Transactions, ACID, and Isolation Levels

A transaction groups several reads and writes so they either all commit or all roll back, with the ACID guarantees of atomicity, consistency, isolation, and durability. Isolation level is the dial that trades concurrency anomalies (dirty reads, non-repeatable reads, phantoms) against throughput, and most databases default to a weaker level than engineers assume. Applied AI and data interviews probe it because pipelines that ignore isolation produce silent, intermittent corruption that no unit test catches.