Global Positioning System Reference
In-Depth Information
Dustdar and Schreiner (2005), the business logic does not lie entirely in
a single monolithic entity, that is the operations and the algorithms that
handle data and information exchange may also result from the composition
of different services. This fundamental characteristic leads to the need
of considering some key issues, such as the necessity to coordinate the
sequence of operations to ensure the accuracy of computation and avoid
inconsistencies, and the necessity to use a transaction protocol (e.g., the
WS-Transaction (OASIS 2009) protocol used in Web services stack) suitable
either for short or long running tasks. Besides these general problems,
another important challenge described in Dustdar and Schreiner (2005) is
related to the choice of the composition strategy that could be automated
or manual, static or dynamic, that is whether the composition takes place
during the design phase of the whole architecture or at run time. In addition,
any composition mechanism, besides providing a dynamic and fl exible
composition model, has to satisfy several non-functional requirements such
as scalability, security and dependability (Milanovic and Malek 2004).
Among the various approaches proposed in literature, two types
of service composition are widely used, namely choreography and
orchestration.
As for service choreography, each service involved in the composition
knows its role in the whole interaction. Choreography implementation is
fairly simple, but discovering the source of a malfunction can be a diffi cult
task. The Web Service Choreography Interface (WSCI) is one of the most
widespread standards for the choreography of services (Barros et al. 2005).
Service orchestration, instead, describes how Web services can interact
by exchanging messages, including the business logic and the execution
order of the interactions, thus benefi ting from loosely coupled services. For
simple orchestrations involving few components, a possible and quite basic
approach consists of the usage of a traditional programming language to
link the various components. However, since programming languages are
mainly focused on the defi nition of classes, methods or structures rather
than on the overall execution process, for complex orchestrations it has been
necessary to develop service composition standards accepted and used by
all the involved entities. Among the current standards and proposals for
Web services orchestration, the Web Services Business Process Execution
Language (WS-BPEL) (Alves et al. 2007) represents a robust and widely
adopted solution. In the following subsections, a brief overview of the
WS-BPEL process is given along with the description of its main features.
Then, in order to build the basis for an integrated solution, requirements
and properties for the OGC service composition are detailed and several
approaches are presented. Finally, an initial analysis of issues related to
the Web services and OGC services orchestration is performed also with
relation to the OGC program for the interoperability.
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