| Enterprise messaging is at the core of an ESB architecture. In this chapter we will explore Message Oriented Middleware (MOM) and what it contributes to an enterprise integration strategy. A MOM is a key part of the ESB architecture, as it provides the underpinnings of the network of virtual channels that an ESB uses to route messages throughout an extended enterprise and beyond. Message Oriented Middleware is a concept that involves the passing of data between applications using a communication channel that carries self-contained units of information (messages). In a MOM-based communication environment, messages are usually sent and received asynchronously. Using message-based communications, applications are abstractly decoupled; senders and receivers are never aware of each other. Instead, they send and receive messages to and from the messaging system. It is the responsibility of the messaging system (MOM) to get the messages to their intended destinations. In a messaging system, an application uses an API to communicate through a messaging client that is provided by the MOM vendor. The messaging client sends and receives messages through a messaging system, as illustrated in Figure 5-1. Figure 5-1. Messaging applications use a messaging API to communicate with each other through a messaging system 
The messaging system is responsible for managing the connection points between multiple messaging clients, and for managing multiple channels of communication between the connection points. The messaging system is usually implemented as a software process, which is commonly known as a message server or a message broker. Message servers are usually capable of being grouped together to form clusters that provide advanced capabilities such as load balancing, fault tolerance, and sophisticated routing using managed security domains. The "server" implementation is the most typical, although some messaging vendors offer alternate architectures where the "server" functionality is built into a more heavyweight client. | 