Managing HP Serviceguard for Linux Ninth Edition, April 2009

Another alternative is for the failure of one component to still allow bringing down
the other components cleanly. If a database SQL server fails, the database should still
be able to be brought down cleanly so that no database recovery is necessary.
The worse case is for a failure of one component to cause the entire system to fail. If
one component fails and all other components need to be restarted, the downtime will
be high.
Be Able to Monitor Applications
All components in a system, including applications, should be able to be monitored
for their health. A monitor might be as simple as a display command or as complicated
as a SQL query. There must be a way to ensure that the application is behaving correctly.
If the application fails and it is not detected automatically, it might take hours for a
user to determine the cause of the downtime and recover from it.
Minimizing Planned Downtime
Planned downtime (as opposed to unplanned downtime) is scheduled; examples
include backups, systems upgrades to new operating system revisions, or hardware
replacements. For planned downtime, application designers should consider:
• Reducing the time needed for application upgrades/patches.
Can an administrator install a new version of the application without scheduling
downtime? Can different revisions of an application operate within a system? Can
different revisions of a client and server operate within a system?
• Providing for online application reconfiguration.
Can the configuration information used by the application be changed without
bringing down the application?
• Documenting maintenance operations.
Does an operator know how to handle maintenance operations?
When discussing highly available systems, unplanned failures are often the main point
of discussion. However, if it takes 2 weeks to upgrade a system to a new revision of
software, there are bound to be a large number of complaints.
The following sections discuss ways of handling the different types of planned
downtime.
Reducing Time Needed for Application Upgrades and Patches
Once a year or so, a new revision of an application is released. How long does it take
for the end-user to upgrade to this new revision? This answer is the amount of planned
downtime a user must take to upgrade their application. The following guidelines
reduce this time.
Minimizing Planned Downtime 301