SQLite
Introduction
SQLite is a in-process library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. The code for SQLite is in the public domain and is thus free for use for any purpose, commercial or private.
Write-Ahead Logging
Some versions of SQLite use write-ahead logging to store new changes to the database before they are written to the main database file, allowing a rollback of the current set of changes before they are committed. Database access uses both files to get the latest changed version even if part of it is still in the "WAL" file. This file is a separate file from the main database, using -wal at the end of the file extension so that it becomes .db-wal or .sqlite-wal. Also, a shared-memory file is often used for a hash-table index of the write-ahead log, with a .db-shm or .sqlite-shm extension.
