how to make sure 2 apps dont get the same row
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  1. #1
    Join Date
    Jul 2003
    Posts
    1

    how to make sure 2 apps dont get the same row

    Hi,

    I am tuning a legacy system which was written on Oracle 7.2 with Powerbuilber as a frontend.

    We are developing and additional web based interface to the db and have upgraded to 8.1.7 for now.

    The problem is that i have an "GoodsInventory" table. When a purchase is made I need to insure that the Powerbuilber (which I preety much cant change) and the new WEb App (made by us so can be changed)
    pull one row of inventory at a time and data integrity is maintained.

    I select the next available row of inventory and update it with the order details. i want to be sure only one app does this at a time (powerbiler/WEb app)

    I have tried SELECT * FROM INVENTORY FOR UPDATE OF INV_STATUS NOWAIT

    (INV_STAT is the status flag which i update from A (available) to S (sold) once the row is taken)

    but that wont help. I was thinking is i could write a pl/sql block to allocate the next avalible row??

    Any suggestions here??

  2. #2
    Join Date
    Jan 2003
    Location
    Hull, UK
    Posts
    220
    Hi,

    U can have checksum calculated using this OWA_OPT_LOCK package.

    first u get the original checksum

    DECLARE
    v_rowid ROWID;
    BEGIN
    SELECT rowid
    INTO v_rowid
    FROM emp
    WHERE empno = 1000;
    DBMS_OUTPUT.PUT_LINE(owa_opt_lock.checksum('SCOTT','EMP', v_rowid));
    END;

    then u can write a procedure

    update_dept(p_empno IN emp.empno%TYPE,
    p_old_checksum IN NUMBER,
    p_new_dept IN emp.deptno%TYPE)

    v_rowid ROWID;
    v_new_checksum NUMBER;
    BEGIN
    SELECT rowid
    INTO v_rowid
    FROM emp
    WHERE empno = p_empno
    FOR UPDATE;

    v_new_checksum := owa_opt_lock.checksum('SCOTT','EMP', v_rowid);

    IF v_new_checksum = p_old_checksum THEN
    UPDATE emp
    SET deptno = p_new_dept
    WHERE rowid = v_rowid;
    COMMIT;
    ELSE
    ROLLBACK;
    RAISE_APPLICATION_ERROR(-20000, 'Data has changed since you retrieved it.');
    END IF;
    END;

    the above method is one of the ways....

    have a look at this

    http://www.oracle-base.com/Articles/...gMethods9i.asp

    HTH

    Srini

  3. #3
    Join Date
    Jul 2003
    Posts
    59
    Advanced Queueing can do this for you. Basically you generate data into a table from producers, and there are consumers who consume the rows, and never walk on each other.

    Check the docs:

    http://otn.oracle.com/pls/tahiti/tah...&word=queueing

  4. #4
    Join Date
    May 2000
    Location
    ATLANTA, GA, USA
    Posts
    3,135
    A transaction can be written in 2 ways - pessimistic and optimistic.
    Your transaction type is pessimistic.
    Change it to optimistic type.

    For example,
    select the row and store inv_stat column value in a variable.
    Then in the update statement add a where clause and the check the for the old value. If the update succeeds, then it means no transaction has changed the row. If it fails, then raise an error.

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