ESB ??

I had talked about ESB Myths some time back. Today, I thought let me take one step backward and find out

What is ESB?

One defination goes as
"An Enterprise Service Bus (ESB) is a flexible connectivity infrastructure for integrating applications and services. An ESB can power your service-oriented architecture (SOA) by reducing the number, size, and complexity of interfaces between those applications and services."

Another says
"The Enterprise Service Bus (ESB) provides a new way to build and deploy enterprise service-oriented architectures (SOA). ESB is a concept that is increasingly gaining the attention of architects and developers, as it provides an effective approach to solving common problems such as service orchestration, application data synchronization, and business activity monitoring."

Another says
"An enterprise service bus (ESB) is a pattern of middleware that unifies and connects services, applications and resources within a business. Put another way, it is the framework within which the capabilities of a business' applications are made available for reuse by other applications throughout the organization and beyond."

Another says
"An ESB is software infrastructure that simplifies the integration and flexible reuse of business components using a service-oriented architecture. An ESB makes it easy to dynamically connect, mediate and control services and their interactions."

Another one says
"An enterprise service bus refers to a category of middleware infrastructure products or technologies, based on Web services standards, that enable a service-oriented architecture via an event-driven and XML-based¹ messaging engine (the bus). An enterprise service bus generally provides an abstraction layer on top of an Enterprise Messaging System which allows integration architects to exploit the value of messaging without writing code."

I feel the last defination describes the ESB better. ESB is an abstract layer sitting on top of a messaging system, which makes use of Web Services to achieve connectivity.

Although the exact definition of an ESB varies, most agree that the following are characteristics of an ESB::

* it requires the clear separation of message headers and message body
* it is usually operating system and language independent; it should work between Java and .Net applications, for example
* it (often) uses XML and Web services to transport messages
* it includes adapter standards (such as J2C) for incorporating existing applications into the bus
* it includes support for asynchronous processing
* it includes intelligent, content-based routing
* it includes a standardized security model to authorize, authenticate, and audit use of the ESB
* it includes transformation services (such as XSLT) between the format of the sending application and the receiving application, including the transformation of data/message formats
* it includes validation against schemas for sending and receiving messages
* it can uniformly apply business rules, enrichment of the message from other sources, splitting and combining of multiple messages, and the handling of exceptions
* it can conditionally route or transform messages based on a central policy
* it is monitored for message latency and other characteristics described in a Service Level Agreement
* it (often) facilitates "service classes," responding appropriately to higher and lower priority users
* it supports queuing, holding messages if applications are temporarily unavailable
* it handles a "publish and subscribe" messaging model, including event handling
* it handles business events from disparate sources