three. Both cases require to provide the current
node value by an initial publication of course. In
case of no change (b), the subscription is renewed
by the subscriber after the lease has expired. If
a change of the EDT node value occurs during a
valid lease (c), the lease period is automatically
extended on both sides via the publication mes-
sage. In such simple scenario, the lease-based
approach already saves more than 60 percent of
the messages.

Figure 11. Sequence of information exchange between a single subscriber and a single publisher dur-
ing four detection intervals i n to i n+3 . (a) displays the performance of the ACK-based variant, which is
constant regardless of the existence of events. (b) and (c) illustrate the lease allocation applying a lease
factor of three, i.e., the subscription is valid for three evaluation intervals. In case of no change, see (b),
the subscription is renewed by the subscriber whereas a change of the EDT node value allows to extend
the lease on both sides via the publication message (c).