Two phase commit

2PC - A feature of transaction processing systems that enables databases to be returned to the pre-transaction state if some error condition occurs. A single transaction can update many different databases. The two-phase commit strategy is designed to ensure that either all the databases are updated or none of them, so that the databases remain synchronized.

Database changes required by a transaction are initially stored temporarily by each database. The transaction monitor then issues a "pre-commit" command to each database which requires an acknowledgment. If the monitor receives the appropriate response from each database, the monitor issues the "commit" command, which causes all databases to simultaneously make the transaction changes permanent.

Commit request phase

or voting phase

1. The coordinator sends a query to commit message to all cohorts and waits until it has received a reply from all cohorts.
2. The cohorts execute the transaction up to the point where they will be asked to commit. They each write an entry to their undo log and an entry to their redo log.
3. Each cohort replies with an agreement message (cohort votes Yes to commit), if the cohort's actions succeeded, or an abort message (cohort votes No, not to commit), if the cohort experiences a failure that will make it impossible to commit.

Commit phase

or Completion phase

Success

If the coordinator received an agreement message from all cohorts during the commit-request phase:

1. The coordinator sends a commit message to all the cohorts.
2. Each cohort completes the operation, and releases all the locks and resources held during the transaction.
3. Each cohort sends an acknowledgment to the coordinator.
4. The coordinator completes the transaction when all acknowledgments have been received.

Failure

If any cohort votes No during the commit-request phase (or the coordinator's timeout expires):

1. The coordinator sends a rollback message to all the cohorts.
2. Each cohort undoes the transaction using the undo log, and releases the resources and locks held during the transaction.
3. Each cohort sends an acknowledgement to the coordinator.
4. The coordinator undoes the transaction when all acknowledgements have been received.

http://en.wikipedia.org/wiki/Two-phase_commit_protocol