DURABILITY

Let’s understand durability using a simple accounts table
Initially, we have:

• Alice = 1000
• Bob = 500 lets perform a money transfer of 300 from Alice to Bob:

BEGIN;

UPDATE accounts
SET balance = balance - 300
WHERE name = 'Alice';

UPDATE accounts
SET balance = balance + 300
WHERE name = 'Bob';

COMMIT;

At this point we deducted money from Alice added it to Bob and committed the transaction. So the updated balances become
Alice = 700
Bob = 800
this is durability

Now we simulate system restart reconnect and check data:
SELECT * FROM accounts;
Result:
Alice = 700
Bob = 800
even after restart data is still there

Final understanding
Once a transaction is committed its changes are permanently saved in the database. Thats exactly what durability means

What about failures?
If a failure happens before COMMIT:
The transaction isnt complete so none of the changes are saved. The data stays as it was.
If a failure happens after COMMIT:
The changes are already written and secured so they wont be lost.

Because of this Money wont just disappear Balances wont become incorrect after a crash The system remains reliable