Service-Oriented Architecture - Guide to Cloud Computing for Business and Technology Managers - page 155

in a cost-effective and agile manner that would combine the best of custom
solutions as well as packaged applications while simultaneously reducing
lock-in to any single IT vendor.
A Service-oriented architecture (SOA) is a style of organization that guides
all aspects of creating and using business services throughout their life cycle
(from conception to retirement), as well as defining and provisioning the IT
infrastructure that allows different applications to exchange data and partici-
pate in business processes regardless of the operating systems or program-
ming languages underlying these applications. The key organizing concept of
an SOA itself is a service. The processes, principles, and methods defined by
SOA are oriented toward services; the development tools selected by an SOA
are oriented toward creating and deploying services; and the runtime infra-
structure provided by an SOA is oriented to executing and managing services.
A service is a sum of constituting parts including a description, the imple-
mentation, and the mapping layer (termed as transformation layer) between
the two. The service implementation, termed as the executable agent, can be
any environment for which Web Service support is available. The description
is separated from its executable agent; one description might have multiple
different executable agents associated with it and vice versa. The executable
agent is responsible for implementing the Web Service processing model as
per the various Web Service specifications and runs within the execution
environment, which is typically a software system or programming lan-
guage environment. The description is separated from the execution envi-
ronment using a mapping or transformation layer often implemented using
proxies and stubs. The mapping layer is responsible for accepting the mes-
sage, transforming the XML data to be native format, and dispatching the
data to the executable agent.
Web Service roles include requester and provider; a requester can be a pro-
vider and vice versa. The service requester initiates the execution of a service
by sending a message to a service provider, which executes the service and
returns the results, if any specified, to the requester.
7.1.1 Services
Services are coarse-grained, reusable IT assets that have well-defined inter-
faces (or service contracts) that clearly separate the service accessible inter-
face from the service technical implementation. This separation of interface
and implementation serves to decouple the service requesters from the ser-
vice providers so that both can evolve independently as long as the service
contract remains unchanged.
A service is a location on the network that has a machine-readable descrip-
tion of the messages it receives and optionally returns. A service is therefore
defined in terms of the message exchange patterns it supports. A schema for
the data contained in the message is used as the main part of the contract