The Four Guarantees
ACID is a set of properties that guarantee database transactions are processed reliably, even in the face of errors, power failures, or concurrent access.
ACID Properties
┌──────────────────────────────────────────────────────────┐
│ ACID Properties │
├────────────┬─────────────────────────────────────────────┤
│ Atomicity │ All or nothing — entire transaction │
│ │ succeeds or entire transaction fails │
├────────────┼─────────────────────────────────────────────┤
│ Consistency│ Database moves from one valid state to │
│ │ another — constraints always satisfied │
├────────────┼─────────────────────────────────────────────┤
│ Isolation │ Concurrent transactions don't interfere │
│ │ with each other — as if running sequentially│
├────────────┼─────────────────────────────────────────────┤
│ Durability │ Once committed, changes survive crashes │
│ │ — data is permanently stored │
└────────────┴─────────────────────────────────────────────┘
Example: Bank Transfer
BEGIN;
UPDATE Accounts SET Balance = Balance - 500 WHERE Name = 'Alice';
UPDATE Accounts SET Balance = Balance + 500 WHERE Name = 'Bob';
COMMIT;
Atomicity: Both UPDATEs succeed, or neither does
Consistency: Total balance (Alice + Bob) stays the same
Isolation: Other transactions can't see partial results
Durability: After COMMIT, changes survive a power failure