A database does not need a write-ahead log (WAL) to achieve durability. A database can write its long-term data structure durably to disk before returning to a client. Granted, this is a bad idea! And granted, a WAL is critical for durability by design in most databases. But I think it's helpful to understand WALs by understanding what you could do without them. So let's look at what terrible design we can make for a durable database that has no write-ahead log. To motivate the idea of, and buil...