INDEPODENCY SITUATION

Let me explain duplicate transaction (same request multiple times)

First, we have accounts table

Alice = 1000
Bob = 500

since, now we try same transfer again and again.

First time transfer:

BEGIN;

UPDATE accounts
SET balance = balance - 200
WHERE name = 'Alice';

UPDATE accounts
SET balance = balance + 200
WHERE name = 'Bob';

COMMIT;

After first transfer:

Alice = 800
Bob = 700

since, 200 is transferred successfully.

Now we run the same transfer again (duplicate request).

BEGIN;

UPDATE accounts
SET balance = balance - 200
WHERE name = 'Alice';

UPDATE accounts
SET balance = balance + 200
WHERE name = 'Bob';

COMMIT;

After second time:

Alice = 600
Bob = 900

since, same operation is applied again.. so money is transferred again.

Final observation:

since, database does not know it is duplicate request

it will execute again and again

so duplicate transactions are allowed

Problem:

since, same request runs multiple times

money will be deducted multiple times

this leads to incorrect balance

THE FINAL THING IS:

since, database only ensures correctness of each transaction

but it does not prevent duplicate requests

so,

this should be handled in application level

like,

unique transaction id
checking if transaction already processed

because of this,

same request will not be applied again
no duplicate deduction
system remains correct

this is how real payment apps avoid duplicate transactions.

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