How do relational databases ensure transactional consistency?

Relational databases ensure transactional consistency through ACID properties (Atomicity, Consistency, Isolation, Durability) and mechanisms like locking, isolation levels, and write-ahead logging. Transactions—groups of operations treated as a single unit—must either fully complete or have no effect, maintaining data integrity even during failures. For example, transferring money between bank accounts requires deducting from one account and crediting another. If either operation fails, the transaction rolls back, ensuring the system remains consistent. Databases enforce rules like foreign key constraints and unique indexes to prevent invalid data, ensuring logical consistency even before transactions commit.

Locking mechanisms prevent conflicting data access. When a transaction modifies data, it acquires an exclusive lock, blocking other transactions from reading or writing the same data until the lock is released. For reads, shared locks allow concurrent access but prevent writes. For instance, if two transactions try to update the same account balance simultaneously, the database queues the second transaction until the first completes, avoiding inconsistent intermediate states. Isolation levels (e.g., Read Committed, Serializable) control how strictly locks are applied. A higher isolation level like Serializable prevents phenomena like dirty reads (reading uncommitted data) but may reduce concurrency. Developers choose levels based on their application’s tolerance for consistency versus performance trade-offs.

Write-ahead logging (WAL) and recovery processes further ensure consistency. Before any data change is written to disk, it’s logged in a transaction log. If a crash occurs, the database uses this log to redo incomplete transactions or undo failed ones during recovery. For example, if a power outage interrupts a transaction halfway, the database replays the log to restore the system to its last consistent state. Checkpoints periodically sync memory and disk data, reducing recovery time. These mechanisms guarantee that committed transactions persist and uncommitted ones leave no trace, maintaining consistency across both normal operation and failure scenarios.