What is the role of logs in relational databases?

Logs in relational databases play a critical role in ensuring data integrity, enabling recovery from failures, and maintaining transactional consistency. When a database processes transactions—like inserting, updating, or deleting data—it records every change in a log file before applying it to the actual data files. This approach, often called write-ahead logging (WAL), ensures that no committed transaction is lost if the system crashes. Logs also enforce the ACID properties (Atomicity, Consistency, Isolation, Durability) by providing a way to undo incomplete transactions or redo committed ones during recovery. Without logs, databases would risk data corruption or loss in unexpected shutdowns or errors.

A database log typically stores details such as the transaction ID, the type of operation, the data before modification (for rollback), and the data after modification (for redo). For example, if a user updates a row in a table, the log records the old value (to undo the change if needed) and the new value (to reapply it after a crash). This dual recording allows the database to handle two key scenarios: rolling back uncommitted transactions (e.g., if a user cancels an operation) and reapplying committed transactions that weren’t fully written to disk before a failure. Logs are sequential and append-only, making them efficient to write and durable even under heavy workloads.

To optimize recovery, databases use checkpoints—periodic snapshots of the database state—to limit how much log data needs to be processed after a crash. For instance, SQL Server’s transaction log and Oracle’s redo/undo logs work with checkpoints to reduce recovery time. Beyond crash recovery, logs enable features like replication (by shipping logs to replicas) and auditing (by reviewing historical changes). For example, PostgreSQL’s Write-Ahead Log (WAL) is used for both replication and point-in-time recovery. By providing a reliable record of changes, logs act as the backbone of a database’s ability to maintain reliability and consistency in real-world scenarios.