CA 37 – Durability (ACID)

This one is about Durability, means once data is saved (commit), it should not disappear even if system crashes. First I checked initial data: SELECT * FROM accounts; Alice -> 1000 Bob -> 500 Then I did a transfer: BEGIN; UPDATE accounts SET balance = balance - 300 WHERE name = 'Alice'; UPDATE accounts SET balance = balance + 300 WHERE name = 'Bob'; COMMIT; After commit I checked again: SELECT * FROM accounts; Alice -> 700 Bob -> 800 So transfer done. Now imagine system crash or restart. After reconnecting I ran same query again: SELECT * FROM accounts; Still: Alice -> 700 Bob -> 800 So data is not lost. That is durability. Then I thought what if crash happens before commit → changes not saved. If crash happens after commit → data stays safe. So database makes sure once COMMIT happens, data is stored properly (in disk/logs). So simple idea: Before commit → not permanent After commit → permanent That is durability, even crash also cannot remove committed data.