RDF/IMP, IMPX, and ZLT System Management Manual

Network Transactions
HP NonStop RDF/IMP, IMPX, and ZLT System Management Manual—524388-002
13-9
The Effects of Undoing Network Transactions
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.
The Effects of Undoing Network Transactions
Within an RDF network environment, phase 3 undo processing usually results in other
transactions being undone on every system in the network because the RDF product is
designed to make the safest, and most conservative, assumptions regarding all
possible interrelationships between transactions. This is best illustrated by example.
Consider an RDF network consisting of two RDF subsystem configurations (primary
system \A protected by backup system \X, and primary system \B protected by backup
system \Y). Assume that network transactions originate on both \A and \B, and that
they update data on both \A and \B. Assume further that each system also executes
local, non-network, transactions.
More specifically, assume that system \A (the network master) executes the following:
1. T
10
(network transaction started on \A)
2. T
11
(non-network transaction)
3. T
11
commit
4. T
10
commit
5. T
12
(network transaction started on \A)
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.