mysql存储过程之事物管理(三)
SELECT balance
INTO from_account_balance
FROM account_balance
WHERE account_id=from_account
FOR UPDATE;
IF from_account_balance>=tfer_amount THEN
UPDATE account_balance
SET balance=balance-tfer_amount
WHERE account_id=from_account;
UPDATE account_balance
SET balance=balance+tfer_amount
WHERE account_id=to_account;
COMMIT;
SET status=0;
SET message='OK';
ELSE
ROLLBACK;
SET status=-1;
SET message='Insufficient funds';
END IF;
END;
乐观锁的例子：
Java代码
CREATE PROCEDURE tfer_funds
(from_account INT, to_account INT, tfer_amount NUMERIC(10,2),
OUT status INT, OUT message VARCHAR(30) )
BEGIN
DECLARE from_account_balance NUMERIC(8,2);
DECLARE from_account_balance2 NUMERIC(8,2);
DECLARE from_account_timestamp1 TIMESTAMP;
DECLARE from_account_timestamp2 TIMESTAMP;
SELECT account_timestamp,balance
INTO from_account_timestamp1,from_account_balance
FROM account_balance
WHERE account_id=from_account;
IF (from_account_balance>=tfer_amount) THEN
-- Here we perform some long running validation that
-- might take a few minutes */
CALL long_running_validation(from_account);
START TRANSACTION;
-- Make sure the account row has not been updated since
-- our initial check
SELECT account_timestamp, balance
INTO from_account_timestamp2,from_account_balance2
FROM account_balance
WHERE account_id=from_account
FOR UPDATE;
IF (from_account_timestamp1 <> from_account_timestamp2 OR
from_account_balance <> from_account_balance2) THEN
ROLLBACK;
SET status=-1;
SET message=CONCAT("Transaction cancelled due to concurrent update",
" of account" ,from_account);
ELSE
UPDATE account_balance
SET balance=balance-tfer_amount
WHERE account_id=from_account;
UPDATE account_balance
SET balance=balance+tfer_amount
WHERE account_id=to_account;
COMMIT;
SET status=0;
SET message="OK";
END IF;
ELSE
ROLLBACK;
SET status=-1;
SET message="Insufficient funds";
END IF;
END$$
5，事务设计指南
Java代码
1，保持事务短小
2，尽量避免事务中rollback
3，尽量避免savepoint
4，默认情况下，依赖于悲观锁
5，为吞吐量要求苛刻的事务考虑乐观锁
6，显示声明打开事务
7，锁的行越少越好，锁的时间越短越好