Answer: Database systems handle transaction failures through two mechanisms, rollback and logging.
Database systems handle transaction failures by leveraging the ACID (Atomicity, Consistency, Isolation, Durability) properties to ensure data integrity and system reliability. When a transaction fails, the system employs two critical mechanisms: rollback and logging.
- Rollback or Undo: It is part of the atomicity property, which guarantees that all operations within a transaction are completed successfully or, if not, the transaction is aborted, and the system reverts any changes made back to its original state. This prevents partial updates that could lead to data inconsistency.
- Logging: It supports durability, ensuring that once a transaction is committed, its effects are permanently recorded in the database, even in the event of a system failure. Transaction logs record all changes made during transactions. In case of a failure, these logs are used to redo completed transactions and undo incomplete ones upon system recovery, maintaining data integrity.
- Checkpoints: To minimize rollback time, database systems periodically take snapshots of the database state and store them as checkpoints. If a major failure occurs, the system can recover to the latest checkpoint and then use the transaction log to redo committed transactions since that point.