Editing Two-phase commit protocol (section)

====Presumed abort and presumed commit====
Presumed abort or Presumed commit are common such optimizations.<ref name="weikum2001" /><ref name=Bern2009/><ref name="mohan1983">[[C. Mohan]], Bruce Lindsay (1985): [http://portal.acm.org/citation.cfm?id=850772  "Efficient commit protocols for the tree of processes model of distributed transactions"],''ACM SIGOPS Operating Systems Review'',
19(2),pp. 40-52 (April 1985)</ref> An assumption about the outcome of transactions, either commit, or abort, can save both messages and logging operations by the participants during the 2PC protocol's execution. For example, when presumed abort, if during system recovery from failure no logged evidence for commit of some transaction is found by the recovery procedure, then it assumes that the transaction has been aborted, and acts accordingly. This means that it does not matter if aborts are logged at all, and such logging can be saved under this assumption. Typically a penalty of additional operations is paid during recovery from failure, depending on optimization type. Thus the best variant of optimization, if any, is chosen according to failure and transaction outcome statistics.