How does an ESB differ from an Application Server? Generally an application server will consist of Servlets, EJBS, Webservices, JCA Components (For connecting to databases), and can also have a persistence layer like JPA/Hibernate. Application servers can also be extended to have rules engines like Drools etc. A typical application server architecture would look like:
However, in a typical enterprise, a system may have to deal with multiple other systems, running on multiple clusters and external distributed environments. For example some of these external systems may include a Billing System, an Inventory System, an Accounting System and a CRM, and this is where an ESB Architecture fits in.
An ESB System, provides a set of tools for synchronizing/orchestrating the communication between these external systems. Note: Some of these external systems may not be in Java, and may involve other technologies like .NET, Php/Lamp, or other legacy technologies. Also communicating between these systems may not simply be RMI or JMS Messages in a Java Environment. It may also involve communicating using SOAP/XML messages over a Web service or Restful interface, or simply reading file in different formats over an FTP link.
JBoss ESB
