The data in your message

 

Give the address of the buffer that contains your data in the Buffer parameter of the MQPUT call. We can include anything in the data in your messages. The amount of data in the messages, however, affects the performance of the application that is processing them.

The maximum size of the data is determined by:

• The MaxMsgLength attribute of the queue manager

• The MaxMsgLength attribute of the queue on which you are putting the message

• The size of any message header added by WebSphere MQ (including the dead-letter header, MQDLH and the distribution list header, MQDH)

The MaxMsgLength attribute of the queue manager holds the size of message that the queue manager can process. This has a default of 100 MB for all WebSphere MQ V6 products.

To determine the value of this attribute, use the MQINQ call on the queue manager object. For large messages, we can change this value.

The MaxMsgLength attribute of a queue determines the maximum size of message that we can put on the queue. If you attempt to put a message with a size larger than the value of this attribute, your MQPUT call fails. If you are putting a message on a remote queue, the maximum size of message that we can successfully put is determined by the MaxMsgLength attribute of the remote queue, of any intermediate transmission queues that the message is put on along the route to its destination, and of the channels used.

For an MQPUT operation, the size of the message must be smaller than or equal to the MaxMsgLength attribute of both the queue and the queue manager. The values of these attributes are independent, but you are recommended to set the MaxMsgLength of the queue to a value less than or equal to that of the queue manager.

WebSphere MQ adds header information to messages in the following circumstances:

• When you put a message on a remote queue, WebSphere MQ adds a transmission header structure (MQXQH) to the message. This structure includes the name of the destination queue and its owning queue manager.

• If WebSphere MQ cannot deliver a message to a remote queue, it attempts to put the message on the dead-letter (undelivered-message) queue. It adds an MQDLH structure to the message. This structure includes the name of the destination queue and the reason that the message was put on the dead-letter queue.

• If you want to send a message to multiple destination queues, WebSphere MQ adds an MQDH header to the message. This describes the data that is present in a message, belonging to a distribution list, on a transmission queue. Consider this when choosing an optimum value for the maximum message length.

• If the message is a segment or a message in a group, WebSphere MQ might add an MQMDE.

These structures are described in the Application Programming Reference.

If your messages are of the maximum size allowed for these queues, the addition of these headers means that the put operations fail because the messages are now too big. To reduce the possibility of the put operations failing:

• Make the size of your messages smaller than the MaxMsgLength attribute of the transmission and dead-letter queues. Allow at least the value of the MQ_MSG_HEADER_LENGTH constant (more for large distribution lists).

• Make sure that the MaxMsgLength attribute of the dead-letter queue is set to the same as the MaxMsgLength of the queue manager that owns the dead-letter queue.

The attributes for the queue manager and the message queuing constants are described in the Application Programming Reference.

For information on how undelivered messages are handled in a distributed queuing environment, see WebSphere MQ Intercommunications.

 

Parent topic:

Putting messages on a local queue using the MQPUT call

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