Definition (1-liner to start your answer):
ACID stands for Atomicity, Consistency, Isolation, and Durability — these are the key properties that ensure a transaction in a database is reliable, even in case of failures.
Meaning: A transaction is atomic — it either completes fully or doesn’t happen at all.
Example: When transferring ₹1000 from Account A to B:
- Step 1: Deduct ₹1000 from A
- Step 2: Add ₹1000 to B
If Step 2 fails, Step 1 must roll back so money doesn’t disappear.
Interview tip: ✅ “Atomicity ensures that partial transactions don’t leave the database in an inconsistent state.”
Meaning: The database must remain valid before and after the transaction. All constraints, rules, and relationships must still hold true.
Example: If a constraint says account balance cannot go negative, the DB should reject any transaction violating this.
Interview tip: ✅ “Consistency ensures that only valid data following all constraints is written to the database.”
Meaning: Multiple transactions can occur at the same time without affecting each other’s results.
Example: Two users booking movie tickets — each transaction should see a consistent view of the seats and not double-book.
Interview tip: ✅ “Isolation prevents one transaction’s intermediate data from being visible to another.”
(If interviewer asks further:)
- SQL supports isolation levels like READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE.
Meaning: After a transaction is successfully committed, its changes are permanent — even if the system crashes immediately after.
Example: If a transfer succeeds and then the server crashes, the new balance should still be there when it restarts.
Interview tip: ✅ “Durability guarantees that committed data is safely stored on disk and cannot be lost.”
| Property | Keyword | Meaning |
|---|---|---|
| Atomicity | All or nothing | Entire transaction succeeds or fails |
| Consistency | Validity | Database remains correct |
| Isolation | Independence | Transactions don’t affect each other |
| Durability | Permanence | Committed data won’t be lost |
“ACID stands for Atomicity, Consistency, Isolation, and Durability. Atomicity ensures a transaction happens completely or not at all. Consistency maintains database validity before and after a transaction. Isolation ensures transactions running simultaneously don’t affect each other’s results. And Durability means once a transaction is committed, the data remains safe even after system failures. Together, these properties make database transactions reliable.”
Would you like me to give you a few short practice questions interviewers ask about ACID (like scenario-based ones)?