RDF/IMP, IMPX, and ZLT System Management Manual

Introducing RDF
HP NonStop RDF/IMP, IMPX, and ZLT System Management Manual—524388-002
1-6
Planned Outages
If your database is NonStop SQL/MP and if your applications form the typical
requestor-server environment where the requestors send requests into your primary
system from other locations, then you could start your servers on your primary and
backup systems. You must take care that you only route requestor work to your
primary system. This leaves your servers running essentially in standby mode on your
backup system.
When you lose your primary system because of some unplanned outage, you execute
the RDF Takeover operation. This brings your backup database into a consistent
state, typically within a small number of seconds.
You need to write a program that controls routing requestor work. This program can
monitor one of two mechanisms that report a successful RDF takeover:
•
RDF Event 724 reports the successful completion of an RDF takeover for the RDF
Control Subvolume named in the event.
•
RDFTKOVR file in the RDF Control Subvolume on your backup system. This file is
normally empty (eof = 0). Upon the successful completion of an RDF Takeover
operation, however, the key word "DONE" is written to the file.
This program should be set to monitor either one of these events. When this program
detects a successful RDF Takeover, it can then route requestor work to the servers on
the backup system, and those servers are then ready to resume business on the
backup system.
Of course, what makes this method work is ensuring that no work is ever routed to the
backup system until the takeover has completed.
Planned Outages
RDF can be very useful when a planned shutdown of the primary system is necessary.
For example, you might need to bring the system down to install new hardware or to
perform a system software upgrade. In such a situation, you might determine it is
unacceptable to stop your business applications for the time required.
With RDF, you need only stop the applications momentarily, do a switchover from the
primary system to the backup system, and then restart the applications on the backup
system. When the primary system is ready for use again, you can use RDF to bring the
primary database up-to-date with changes made to the backup database while the
primary system was shut down. After the primary database is consistent with the
backup database, you can perform another switchover, this time from the backup
system to the primary system, and then restart the applications on the primary system.
For instructions on how to perform a switchover, see Carrying Out a Planned
Switchover.