Transactions and ACID

Imagine transferring $500 from your checking account to your savings account. The bank needs to do two things: subtract $500 from checking and add $500 to savings. Now imagine the power goes out between those two steps. The money was subtracted from checking but never added to savings. Your $500 has vanished.

This is the problem transactions solve. A transaction groups multiple operations into a single, all-or-nothing unit. Either all the operations succeed, or none of them do. There's no halfway state where money has disappeared.

This guarantee — and three others like it — are captured in the acronym ACID: Atomicity, Consistency, Isolation, and Durability. They're the foundational promises that make databases trustworthy, and understanding them is essential for building applica