6 ● RECOVERY BLOCKSRECOVERY BLOCKS ARE A WAY OF STRUCTURING BACKWAR...

17.6

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Recovery blocks

Recovery blocks are a way of structuring backward error recovery to cope with unantic-

ipated faults. In backward error recovery, periodic dumps of the state of the system are

made at recovery points. When a fault is detected, the system is restored to its state at the

most recent recovery point. (The assumption is that this is a correct state of the system.)

The system now continues on from the recovery point, using some alternative course

of action so as to avoid the original problem.

An analogy: if you trip on a banana skin and spill your coffee, you can make a fresh

cup (restore the state of the system) and carry on (carefully avoiding the banana skin).

Checking

module

User

Normal

Alternative

Figure 17.3

Components in a recovery block scheme

As shown in Figure 17.3, backward error recovery needs: