What is the perfect strategy to lock tables in MySql? - mysql

Question

I have wrote this procedure which creates new A/c voucher. It runs on MySql 5.0. Now, it's time to implement this in our production. But I am not sure that it meets the requirement. Is this Lock Strategy perfect ? Please help.
It calls this way :
CALL SpAcVoucherCreate(1,'2022/03/31','2831',5000,'A001');
My procedure is as follows :
USE `FinanceDB`;
DROP PROCEDURE IF EXISTS `SpAcVoucherCreate`;
DELIMITER $$
CREATE DEFINER=`root`#`localhost` PROCEDURE `SpAcVoucherCreate`(
V_VOCHNO INT,
V_VOCHDT CHAR(10),
V_ACCODE CHAR(4),
V_AMOUNT DECIMAL(12,2),
V_USER_ID CHAR(5)
)
BEGIN
DECLARE V_ERR_OCCURED BOOLEAN;
SET V_ERR_OCCURED=FALSE;
BEGIN
DECLARE EXIT HANDLER FOR SQLEXCEPTION
SET V_ERR_OCCURED=TRUE;
SET AUTOCOMMIT=0;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
START TRANSACTION;
LOCK TABLES `ACTRANS` WRITE;
INSERT INTO ACTRANS
(VOCHNO,VOCHDT,ACCODE,AMOUNT,USER_ID)
VALUES
(V_VOCHNO,V_VOCHDT,V_ACCODE,V_AMOUNT,V_USER_ID);
IF V_ERR_OCCURED=TRUE THEN
ROLLBACK;
ELSE
COMMIT;
END IF;
UNLOCK TABLES;
SET AUTOCOMMIT=1;
END;
END $$
DELIMITER ;

Answer 1

I find the procedure to be doing several things that are unnecessary.
SERIALIZABLE acts the same as REPEATABLE READ for what you're doing. The only effect of SERIALIZABLE in MySQL is that it makes non-locking SELECT statements into locking SELECT statements as if you had used SELECT...LOCK IN SHARE MODE. Since you only do an INSERT, this difference has no effect.
There's no need to disable autocommit and then do a transaction start and commit/rollback for a single statement. If you had left autocommit enabled, then a successful INSERT would commit, and an unsuccessful INSERT would not commit. It would have the same result as your code.
There would be no need for an SQLEXCEPTION handler to rollback the transaction, because you have only a single statement in the transaction. It either succeeds or it does not succeed.
Regardless of any of the above, you have shown no reason to use LOCK TABLES. It will only block INSERT/UPDATE/DELETE unnecessarily, if multiple clients try to add rows concurrently. If there is a special reason you need to do this, you have not described it.
Your procedure body has an unnecessary BEGIN/END block. It doesn't do anything. Also I believe that DECLARE is only allowed following the first BEGIN, not within other blocks in the body.
There is probably no reason to use a stored procedure at all, since it only accomplishes a single INSERT operation. Why not just do the INSERT directly in the client? I suppose if you want to restrict the privilege to do inserts to the procedure, if the user doesn't have that privilege, that would be a reason.
I also agree with the comment above that you are using a version of MySQL that is suspiciously out of date. MySQL 5.0 passed its end of life date in 2012, according to https://endoflife.software/applications/databases/mysql. You are missing many bug fixes, security patches, and of course modern features.
Update:
The comment from SolarFlare is correct, one is not allowed to use LOCK or UNLOCK statements in stored procedures in MySQL:
mysql> create procedure p()
-> begin
-> lock tables mytable write;
-> insert into mytable () values ();
-> unlock tables;
-> end//
ERROR 1314 (0A000): LOCK is not allowed in stored procedures