Three-phase commit protocol: Difference between revisions

Revision as of 10:54, 11 February 2016

Revision as of 04:47, 2 March 2016

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===Coordinator===

#The coordinator receives a transaction request. If there is a failure at this point, the coordinator aborts the transaction (i.e. upon recovery, it will consider the transaction aborted). Otherwise, the coordinator sends a '''canCommit?''' message to the cohorts and moves to the waiting state.

#If there is a failure, timeout, or if the coordinator receives a '''No''' message in the waiting state, the coordinator aborts the transaction and sends an '''abort''' message to all cohorts. Otherwise the coordinator will receive '''Yes''' messages from all cohorts within the time window, so it sends '''preCommit''' messages to all cohorts and moves to the prepared state.

#If the coordinator succeeds in the prepared state, it will move to the commit state. However if the coordinator times out while waiting for an acknowledgement from a cohort, it will abort the transaction. In the case where all acknowledgements are received, the coordinator moves to the commit state as well.

===Cohort===

#The cohort receives a '''canCommit?''' message from the coordinator. If the cohort agrees it sends a '''Yes''' message to the coordinator and moves to the prepared state. Otherwise it sends a '''No''' message and aborts. If there is a failure, it moves to the abort state.

#In the prepared state, if the cohort receives an '''abort''' message from the coordinator, fails, or times out waiting for a commit, it aborts. If the cohort receives a ''' preCommit''' message, it sends an '''[[acknowledgement (data networks)|ACK]]''' message back and awaits a final '''commit''' or '''abort'''.

#If, after a cohort member receives a ''' preCommit''' message, the coordinator fails or times out, the cohort member goes forward with the commit.

==Motivation==

A [[Two-phase commit protocol]] cannot dependably recover from a failure of both the '''coordinator''' and a cohort member during the '''Commit phase'''. If only the '''coordinator''' had failed, and no cohort members had received a '''commit''' message, it could safely be inferred that

no '''commit''' had happened. If, however, both the '''coordinator''' and a cohort member

failed, it is possible that the failed cohort member was the first to be notified, and had

actually done the '''commit'''. Even if a new '''coordinator''' is selected, it cannot

confidently proceed with the operation until it has received an agreement from

'''all''' cohort members ... and hence must block until all cohort members respond.

The Three-phase commit protocol eliminates this problem by introducing the '''Prepared to commit'''

state. If the '''coordinator''' fails before sending '''preCommit''' messages, the '''cohort''' will

unanimously agree that the operation was '''aborted'''. The '''coordinator''' will not send out a '''doCommit'''dpCommit

message until '''all''' cohort members have '''ACK'''ed that they are '''Prepared to commit'''.

This eliminates the possibility that '''any''' cohort member actually completed the

transaction before '''all''' cohort members were aware of the decision to do so

(an ambiguity that necessitated indefinite blocking in the [[Two-phase commit protocol]]).