Monday, August 13, 2007

SOA Design Principles

I've just completed an excellent book titled "SOA: Principles of Service Design" by Thomas Erl. Unfortunately, SOA has become such a hackneyed expression that its virtually lost all meaning. However, Erl does a nice job a describing what it really means to create a service-based architecture. These are some of the design principles he identified:

Service Loose Coupling

Coupling refers to a connection or relationship between two things. A measure of coupling is thus comparable to a level of dependency. In SOA, the objective is to reduce ("loosening") dependencies between the service contract, it's implementation, and its service consumers. This makes a service more reusable and flexible. A well-defined meta-data contract, such as provided through a web service WSDL, defines the interfaces with service.

Service Abstraction

Sometimes known as "data hiding", this principle emphasizes the need to hide as much as possible the underlying details of a service. This works in conjunction with Service Loose Coupling to help make services more reusable. This also permits changes to be made to the service without unduly impacting clients. In essence, the service should be a "black-box" that performs its operations without any understanding of the mechanics behind it.

Service Reusability

A service should not be specific to a single functional context. Instead, it should be "agnostic", and support numerous usage scenarios, without having functional dependencies. One challenge with service reusability is that it often promotes more granular-type services, because a service with narrower focus is often more easily reused. This can have performance implications and must be weighed carefully.

Service Autonomy

A service should exercise a high level of control over its underlying runtime environment. In other words, it should managed within its own environment, and be able to support a high degree of reliability and performance. The client should no visibility or concern over the environment in which the service is run.

Service Statelessness

Services should minimize resource consumption by deferring the management of state information, when necessary. A service that maintains state also likely reduces reusability, as it then is likely imposing a certain process or functional dependency on the service.

Service Discoverability

Services should be supplemented with communication meta-data by which they can be effectively discovered and interpreted. In other words, services should be defined within a common registry that enables users to readily understand its purpose and requirements. Obviously, this need becomes more pronounced as an organization grows and spawns multiple development groups or organizations.

Service Composability

Services should be effective composition participants, regardless of the size and complexity of the composition. For instance, this assumes that it will include comprehensive exception handling features and support multiple run-time scenarios. It also requires that the service be capable of handling multiple, concurrent transactions and be highly performing. Using a consistent invocation pattern is also instrumental in this.

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