9. Write Ahead Logging (WAL)

Transaction logs are an essential part of databases because they ensure that no data is lost even when a system failure occurs. They are a history log of all changes and actions in a database system. This ensures that no data is lost due to failures, such as a power failure or a server crash.

The log contains sufficient information about each transaction that has already been executed, so the database server can recover the database cluster by replaying the changes and actions in the transaction log in the event of a server crash.

In the field of computer science, WAL is an acronym for Write-Ahead Logging, which is a protocol or rule that requires both changes and actions to be written to a transaction log. However, in PostgreSQL, WAL is also an acronym for Write Ahead Log. In PostgreSQL, the term WAL is used interchangeably with transaction log, and it also refers to the implemented mechanism for writing actions to a transaction log (WAL). Although this can be confusing, this document will adopt the PostgreSQL definition.

The WAL mechanism was first implemented in version 7.1 to mitigate the impacts of server crashes. It also made possible the implementation of the Point-in-Time Recovery (PITR) and Streaming Replication (SR), both of which are described in Chapters 10 and 11, respectively.

Although understanding the WAL mechanism is essential for system integrations and administration using PostgreSQL, the complexity of this mechanism makes it impossible to summarize its description in brief. Therefore, the complete explanation of WAL in PostgreSQL is as follows:

• The logical and physical structures of the WAL (transaction log).
• The internal layout of WAL data.
• Writing of WAL data.
• WAL writer process.
• The checkpoint processing.
• The database recovery processing.
• Managing WAL segment files.
• Continuous archiving.