Prevent concurrency bugs on transactions on Oracle

Question

I have a procedure where a create loans for clients. Clients can get only 10000 max. I first read the amount requested by the client on old loans, if the client has requested more than 10000 I raise an error else I insert a new loan.

create or replace NONEDITIONABLE PROCEDURE CREATE_LOAN
( 
    p_client_id INT,
    p_requested_loan_amount DECIMAL
)
IS
    v_available_amount DECIMAL DEFAULT 0;
BEGIN
    --Get available amount for the client( Clients cant require more thant 10000)
    SELECT 10000 - COALESCE(SUM(l.amount), 0) INTO v_available_amount
    FROM loans l
    WHERE l.client_id = p_client_id
    AND (l.state_id = 1 OR l.state_id = 2);
    
    --If the cliente requested more than 10000 in older loans raise error
    IF  p_requested_loan_amount > v_available_amount
    THEN
        raise_application_error( -20001, 'Not enough available amount.' );
    END IF; 
    
    --Else insert new loan
    INSERT INTO loans (amount, state_id, client_id)
    VALUES(p_requested_loan_amount, 1, p_client_id);
END;

How can I prevent two concurrent transaction from reading the old loans at the same time and both beliving that they have available amount before they insert. If I were using Sql Server I could increase the isolation level and that would prevent the problem, but it does not work the same way on Oracle.

Answer 1

In your SELECT statement, use the FOR UPDATE clause. This will get a row lock and stop another transaction from locking that row too.

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