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What does the term ESB actually mean?

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Nick Allen, Program Manager at Microsoft in the connected systems division, asked in his blog:

What does the term ESB- Enterprise Service Bus- actually mean? That question has been the topic of an ongoing debate for several years now that doesn't seem to have any sign of stopping. When I first read about ESBs in 2003, I didn't expect to still be trying to understand them more than four years later.

This question was visited last year by Miko Matsumura in a two part article (part1, part2). This time Nick went on to search for every major vendor's ESB definition. However,

Microsoft [...] dodges the question of definition (the [ESB Guidance] site plays up the ESB means different things to different people angle).

For Anne Thomas Manes, Research Director with Burton Group,

ESBs are essentially integration systems, not SOA systems. SOA is about tearing down application silos, but integration systems reinforce those silos. [...] an ESB is especially good for bridging to legacy applications, and therefore it is a useful component in a services infrastructure

Anne's definition seems to be in line with Ron Ten-Hove from Sun Microsystems and JBI Spec Lead (not cited by Nick):

An Enterprise Service Bus (ESB) is a distributed middleware system for integrating enterprise IT assets using a service-oriented approach.

Or with the one from Gartner (not cited by Nick):

A Web-services-capable infrastructure that supports intelligently directed communication and mediated relationships among loosely coupled and decoupled business components.

This seems to be the common themes of BEA's, IBM's, Oracle's and other's as noted by Nick: Common Communication and Integration Services for Business Components.  There is a notable difference between vendors, for some (IBM, Microsoft), an ESB is a pattern that associates a series of capabilities and for others it is a product. This has been the stickiest point of the discussion. In 2005, Rich Turner, Product Manager for Microsoft's Identity Platform, noted that:

An ESB [product] is smart-plumbing to which to attach dumb nodes. [...] The WS route makes the nodes themselves smart, reducing the need for underlying smart-plumbing, and ensuring open communications across any platform and device.

Rich's point is even more relevant since this year, the WS-TX and WS-RX specifications have been ratified, enabling, at last, a secure, reliable and transacted communication mechanism based on standard web service protocols offering a viable alterative to proprietary ones. Rourke McNamara distinguishes ESB implementations (ESBi) from ESB products (ESBp):

ESB products (ESBp) are designed to help companies build ESBi. The ESB market is mature and the definition of what makes an ESB product (ESBp) has been made clear over the course of 2006. This year has seen every major SOA vendor release of refine their ESB offering or offerings.

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