Availability Guide for Application Design

What Is Application Availability?
Availability Guide for Application Design—525637-004
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Cost Reduction
Cost Reduction
In addition to preventing additional costs incurred if the application goes offline,
applications with a higher level of availability also decrease the day-to-day operational
costs of the application. Fewer calls for help could mean that fewer operators are
needed to support the application. Moreover, applications designed for ease of
maintenance and speed of repair use automated procedures where possible, and are
geared towards efficient use of operations staff. Use of such clearly defined
maintenance procedures further reduces the need for operations staff.
Life-Threatening Situations
Many computer systems are installed not for financial reasons but because the power
and efficiency of a computer system can save lives. An example of such an application
is an emergency response and dispatch system; when someone calls the emergency
response number, the application determines the location of the caller using
information linked to the telephone number and alerts the local fire or police
department to send help as soon as possible. Clearly, interruption of such a service at
any time could cost lives.
Another example of an application designed to save lives is an air traffic control
system. Application failure in this case could cost hundreds of lives.
Many other types of applications, while not specifically designed to save lives, can also
lead to fatalities in the event of an outage. Examples include street traffic flow control,
chemical plant process control, and the telephone system, without which the
emergency response system could not function.
Measuring Availability
HP measures availability in terms of the number of minutes of downtime each year.
This approach contrasts with the traditional percentage of uptime. The following
paragraphs explain HP’s reasons for this new approach.
As stated earlier in this section, availability is measured in terms of reliability and
maintainability. Traditional practice uses a formula similar to the following:
In the formula, uptime is the mean average time for which the application is available.
Repair time is the mean average of the time it takes to get the application back online
following an outage.
Availability increases as uptime increases or as repair time decreases. Availability
equal to 1 is a perfect score meaning that either the application never breaks or, when
Availability
=
Uptime
Uptime + Repair Time
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