With the paradigm of service-oriented architecture that we shall explore later in the book, the Messages Broker pattern started showing some disadvantages.

First off, Message Brokers have been always implemented as a monolithic application that performs all required functionalities of a hub in the hub-and-spoke architecture. They implement routing, validation, and transformation, as well as error handling, logging, and all other message processing activities. This made the message broker a single point of failure in the solutions.

In early 2000s, the notion of Enterprise Service Bus (ESB) emerged, which used the bus paradigm instead of the hub. Bus architecture relies on distributed services that can be added and removed seamlessly, making it more dynamic. Significant componentization makes ESB more reliable than message hubs.

The notion of ESB has been changing constantly since its appearance on the market. It is still an evolving notion, and its definitions...