WAL vs rollback journal
There are 2 opposite ways to rollback the database to a previous state: "Rollback Journal" and "Write-ahead log".
- make a copy of the original database content and save it in a separate file, i.e. rollback journal, then write the new values into database directly.
- A COMMIT occurs when the rollback journal is deleted.
- rollback: copy the content in rollback journal back to database.
- changes are appended to the write-ahead log file.
- A COMMIT occurs when a special "commit" record is appended to the WAL. (the original database may not be altered!)
- multiple transactions can be committed before a "checkpoint", when the database will be updated according to write-ahead log.