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SQL Database Tutorial• 7 min read
ACID Transactions & Concurrency Control
Atomicity, Consistency, Isolation, Durability, row locks, and isolation levels
A database transaction is a logical unit of work consisting of multiple operations that must succeed or fail as a single indivisible unit, adhering to the ACID guarantees.
The 4 Pillars of ACID
1Atomicity: "All or Nothing." If 9 out of 10 statements succeed but the 10th fails, the entire transaction is rolled back with zero partial writes.
2Consistency: Any transaction will bring the database from one valid state to another, respecting all constraints and foreign keys.
3Isolation: Concurrent transactions cannot interfere with each other or see intermediate uncommitted states.
4Durability: Once a transaction commits, its writes are permanent on disk even during a power outage or OS crash.
Atomic Bank Account Transfer Transaction
BEGIN TRANSACTION;
-- 1. Deduct funds from Alice
UPDATE BankAccounts
SET Balance = Balance - 500
WHERE AccountID = 'ACC_ALICE';
-- 2. Credit funds to Bob
UPDATE BankAccounts
SET Balance = Balance + 500
WHERE AccountID = 'ACC_BOB';
-- If any step raises an error: ROLLBACK;
-- Otherwise, commit permanently:
COMMIT;ADVERTISEMENT
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Test Yourself With Exercise
What does Atomicity in ACID transactions guarantee?