Just what is an ESB, anyway?

"We define it as a vertical bus that connects end-points together using network protocols and applications and languages that already exist in the enterprise," Newcomer said. "We view it as an incremental technology rather than a foundational technology."

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The ESB concept itself is subject to multiple definitions, said Shawn Willett, principal analyst at Current Analysis. "The problem with the ESB market is that people define it differently," Willett said. "I think it's a difficult market because some of the big players don't see an ESB as a product per se; they just see it as an architectural element." Willet places himself amongst those who believe an ESB is, indeed, a product.

An ESB, Willett said, can be defined as a lower cost option for integration that is reliant on Web services standards and endpoints. But do you need to deploy an ESB when setting up an SOA?

"I think if you want to do it right, you do," Willett said.

An SOA is usually defined as an IT architecture that links loosely coupled, easily changeable services based on standards such as Web services.

One believer in the ESB is Rotech Healthcare, an Orlando, Fla.-based company that offers respiratory medicines and home medical equipment. Rotech uses Sonic ESB to integrate Web-based and legacy systems and to tie databases into the company's SOA, said Albert Prast, CIO of Rotech.

Using an ESB makes it easier to build applications that depend on one another without using older technologies such as ftp, Prast said. The ESB provides the glue in a system that uses predefined objects that hand packets of data from one system to another while ensuring transactional integrity, he said. "It allows you to create reusable components that are well-defined, well-documented, secure, and very, very reliable," Prast said.

BEA is a believer as well. "It's a core infrastructure for an SOA," said Kelly Emo, senior product marketing manager for the company's Quicksilver project. The ESB provides a unification layer for dynamic mediation between services, she said.

The ESB connects service consumers to back-end service endpoints, sets up routing and transformation services and manages interactions, Emo said. This is unlike traditional integration, which required too much coding, she said.

"We're seeing this as a rapidly growing market," Emo said.

BEA is promising that its ESB will be different than others because it is not focused on messaging and mediation, Emo said. The company, though, does not see an ESB as a requirement for an SOA. "[However], it makes it a lot easier," Emo said.

The Sonic ESB product offers messaging, mediation and control of interactions between services in an SOA, Chappell said. "Services in an SOA allow flexible interaction between reusable business components," he said.

"Messaging is a core component of an ESB, but there are other core components of an ESB that are equally important," such as service hosting and mediation between disparate data types, Chappell explained.