computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...

, write-ahead logging (WAL) is a family of techniques for providing atomicity and

durability Durability is the ability of a physical product to remain functional, without requiring excessive maintenance or repair, when faced with the challenges of normal operation over its design lifetime. There are several measures of durability in us ...

(two of the ACID properties) in

database system In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases spa ...

s. 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 Stable storage is a classification of computer data storage technology that guarantees atomicity for any given write operation and allows software to be written that is robust against some hardware and power failures. To be considered atomic, upon ...

, before the changes are written to the database. The main functionality of a write-ahead log can be summarized as: * Allow the page cache to buffer updates to disk-resident pages while ensuring durability semantics in the larger context of a database system. * Persist all operations on disk until the cached copies of pages affected by these operations are synchronized on disk. Every operation that modifies the database state has to be logged on disk before the contents on the associated pages can be modified * Allow lost in-memory changes to be reconstructed from the operation log in case of a crash. In a system using WAL, all modifications are written to a

log Log most often refers to: * Trunk (botany), the stem and main wooden axis of a tree, called logs when cut ** Logging, cutting down trees for logs ** Firewood, logs used for fuel ** Lumber or timber, converted from wood logs * Logarithm, in mathe ...

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. After a certain amount of operations, the program should perform a checkpoint, writing all the changes specified in the WAL to the database and clearing the log. WAL allows updates of a database to be done

in-place In computer science, an in-place algorithm is an algorithm which transforms input using no auxiliary data structure. However, a small amount of extra storage space is allowed for auxiliary variables. The input is usually overwritten by the output ...

. Another way to implement atomic updates is with

shadow paging In computer science, shadow paging is a technique for providing atomicity and durability (two of the ACID properties) in database systems. A ''page'' in this context refers to a unit of physical storage (probably on a hard disk), typically of ...

, 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. Modern file systems typically use a variant of WAL for at least file system metadata; this is called journaling.

References

* Database algorithms {{database-stub