Hello all,

I just discovered PLpgSQL yesterday (yay!) as we began development of our 
more robust database backend, but I'm seeing some odd behavior in the use 
of LOCK TABLE. The problem I'm seeing is that two database transactions, 
initiated via JDBC, are able to obtain simultaneous exclusive table locks 
on the same table. If I lock the table in PSQL, the JDBC calls do indeed 
block until I end my transaction in PSQL. However, two JDBC calls don't 
appear to block each other. :(

There may indeed be a better solution than locking a table for what I'm 
doing, so please chime in if I'm missing something there. My next attempt 
will be to wrap the whole select-update in a loop, but I'm afraid of 
creating a deadlock situation.

I used PLpgSQL to implement a next_id function that allows IDs to be 
allocated in continuous blocks (thus I believe I cannot use sequences). The 
client application generates its own IDs but tracks them in the database. 
To minimize DB calls the client allocates IDs in blocks and doles them out 
one-at-a-time. Thus, 20 calls to IDFactory.nextID() in Java will result in 
only 2 calls to the database function next_id_block(10).

As well each object type (there are 6) that needs IDs gets its own 
IDFactory, *and* there are multiple Java clients accessing the same 
database. Therefore, it's quite possible for two IDFactories to call 
next_id_block(count) at the same time. Thus I'm locking the table in 
exclusive mode to force synchronous access. I've also tried access 
exclusive mode to no avail.

Here's the table definition:

     create table idfactory
       name              varchar(20)   not null    primary key,
       next_id           integer       not null    default 1,
       change_num        smallint      not null    default 1
     ) ;

This is the psuedo-code algorithm I've implemented in PLpgSQL:

     next_id_block ( count )
     (1)   lock idfactory table

     (2)   read idfactory row
     (3)   update idfactory row
               increment next_id by count
               increment change_num by 1
           where change_num is equal to that read in (2)

     (4)   FAIL if (3) updated 0 rows

     (5)   return next_id read in (2)

My intent is that by locking the idfactory table, I can assure that no one 
else can update the row between it being read in step 2 and updated in step 
3. I've tried calling this function from JDBC with auto-commit on as well 
as with it off and the connection set to both transaction levels. The 
reality, however, is that some threads are trying to update the row 
concurrently and failing (0 rows updated since the change_num value no 
longer matches.

I'll try to illustrate what seems to be happening in the case of two threads.

     Time    Thread 1      Thread 2
      1      lock
      2      read 1, 1
      3                    lock
      4                    read 1, 1
      5      write 11, 2
      6                    write 11, 2
      7      return 1
      8                    FAIL

It's my understanding that thread 2 should block at T3 since thread 1 
locked the table at T1. Thread 2 shouldn't continue until thread 1's 
transaction is ended (either by commit or abort). True? The truly odd part 
is that if I start up PSQL, begin a transaction, and then lock the table, 
all the threads calling the function block until I end the transaction, 
just as I'd expect. However, the threads won't block each other!

Here's the stored function itself:

     create function next_id_block (
       varchar , integer
     returns bigint
     as '
       -- Parameters
       name_key          alias for $1 ;
       block_size        alias for $2 ;

       -- Constants
       FIRST_ID          constant bigint := 1 ;

       -- Locals
       id_rec            record ;
       new_last_id       bigint ;
       num_rows          integer ;
       -- To avoid a retry-loop, lock the whole table for the transaction
       lock table idfactory in access exclusive mode ;

       -- Read the current value of next_id
       select into id_rec * from idfactory where name = name_key ;

       -- Increment it by block_size
       new_last_id := id_rec.next_id + block_size ;
       update idfactory
           set next_id = new_last_id,
               change_num = change_num + 1
           where name = name_key and change_num = id_rec.change_num ;

       -- Error if filter not found
       get diagnostics num_rows = ROW_COUNT ;
       if num_rows != 1 then
         raise exception ''Failed to update idfactory.next_id to % for % at 
             new_last_id, name_key, id_rec.change_num;
         return -1 ;
       end if ;

       return id_rec.next_id ;
     END ;
     ' language 'plpgsql' with (isstrict) ;

Finally, here's the JDBC code I'm using to call it:

     protected void allocate ( int count )
       PreparedStatement     stmt = null;
       ResultSet             result = null;
       long                  newNextID = INVALID_ID;

         stmt = conn.prepareStatement("select next_id_block(?, ?)");

         stmt.setString(1, name);
         stmt.setInt(2, count);

     //    conn.setAutoCommit(false);
         result = stmt.executeQuery();

         if ( ! result.next() )
           throw new SQLException("Function next_id_block failed");

         // Pull out the new value and close the result set.
         newNextID = Nulls.getLong(result, 1);

         try { result.close(); result = null; }
         catch ( SQLException ignore ) { }

     //    conn.commit();

         // Null values are not allowed.
         if ( Nulls.is(newNextID) )
           throw new SQLException("Function next_id_block returned null");

         nextID = newNextID;
         lastID = nextID + count;
       catch ( SQLException e )
         if ( result != null )
           try { result.close(); }
           catch ( SQLException ignore ) { }

Anyway, this was rather long, but I wanted to provide all the information 
necessary up front. Thank you for any thoughts or ideas you might have.


David Harkness
MEconomy, Inc.

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