What is an ESB?
An Enterprise Service Bus (ESB) is fundamentally an architecture. It is a set of rules and principles for integrating numerous applications together over a bus-like infrastructure. ESB products enable users to build this type of architecture, but vary in the way that they do it and the capabilities that they offer. The core concept of the ESB architecture is that you integrate different applications by putting a communication bus between them and then enable each application to talk to the bus. This decouples systems from each other, allowing them to communicate without dependency on or knowledge of other systems on the bus. The concept of ESB was born out of the need to move away from point-to-point integration, which becomes brittle and hard to manage over time. Point-to-point integration results in custom integration code being spread among applications with no central way to monitor or troubleshoot. This is often referred to as "spaghetti code" and does not scale because it creates tight dependencies between applications.
Why use an ESB?
Increasing organizational agility by reducing time to market for new initiatives is one of the most common reasons that companies implement an ESB as the backbone of their IT infrastructure. An ESB architecture facilitates this by providing a simple, well defined, "pluggable" system that scales really well. Additionally, an ESB provides a way to leverage your existing systems and expose them to new applications using its communication and transformation capabilities.