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Protocol Composability
Protocols are said to compose when they can be used either independently or in combination. Numerous domain-specific communication protocols are effectively "silos" in which protocol designers find themselves coining new mechanisms for dealing with security, reliability, error reporting and other features. Given the broad applicability of Web services, this approach to defining an entire protocol for each vertical domain breaks when domains overlap and becomes extremely costly in terms of both initial development and ongoing support costs.To avoid these costs, the Web services protocol suite is designed as a family of composable protocol building blocks. By design, each of the infrastructure protocols defines a fine grained unit of functionality. For example, the basics of signing and encrypting message contents is generic enough that it is specified once (in WS-Security) and then leveraged by various infrastructure and application protocols.Web service protocol composition exploits the modular architecture of SOAP. SOAP's architecture anticipates the composition of infrastructure protocols through the use of a flexible and extensible message header mechanism. One advantage of this approach is that the protocol surface area for a particular application is based on the actual features used by that application. A given protocol imposes absolutely no semantic effect to applications that do not use it. Software operating on computing devices of various scales can use the exact protocols they need, maximizing the applicability of the architecture. A second advantage is that new protocols can be introduced at any time to complement the existing ones and extend functionality. The ability to innovate is thus built into the architecture. With this flexibility and extensibility, getting a coherent and comprehensive view of the spectrum of available protocols can be a challenge. Addressing this challenge is a goal of this book.In addition to specifications defining building blocks protocols at the core of the Web services architecture, system interoperability requires additional specifications more closely related to protocol composition and application processing. A three-layer hierarchy of specifications has served us well to date: building block protocols, specification profiles, and application protocols. The nature of the building block protocols has been discussed already. Specification profiles are provided to define usage constraints and the best practices for use of the building block protocols when used in various combinations. Application specifications define specific business functions that are developed using Web services, such as purchase orders or Interoperability Profiles" (in Chapter 3), "The WS-I Basic Security Profile" (in Chapter 4), and "The Device Profile for Web Services Specification" (in Chapter 5).
