How to synchronize between processes

# lock across process
e.g. Server handles request from different clients, there clients are essentially different processes, they could potentially write to the same piece
of data(e.g. same records in table), so we need to make sure these access well synchronized.

Since normal locking mechnism applies only to inter process(inter-threads), for normal lock from JDK won't work here.

In this case, we need to use some medium from outside, which could be database records or some file on our hard disk.

## Using database records to be mutual exclusive

CREATE OR REPLACE PROCEDURE BBDEV_DBO.Prcubblock(txtType IN CHAR, success OUT INT) IS
intRecCount INT;
BEGIN
DELETE FROM UBBMSTLOCK WHERE dtlastmodified <= SYSDATE - 1/48;
SELECT COUNT(*) INTO intRecCount FROM UBBMSTLOCK;
IF (intRecCount = 0) THEN
INSERT INTO UBBMSTLOCK VALUES (' ', 0, SYSDATE);
END IF;
IF (txtType = 'U') THEN
UPDATE UBBMSTLOCK l
SET l.txtlocktype = 'U', l.dtlastmodified = SYSDATE
WHERE l.txtlocktype = ' ' OR l.txtlocktype = 'U';
success := SQL%Rowcount;
ELSIF txtType = 'S' THEN
UPDATE UBBMSTLOCK l
SET l.txtlocktype = 'S', l.dtlastmodified = SYSDATE
WHERE l.txtlocktype = ' ' OR l.txtlocktype = 'S';
success := SQL%Rowcount;
IF (success > 0) THEN
UPDATE UBBMSTLOCK l
SET l.Intlockcount = l.Intlockcount + 1, l.dtlastmodified = SYSDATE
WHERE l.txtlocktype = 'S';
END IF;
ELSE
success := 0;
END IF;
COMMIT;
EXCEPTION
WHEN OTHERS THEN
ROLLBACK;
success := 0;
END;
/