Jorge Manuel de Oliveira Vicente Tese de Mestrado em Engenharia ...

• Allow quality to be quantified
• Reflect users’ pain points and priorities
• Include availability, performance, and accuracy metrics
• Take into account security features and systems
• Be realistic and affordable
The best way to measure service levels is from the user’s perspective. How available were
the services that users need to do their jobs and how responsive were the services? Whichever
way these user perceptions are measured, the SLA will need to document each SLI used to
measure the SLOs, and to specify the data source for each.
For example, a Web-based application may define the user response time as the key SLI. It
may be the case that the response time is to be measured from the time the “submit” button is
pressed to the time a refreshed screen showing confirmation is received. In this case, the user
perception is translated into a performance metric that must be measured by an underlying
technology, such as a synthetic transaction, where a transaction is created to mimic user
behavior for the purpose of measuring results at a given point in time [15].
Availability and Performance Metrics
Performance and availability metrics are essential for SLAs since they provide necessary
metrics for measurement.
Availability is usually a simple thing to measure—was the service available when it was
supposed to be or not? If it was available, but did not perform properly, then that is a
performance problem, not an availability problem. In addition, look at the availability from the
perspective of the user, not the service provider. In the SLA, do not include separate metrics for
the network, servers, and applications. Rather, represent availability by such things as total
hours of availability, hours or minutes of outage, or mean time to repair.
Performance is significantly more difficult to measure than availability. The trick is deciding
which aspect of performance is appropriate for the SLA that you are writing and maintaining the
end-user perspective—that is, what the SLA is about. And, as with availability, keep it simple—
only go as deep as is necessary to accurately reflect the user experience.
When discussing performance for an SLA, the critical question is whether the service
functioned as well as it was supposed to. Usually, performance has a speed dimension to it.
Speed may be represented by response time, data transfer rate, or other measures. Another
common performance characteristic is volume. Volume may be reflected in the amount of data
transferred, the number of transactions processed, and so forth.
Unfortunately, as the size and complexity of the infrastructure for providing a service
increases, so does the challenge of assessing the availability and performance of service.
11