A figure shows the basic mechanism of program-to-program communication using a synchronous communication model.

In Figure 1, program A prepares a message and puts it on Queue 1. Program B gets the message from Queue 1 and processes it. Both Program A and Program B use an application programming interface (API) to put messages on a queue and get messages from a queue. The WebSphere® MQ API is called the Message Queue Interface (MQI).

When Program A puts a message on Queue 1, Program B might not be running. The queue stores the message safely until Program B starts and is ready to get the message. Likewise, at the time Program B gets the message from Queue 1, Program A might no longer be running. Using this model, there is no requirement for two programs communicating with each other to be executing at the same time.

There is clearly a design issue, however, about how long Program A should wait before continuing with other processing. This design might be desirable in some situations, but when the wait is too long, it is not so desirable any more. Asynchronous communication is designed to handle those situations.

Figure 1. Synchronous application design model