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A Service-Oriented ArchitectureCompanies have long sought to integrate existing systems in order to implement information technology (IT) support for business processes that cover all present and prospective systems requirements needed to run the business end-to-end. A variety of designs can be used to this end, ranging from rigid point-to-point electronic data interchange (EDI) interactions to web auctions. By updating older technologies, such as Internet-enabling EDI-based systems, companies can make their IT systems available to internal or external customers; but the resulting systems have not proven to be flexible enough to meet business demands. A flexible, standardized architecture is required to better support the connection of various applications and the sharing of data. SOA is one such architecture. It unifies business processes by structuring large applications as an ad hoc collection of smaller modules called services. These applications can be used by different groups of people both inside and outside the company, and new applications built from a mix of services from the global pool exhibit greater flexibility and uniformity. One should not, for example, have to provide redundantly the same personal information to open an online checking, savings or IRA account, and further, the interfaces one interacts with should have the same look and feel and use the same level and type of input data validation. Building all applications from the same pool of services makes achieving this goal much easier and more deployable to affiliate companies. An example of this might be interacting with a rental car company's reservation system even though you are doing so from an airline's reservation system.

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.

Relative to typical practices of earlier attempts to promote software reuse via modularity of functions, or by use of predefined groups of functions known as classes, SOA's atomic-level objects are often 100 to 1,000 times larger, and are associated by an application designer or engineer using orchestration. In the process of orchestration, relatively large chunks of software functionality (services) are associated in a non-hierarchical arrangement (in contrast to a class hierarchy) by a software engineer, or process engineer, using a special software tool which contains an exhaustive list of all of the services, their characteristics, and a means to record the designer's choices which the designer can manage and the software system can consume and use at run-time.

Underlying and enabling all of this is metadata which is sufficient to describe not only the characteristics of these services, but also the data that drives them. 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.

The goal of SOA is to allow fairly large chunks of functionality to be strung together to form ad hoc applications which are built almost entirely from existing software services. The larger the chunks, the fewer the interface points required to implement any given set of functionality; however, very large chunks of functionality may not be granular enough to be easily reused. Each interface brings with it some amount of processing overhead, so there is a performance consideration in choosing the granularity of services. The great promise of SOA is that the marginal cost of creating the n-th application is zero, as all of the software required already exists to satisfy the requirements of other applications. Only orchestration is required to produce a new application.

The key is that there are no interactions between the chunks specified within the chunks themselves. Instead, the interaction of services (all of which are unassociated peers) is specified by humans in a relatively ad hoc way with the intent driven by newly emergent business requirements. Thus the need for services to be much larger units of functionality than traditional functions or classes, lest the sheer complexity of thousands of such granular objects overwhelm the application designer. The services themselves are developed using traditional languages like Java, C#, C++, C or COBOL.

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.

Increasing numbers of third-party software companies are offering software services for a fee. In the future, SOA systems may consist of such third-party services combined with others created in-house. This has the potential to spread costs over many customers, and customer uses, and promotes standardization both in and across industries. In particular, the travel industry now has a well-defined and documented set of both services and data, sufficient to allow any reasonably competent software engineer to create travel agency software using entirely off-the-shelf software services. Other industries, such as the finance industry, are also making significant progress in this direction.

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.