What are ACID Properties in databases?

ACID Properties stand for Atomicity, Consistency, Isolation, and Durability. They define the key guarantees of database transactions to ensure data integrity and reliability.

Why are ACID Properties important?

ACID Properties are important because they ensure database transactions are reliable, prevent data corruption, and maintain integrity even in case of system crashes or failures.

Can you give a real-world example of ACID Properties?

A banking transaction is a classic example: transferring money from one account to another must be atomic (either complete or fail), consistent (balances remain valid), isolated (no other transaction interferes), and durable (changes persist even after a crash).

ACID Properties Interactive Demo

🧪 ACID Properties Overview

ACID: Four guarantees for database transactions - Atomicity, Consistency, Isolation, Durability

🧪 ACID = Transaction Chemistry

⚛️

Atomicity: All or nothing (Complete transaction or none)

🎯

Consistency: Data always in a valid state (e.g., $1000 + $200 = $1200)

🔒

Isolation: Transactions don’t interfere with each other

💾

Durability: Committed changes are permanent

Click the buttons above to explore each ACID property!

💳 Community Account Transaction

Live Demo: Simulate transactions in a community account to see ACID properties in action!

🏦 Community Account
Balance: $10000
Manager: Bhide
Status: Ready

Transaction Log (ACID Tracking):

Ready to track ACID properties...

🔄 Isolation Demo: Concurrent Transactions

Scenario: Two users attempt to withdraw from the community fund simultaneously!

👨 User 1 Transaction

Withdraw Amount:

Ready to start transaction...

👨‍🦲 User 2 Transaction

Withdraw Amount:

Ready to start transaction...

Try concurrent transactions to see Isolation in action!

⚠️ What Happens When ACID is Violated?

-- ❌ Atomicity Violation Example
BEGIN TRANSACTION
  UPDATE accounts SET balance = balance - 1000 WHERE account_id = 'user1';
  -- System crash occurs! 💥
  UPDATE accounts SET balance = balance + 1000 WHERE account_id = 'user2';
COMMIT;
-- Result: User1 lost $1000, User2 didn't receive it!
-- System lost money 😱

-- ✅ Proper Atomicity with Rollback
BEGIN TRANSACTION
  UPDATE accounts SET balance = balance - 1000 WHERE account_id = 'user1';
  -- System detects failure
  ROLLBACK; -- Revert all changes
-- Result: Both accounts unchanged, data safe!
-- Either full success or full failure

Click to see atomicity violation...

-- ❌ Consistency Violation Example
-- Rule: Account balance cannot be below $500
UPDATE accounts SET balance = 200 WHERE account_id = 'user1';
-- Business rule violated! Balance < $500
-- Rule: Total money in system must remain constant
UPDATE accounts SET balance = balance - 1000 WHERE account_id = 'user1';
-- No corresponding addition! Money lost!

-- ✅ Proper Consistency Checks
BEGIN TRANSACTION
  -- Check constraints before update
  IF (balance - 1000) >= 500 THEN
    UPDATE accounts SET balance = balance - 1000 WHERE account_id = 'user1';
    UPDATE accounts SET balance = balance + 1000 WHERE account_id = 'user2';
    COMMIT;
  ELSE
    ROLLBACK; -- Maintain consistency
  END IF;

Click to see consistency violation...

-- ❌ Isolation Violation (Dirty Read)
-- Transaction 1 (User1)
BEGIN TRANSACTION
  UPDATE accounts SET balance = 15000 WHERE account_id = 'community';
  -- Transaction 2 (User2) reads uncommitted data
  -- User2 sees balance = 15000 (dirty read!)
ROLLBACK; -- Transaction 1 fails
-- Problem: User2 made decisions based on wrong data!

-- ✅ Proper Isolation with Locking
-- Transaction 1 (User1)
BEGIN TRANSACTION
  SELECT balance FROM accounts WHERE account_id = 'community' FOR UPDATE;
  -- Row locked, User2 must wait
  UPDATE accounts SET balance = 15000 WHERE account_id = 'community';
COMMIT; -- Lock released
-- User2 can now safely read committed data

Click to see isolation violation...

-- ❌ Durability Violation Example
BEGIN TRANSACTION
  UPDATE accounts SET balance = balance + 5000 WHERE account_id = 'community';
  COMMIT; -- Transaction successfully committed
-- Power failure occurs! 💡⚡
-- Data was only in memory, not written to disk
-- After restart: Changes lost! 😱

-- ✅ Proper Durability with Write-Ahead Logging
BEGIN TRANSACTION
  -- Write to log file first (WAL)
  LOG: "UPDATE accounts SET balance = balance + 5000 WHERE account_id = 'community'"
  -- Flush log to disk
  UPDATE accounts SET balance = balance + 5000 WHERE account_id = 'community';
COMMIT; -- Guaranteed permanent
-- Even after a crash, changes can be recovered from log

Click to see durability violation...

🛠️ How Databases Implement ACID

Property	Implementation Technique	Database Feature	Performance Impact
⚛️ Atomicity	Transaction logs, Rollback mechanisms	BEGIN/COMMIT/ROLLBACK statements	Low - minimal overhead
🎯 Consistency	Constraints, Triggers, Validation rules	CHECK constraints, Foreign keys, Triggers	Medium - validation overhead
🔒 Isolation	Locking, MVCC (Multi-Version Concurrency Control)	Row/Table locks, Isolation levels	High - may reduce concurrency
💾 Durability	Write-Ahead Logging (WAL), Disk syncing	Transaction logs, Checkpoint mechanisms	Medium - disk I/O overhead

🌍 Real-World ACID Scenarios

Scenario	ACID Requirement	What Could Go Wrong	Solution
🏦 ATM Withdrawal	All ACID properties required	Money deducted but cash not dispensed	Atomic transaction with rollback
🛒 E-commerce Purchase	Atomicity + Consistency	Payment taken but order not created	Two-phase commit across systems
📊 Analytics Updates	Consistency + Durability	Reports show incorrect numbers	Batch updates with validation
🎮 Gaming Leaderboard	Isolation + Consistency	Multiple players claim the same rank	Optimistic locking with retry
📧 Email System	Durability + Atomicity	Email lost during sending	Message queuing with persistence