Two-phase commit ensures that distributed transactions occur consistently across all databases. Two-phase commit protects against inconsistency by making sure that all databases commit the transaction, or that none commit. To ensure database integrity across all involved databases, the database engine commits database updates in two distinct phases. During the first phase, the database engine checks each database involved in a transaction to verify that it is ready to commit the transaction. During the second phase, the database engine directs the databases to commit the transaction and then verifies that they committed it properly. If there is an inconsistency, the database engine displays error messages and allows you to complete or roll back the inconsistent transaction to return the data to a consistent state.
