Since releasing my latest book, Enterprise Service Bus (O'Reilly Media, 2004), I have been doing a fair amount of visiting corporations, conducting seminars, and generally discussing with enterprise architects the subject of enterprise service-oriented architecture (SOA) and how an enterprise service bus (ESB) backbone can be leveraged to provide a framework for an enterprise SOA. Along the way, I have been asked many questions about the nature of an ESB. I have also fended off some misconceptions that have been growing in the general IT population regarding what an ESB is and when, where, and how it can be used. I have gathered together the most popular questions and misconceptions, and offer some clarity in the form of a "top ten" list.

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 model allows the independent scalability of integration components, which are plugged into your SOA as event-driven services on an as needed basis.

In order for an integration broker to be truly capable of supporting an SOA, and to be considered a true ESB, it would need to have its base functions broken up into their constituent parts, which would then be capable of being separately deployed across the bus while working together in harmony as necessary.

Let's use an example of an XSLT-based transformation engine that accepts an incoming XML document and applies an XSLT style sheet to it in order to produce an outgoing document in another XML format. I can tell you that there is nothing that can chew up computing resources more than the parsing and manipulation of XML. If this particular XSLT transformation sits between two popular applications that communicate regularly with each other, then that individual transformation can become a performance and scalability bottleneck. If you are using a monolithic hub-and-spoke integration broker approach, in order to remove the bottleneck and scale up the deployment you would need to either install that integration broker on one big powerful machine, or install the integration broker across multiple machines - just to support that one transform scenario! All the while, the other integration broker capabilities, such as the execution of routing rules, are competing for the same computing resources as the transformation operation.

In contrast to the monolithic hub-and-spoke architecture of an integration broker, the foundational core of an ESB provides a distributed services architecture. This architecture is built for integration and has the ability for integration broker functionality, such as message routing, data transformation, and application adapters to be selectively deployed on an as-needed basis.These are separate integration services that are a natural part of an SOA processing pipeline across the bus.

An individual XSLT transformation can be deployed as a service in its own ESB service container, and multiple instances of that container can be load-balanced across many machines. If the ESB container implementation is cross-platform, then you can be flexible as to what kinds of machines you spread the transform service across - Linux boxes, Solaris boxes, Windows boxes, and so on. And for those of you who don't find solace in the architectural purity of this discussion, consider this: the ESB vendors who are leading the charge in defining and delivering ESB products are also putting forth a license model where there is no additional cost for deploying as many of these lightweight ESB service containers as necessary to get the job done.

The integration services provided by the ESB can be combined with other services into SOA-based processing pipelines that can span business boundaries. The distributed services in an ESB can be combined with itinerary-based routing (see Myth #7) to allow self-directed, message-oriented service interactions, which allow different parts of the ESB to operate independently of one another, without relying on a centralized routing engine.