R – Implementing ESB (Enterprise Service Bus) in a functional language

Messaging tends to concentrate on the reliable exchange of messages around a network; using queues as a reliable load balancer and topics to implement publish and subscribe.

An ESB typically tends to add different features above and beyond messaging such as orchestration, routing, transformation and mediation.

I'd recommend reading about the Enterprise Integration Patterns which gives an overview of common patterns you'll tend to use in integration problems which are all based above a message bus (though can be used with other networking technologies too).

For example using open source; Apache ActiveMQ provides a loosely coupled reliable exchange of messages. Then you can use Apache Camel to implement the Enterprise Integration Patterns for smart routing, transformation, orchestration, working with other technologies and so forth.

Let me try to clarify those terms to you:

Bus in context of ESB architecture should not be considered as simple queue for message dispatching. To allow integration of different services, ESB provides much more. Important additional functionalities of ESB:

• Routing. Messages can be routed to different services, depending on message content or endpoint specification.
• Message Transformations/Mediations between different formats
• Transport protocol conversion. ESB should be able to seamlessly integrate applications that use different transport protocols (JMS, HTTP/S, pure TCP, etc.)
• Message enhancement. Messages can be enriched with missing data before further processing.
• Security
• Management and Monitoring

Those functionalites are provided by services that operate within ESB. Services connect to each other via endpoints - uniform, unique "addresses". Messages dispatched between endpoints are using unified transport (method/protocol that encapsulates message's payload). Application that natively use different transport, need to connect to ESB via suitable adapter - service that will provide necessary transport conversion. This way applications that use ESB are decoupled from each other and don't need to provide conversions themselves.

Of course, those are only very brief descriptions of terms. Remember, Enterprise Service Bus is only catch-term for specific kind of architecture (or concept), but it is not standardized in any way. So specific implementations can be very different from each other. If you are interested in standardized ESB, you can take a look at JBI (Java Bussiness Integration). There are several open-source implementations of JBI avalable, among them Apache ServiceMix, Mule, OpenESB. Very good introduction to ESB technologies is presented in "Open Source ESBs in Action" book published by Manning.