Enterprise Service Bus (ESB) - Guide to Cloud Computing for Business and Technology Managers - page 191

2. Service enablement : The service development methodology should
determine which discrete application elements need to be exposed
as services.
3. Service orchestration : Distributed services need to be configured and
orchestrated in a unified and clearly defined distributed process.
4. Service deployment : Emphasis should also be placed on the produc-
tion environment that addresses security, reliability, and scalability
concerns.
5. Service management : Services must be audited, maintained, and
reconfigured, and corresponding changes in processes must be
made without rewriting the services or underlying application.
9.1.1 Evolution of ESB
Conceptually, the ESB has evolved from the store and forward mechanism
found in middleware products and now is a combination of EAI, Web
Services, XSLT, and orchestration technologies, such as BPEL. To achieve its
operational objectives, the ESB draws from traditional EAI broker functional-
ity in that it provides integration services such as connectivity and routing of
messages based on business rules, data transformation, and adapters to appli-
cations. These capabilities are themselves SOA based in that they are spread
out across the bus in a highly distributed fashion and hosted in separately
deployable service containers. This is a crucial difference from traditional
integration brokers, which are usually heavyweight, highly centralized, and
monolithic in nature. The ESB approach allows for the selective deployment
of integration broker functionality exactly where it is needed with no addi-
tional overbloating where it is not required.
To surmount problems of system heterogeneity and information model
mismatches in an SOA implementation, an EAI middleware supporting hub-
and-spoke integration patterns could be used. The hub-and-spoke approach
introduces an integration layer between the client and server modules that
must support interoperability among and coexist with deployed infra-
structure and applications, and not attempt to replace them. However, this
approach has its own drawbacks as a hub can be a central point of failure and
can quickly become a bottleneck.
A scalable distributed architecture such as an SOA needs to employ a
constellation of hubs. The requirements to provide an appropriately capable
and manageable integration infrastructure for Web Services and SOA are
coalescing into the concept of the Enterprise Service Bus (ESB), which will
be the subject of this section. The two key ideas behind this approach are to
loosely couple the systems taking part in the integration and break up the
integration logic into distinct, easily manageable pieces.
Figure 9.1 above shows a simplified view of an ESB that integrates a J2EE
application using JMS, a .NET application using a C# client, an MQ application