Recovery

/ˈdʌbəl-juː-eɪ-ɛl/

n. “The journal that keeps your database honest.”

WAL, or Write-Ahead Logging, is a technique used by many relational databases, including PostgreSQL and SQLite, to ensure data integrity and durability. The core idea is simple: before any changes are made to the main database files, the changes are first recorded sequentially in a separate log file. This guarantees that in the event of a crash or power failure, the database can replay the log to recover to a consistent state.

WAL serves several key purposes:

• Durability: Ensures committed transactions are not lost even if the system crashes immediately after.
• Crash Recovery: Allows the database to restore consistency by replaying or rolling back operations recorded in the log.
• Concurrency: Improves performance by decoupling the writing of log entries from updates to the main database files.

Here’s a conceptual example of how WAL works in PostgreSQL:

-- User executes a transaction to update balance
BEGIN;

UPDATE accounts
SET balance = balance - 100
WHERE username = 'Alice';

-- Before this update is written to the main database,
-- a log entry is written to the WAL file
-- WAL entry: "Subtract 100 from Alice's balance"

COMMIT;
-- WAL ensures that this update can be replayed if a crash occurs

In this example, the database records the intended change in the WAL first. If the system crashes before writing the update to the main storage, the database can replay the WAL entry to complete the transaction. This method prevents data corruption and ensures transactional consistency.

Operationally, WAL is a backbone feature in high-availability and replication setups. Databases often ship WAL logs to standby servers to keep replicas synchronized, enabling real-time failover.

In essence, WAL is the safety net for modern databases — quietly ensuring that no committed transaction is ever truly lost, even in the worst-case scenarios.