Saturday, March 19, 2011

Atomicity

We've already covered some of this in previous postings, but in the next four entries let's explicitly look at all ACID properties. And of course that means we start here with ... A is for Apple, Aardvark and Atomic, i.e., the transaction completes successfully (commits) or if it fails (aborts) all of its effects are undone (rolled back).

In order to ensure that a transaction has an atomic outcome, the two-phase commit protocol that we discussed earlier, is typically used. This protocol is used to guarantee consensus between participating members of the transaction. When each participant receives the coordinator’s phase 1 message, they record sufficient information on stable storage to either commit or abort changes made during the action. After returning the phase 1 response, each participant that returned a commit response must remain blocked until it has received the coordinator’s phase 2 message. Until they receive this message, these resources are unavailable for use by other actions. If the coordinator fails before delivery of this message, these resources remain blocked. However, if failed machines eventually recover, crash recovery mechanisms can be employed to unblock the protocol and terminate the transaction.
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