| 摘要 |
A database durability implementation records only committed transactions in a log file. A pair of log files and a pair of snapshot files are maintained. When a snapshot of the database is completed, the 'current' log becomes the 'prior' log and the other log becomes the 'current' log. After the next snapshot is completed, the prior log and prior snapshot may be deleted. Transactions that are not committed are not recorded in the current log, thereby avoiding the need to undo aborted transactions. If a given change is reflected in a completed snapshot, it does not appear in either of the logs; if the change is not yet reflected in a completed snapshot, it is guaranteed to be stored in one of the logs. During recovery, the system assesses both snapshots. The most recent of the completed snapshots is used, and the corresponding log(s) is (are) applied. |
