While a complete database is held in a single disk file, SQLite does make use of many temporary files during the course of processing a database. This tag is about the manner in which SQLite records the logs and other supporting data in temporary files.
SQLite implements isolation and concurrency control (and atomicity) using transient journal files that appear in the same directory in as the database file. There are two major "journal modes". The older "rollback mode" corresponds to using the "DELETE", "PERSIST", or "TRUNCATE" options to the journal_mode pragma.
In rollback mode, changes are written directly into the database file, while simultaneously a separate rollback journal file is constructed that is able to restore the database to its original state if the transaction rolls back. Rollback mode (specifically DELETE mode, meaning that the rollback journal is deleted from disk at the conclusion of each transaction) is the current default behavior.
Since version 3.7.0 (2010-07-21), SQLite also supports "WAL mode". In WAL mode, changes are not written to the original database file. Instead, changes go into a separate "write-ahead log" or "WAL" file. Later, after the transaction commits, those changes will be moved from the WAL file back into the original database in an operation called "checkpoint". WAL mode is enabled by running "PRAGMA journal_mode=WAL".
Credits: sqlite.org
