Availability Guide for Application Design

Availability Guide for Application Design—525637-004
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1 What Is Application
Availability?
Application availability means that the application is always available to the end user.
While the system manager for a large computer system—such as an enterprise
server—might consider availability only in terms of keeping the server up and running,
the end user might not care that the server has not been down in many months if the
network that connects the end user to the server is constantly failing. From the end
user’s perspective, if the application cannot be used to get the job done, then the
system is down.
Two factors determine the availability of an application:
•
Reliability of the hardware, system software, and application software across the
server computer system, the network, and the client computer system
•
Maintainability of the hardware, system software, and application software across
the server computer system, the network, and the client computer system
Reliability is the measure of how long the application is available before it becomes
unavailable. Hewlett-Packard (HP) also considers an application to be unavailable if it
is running so slowly that it cannot do the job to the user’s satisfaction, as defined by a
service-level agreement.
Maintainability is also critical because even the most reliable computer systems go
offline from time to time when a data file is full, an extended power outage has
occurred, and so on. In addition, most computer systems require that the application
be brought down when major upgrade tasks are performed. Maintainability is therefore
the measure of how quickly the downed system can be brought back online and the
application once again made available to the end user community.
Applications that are required to execute with high levels of availability must be
designed to be both reliable and maintainable. This guide provides suggestions on how
to achieve these goals.
Who Ensures Application Availability?
Making sure that applications are always available is the joint responsibility of:
•
The designers and developers of the application
•
Those who administer and maintain the application and the systems on which it
runs
This guide is concerned with designing applications for availability. For information on
what system managers, operators, and system administrative support personnel need
to be aware of, refer to the manuals listed in Where Else Can You Find Related
Information? on page xv.