Logging and Buffering

For the purposes of transaction rollback and restart recovery, a transaction log is maintained during normal transaction processing. The log is shared by all transactions, and it keeps, in chronological order, a record of each update on the database. The log record for an update action makes possible the redoing of the update on the previous version of an updated page when that update has been lost due to a failure. The log record for a forward-rolling update action also makes possible the undoing of the update in a backward-rolling transaction or in a transaction that must be aborted due to a failure. The log records are buffered in main memory before they are taken onto disk, but, unlike database pages, the log records are flushed onto the log disk whenever some transaction commits, so that each committed transaction is guaranteed to have every one of its updates recorded either on the disk version of the database or on the log disk.