5 Principles of SOA

In many customer engagements, I need to establish a basic set of principles of SOA. The following sections introduce fundamental principles that a Service-oriented Architecture (SOA) should expose.These are not introduced as an absolute truth, but rather as a frame of reference for SOA-related discussions. You’ll note that the first four are based on Don Box’s four tenets, although over time they may have acquired a slight personal spin.

1. Explicit Boundaries

Everything needed by the service to provide its functionality should be passed to it when it is invoked. All access to the service should be via its publicly exposed interface; no hidden assumptions must be necessary to invoke the service. “Services are inextricably tied to messaging in that the only way into and out of a service are through messages”. A service invocation should – as a general pattern – not rely on a shared context; instead service invocations should be modeled as stateless. An interface exposed by a service is governed by a contract that describes its functional and non-functional capabilities and characteristics. The invocation of a service is an action that has a business effect, is possibly expensive in terms of resource consumption, and introduces a category of errors different than those of a local method invocation or remote procedure call. A service invocation is not a remote procedure call.

2. Shared Contract and Schema, not Class

3. Policy-driven

To interact with a service, two orthogonal requirement sets have to be met:

1. the functionality, syntax and semantics of the provider must fit the consumer’s requirements,
2. the technical capabilities and needs must match.

4. Autonomous

Related to the explicit boundaries principle, a service is autonomous in that its only relation to the outside world – at least from the SOA perspective – is through its interface. In particular, it must be possible to change a service’s runtime environment, e.g. from a lightweight prototype implementation to a full-blown, application server-based collection of collaborating components, without any effect on its consumers. Services can be changed and deployed, versioned and managed independently of each other. A service provider can not rely on the ability of its consumers to quickly adapt to a new version of the service; some of them might not even be able, or willing, to adapt to a new version of a service interface at all (especially if they are outside the service provider’s sphere of control).

5. Wire formats, not Programming Language APIs

Services are exposed using a specific wire format that needs to be supported. This principle is strongly related to the first two principles, but introduces a new perspective: To ensure the utmost accessibility (and therefore, long-term usability), a service must be accessible from any platform that supports the exchange of messages adhering to the service interface as long as the interaction conforms to the policy defined for the service. For example, it is a useful test for conformance to this principle to consider whether it is possible to consume or provide a specific service from a mainstream dynamic programming language such as Perl, Python or Ruby. Even though none of these may currently play any role in the current technology landscape, this consideration can serve as a litmus test to assess whether the following criteria are met:

• All message formats are described using an open standard, or a human readable description
• It is possible to create messages adhering to those schemas with reasonable effort without requiring a specific programmer’s library
• The semantics and syntax for additional information necessary for successful communication, such as headers for purposes such as security or reliability, follow a public specification or standard
• At least one of the transport (or transfer) protocols used to interact with the service is a (or is accessible via a) standard network protocol