Appendix A. WebSphere MQ overview 723
Extending queue manager facilities
Messages and queues
Messages and queues are the basic components of a message queuing system.
A message is a string of bytes that is meaningful to the applications that use
Messages are used to transfer information from one application program to
another, or between different parts of the same application. The applications
can be running on the same platform, or on different platforms.
WebSphere MQ messages have two parts:
– Application data
The content and structure of the application data is defined by the
application programs that use it.
– Message descriptor
The message descriptor identifies the message and contains additional
control information, such as the type of message and the priority assigned
to the message by the sending application. The format of the message
descriptor is defined by WebSphere MQ.
The default maximum message length is 4 MB, but may be increased to 100
MB. In practice, the message length might be limited by:
– The maximum message length defined for the receiving queue
– The maximum message length defined for the queue manager
– The maximum message length defined by the queue
– The maximum message length defined by either the sending or receiving
– The amount of storage available for the message
It might take several messages to send all the information that an application
Application programs send and receive messages using MQI calls. For
example, to put a message onto a queue, an application:
a. Opens the required queue by issuing an MQI MQOPEN call.
b. Issues an MQI MQPUT call to put the message onto the queue.
Another application can retrieve the message from the same queue by
issuing an MQI MQGET call.