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APDEX

Many Uses of Apdex

Apdex is a simple formula that converts many performance values into an easy to understand 0-to-1 performance index. Defining the target application response time T and accurately interpreting the result (the score) requires some methodology–and that methodology should reflect how you plan to use Apdex. There are three ways to use Apdex: to do tactical diagnostics, to support process, and to link performance to your business. These three uses govern how you make Apdex parameter choices.

Tactical Diagnosis
This is the simplest use of Apdex.  The Apdex parameters are completely under your control without requiring agreement from any other group.  It allows you to experiment with T values to see the sensitivity of your applications to the Apdex formula.  Once you have a T value set you can use the Apdex scores to sort many measurements to determine which applications need attention.  This is an alternative to alarm triggers that are often too sensitive and provide false alarms.

When used for diagnostics the Apdex T usually floats as needed to get the job done within a specific investigation.  The Apdex scores provide first level performance problem diagnostics.  Of course the tool that supplies the real-time data must have drill-down capabilities to continue on the diagnostic path. 

Process Support
Sophisticated performance management requires process.  The four fundamental ITIL-based processes that apply to application performance are: incident management, availability management, capacity management, and finally service assurance.  When applied to these processes Apdex takes on management properties that require you to involve other groups in your organization so Apdex scores will have meaning across groups.

When supporting incident management, the Apdex T and the “trigger” score should be meaningful (show actual incidents), yet not too aggressive (which creates false positives).  This said, we find that organizations tend to run Apdex parameters “hot” so Apdex scores vary a lot over a day.

By the time you progress to performance assurance, Apdex is incorporated in executive reporting and/or possibly is the foundation for SLAs.  For this use the Apdex T and “acceptable” score should be more relaxed.  Apdex scores should not swing wildly over the course of a day if in fact performance was consistent in the view of the users.

Think of it as similar to tracking human health.  Doctors in a hospital emergency room want monitoring instruments to be sensitive to the slightest patient changes.  In contrast, doctors at the U.S. government’s Centers for Disease Control (CDC) need to assess the health of the entire US population, so their parameters must be set to discover trends.  If parameters are too “hot” the results become very “noisy” and it is very hard to see long term trends.

Business/Performance Linkage
The most advanced use of Apdex is to report performance as it relates to the business.  Here Apdex is an element of dialog among IT staff, business managers, user representatives, and executive staff.  Apdex parameters are now carefully analyzed, presented with supporting evidence that shows business linkage, agreed upon by appropriate members of the organization, and documented.  The Apdex methodology brings structure, context, and an open standard to the dialog among key participants.  Apdex helps achieve consensus across groups about how to link application performance to the needs of the business.

Once company executives rely on Apdex reports then longer term performance management issues arise. For example how should Apdex parameters evolve over time?  This is like how Dow Jones decides to replace one of the 30 companies in the Dow Jones Industrial Average or showing the price of a stock before and after a share split.  The basis has changed.  How should you make the change?  Such a change shifts Apdex scores on the day the change is implemented.  How do you communicate the change in charts going forward?  Eric Goldsmith, Operations Architect at AOL has dealt with these issues, and described them in a fascinating presentation on the topic in the 2007 Apdex Symposium.

In a nutshell, you can use Apdex for simple tactical purposes within the network group as well as complex strategic executive level purposes.  The good news is that you can walk before you run, starting with simple uses and working your way up to sophisticated uses, leveraging your Apdex experience along the way.

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