ransaction Rollback

We roll back a failed transaction, Ti, by using the log. The system scans the log backward; for every log record of the form <Ti, Xj, V1, V2> found in the log, the system restores the data item Xj to its old value V1. Scanning of the log terminates when the log record <Ti, start> is found.

Scanning the log backward is important, since a transaction may have updated a data item more than once. As an illustration, consider the pair of log records