What is an ESB?

Vendors position ESB as the latest generation of application integration technology, succeeding traditional integration brokers such as messaging middleware. Although specific implementations vary, ESBs share a common set of characteristics.

Stuart Ransom, a vice president at Sonic Software, described the minimum requirements — the ESB table stakes as he put it — as messaging, transformation and routing.

That's consistent with Gartner's ESB definition, which the firm developed in late 2002.

The messaging layer — the B in ESB — provides an intermediary through which applications can communicate. Transformation involves mapping data to a format that both sending and receiving applications can use. Routing helps deposit a message at the proper destination.

How is ESB technology related to SOA?

An ESB supports applications deployed as reusable software components, often referred to as services. Thus, proponents view the technology as a critical element of an SOA, which is a collection of services that an organization can coordinate to support its business processes.

"ESB forms the core platform on top of which an SOA can be deployed," said Atul Saini, chief executive officer, chief technology officer and chairman of Fiorano Software, which develops ESB products.

Web services standards have emerged as a way to achieve interoperability among SOA services. Those standards include Extensible Markup Language for the message format, Simple Object Access Protocol for message transport and Web Services Description Language for interface descriptions.

Ransom said support for Web services is a central ESB requirement. Several ESB vendors employ Web services standards as the means for linking service-based applications. By using Web services, an ESB can deal with applications devised from the beginning as services and with applications accessible via Web services. In addition, ESB vendors often provide optional adapters for older applications that don't support Web services.

Ransom describes ESB as an integration layer "between existing legacy applications and new applications being built."

Although the technology can provide an SOA launching pad, industry observers say vendors could build such an architecture without an ESB.

SOAs "are increasing in popularity, but very few customers have adopted an enterprise service bus," said Michael Beckley, vice president of product strategy at Appian, a provider of business process management products and services.