Is there a best way to avoid execution of process more than once in Oracle?

Question

Let\'s say that I have a procedure called myproc. This is a complex process and i cannot allow two instances executing at the same time the proc.

Answer 1

Use dbms_lock.allocate_unique along with dbms_lock.request. The usage notes says:

The first session to call ALLOCATE_UNIQUE with a new lock name causes a unique lock ID to be generated and stored in the dbms_lock_allocated table. Subsequent calls (usually by other sessions) return the lock ID previously generated.

I think this could be what you're after.

Answer 2

You can create a table processes. You also ensure that each process has some sort of unique identifier - for instance a hash of the owner, object_name from dba_objects so you could create this dynamically in your package.

You then create a function to lock each row individually as a process is run.

As @Sergio pointed out in the comments this would not work if for some reason you needed to commit in the middle of the process - unless, of course, you re-selected after each commit.

function locking ( Pid ) return number is

   l_locked number := 0;

begin

   select 1
     into l_locked
     from processes
    where id = Pid
         -- exit immediately if the proc is running
      for update nowait
          ;

   return l_locked;

   exception when others then
      return 0;

end;

This has the benefit of locking that row in processes for you until the session that's currently running your procedure has finished.

You then wrap this in your procedure:

-- if we failed to lock locking will have thrown an error
-- i.e. we have 0 here.
if locking( 123 ) = 0 then
   exit;
end if;

As long as each procedure has a unique id - the important bit - your procedure will exit cleanly.

---

It might not apply in your situation but, my normal way of doing this is to use mod. Though it doesn't stop 2 of the same process running it does ensure that when you have more than 1 you only run them on different data. Something like as follows:

procedure my_procedure ( PNumerator number, PDenominator number ) is

    cursor c_my_cursor ( CNumerator number, CDenominator number ) is
     select columns
       from my_table
      where mod( ascii(substr(id, -1)), CDenominator ) = CNumerator
            ;

begin
   open c_my_cursor( PNumerator, PDenominator );
   ...    
end;