Service-oriented architecture (SOA)

We have been fiddling with object-oriented, component-based, aspect-oriented, and agent-based software development processes. However, with the arrival of service paradigms, software packages and libraries are being developed as a collection of services. That is, software systems and their subsystems are increasingly expressed and exposed as services. Services are capable of running independently of the underlying technology. Also, services can be implemented using any programming and script languages.

Services are self-defined, autonomous, and interoperable, publicly discoverable, assessable, accessible, reusable, and compostable. Services interact with one another through messaging. There are service providers/developers and consumers/clients. There are service discovery services that innately leverage both private and public service registries and repositories. Client services can find their serving services dynamically through service discovery services.

Every service has two parts: the interface and the implementation. The interface is the single point of contact for requesting services. Interfaces give the required separation between services. All kinds of deficiencies and differences of service implementation get hidden by the service interface. To make the service interface easy to use by other services, it is a good idea to use a schema definition that defines the structure of the messages. When a service is used by multiple other services, formalizing the service with a contract is paramount. A contract bounds the service with schemas, a clear message exchange pattern, and policies. Policies define the QoS attributes, such as scalability, sustainability, security and so on. SOA differs from the client/server architecture in the sense that services are universally available and stateless, while client/server architecture requires tight coupling among the participants.

Precisely speaking, SOA enables application functionality to be provided as a set of services, and the creation of personal as well as professional applications that make use of software services.