Editing Two-phase commit protocol (section)

===Commit (or completion) phase===

====Success====
If the coordinator received an agreement message from all participants during the commit-request phase:
#The coordinator sends a commit message to all the participants.
#Each participant completes the operation, and releases all the locks and resources held during the transaction.
#Each participant sends an acknowledgement to the coordinator.
#The coordinator completes the transaction when all acknowledgements have been received.

====Failure====

If any participant votes No during the commit-request phase (or the coordinator's timeout expires):
#The coordinator sends a rollback message to all the participants.
#Each participant undoes the transaction using the undo log, and releases the resources and locks held during the transaction.
#Each participant sends an acknowledgement to the coordinator.
#The coordinator undoes the transaction when all acknowledgements have been received.

====Message flow====
<pre>
Coordinator                                          Participant
                             QUERY TO COMMIT
                 -------------------------------->
                             VOTE YES/NO             prepare*/abort*
                 <-------------------------------
commit*/abort*               COMMIT/ROLLBACK
                 -------------------------------->
                             ACKNOWLEDGEMENT          commit*/abort*
                 <--------------------------------  
end
</pre>
An * next to the record type means that the record is forced to stable storage.<ref name="mohan1986">[[C. Mohan]], Bruce Lindsay and R. Obermarck (1986): [http://dl.acm.org/citation.cfm?id=7266  "Transaction management in the R* distributed database management system"],''ACM Transactions on Database Systems (TODS)'', Volume 11 Issue 4, Dec. 1986, Pages 378 - 396</ref>