17.7. Optimizations to the Protocol

Presumed Abort

If a transaction is going to roll back, the coordinator may record this information locally and tell all enlisted participants. Failure to contact a participant has no effect on the transaction outcome. The coordinator is informing participants only as a courtesy. Once all participants have been contacted, the information about the transaction can be removed. If a subsequent request for the status of the transaction occurs, no information will be available and the requester can assume that the transaction has aborted. This optimization has the benefit that no information about participants need be made persistent until the transaction has progressed to the end of the prepare phase and decided to commit, since any failure prior to this point is assumed to be an abort of the transaction.

One-Phase

If only a single participant is involved in the transaction, the coordinator does not need to drive it through the prepare phase. Thus, the participant is told to commit, and the coordinator does not need to record information about the decision, since the outcome of the transaction is the responsibility of the participant.

Read-Only

When a participant is asked to prepare, it can indicate to the coordinator that no information or data that it controls has been modified during the transaction. Such a participant does not need to be informed about the outcome of the transaction since the fate of the participant has no affect on the transaction. Therefore, a read-only participant can be omitted from the second phase of the commit protocol.