ACID is not a list of features — it is a set of guarantees that collectively ensure a database never leaves data in a corrupt intermediate state, no matter what goes wrong. Each property targets a specific failure mode: Atomicity blocks partial writes, Consistency enforces business rules, Isolation prevents concurrent interference, and Durability makes commits permanent even through crashes. Together they let you write multi-step operations — transfers, checkouts, reservations — and trust the result is always correct.

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A — ATOMICITY: All or Nothing

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A transaction is indivisible. Either every statement commits

or none of them do. There is no in-between visible state.

BEGIN TRANSACTION

│

Step 1 ──► Step 2 ──► Step 3

│ │ │

OK ✓ OK ✓ FAIL ✗

│

ROLLBACK ↩

┌───────────────────────────┐

│ DB is back to pre-tx state │

│ as if nothing happened │

└───────────────────────────┘

Mechanism: The database engine writes every change to an

undo log before applying it. On failure, it reads the undo

log backwards and reverts each row to its original value.

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C — CONSISTENCY: Valid State → Valid State

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Every transaction must leave the database in a state that

satisfies all defined constraints (PK, FK, CHECK, NOT NULL,

triggers, and application-level invariants).

BEFORE TX: AFTER TX (COMMIT):

┌─────────────────────┐ ┌─────────────────────┐

│ acc A: balance=500 │ ─────► │ acc A: balance=400 │

│ acc B: balance=200 │ │ acc B: balance=300 │

│ CHECK (balance >= 0)│ │ CHECK still holds ✓ │

└─────────────────────┘ └─────────────────────┘

Total = 700 Total = 700 ← invariant

If a transfer would violate CHECK (balance >= 0):

┌─────────────────────┐

│ acc A: balance=500 │ ─────► ROLLBACK (constraint

│ Transfer: -600 │ violated, state unchanged)

└─────────────────────┘

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I — ISOLATION: Concurrent Transactions Don't Interfere