Key points for two phase commit:
- Multiple party need finish an action together in a distributed environment. Assume there is an coordinator to coordinate the action for all parties and everyone can write “redo/undo” log.
- Assume that every party can recover/revert the action as long as the log is written to disk.
Commit request phase
or voting phase
- The coordinator sends a query to commit message to all cohorts and waits until it has received a reply from all cohorts.
- The cohorts execute the transaction up to the point where they will be asked to commit. They each write an entry to their undo logand an entry to their redo log.
- Each cohort replies with an agreement message (cohort votes Yes to commit), if the cohort’s actions succeeded, or an abortmessage (cohort votes No, not to commit), if the cohort experiences a failure that will make it impossible to commit.
or Completion phase
If the coordinator received an agreement message from all cohorts during the commit-request phase:
- The coordinator sends a commit message to all the cohorts.
- Each cohort completes the operation, and releases all the locks and resources held during the transaction.
- Each cohort sends an acknowledgment to the coordinator.
- The coordinator completes the transaction when all acknowledgments have been received.
If any cohort votes No during the commit-request phase (or the coordinator’s timeout expires):
- The coordinator sends a rollback message to all the cohorts.
- Each cohort undoes the transaction using the undo log, and releases the resources and locks held during the transaction.
- Each cohort sends an acknowledgement to the coordinator.
- The coordinator undoes the transaction when all acknowledgements have been received.