High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 158

still providing publications? On the other side,
the publishing node requires to know whether
there is still a subscriber awaiting information
about EDT node values. These problems could
indeed be solved by using a simple ACK-scheme
for every transmission to inform the sender about
the success. Unfortunately, pure ACK-based com-
munication is inefficient for WSNs as mentioned.
Furthermore, publish/subscribe is designed to
achieve large periods without any transmission,
which is not possible with ACK-based schemes.

associated lease factor and the EDT evaluation
interval. Both factors are given by the binary
event specification. The lease factor enables fine-
tuned and customized lease intervals. It adapts the
lease period to the monitored phenomenon and
to expected conditions in the sensor network. For
example, sensor networks, which are subject to
permanently changing situations or node mobility,
require a high adaptiveness. Those should apply
short lease intervals. In contrast to that, sensor net-
works deployed at rather fixed network structures
could make use of larger lease intervals to save
energy and extend the overall network lifetime.
If the lease factor is chosen to be one, i.e., the
leasing time is one detection interval, this scheme
converges to ACK-based approaches.
On receiving a matching subscription, the node
determines the expiration date of the respective
publication. The expiration date is assigned to
the corresponding EDT node together with the
toPublish flag. After initially publishing the
current EDT node value, any further change is
published as long as this flag is set. Consequently,
the flag is automatically removed from the EDT
node when the expiration date is reached, i.e.,
the lease has expired. Similar to the toPublish
flag, the subscriber assigns an expiration date to
the requested undecidable EDT node. Even if no
publisher responds to the subscription, the node
sends no new subscription before this expiration
date has expired. That assures to renew the publish/
subscribe relations with respect to the configured
adaptation rate only. Other approaches usually
try to subscribe at each detection interval again,
which heavily drains the power resources.
To save more energy, new and renewed leases
are distinguished. The initial respond of the pub-
lisher for renewals can be saved, since it is not
necessary if no change has occurred. If earlier
agreed leases are to be renewed only, the publisher
does not respond with the initial node value but
extends the lease period and continues providing
state changes until the newly assigned expira-
tion date is reached. In addition, publisher and

Leasing of Publication Time

Due to assumed conditions in WSNs, the publish/
subscribe scheme must adapt frequently to reach a
certain level of robustness in phenomenon detec-
tion. Certainly, the overhead needed for adaptation
must be kept as small as possible but still allow
for balancing the adaptiveness with respect to
the application. Accordingly, publications and
subscriptions should either be removable or be
valid for certain time periods only. The latter is
much more suitable for WSNs where unforeseen
changes leave no chance for appropriate responses
or un-subscriptions. Therefore, an adaptive lease
procedure limits the validity of publications and
subscriptions. It allows to subscribe a data inter-
est for a certain lease period only, after which
the publish/subscribe relation has to be renewed.
Such lease-based publish/subscribe requires
significantly less transmissions than ACK-based
variants and enables lease intervals assigned per
phenomenon. Usage of lease-based approaches is
also well known in other application areas, e.g.,
for labeling of references and objects in automatic
garbage collection or for allocation of resources
like the IP addresses from servers using the
Dynamic Host Configuration Protocol (DHCP).
A lease-based subscription specifies a certain
time interval determining the validity period
of subscriptions during which associated pub-
lications have to be sent. This lease period is
determined as the product of the phenomenon-