By Dave Chappell
Myth #1. ESB is just a new name for EAI.
While many IT architecture groups are focusing on building SOAs, they still inevitably beg the question of "how is ESB different from EAI?" An ESB is an infrastructure for building an enterprise SOA, and is capable of being used in a more general way than a conventional EAI broker. According to Forrester Research, an ESB helps enterprises obtain the value of SOA by increasing connectivity, adding flexibility that speeds change, and providing greater control over use of the important resources that it binds.
An ESB can be used to handle integration projects that have traditionally been relegated to EAI tools. However, an ESB can also be used for establishing B2B relationships across companies.
An ESB provides EAI capabilities, but is based on a fundamentally different architecture that is providing the basis of an industry transition from traditional integration to coordinated service interaction. EAI brokers are historically implemented as a monolithic stack, using centralized hub-and-spoke architecture.
An ESB provides the same base functionality as an EAI broker - connectivity, application adapters, routing of messages based on rules, and data transformation engine - yet, in an ESB, 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 allows the selective deployment of integration broker functionality exactly where you need it, with no additional over-bloating where it's not needed. The distributed nature of the ESB container allows the independent scalability of integration components, which are plugged into your SOA as event-driven services on an as needed basis.
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http://soa.sys-con.com/node/48035