Safety of messages

In addition to the usual recovery features of WebSphere MQ, distributed queue management ensures that messages are delivered properly by using a syncpoint procedure coordinated between the two ends of the message channel. If this procedure detects an error, it closes the channel to allow you to investigate the problem, and keeps the messages safely in the transmission queue until the channel is restarted.

The syncpoint procedure has an added benefit in that it attempts to recover an in-doubt situation when the channel starts up. (In-doubt is the status of a unit of recovery for which a syncpoint has been requested but the outcome of the request is not yet known.) Also associated with this facility are the two functions:

  1. Resolve with commit or backout
  2. Reset the sequence number

The use of these functions occurs only in exceptional circumstances because the channel recovers automatically in most cases.

Fast, nonpersistent messages

In WebSphere MQ for AIX, iSeries, HP-UX, Linux, Solaris, Windows systems, and z/OS without CICS and MQSeries V5.1 for Compaq Tru64 UNIX, and OS/2 Warp, the nonpersistent message speed (NPMSPEED) channel attribute can be used to specify that any nonpersistent messages on the channel are to be delivered more quickly. For more information about this attribute, see Nonpersistent message speed (NPMSPEED).

If a channel terminates while fast, nonpersistent messages are in transit, the messages may be lost and it is up to the application to arrange for their recovery if required.

If the receiving channel cannot put the message to its destination queue then it is placed on the dead letter queue, if one has been defined. If not, the message is discarded.

In MQSeries for Compaq OpenVMS Alpha fast messages are enabled differently. For channels of type sender, server, receiver or requester, set the description field at both ends of the channel as follows: 

  DESCR('>>> description') +

Specifying >>> as the first characters in the channel description defines the channel as fast for nonpersistent messages.

Note:
If the other end of the channel does not support the option, the channel runs at normal speed.

Undelivered messages

For information about what happens when a message cannot be delivered, see What happens when a message cannot be delivered?.


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