High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 161

Figure 12. Sequence of information exchange for the same scenario as shown before in Figure 11 while
applying the reliable mode for the lease-based publish/subscribe scheme in (b) and (c). (a) displays the
performance of the ACK-based variant for comparison. Even with an enabled reliable mode, the lease-
based approach requires 50 percent less messages.

otherwise subscribing nodes may possibly not
have the ability to generate a final evaluation
result at all. In that case, local event detection
would depend on the results of the publishing
node in either way.
However, avoidance of fault propagation is
of utmost importance in distributed systems like
WSNs. In consequence of using the publish/sub-
scribe collaboration scheme published faulty val-
ues could possibly propagate through the network
from node to node. This happens when a value
of an EDT node, which was formerly obtained
through subscriptions, is published further. In the
worst case, faulty values could be distributed to

all nodes in the entire network. Hence, stable or
even reliable application becomes impossible.
A simple condition circumvents this problem.
The implementation of EDTs must not allow
EDT nodes to assign values by subscriptions
and to publish those values at the same time. In
the presented implementation this is guaranteed
by mutual exclusion of the undecidable and the
toPublish flags. This assures that faulty values
cannot propagate beyond the defined collabora-
tion region of the publishing node.
Another side effect occurs if a subscriber has
several publishers available at the same time.
These may deliver different Boolean values for the