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  Structure of Recovery manager and Two Phase commit protocol

Structure of Recovery manager:

Recovery management comprises of two components.

a) Recovery manager that is accountable for tracking transactions and the coordination of transaction COMMIT and ABORT and system CHECKPOINT and RESTART (see below).

b) Log manager that is utilized by recovery manager and other components to record information in the system log for the transaction or system.

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         Relationship among Log manager and component actions.

The function of the recovery system is two-fold: First the recovery system permits an in-progress transaction to be ‘undone’ in the event of a ‘minor’ error without affecting other transactions. Illustrations of such errors are operator cancellation of the transaction, deadlock, timeout, protection or integrity violation, resource limit.

Second in the occurrence of a ‘serious’ error the recovery subsystem reduces the amount of work that is lost as well as by restoring all data to its most recent committed state. It does this by regularly recording copies of key portions of the system state in non-volatile storage as well as by continuously maintaining a log of changes to the state as they occur. In the event of a catastrophe the mainly recent transaction consistent version of the state is reconstructed from the current state on non-volatile storage by utilizing the log to:

• Undo whichever transactions those were incomplete at the time of the crash.
• Redo whichever transactions that completed in the interval between the checkpoints and the crash.

In the case that on-line non-volatile storage doesn’t survive one must start with an archival version of the state with reconstruct the most recent consistent state from it. This process needs:

• Periodically making absolute archive copies of objects within the system.

• Running a alter accumulation utility against the logs written since the dump. This utility produces a lot smaller list of updates that will bring the image dump up to date. As well this list is sorted by physical address thus that adding it to the image dump is a sequential operation.

• The change accretion is merged with the image to reconstruct the most recent consistent state.

Other reasons for keeping a lag of the actions of transactions comprise auditing and performance monitoring since the log is a trace of system activity.

There are three separate recuperation mechanisms:

1. Incremental log of updates to the state.
2. Current on-line version of the state.
3. Archive versions of the state.

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