Phases of the Two-Phase Commit Protocol

In a two-phase commit transaction, the coordinator sends all the data modification instructions (for example, inserts) to all the participants. Then, the coordinator starts the two-phase commit protocol. The two-phase commit protocol has two parts, the precommit phase and the postdecision phase.

Precommit Phase

During the precommit phase, the coordinator and participants perform the following dialog:

1. Coordinator. The coordinator directs each participant database server to prepare to commit the transaction.
2. Participants. Every participant notifies the coordinator whether it can commit its transaction branch.
3. Coordinator. The coordinator, based on the response from each participant, decides whether to commit or roll back the transaction. It decides to commit only if all participants indicate that they can commit their transaction branches. If any participant indicates that it is not ready to commit its transaction branch (or if it does not respond), the coordinator decides to end the global transaction.

Postdecision Phase

During the postdecision phase, the coordinator and participants perform the following dialog:

1. Coordinator: The coordinator writes the commit record or rollback record to the coordinator's logical log and then directs each participant database server to either commit or roll back the transaction.
2. Participants: If the coordinator issued a commit message, the participants commit the transaction by writing the commit record to the logical log and then sending a message to the coordinator acknowledging that the transaction was committed. If the coordinator issued a rollback message, the participants roll back the transaction but do not send an acknowledgment to the coordinator.
3. Coordinator: If the coordinator issued a message to commit the transaction, it waits to receive acknowledgment from each participant before it ends the global transaction. If the coordinator issued a message to roll back the transaction, it does not wait for acknowledgments from the participants.