Transactions & concurrency

A transaction is a unit of work that must be treated as a whole: either all of it happens, or none of it does. A bank transfer that debits one account but never credits the other would be a disaster, and the database must make that impossible even when the power fails mid-transfer. The four guarantees are ACID: Atomicity (all or nothing), Consistency (valid state to valid state), Isolation (concurrent transactions behave as if serial) and Durability (committed work survives crashes). Worked example — the lost update: two transactions both read a balance of 100, both add 50, and both write back 150, so one deposit is lost and the balance should have been 200. Concurrency control exists to stop exactly this.