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In computer science, write-ahead logging is a family of techniques for providing atomicity and durability in database systems. A write ahead log is an append-only auxiliary disk-resident structure used for crash and transaction recovery. The changes are first recorded in the log, which must be written to stable storage, before the changes are written to the database.
The main functionality of a write-ahead log can be summarized as:
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 need to know whether the operation it was performing succeeded, succeeded partially, 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.