Transactions, ACID, and Isolation Levels

A transaction bundles multiple reads and writes so the whole set either commits together or rolls back together, backed by the ACID guarantees of atomicity, consistency, isolation, and durability. The isolation level is the knob that balances concurrency anomalies (dirty reads, non-repeatable reads, phantoms) against throughput, and most databases ship with a weaker default than engineers expect. AI, ML, and data interviews probe it because pipelines that overlook isolation yield silent, intermittent corruption that no unit test will catch.