SOA Overview

Service-oriented architecture (SOA) Definition : A service-oriented architecture can be defined as a group of services, which communicate with each other.
The process of communication involves either simple data passing or it could involve two or more services coordinating some activity. Some means of connecting services to each other is needed.
SOAs build applications out of software services. Services are intrinsically unassociated units of functionality, which have no calls to each other embedded in them. They typically implement functionalities most humans would recognize as a service, such as filling out an online application for an account, viewing an online bank statement, or placing an online booking or airline ticket order. Instead of services embedding calls to each other in their source code, protocols are defined which describe how one or more services can talk to each other. This architecture then relies on a business process expert to link and sequence services, in a process known as orchestration, to meet a new or existing business system requirement.

XML has been used extensively in SOA to create data which is wrapped in a nearly exhaustive description container. Analogously, the services themselves are typically described by WSDL, and communications protocols by SOAP. Whether these description languages are the best possible for the job, and whether they will remain the favorites in the future, is an open question. What is certain is that SOA is utterly dependent on data and services that are described using some implementation of metadata which meets the following two criteria: the metadata must be in a form which software systems can use to configure themselves dynamically by discovery and incorporation of defined services, and also to maintain coherence and integrity. The metadata must also be in a form which system designers can understand and manage at a reasonable cost and effort.
SOA services are loosely coupled, in contrast to the functions a linker binds together to form an executable, a dynamically linked library, or an assembly. SOA services also run in "safe" wrappers such as Java or .NET, and other programming languages that manage memory allocation and reclamation, allow ad hoc and late binding, and provide some degree of indeterminate data typing.
SOA is an architecture that relies on service-orientation as its fundamental design principle.In a SOA environment, independent services can be accessed without knowledge of their underlying platform implementation.
SOA relies on services exposing their functionality via interfaces which other applications and services read to understand how the service can be utilized.