Step 2: Gauging HA Primary Variables
It is now time to properly place your arrows (as relative value indications) on the primary variable gauges (refer to Figure 1.8). You should place the assessment arrow on each of the 10 variables as accurately as possible and as rigorously as possible. Each variable continuum should be evenly divided into a scale of some kind, and an exact value should be determined or calculated to help place the arrow. For example, the cost of the downtime (per hour) variable could be a scale from $0/hr at the bottom (left) to $500,000/hr at the top (right) for Company X. The $500,000/hr top scale value would represent what might have been the peak order amounts ever taken in by the online order entry system for Company X and thus would represent the known dollar amount being lost for this period. Remember that everything is relative to the other systems in your company and to the perceived value of each of these variables. In other words, some companies won’t place much value on the end-user tolerance of downtime variable if the application is for internal employees. So, adjust accordingly.
For each of the primary variable gauges, you need to follow these steps:
Assign relative values to each primary variable (based on your company’s characteristics).
Place an arrow on the perceived (or estimated) point in each gauge that best reflects the system being assessed.
As another example, let’s look at the first HA primary variable, total uptime requirement percentage. If you are assessing an ATM system, the placement of the assessment arrow would be at a percentage of 98% or higher. Remember that five 9s means a 99.999% uptime percentage. Also remember that this uptime requirement is for the “planned” time of operation, not the total time in a day (or a year)—except, of course, if the system is a 24×7×365 system. Very often the service level agreement that is defined for this application will spell out the uptime percentage requirement. Figure 3.1 shows the placement of the assessment arrow at 99.999% for the ATM application example—at the top edge of extreme availability (at least from an uptime percentage point of view).
Figure 3.1 HA primary variables gauge—ATM uptime percentage example.