ACID Properties

These are the 4 properties that guarantee database transactions are processed reliably.

A — Atomicity: "All or nothing"

A transaction either completes fully or doesn't happen at all. If any step fails, the entire transaction is rolled back.

Example: Transfer ₹500 from Account A → B

• Debit A: ✅
• Credit B: ❌ (system crashes)

Atomicity ensures the debit is also rolled back. Money doesn't vanish.

C — Consistency: "DB must go from one valid state to another valid state"

A transaction must leave the database in a consistent state — all rules, constraints, and cascades must hold.

Example: If a rule says "balance can't go negative", a transaction that would cause that must fail entirely.

I — Isolation: "Concurrent transactions don't interfere with each other"

Even if 100 transactions run simultaneously, each should behave as if it's running alone.

Example: Two people booking the last seat on a flight simultaneously — isolation ensures only one succeeds.

Handled via isolation levels (Read Uncommitted → Serializable).

D — Durability: "Once committed, data is permanent"

Even if the system crashes right after a commit, the data will not be lost. Typically ensured via write-ahead logs (WAL) or disk writes.

Summary Table

Property	Core Idea	Guaranteed By
Atomicity	All or nothing	Rollback / Undo logs
Consistency	Valid state always	Constraints, Triggers
Isolation	No interference	Locks, MVCC
Durability	Committed = permanent	WAL, Disk flush

Interview Follow-up: "Which property is hardest to achieve?" → Isolation, because it has a direct tradeoff with performance, which is why multiple isolation levels exist.