12. What is ACID?

 

What is ACID?

 

ACID stands for:

1. Atomicity:-  All or nothing                               
2. Consistency:- Data stays valid and rules are followed      
3. Isolation:- Transactions don’t interfere with each other
4. Durability:- Committed data stays, even after failure     

 These four properties ensure that transactions in databases behave correctly, even if something goes wrong like a crash or failure.

 

1. Atomicity – All or Nothing 

•  A transaction is a single unit of work.
•  Either everything inside the transaction happens, or nothing happens.
•  If one step fails, the entire transaction is rolled back.

 Example:

• Transfer $100 from A to B
• 1. Subtract $100 from Account A ✅
• 2. Add $100 to Account B ❌ (fails) 

Rollback the subtraction too. No changes happen at all.

 

2. Consistency – Valid State 

•  After a transaction, the data must be in a valid state, following all rules and constraints (like foreign keys, uniqueness).
• The database moves from one valid state to another. 

Example:

You can't insert a user with a duplicate email if there's a unique constraint on email. That keeps data consistent.

 

3. Isolation – No Conflict Between Transactions 

•  Multiple transactions can happen at the same time, but they shouldn’t affect each other.
•  The database ensures intermediate results are not visible to other running transactions. 

Example:

If two users try to book the last movie seat at the same time, isolation ensures only one of them gets it.

There are levels of isolation (READ\_COMMITTED, REPEATABLE\_READ, SERIALIZABLE) that control how strict this is.

 

4. Durability – Once Committed, Always Saved 

•  Once a transaction is committed, the changes are permanent, even if there's a power failure or crash.
•  This is achieved using logs (like redo logs) and disk writing mechanisms. 

Example:

You make a payment, and the system crashes. Since the transaction was committed, your payment is not lost.