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Availability Levels and Measurements
Based on the answers to the questions discussed in the previous section, you can specify which category of availability your users require:
High availability. The system or application is available during specified operating hours with no unplanned outages.
Continuous operations. The system or application is available 24 hours a day, 7 days a week, with no scheduled outages.
Continuous availability. The system or application is available 24 hours a day, 7 days a week, with no planned or unplanned outages.
High Availability Level
High availability is the level of availability normally expected by users. At this level, once you commit to a schedule of system availability, there should be no unscheduled or unplanned outages or downtimes. For example, the system is committed to be available from 8:00 a.m. to 5:00 p.m., Monday through Friday. There should be no unplanned outages during this time. Any outage would definitely affect users, since they could be in the middle of important work.
Is an outage pre-announced or not? Remember whose perspective matters—the user's. If you announce an outage an hour in advance, you might consider it planned, but your users may consider it unplanned, since they don't have enough time to adjust their work to cope with the outage.
When an outage will occur and when users are informed about it are both important. For example, telling users at 8:00 a.m. that a downtime will occur in eight hours is more acceptable than telling them at 5 p.m. that an outage will happen at 8:00 a.m. the following day, since the latter example gives users no time to prepare unless they work overtime.
High availability still gives you room to schedule system downtimes, as long as you schedule them outside the committed availability period. For example, you can deliver high availability while retaining the ability to schedule nightly backups. But you must ensure that the system operates reliably during committed periods of availability. The challenge here is to eliminate problems, or at least make them transparent to users or less likely to affect system availability.
Continuous Operations Level
Continuous operations means that a system is committed to constant availability, with no unscheduled downtime. To achieve this level, you must implement high availability and continuous operations techniques that make the system more reliable and eliminate dependence on scheduled maintenance work that would require system downtime.
Continuous Availability Level
The continuous availability level includes the level of performance of the continuous operations level, but the system is committed to being available always, with no scheduled or unscheduled downtime. This level of availability is normally demanded in critical systems that provide essential services to the general public, such as electricity, communication systems, and banking services such as automated teller machines (ATMs). Internet service providers and e-commerce systems also need continuous availability. Obviously, this level of availability is the most difficult and costly to achieve. Users must be aware of this expense, and must be willing to pay for it. One hundred percent continuous availability is almost impossible to achieve consistently.