writeahead

Writeahead, in the context of databases, usually refers to write-ahead logging (WAL), a durability and atomicity mechanism used by many database management systems. The core idea is that any modification to the database state is first written to a log before the actual data pages are updated. This ensures that, in the event of a crash, the log can be used to reconstruct the intended state by redoing committed transactions and undoing uncommitted ones, thereby bringing the database to a consistent point.

Typically, the log is stored on disk in a sequential, append-only fashion. When a transaction makes changes, the system records the details of those changes in the log and only then applies them to the data files. Commit records mark when a transaction’s effects are considered durable. The logs facilitate crash recovery, checkpointing, and point-in-time recovery.

Key concepts associated with WAL include log structure, segments, and transaction identifiers. Checkpoints are periodically created to reduce recovery work by ensuring that in-memory changes are flushed to stable storage. Recovery processes read the log to redo the effects of committed transactions and undo uncommitted ones, restoring a consistent database state after a crash.

WAL supports different flush strategies, such as synchronous commits (ensuring durability on commit) and asynchronous commits (which may improve performance at the cost of potential risks during a crash). It also enables online backups and replication, since the log provides a complete change history.

Well-known implementations include PostgreSQL, which uses WAL for crash recovery and replication, and SQLite, which employs a write-ahead journaling mode. While WAL introduces extra I/O for log writes, it typically improves recoverability and concurrent transaction processing.