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environments made available. Finally, a case study that will demonstrate
how everything has come to work together in a real-life project will be
given before we conclude.
8.2
In a service-oriented architecture, computing logic and resource are
demarcated in a unit of
service
. Services as autonomous self-administra-
tive modules are loosely coupled in highly distributed computing envi-
ronments where service interactions wholly depend on the exchange of
self-containing messages across networks, while a service itself exposes
nothing but functional interface to the outside world. It is an intrinsic
requirement for a service-oriented architecture (SOA) that services as
composable and reusable building blocks should be coordinated in high-
value logic to achieve advanced application objectives, while a service on
its own is simply less likely to be capable of meeting such requirements.
Enforcement of open standards of SOA helps eliminate implementation
diversities, like differences between platforms, implementation languages,
network protocols, security mechanisms, open or proprietary technolo-
gies, and so on, in the heterogeneous computing environment. It is now
possible for services to be treated equally in a standardized and consistent
manner to serve application requests despite underlying differences.
Service orchestration
simply provides the means of connecting the services
through a central point of control and communication. An orchestration
engine drives the execution of application workl ow and service invoca-
tions according to the orchestration logic that has been dei ned. BPEL is a
standard way of documenting such orchestration logic.
By focusing on service-level activities, l exibility, and simplicity, BPEL
accommodates the creation, modii cation, merging, and division of large,
complex application logic through the control of single or multiple process
dei nition documents, which is much preferred by developers, architects,
or domain experts whoever hold overviews of applications but rather not
to drill down to implicit programmatic levels. The fact is, BPEL, as a pur-
pose standard, rel ects the intrinsic nature of interoperability, compos-
ability, reusability, and extensibility of servic orientation. It is an inseparable
facet of designing and building a service-oriented architecture. To under-
stand the context in which BPEL exists, the problem scope it targets, the
essential models of service orchestration it promotes, as well as those
technical backgrounds it relies on, helps us appreciate the essence of ser-
vice-orientated architecture, around which grid services are eventually
built. We will develop our discussions with these essential principles in
mind throughout the chapter.
BPEL and Grid Service Orchestration
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