IBM Websphere - MQ Introduction

 IBM Websphere - MQ Introduction

WebSphere MQ, previously known as MQ Series, is a tool to transfer message and data from one system to another, a means of program-to-program communication. Basically, one program puts a message in a queue and the other program reads from the queue. This can be synchronous or asynchronous. It's time independant, communicating applications do not have to be active at the same time.

MQSeries runs on a variety of platforms. The MQSeries products enable programs to communicate with each other across a network of unlike components, such as processors, subsystems, operating systems and communication protocols. MQSeries programs use a consistent application program interface (API) across all platforms.

The figure shows the main parts of an MQSeries application at run time. Programs use MQSeries API calls, that is the Message Queue Interface (MQI), to communicate with a queue manager (MQM), the run-time program of MQSeries. For the queue manager to do its work, it refers to objects, such as queues and channels. The queue manager itself is an object as well.

What is Messaging and Queuing?

Message queuing is a method of program-to-program communication. Programs within an application communicate by writing and retrieving application-specific data (messages) to/from queues, without having a private, dedicated, logical connection to link them. Messaging means that programs communicate with each other by sending data in messages and not by calling each other directly. Queuing means that programs communicate through queues. Programs communicating through queues need not be executed concurrently.

This figure shows how two programs, A and B, communicate with each other. We see two queues; one is the "output" queue for A and at the same time the "input" queue for B, while the second queue is used for replies flowing from B to A. The squares between the queues and the programs represent the Message Queuing Interface (API) the program uses to communicate with MQSeries' run-time program, the queue manager. As said before, the API is a simple multi platform API consisting of 13 calls.

About the queue manager

The heart of MQSeries is the message queue manager (MQM), MQSeries' run-time program. Its job is to manage queues and messages for applications. It provides the Message Queuing Interface (MQI) for communication with applications. Application programs invoke functions of the queue manager by issuing API calls. For example, the MQPUT API call puts a message on a MQSeriesqueue to be read by another program using the MQGET API call. This scenario is shown in the next figure.

A program may send messages to another program that runs in the same machine as the queue manager (shown above), or to a program that runs in a remote system, such as a server or a host. The remote system has its own queue manager with its own queues. This scenario is shown in the next figure.

The queue manager transfers messages to other queue managers via channels using existing network facilities, such as TCP/IP, SNA or SPX. Multiple queue managers can reside in the same machine. They also need channels to communicate. Application programmers do not need to know where the program to which they are sending messages runs. They put their messages on a queue and let the queue manager worry about the destination machine and how to get the messages there. MQSeries knows what to do when the remote system is not available or the target program is not running or busy. For the queue manager to do its work, it refers to objects that are defined by an administrator, usually when the queue manager is created or when a new application is added. MQSeries for Windows provides graphical user interfaces; other platforms use the command line interface or panels.