Chapter 3. Measuring Availability

Measuring availability is important to keeping your system highly available. Only by measuring availability can you understand how your application is performing now and examine how your application’s availability changes over time.

The most widely held mechanism for measuring the availability of a web application is calculating the percent of time it’s accessible for use by customers. We can describe this by using the following formula for a given period:

Site availability percentage equals StartFraction t o t a l normal bar s e c o n d s normal bar i n normal bar p e r i o d minus s e c o n d s normal bar s y s t e m normal bar i s normal bar d o w n Over t o t a l normal bar s e c o n d s normal bar i n normal bar p e r i o d EndFraction

Let’s consider an example. Suppose that over the month of April, your website was down twice; the first time it was down for 37 minutes, and the second time it was down for 15 minutes. What is the availability of your website?

Example 3-1. Availability percentage
Total Number of Seconds Down equals left-parenthesis 37 plus 15 right-parenthesis asterisk 60 equals 3 comma 120 s e c o n d s
Total Number of Seconds in Month equals 30 d a y s asterisk 86 comma 400 s e c o n d s slash d a y equals 2 comma 592 comma 000 s e c o n d s
Site availability percentage equals StartFraction t o t a l normal bar s e c o n d s normal bar i n normal bar p e r i o d minus s e c o n d s normal bar s y s t e m normal bar i s normal bar d o w n Over t o t a l normal bar s e c o n d s normal bar i n normal bar p e r i o d EndFraction
Site availability percentage equals StartFraction 2 comma 592 comma 000 s e c o n d s minus 3 comma 120 s e c o n d s Over 2 comma 592 comma 000 s e c o n d s EndFraction
Site availability percentage equals 99.8795

Your site availability is 99.8795%.

You can see from this example that it only takes a small amount of outage to have ...

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