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  Two Phase commit protocol and Recovery Management

Two Phase commit protocol:

Generals paradox:

In order to comprehend that the two-phase commit protocol solves some problem it is useful to analyze this generals paradox. There are two generals on campaign. They have an objective (a hill) that they desire to capture. If they concurrently march on the objective they are assured of success. If merely one marches he will be annihilated.

The generals are encamped merely a short distance apart however due to technical difficulties they can communicate merely via runners. These messengers have a flaw each time they venture out of camp they stand some chance of getting lost (they aren’t very smart.)

The problem is to discover some protocol that allows the generals to march together even though some messengers get lost. There is a plain proof that- no fixed length protocol exists: Let P is the express such protocol. Suppose the last messenger in P gets lost. Afterwards either this messenger is useless or one of the generals doesn't get a needed message. By the minimality of P the last message isn’t useless so one of the general doesn't march if the last message is lost. This contradiction establishes that no such protocol P exists.

The generals paradox (which as you now see isn’t a paradox) has strong analogies to problems faced by a data recovery management when doing commit processing. Imagine that one of the generals is a computer in Tokyo as well as that the other general is a cash-dispensing terminal in Fuessen Germany. The goal is to-

•    Open a cash drawer with a million marks in it (at Fuessen) in addition to
•    Debit the appropriate account in the non-volatile storage of the Tokyo computer.

If merely one thing happens either the Germans or the Japanese will destroy the general that didn’t ‘march’.

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