18.3. Detailed Discussion: Availability Duration and Performance

Availability as a monitoring mechanism has two important facets: the immediate effect of when it changes and then the historic perspective on how changes in availability reflect resource performance.

An historic perspective introduces the idea of availability duration. How long was a resource in a particular state? How often does it change?

The idea of availability duration is important to get an accurate picture of how a resource is performing. There are several ways that JBoss ON breaks out that information:

The last element is particularly important in assessing the resource's performance in light of its availability. The mean time between failures is the time between when a resource comes up and when it next goes down — it is the mean^[4] of all of its up periods. This gives an idea of how stable a system is. The mean time to recovery gives an idea of how long the resource stays down, which indicates its resilience or fault tolerance. A low MTBF and high MTTR indicate some potential maintenance problems or application instability on a resource.

From a monitoring perspective, the historic perspective is critical, particularly when planning equipment replacements and upgrades.

From an alerting perspective — from an immediate response perspective — only availability changes matter.

The first and most obvious alert condition issues an alert based solely on a state change.

However, resources can cycle or can have a few seconds or minutes where they are inaccessible but that doesn't affect the overall performance of the resource or of whatever function it performs. A resource hits a certain state and has to stay there for a certain amount of time before the state becomes important.