10.2.9. About the 2-Phase Commit Protocol
In the first phase, the transaction participants notify the transaction coordinator whether they are able to commit the transaction or must roll back.
In the second phase, the transaction coordinator makes the decision about whether the overall transaction should commit or roll back. If any one of the participants cannot commit, the transaction must roll back. Otherwise, the transaction can commit. The coordinator directs the transactions about what to do, and they notify the coordinator when they have done it. At that point, the transaction is finished.