Native Web service support for service-orientation principles

Having now worked through the individual principles of service-orientation in some detail, it becomes evident that Web services provide inherent support for some of these principles. It is important to recognize specifically which principles are built into the structure of common Web services because this allows us to place an emphasis on those that require a conscious effort to realize.

Table 8.2 recaps the principles of service-orientation and explains to what extent they are natively supported by Web services.

Table 8.2. A look at which service-orientation principles are automatically supported by Web services.

Service-Orientation Principle	Web Service Support
service reusability	Web services are not automatically reusable. This quality is related to the nature of the logic encapsulated and exposed via the Web service.
service contract	Web services require the use of service descriptions, making service contracts a fundamental part of Web services communication.
service loose coupling	Web services are naturally loosely coupled through the use of service descriptions.
service abstraction	Web services automatically emulate the black box model within the Web services communications framework, hiding all of the details of their underlying logic.
service composability	Web services are naturally composable. The extent to which a service can be composed, though, generally is determined by the service design and the reusability of represented logic.
service autonomy	To ensure an autonomous processing environment requires design effort. Autonomy is therefore not automatically provided by a Web service.
service statelessness	Statelessness is a preferred condition for Web services, strongly supported by many WS-* specifications and the document-style SOAP messaging model.
service discoverability	Discoverability must be implemented by the architecture and even can be considered an extension to IT infrastructure. It is therefore not natively supported by Web services.

It turns out that half of the principles of service-orientation are natural characteristics of common Web services. This underlines the synergy of the marriage between SOA and the Web services technology platform and gives us a good indication as to why Web services have been so successful in realizing SOA.

It also highlights the principles that require special attention when building service-oriented solutions. The four principles identified as not being automatically provided by Web services are:

• service reusability
• service autonomy
• service statelessness
• service discoverability

Chapters 11 through 15 discuss service modeling and design in detail and provide guidelines to ensure that these important principles are taken into consideration when building services for use within SOA.

These processes further emphasize the other four principles as wellthough they may be automatically implemented through the use of Web services, that does not mean they will necessarily be properly realized. For example, the fact that Web services require the use of service contracts has no bearing on how well individual service descriptions are designed.

SUMMARY OF KEY POINTS

• Service abstraction, composability, loose coupling, and the need for service contracts are native characteristics of Web services that are in full alignment with the corresponding principles of service-orientation.
• Service reusability, autonomy, statelessness, and discoverability are not automatically provided by Web services. Realizing these qualities requires a conscious modeling and design effort.