Services as a Paradigm of Computation - Formal Methods and Hybrid Real-Time Systems - page 525

2.3 Web Services

Currently, the most prominent kind of services are Web services . A Web service is
a functionality (e.g., a standard business function) provided at a unique network
address, given as a URI. This functionality is described in a standard definition
language, and available via various transport protocols, formats and profiles for
quality of service. Today's implemented Web Services rely on highly distributable
communication- and integration backbones (sometimes called “the Big Basic
Bus”, BBB). Each Web Service is addressed by its unique URI and is usable by
other services along its - publically known - interface. A Web Service is assumed
as “always on”: A user does not have to create it, nor to care about destruction,
etc. Several instances of a Web Service may exist concurrently, mimicking for
each user exclusive access to the service. The language WS-BPEL established
itself as a quasi-standard for the implementation of Web services.

2.4 Service-Oriented Architectures

Though independent of other services, a service is typically constructed with
respect to other services: Purpose and use of a service is its communication
with other services. Partners to communicate with may reside anywhere in the
real world. For a Web service, any service on the web is a potential candidate
to serve as a partner. A fundamental problem then is service discovery , i.e.
for a service P the problem to identify proper communication partners, and
to establish communication with those partners. A service-oriented architecture
(SOA) solves this problem by help of a scenario that assumes

- agents called providers : A provider offers services to the public, to be used by
(i.e., to be composed with) other services. To this end, a provider publishes
information about the services he offers.
- agents called requesters : A requester requests services, i.e. wants to find
services it can use, as they are published by providers.
- agents called brokers : A broker collects information about the services pro-
vided by providers and the services wanted by requesters. Upon detecting
services that would properly fit, the broker informs the requester about the
provider, such that they can directly bind their services. In more elaborated
variants, a broker may itself compose two or more provider services, and
offer the composed service to a requester. Even more, a broker may observe
that a provider service only “almost” fits to a requester service. In this case
the broker may construct an adapter service to bridge the gap. Figure 1
shows the conventional outline of SOA, indicating the three agents and their
pairwise activities.

SOC and SOA can be conceived as virtualization , viz. abstraction, from tech-
nical implementation details of services. SOA is an architectural style to realize
SOC.Thiscanbeconceivedananalogytothe client/server architectural style
that realizes distributed computing.