Originally published on tamiz.pro.
Introduction
In 2024, a developer discovered a 16-year-old concurrency bug in SQLite's Write-Ahead Logging (WAL) mechanism using TLA+. This case study reveals how formal verification tools can unravel complex, long-standing issues in critical systems.
Understanding the SQLite WAL System
SQLite's WAL protocol enables concurrent reads and writes by maintaining a separate log file. Writers append changes to the WAL, while readers access the original database and WAL in tandem. Over 16 years, subtle race conditions in WAL's state transitions could evade traditional testing.
