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Disaster recovery

 

Disaster recovery planning is the responsibility of individual installations, and the functions performed may include the provision of regular system ‘snapshot’ dumps that are stored safely off-site. These dumps would be available for regenerating the system, should some disaster overtake it. If this occurs, we need to know what to expect of the messages, and the following description is intended to start you thinking about it.

First a recap on system restart. If a system fails for any reason, it may have a system log that allows the applications running at the time of failure to be regenerated by replaying the system software from a syncpoint forward to the instant of failure. If this occurs without error, the worst that can happen is that message channel syncpoints to the adjacent system may fail on startup, and that the last batches of messages for the various channels will be sent again. Persistent messages will be recovered and sent again, nonpersistent messages may be lost.

If the system has no system log for recovery, or if the system recovery fails, or where the disaster recovery procedure is invoked, the channels and transmission queues may be recovered to an earlier state, and the messages held on local queues at the sending and receiving end of channels may be inconsistent.

Messages may have been lost that were put on local queues. The consequence of this happening depends on the particular WebSphere MQ implementation, and the channel attributes. For example, where strict message sequencing is in force, the receiving channel detects a sequence number gap, and the channel closes down for manual intervention. Recovery then depends upon application design, as in the worst case the sending application may need to restart from an earlier message sequence number.

 

Parent topic:

Problem determination in DQM

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