RDF System Management Manual for H-Series RVUs (RDF 1.8)

An alternative, and preferred, approach is to configure a takeover trigger to send a message to
the requester routing program.
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.
Features
In providing backup protection for online databases, RDF offers many advantages:
• Continuous Availability
RDF maintains an online copy of your production database on one or more backup systems.
If the primary system should go down, the backup database(s) will be consistent and you
can resume your business processing on a backup system with minimal interruption and
data loss.
• Fault tolerance
You can restart RDF after a system failure. Single processor failures do not bring the
subsystem down. If a double processor failure occurs, RDF goes down, but it can be restarted
with no loss of data (issue a START RDF command after the processors have been restored).
• High performance
RDF can typically replicate data from the primary RDF node as fast as the customer
application is capable of generating it.
• Flexibility in protection
You can run RDF with updating on the backup system either enabled or disabled.
RDF is also very flexible with regard to system interrelationships and to disk usage
requirements on backup systems. Besides the most basic configuration of a single primary
system protected by a single backup system, you can have configurations such as these (see
Figure 1-2):
— Multiple primary systems protected by one backup system.
— Reciprocal protection between two systems, where each is the backup to the other
(different databases on the two systems).
— A single primary system whose database changes are replicated to databases on multiple
backup systems. Such an environment makes possible simultaneous read-only access
to all of the backup databases (this is desirable for query-intensive applications such as
telephone directory assistance).
— Triple contingency—a special instance of the database replication feature whereby a
single primary system is protected by two identical backup systems. This feature allows
40 Introducing RDF