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High Availability

Fundamental Approaches
There are three fundamental approaches to increasing availability beyond that of basic components: Redundancy, Isolation and Parallelism.

Redundancy uses multiple lower-availability components to provide a higher availability system.

Employing replication, high availability systems commonly utilize dual power supplies and mirrored disks (where two or more disks contain exactly the same information).

Error-correcting components provide a measure of redundancy—and higher availability—without more costly duplication.

Redundant components do no good if they are both subject to a common source of failure.

Two replicated computer systems in the same room, for example, do not increase availability in the event of a fire.

Locating systems in separate rooms or mirroring disks on separate controllers amounts to physical isolation. Using messaging for inter-process communication is a form of logical isolation (failure in one doesn't necessarily cause the other to hang as synchronous procedure calls often do).

Many tasks have subtasks with differing availability requirements. Often the subtasks can be queued for later processing or moved to another system for processing.

Machines (ATMs), for instance, use deferred processing to improve availability. Initial account verification happens while the customer is at the machine, but the actual processing of the withdrawal is performed later.

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