Multicast message reliability

Use this information to learn how to set the IBM MQ Multicast subscription and message history.

A key element of overcoming transmission failure with multicast is the buffering of transmitted data (a history of messages to be kept at the transmitting end of the link) by IBM MQ. This process means that no buffering of messages is required in the putting application process because IBM MQ provides the reliability. The size of this history is configured via the communication information (COMMINFO) object, as described in the following information. A bigger transmission buffer means that there is more transmission history to be retransmitted if needed, but due to the nature of multicast, 100% assured delivery cannot be supported.

MSGHIST

This value is the amount of message history in kilobytes that is kept by the system to handle retransmissions in the case of NACKs (negative acknowledgments).

A value of 0 gives the least level of reliability. The default value is 100 KB.

The IBM MQ Multicast new subscription history is controlled in the communication information (COMMINFO) object by the NSUBHIST attribute:

NSUBHIST

The new subscriber history controls whether a subscriber joining a publication stream receives as much data as is currently available, or receives only publications made from the time of the subscription.

NONE

A value of NONE causes the transmitter to transmit only publication made from the time of the subscription. NONE is the default value.

ALL

A value of ALL causes the transmitter to retransmit as much history of the topic as is known. In some circumstances, this situation can give a similar behavior to retained publications. Note: Using the value of ALL might have a detrimental effect on performance if there is a large topic history because all the topic history is retransmitted.

Parent topic: Administer multicast

Related information

• DEFINE COMMINFO
• ALTER COMMINFO