WALjournal

WALjournal is a type of file used in database systems, particularly in those employing a Write-Ahead Logging (WAL) mechanism. The primary purpose of a WAL file is to ensure data durability and recoverability. Before any changes are made to the main database data files, they are first recorded in the WAL journal. This log entry contains the modifications that are about to be made.

The write-ahead logging protocol dictates that log records must be written to persistent storage before the corresponding data changes are applied to the database. If the database system crashes before the data pages are updated, the WAL journal can be replayed upon restart. This replay process reapplies the logged changes, bringing the database back to a consistent state without losing any committed transactions.

WALjournals are typically sequential files, and as transactions are committed and data is flushed to disk, older portions of the journal may be truncated or archived. The management of WAL files is crucial for database performance and reliability. Different database systems may have variations in how they implement and manage their WALjournals, but the core principle of logging changes before applying them remains consistent. This technique is a fundamental component of many modern relational and even some non-relational database systems.