Durability

The problem
We are transferring money between accounts
After transaction is committed data should not be lost
Even if system crashes data must remain correct

Initial Data
Alice has 1000
Bob has 500

Transaction Flow
Transfer 200 from Alice to Bob

Step 1
Start transaction
Step 2
Deduct 200 from Alice
Step 3
Add 200 to Bob
Step 4
Commit transaction

After Commit

Alice balance becomes 800
Bob balance becomes 700
Changes are saved in database
After System Restart
Reconnect to database
Check balances again
Alice still has 800
Bob still has 700

Changes are not lost

Database follows durability property
Once transaction is committed data is permanently stored
Even if system crashes
Committed data remains

Failure before commit

Transaction is not completed
Changes are rolled back
Final state
Alice 1000
Bob 500

Failure after commit

Transaction already saved
Data remains unchanged
Final state
Alice 800
Bob 700

Commit makes data permanent
Database ensures committed data is never lost