RDF/IMP and IMPX System Management Manual (RDF 1.4+)

Network Transactions
HP NonStop RDF/IMP and IMPX System Management Manual—524388-001
13-8
Takeover and File Recovery
Takeover and File Recovery
When a takeover operation completes in an RDF network environment, the purger logs
two events: one reports a safe MAT position (indicating that all committed data up to
that location was successfully applied to the backup database), and the second (888 or
858) reports whether or not a File Recovery position is available for use on the primary
system. The RDF event 888 reports that a File Recovery is available and it includes
the exact sno and rba to be used for a File Recovery operation on the primary system.
If, however, “kept-commits” have been encountered during phase 2 processing, a File
Recovery position is not available; this is reported in RDF event 858.
If an RDF event 888 is reported, then the specified File Recovery position is based on
both phase 1 and phase 3 processing. Each system logs its own File Recovery
position. While that position may differ from one backup system to the next, the logged
position for any single system is correct. If you supply the returned File Recovery
position to the TMF file recovery process on the primary system, the process recovers
the files on the primary database up to that point. If you use File Recovery to a MAT
position on all primary systems in the RDF network, in each case using the returned
File Recovery positions, then your primary distributed database will be consistent
across the RDF network.
You would use the File Recovery position with File Recovery in situations such as the
following. Assume you have had an outage of your primary system, you have
executed the RDF takeover operation on your backup system, and you have resumed
business transactions on your backup system. Assume further that the former primary
system has been repaired, it is back online, and you want to switch your business
transactions from the active backup database back to the former primary database.
To do so, you merely execute a planned RDF switchover from the backup to the newly
restored primary.
The problem with doing a planned switchover from backup to primary after an RDF
takeover operation is that some transactions might have committed on the primary
system immediately prior to the unplanned outage, and the outage brought down the
extractor before it could send that data to the backup system. In such a case, when
you bring the primary system back up the two databases are no longer synchronized
because the primary database contains committed transactions that are not in the
backup database. Such transactions cannot be recovered.
In the past you would have had to synchronize your entire primary and backup
databases. That could be a lengthy task. Now you can simply use TMF file recovery
to a MAT position. If you execute this operation on your primary system using the MAT
position specified in the RDF event 888 message (see the description of message 888
in Appendix C), it brings the primary database into the exact same state that the
backup database was in upon completion of the RDF takeover. Thus, after file
recovery has completed, you can execute a normal planned switchover from backup to
primary.
Note. Due to the order transactions that commit on individual systems, file recovery might not
always be possible. If an 888 message is generated, however, it can be trusted.