Services as a Paradigm of Computation - Formal Methods and Hybrid Real-Time Systems - page 524

- What kind of formal representations of such systems are reasonable?
- How can the expressive power of formal description techniques of such sys-
tems be compared?
- Is there a “most general” class of “representable” such systems, in analogy
to the class of computable functions in the classical setting?

2 Service-Oriented Computing and Service-Oriented

Architectures

The above discussion focused communicating agents as a basic construct of the
new paradigm. We suggest services as an adequate concretion for such agents.
Services provide viable means to implement communicating agents. Furthermore,
there exists a rich theory to handle services, and to answer the above questions.
Details will be given in the next sections.

2.1 The New Paradigm in Practical Applications

Governed by practical needs, and not caring too much about theoretical aspects,
systems following the new paradigm have been implemented for decades. Exam-
ples include operating systems, technical control systems, workflows etc. But only
nowadays such systems are conceived as following a new paradigm. This may
be due to the emerging problems that arise in the course of automatic composi-
tion of such systems, as required for computer based interorganizational business
processes and new software architectures, such as “programming-in-the-world”
or “programming on demand”.
In this context, systems that fit into the new paradigm are usually denoted as
services . Their implementation in the framework of existing technologies evoked
the concepts of service-oriented computing (SOC) and, as a principle of using
SOC, the term of service-oriented architectures (SOA). We expand on those
aspects in the sequel.

2.2 Service-Oriented Computing

Trying to identify what many different views, descriptions and definitions for
services have in common, we can conclude that a service is a well defined,
self-contained module that provides some concrete functionality to its environ-
ment. Consequently, the minimal requirements of a service include an inter-
face and an internal control . An interface can usually be conceived as a set of
ports , with each port capable to store messages . The internal control triggers
the actions of the service. An action either sends a messages to a port or re-
ceives one from a port or operates locally. Hence, asynchronous communication
is the usual communication mode of services. But other modes may be sup-
ported as well, such as synchronous (handshake) communication or lock step
communication.