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A rollback is the operation of restoring a database to a previous state by canceling a specific transaction or transaction set. Rollbacks are either performed automatically by database systems or manually by users.
When a database user changes a data field but has not yet saved the change, the data is stored in a temporary state or transaction log. Users querying the unsaved data see the unchanged values. The action of saving the data is a commit; this allows subsequent queries for this data to show the new values.
However, a user may decide not to save the data. Under this condition, a rollback command manipulates the data to discard any changes made by the user, and does so without communicating this to the user. Thus, a rollback occurs when a user begins changing data, realizes the wrong record is being updated and then cancels the operation to undo any pending changes.
Rollbacks also may be issued automatically after a server or database crash, e.g. after a sudden power loss. When the database restarts, all logged transactions are reviewed; then all pending transactions are rolled back, allowing users to reenter and save appropriate changes.