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Clarified wording in "Atomic Commit Convention". |
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During the <u>voting</u> phase each node writes the changes in the atomic commit to its own disk. The nodes then report their status to the coordinator. If any node does not report to the coordinator or their status message is lost the coordinator assumes the node’s write failed. Once all of the nodes have reported to the coordinator the second phase begins.
During the <u>commit</u> phase the coordinator sends a commit message to each of the nodes to record in their individual logs. Until this message is added to a node's log, any changes made will be recorded as incomplete. If any of the nodes reported a failure the coordinator will instead send a rollback message. This will remove any changes the nodes have written to disk.<ref>{{cite book |last=Elmasri |first=Ramez |title=Fundamentals of Database Systems 5th Edition |page=688 |publisher=Addison Wesley |year=2006}}</ref><ref>{{cite book |first1=Philip A. |last1=Bernstein |first2=Vassos |last2=Hadzilacos |first3=Nathan |last3=Goodman |title=Concurrency Control and Recovery in Database Systems |chapter=Chapter 7 |publisher=Addison Wesley Publishing Company |year=1987 |url=http://research.microsoft.com/en-us/people/philbe/ccontrol.aspx}}</ref>
The three-phase commit protocol seeks to remove the main problem with the two phase commit protocol which occurs if a coordinator and another node fail at the same time during the commit phase neither can tell what action should occur. To solve this problem a third phase is added to the protocol. The <u>prepare to commit</u> phase occurs after the <u>voting</u> phase and before the <u>commit</u> phase.
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