An introduction to web applications architecture

Loose coupling and encapsulation: coupling between service providers and consumers should be low, and confined to reliance to service public interfaces, with an understanding that such interfaces should disclose as little as possible of the underlying implementation details, which is known as encapsulation. The interfaces should be based on standard communication protocols, rather than proprietary ones, with open standards recommended to foster the highest degree of interoperability.
Abstraction: both the encapsulated implementation, the implementation technology and the physical location of services should be fully transparent to consumers, which should be reliant solely on public interfaces, service contract descriptions and SOA infrastructures to locate and invoke required services.
Autonomy: the more autonomous a service is, the more control it will have over its own implementation and run-time environment, and so the greater will be its flexibility and potential for evolution. Both implementation and run-time environment can be modified without affecting consumers.
Statelessness: excessive state information can compromise the availability of a service and limit its scalability; hence, services should remain stateless as far as is realistic to allow them to do their work.
Standardised interfaces and contracts: to support the service collaboration model and automate service interaction, services need to be described in a similar way using commonly understood standards. In particular, a service description may include descriptions of the service technical interface (that is, the operations which can be invoked and their parameters), the quality of service provided, and a service-level agreement which details both service characteristics (e.g. response time or availability) and cost to the consumer of invocation. Additional metadata may cover a range of information, for instance including user satisfaction rating or future development plans.
Reusability: services must be designed with reuse in mind, so generality of the service in terms of potential reuse across projects and systems is an important design principle. This impacts on both the granularity of the service (the service should strive to support generic business processes and tasks and close alignment with the business) and on the choice of standards and implementation technology, which should foster the highest possible degree of interoperability.