A write-ahead log (WAL) is one of those database concepts that sounds deceptively simple. You write a record to disk before applying it to your in-memory state. If you crash, you replay the log and recover. Done.Except your disk is lying to you.PostgreSQL, SQLite, RocksDB, Cassandra... every production system that claims to be durable relies on a WAL. It's the fundamental contract: "Write here, and I promise your data survives." But making that promise actually stick requires understanding all t...