RDF/IMP and IMPX System Management Manual (RDF 1.3+)

Network Transactions
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
13-8
The Effects of Undoing Network Transactions
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 2 undo processing usually results in at least
some other transactions being undone on every system in the network. That is 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)
6. T
12
commit
7. T
13
(network transaction started on \B)
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.