Patterns: Implementing Self-Service in an SOA Environment

Chapter 3. SOA and the Enterprise Service Bus 37

3.2.1 SOA infrastructure requirements

The Enterprise Service Bus (ESB) is emerging as a middleware infrastructure

component that supports the implementation of SOA within an enterprise. The

need for an ESB can be seen by considering how it supports the concepts of

SOA implementation by:

򐂰 Decoupling the consumer’s view of a service from the implementation of a

service

򐂰 Decoupling technical aspects of service interactions

򐂰 Integrating and managing services in the enterprise

Decoupling the consumer’s view of a service from the actual implementation

greatly increases the flexibility of the architecture. It allows the substitution of one

service provider for another (for example, because another provider offers the

same services for lower cost or with higher standards) without the consumer

being aware of the change or without the need to alter the architecture to support

the substitution.

This decoupling is better achieved by having the consumers and providers

interact through an intermediary. Intermediaries publish services to consumers

.

The consumer binds to the intermediary to access the service, with no direct

coupling to the actual provider of the service. The intermediary maps the request

to the location of the real service implementation.

In an SOA, services are described as being loosely coupled. However, at

implementation time, there is no way to loosely couple a service or any other

interaction between systems. The systems must have some common

understanding to conduct an interaction. Instead, to achieve the benefits of loose

coupling, consideration should be given to how to couple or decouple various

aspects of service interactions, such as the platform and language in which

services are implemented, the communication protocols used to invoke services,

the data formats used to exchange input and output data between service

consumers and providers.

Further decoupling can be achieved by handling some of the technical aspects of

transactions outside of applications. This could apply aspects of interactions

such as:

򐂰 How service interactions are secured

򐂰 How the integrity of business transactions and data are maintained, for

example through reliable messaging, the use of transaction monitors, or

compensation techniques

򐂰 How the invocation of alternative service providers is handled in the event

that the default provider is unavailable