ACID Properties

What is ACID?

ACID is a set of 4 rules that make databases reliable. Every serious database follows these rules.

A - Atomicity
C - Consistency
I - Isolation
D - Durability

Think of ACID like safety rules at a bank. They protect your money (data) from mistakes and problems.

How ACID Works

A - Atomicity

What it means: A transaction must complete fully or not at all. There is no middle state.

Real-life example: Sending money to a friend

Imagine you transfer 100 dollars to your friend. Two things must happen:

Your account loses 100 dollars
Your friend's account gains 100 dollars

What if step 1 works but step 2 fails? You would lose money and your friend would get nothing. That is a disaster.

Atomicity prevents this. If step 2 fails, step 1 is automatically reversed. Either both happen or neither happens.

BEGIN;
UPDATE accounts SET balance = balance - 100 WHERE user_id = 1;
UPDATE accounts SET balance = balance + 100 WHERE user_id = 2;
COMMIT;

If the second UPDATE fails, the first UPDATE is cancelled. Your money is safe.

C - Consistency

What it means: Data must always follow the rules you set. The database checks these rules before saving.

Real-life example: Age must be realistic

You create a rule that age must be between 0 and 120. The database will reject any data that breaks this rule.

CREATE TABLE users (
  name VARCHAR(100),
  age INTEGER CHECK (age >= 0 AND age <= 120)
);

INSERT INTO users (name, age) VALUES ('John', 25);   -- Works
INSERT INTO users (name, age) VALUES ('Jane', -5);   -- Rejected
INSERT INTO users (name, age) VALUES ('Bob', 150);   -- Rejected

The database moves from one valid state to another valid state. It never allows invalid data.

I - Isolation

What it means: Multiple users can work on the database at the same time without affecting each other.

Real-life example: Two people buying the last ticket

Imagine there is 1 concert ticket left. Two people try to buy it at the exact same moment.

Without isolation:

Person A sees 1 ticket available
Person B sees 1 ticket available
Both click buy
Both get the ticket (but there was only 1)

With isolation:

Person A starts buying
Person B must wait
Person A completes the purchase
Person B sees 0 tickets available

The database handles this automatically. It makes sure transactions do not interfere with each other.

D - Durability

What it means: Once you save data, it stays saved. Even if the power goes out or the server crashes, your data is still there.

Real-life example: Saving a document

When you click Save on a document, you expect it to still be there tomorrow. Even if your computer crashes right after saving.

Databases work the same way. Once a transaction is committed, the data is written to permanent storage.

INSERT INTO orders (customer, total) VALUES ('John', 250);
COMMIT;
-- Server crashes here
-- When server restarts, this order is still in the database

The database uses special techniques like write-ahead logging to ensure data survives crashes.

Why ACID Matters

Without ACID, bad things happen:

Without Atomicity: Money disappears during transfers Without Consistency: Invalid data enters your database Without Isolation: Users overwrite each other's work Without Durability: Data is lost after crashes

ACID is especially important for:

Banking and financial systems
E-commerce and order processing
Healthcare records
Any system where data accuracy matters

Summary

ACID keeps your database safe and reliable:

Atomicity: Transactions complete fully or not at all
Consistency: Data always follows your rules
Isolation: Multiple users do not interfere with each other
Durability: Saved data stays saved forever

These 4 properties are why you can trust databases with important data like money, medical records, and business transactions.