Product managers therefore typically attach an availability constraint (nonfunctional requirement) to each functional requirement of the product. If one of the functions of the product is to generate reports, for example, a product manager should specify how likely it should be at any particular time that a user will be able to use this functionality.
The question with nonfunctional requirements is always the metric - how you measure them. How do you measure availability? Here are some options:
- mean-time-between-failures (MTBF) - the average amount of time elapsed between failures to deliver the functionality.
- failure rate - the frequency that the product fails to deliver the functionality. Failure rate is the reciprocal of MTBF, and often is expressed in terms of failures per hour.
- uptime - percentage of the time that the functionality is available.
- downtime - percentage of the time that the functionality is not available.
- mean-time-between-system-abort (MTBSA) - the average amount of time elapsed between complete "reboots" of the system.
- mean-time-between-critical-failure (MTBCF) - distinguishes between critical and noncritical failures.
- maintenance free operating period (MFOP) - the average amount of time that the functionality is available without any special intervention or system maintenance.
UPDATE: Scott Sehlhorst adds a number of important observations in this entry's comments. One thing he notes is that I neglected to mention MTTR:
- mean-time-to-repair (MTTR) - the average amount of time it takes to repair the system after its functionality becomes unavailable. For hardware products, it usually refers to the time to replace a module or part. For software products, it can refer to the amount of time it takes to reboot or restart the system.