Thread safe operations on table in pl/sql procedure?

Question

I am developing java application which will be running in N localizations simultaneously, each application will have M threads. Every thread takes an unique ID with status NOT_TAKEN from table Queue and changes its status to TAKEN.
Problem:
How to prevent thread from situation like this:
1. Thread A select first ID with status NOT_TAKEN
2. In the same time thread B select first (so it will be the same ID which selected thread A) ID with status NOT_TAKEN.
3. Thread A changes status of ID to TAKEN
4. Thread B changes status of ID to TAKEN
After this operation thread A and B are using the same ID.
What I did:
I have written pl/sql procedure which lock table Queue in exclusive mode, selects first ID, changes its status to TAKEN and unlocks the table. Because it is lock in exclusive mode so only one thread can run this procedure simultaneously.
Question:
How optimally should it be solved, because mine solution prevents from doing all other Updates on Queue table while it is locked, like changing status from TAKEN to OPERATION_DONE so it has performance issue.

Answer 1

As Centinul has said, you need to lock just one row.
I would just add NOWAIT to the select statement to let the Java thread go and try again, instead of waiting for the other threads.
Example: (not tested)
-- Assuming structure of your QueueTable: ( IDCol is PK , StatusCol is VARCHAR2, ... )
-- or make it part of the package....
CREATE OF REPLACE
FUNCTION updateQueue( nQID QueueTable.IDCol%TYPE) RETURN VARCHAR2 AS
   eLocked EXCEPTION;
   PRAGMA EXCEPTION_INIT(eLocked,-54);
   CURSOR curQueueTable IS SELECT 1 CNTR FROM QueueTable WHERE IDCol=nQID AND StatusCol='NOT_TAKEN' FOR UPDATE OF StatusCol NOWAIT;
   recQueueTable curQueueTable%ROWTYPE;
   cRtn VARCHAR2(1);
BEGIN
   cRtn := 'Y';
   BEGIN
      OPEN curQueueTable;
      FETCH curBuffSummary INTO recQueueTable;
      CLOSE curQueueTable;
      IF recQueueTable.CNTR IS NOT NULL AND recQueueTable.CNTR = 1 THEN
          UPDATE QueueTable SET StatusCol = 'TAKEN' WHERE IDCol=nQID;
      ELSE
          -- Already updated
          cRtn := 'N';
      END IF;
         -- You can control your transaction here as well
         -- COMMIT;
         -- But if realy should be done in the Java thread.
    EXCEPTION
       WHEN eLocked OR STANDARD.TIMEOUT_ON_RESOURCE THEN
       -- Thread could not get exclusice row lock. Ignore error.
         cRtn := 'N';
         NULL;
      WHEN OTHERS THEN
         -- Handle other errors...
         -- NULL; just kidding...
         RAISE;
   END;
   RETURN cRtn;
END; Edited by: thomaso on Sep 18, 2009 10:30 AM