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Portal Server and High Availability
Chapter 5 Creating Your Portal Design 85
System Availability
System availability is often expressed as a percentage of the system uptime. A basic
equation to calculate system availability is:
Availability = uptime / (uptime + downtime) * 100
For instance, a service level agreement uptime of four digits (99.99 percent) means
that in a month the system can be unavailable for about seven hours. Furthermore,
system downtime is the total time the system is not available for use. This total
includes not only unplanned downtime, such as hardware failures and network
outages, but also planned downtime, preventive maintenance, software upgrade,
and patches.
If the system is supposed to be available seven days a week, 24 hours a day, the
architecture needs to include redundancy to avoid planned and unplanned
downtime to ensure high availability.
Degrees of High Availability
High availability is not just a switch that you can turn on and off. Various degrees
of high availability refer to the ability of the system to recover from failures and
ways of measuring system availability. The degree of high availability depends on
your specific organization’s fault tolerance requirements and ways of measuring
system availability.
For example, your organization might tolerate the need to reauthenticate after a
system failure, so that a request resulting in a redirection to another login screen
would be considered successful. For other organizations, this might be considered
a failure, even though the service is still being provided by the system.
Session failover alone is not the ultimate answer to transparent failover, because
the context of a particular portal application can be lost after a failover. For
example, consider the case where a user is composing a message in NetMail Lite,
has attached several documents to the email, then the server fails. The user is
redirected to another server and NetMail Lite will have lost the user’s session and
the draft message. Other providers, which store contextual data in the current
JVM™, have the same problem.
Achieving High Availability for Portal Server
Making Portal Server highly available involves ensuring high availability on each
of the following components: