High Level Definition of Event-Based Applications for Pervasive Systems - Advanced Design Approaches to Emerging Software Systems - page 137

configured and observed, even if it requires or-
ganizing collaboration between nodes to deliver
the results.

ESL-defined phenomenon description. By that,
it implements a sensor node configuration that
directly processes and evaluates sensor readings
from the sensors attached to the nodes. It further
enables every node to self-divide event queries
according to its own resources and self-adapt to
the assigned tasks. Simultaneously, the established
EDTs provide the interface for efficient col-
laboration using a lease-based publish/subscribe
approach.
To appropriately distinguish the abstractions
presented to and described by the user and the
hard- and software constructs, we name and define
the following entities.

USER-CENTRIC DESIGN FLOW
OF PERVASIVE APPLICATIONS

A user-centric design flow decouples the processes
of task definition and respective WSN configura-
tion. Such design flow allows users to fully ab-
stract from WSN-related properties. It simplifies
the task definition process to a level that is even
suitable for non-professionals. Therefore, this
chapter introduces an intuitive XML-styled Event
Specification Language (ESL). ESL allows purely
defining the physical phenomenon to be sensed
without regard to any properties of the WSN used.
It features hardware independent description ele-
ments to define complex phenomena and enhances
these by tailor-made application constraints.
Hence, the user remains enabled to define task
for the WSN, but simultaneously the user does
not require programming skills or knowledge of
the actual WSN under configuration anymore. In
addition, the ESL, derived from XML, provides
ideal properties to be implemented and used by
Web services. This offers great potential to real-
ize remote configuration concepts for pervasive
technology via the Internet. To hide necessary
XML-styled descriptions from the user, an inter-
active Graphical User Interface (GUI) supports
the user in application design and automatically
generates the respective XML description of the
application.
Based on that XML description, a novel fully
decentralized mechanism to autonomously set up
distributed event detection, called Event Deci-
sion Tree (EDT), and a cost efficient means to
maintain such EDT, are presented. EDTs are ef-
ficiently constructed on every sensor node using
a tiny Generating Finite State Machine (GFSM)
requiring eight states only. The EDT is a software
construct reflecting the XML-tree structure of the

A phenomenon is a physical condition that can
be measured by sensors. It is not required that a
single sensor can detect or measure that phenom-
enon on its own. This is an abstraction known
real life users.

An event is a software construct announcing a de-
tected phenomenon or that a sensor measurement
meets certain condition. The mere exceeding of a
threshold by the sensor reading is named a primi-
tive event . The combination of primitive events
by logical expressions is named a complex event .

An event specification is the description of the
phenomenon to be sensed including all applica-
tion constraints. It is defined in an XML dialect
namely the Event Specification Language (ESL) .

A binary event specification is the compact rep-
resentation form of the user defined event speci-
fication. It is adjusted to hardware specifics and
sensing features of the sensor nodes in the WSN.
Binary event descriptions are disseminated in the
WSN for direct configuration of sensor nodes.

An Event Decision Tree (EDT) is the on-node
representation of the event specification.