In a system using WAL, all modifications are written to a log before they are applied. Usually both redo and undo information is stored in the log.
The purpose of this can be illustrated by an example. Imagine a program that is in the middle of performing some operation when the machine it is running on loses power. Upon restart, that program might well need to know whether the operation it was performing succeeded, half-succeeded, or failed. If a write-ahead log is used, the program can check this log and compare what it was supposed to be doing when it unexpectedly lost power to what was actually done. On the basis of this comparison, the program could decide to undo what it had started, complete what it had started, or keep things as they are.
WAL allows updates of a database to be done in-place. Another way to implement atomic updates is with shadow paging, which is not in-place. The main advantage of doing updates in-place is that it reduces the need to modify indexes and block lists.
ARIES is a popular algorithm in the WAL family.
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