A [[Twotwo-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 Threethree-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 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 [[Twotwo-phase commit protocol]]).
