In databases, some data changes occasionally. There may be problems when data is changed; for example, an error might have occurred. The error might make the data useless. The database system looks at the data to ensure that it meets certain requirements. It does this by using a transaction. There are two points in time in the database, the time before the data was changed, and the time after the data was changed. If something goes wrong when changing the data, the database system simply puts the database back into the state before the change happened. This is called a "rollback". After all the changes are done successfully, they are "committed". This means that the data makes sense again; committed changes can no longer be undone.