Like the USB hub, the ESB is extensible and scalable. It can span across multiple pieces of hardware, but can still function as a single distributed unit. You should be able to connect ESBs together and share services across the enterprise locally and globally.

Routing
Routing is an important feature of ESB. ESB should support configurable, dynamic routing. Routing allows business processes to be implemented and changed through configuration. This is similar to configuring scanner software for a printer, e-mail application, etc. Once done, a document can be scanned and printed or e-mailed using a touch of a button. The scanner software routes the document to printer or e-mail based on the configuration.

The advent of XML-enabled Content Based Routing (CBR) provided the ability to identify specific fields in the message, which allowed routing of the message to services based on those fields. ESB could examine the message content and route it to the appropriate service.

Transformation
Another concept to understand is transformation. The data in the devices can be represented in multiple formats. For example, when you scan a document, you can use OCR to read it into a MS Word document that represents it in text format, or stores it as a JPG image, or prints it on paper in a hard form. This is analogous to the transformation that happens in the ESB.

Since ESB connects multiple applications and services, it is imperative to support the different formats of data that they require. Thus, an ESB should provide support for transformation of data. A technique to avoid decomposition of data is using a canonical data model across the services. This also reduces the number of transformation types to be supported, as any data type can be transformed to Canonical and to another data type rather than having transformations for every data type. XQuery and XSLT are typically used to perform XML-to-XML transformation.

Business Process
ESB enables moving the business process logic out of applications and into the bus. This way, applications or services can be orchestrated at a higher level compared to hard-coded process logic. This improves the agility of the applications.

AquaLogic Service Bus (ALSB)
Previously code named QuickSilver, AquaLogic Service Bus is an integral part of BEA's AquaLogic product family, which also includes AquaLogic Data Services, AquaLogic Enterprise Security, and AquaLogic service registry. Built on the industry-leading, proven WebLogic Platform that focuses on developers, AquaLogic focuses on application development and integration through configuration and composition.

AquaLogic is much more than an ESB. In addition to providing a robust implementation of the ESB, it also offers service management capabilities. Figure 3 shows the ALSB with standard ESB capabilities and BEA value-add.

ALSB supports UDDI v3 for service registration and discovery. It provides dynamic routing and transformation capabilities across heterogeneous platforms. As depicted in the diagram, it supports multiple protocols.

ALSB also has security and policy enforcement, message tracking and monitoring, SLA alerts, and service-versioning capabilities. This unique combination of service management capabilities with the ESB makes ALSB a very powerful product. ALSB complements other BEA products such as WebLogic Integration and Liquid Data.

Summary
In this article we looked at the analogy of USB to understand the ESB. The success of USB indicates that ESB is also poised to succeed, if executed well. The idea behind both of these technologies is the same: how to provide a standards-based platform to connect the consumers and providers to share and reuse, thus providing value to the customers. The BEA AquaLogic Service Bus combines the ESB capabilities with management and monitoring capabilities.