ACID Properties

Introduction

ACID — Atomicity, Consistency, Isolation, Durability — is the contract a relational DB makes. Lose any of them and you'll lose data, money or both.

Understanding the topic

Core concepts to understand:

• A: all-or-nothing.
• C: only valid states (constraints, types) are persisted.
• I: concurrent transactions don't see each other's partial work.
• D: committed data survives crashes (WAL).
• Trade-offs vary by isolation level.

Syntax reference

Visual workflow / architecture:

Transaction
   │  Atomicity  ─ all or nothing
   │  Consistency ─ valid state
   │  Isolation  ─ as if serial
   │  Durability ─ on disk after COMMIT
   ▼
durable, consistent state

Real-world use

Banks, ledgers, exchanges — every system that handles money depends on ACID guarantees.

Best practices

• Pick a relational DB for any money or critical data.
• Understand isolation levels you set.
• Don't fake durability with async writes for critical data.

Hands-on exercise

Interview preparation — practice these questions:

• Q1. Define each ACID letter.
• Q2. Example of an atomicity violation.
• Q3. How does the WAL provide durability?
• Q4. Trade-off between isolation and concurrency.
• Q5. Are NoSQL DBs ACID? Examples.