What is Write-ahead logs (WAL)?

A type of logs used by storage engine to guarantee transaction ACID. For performance reason, whenever a transaction commits, the storage engine does not changes the data file directly, instead it persists the change by appending it to a log file and then signals the commit success. This process is called write-ahead because the actual change to the data file is batched at a later time, the storage engine just writes ahead to a log file to guarantee that the transaction data persists. If the database crashes in between, the recovery process will redo the changes recorded in the write-ahead logs to apply to the data file, that's why it's also known as redo logs. Another usage for WAL is for change-data-capture (CDC) since it already records all the data changes. e.g. PostgreSQL uses WAL for logical replication.