Task Chaining with Cursor issue on app engine. Exception: Too big query offset. Anyone else get this issue?

Question

Hi,

I'm not sure if anyone else has this problem, but I'm getting an exception "Too big query offset" when using a cursor for chaining tasks on appengine development server (not sure if it happens on live).

The error occurs when requesting a cursor after 4000+ records have been processed in a single query.

I wasn't aware that offsets had anything to do with cursors, and perhaps its just a quirk in sdk for app engine.

To fix, either shorten the time allowed before task is deferred (so fewer records get processed at a time) or when checking time elapsed you can also check the number of records processed is still within range. e.g, if time.time() > end_time or count == 2000.Reset count and defer task. 2000 is an arbitrary number, I'm not sure what the limit should be.

EDIT:

After making the above mentioned changes, the never finishes executing. The with_cursor(cursor) code is being called, but seems to start at the beginning each time. Am I missing something obvious?

The code that causes the exception is as follows:

The table "Transact" has 4800 rows. The error occurs when transacts.cursor() is called when time.time() > end_time is true. 4510 records have been processed at the time when the cursor is requested, which seems to cause the error (on development server, haven't tested elsewhere).

def some_task(trans):
  tts = db.get(trans)
  for t in tts:
    #logging.info('in some_task')
    pass


def test_cursor(request):
  ret = test_cursor_task()


def test_cursor_task(cursor = None):
  startDate = datetime.datetime(2010,7,30)
  endDate = datetime.datetime(2010,8,30)
  end_time = time.time() + 20.0   
  transacts = Transact.all().filter('transactionDate >', startDate).filter('transactionDate <=',endDate)

  count =0
  if cursor:
      transacts.with_cursor(cursor)
  trans =[]
  logging.info('queue_trans')
  for tran in transacts:
    count+=1
    #trans.append(str(tran))   
    trans.append(str(tran.key()))   

    if len(trans)==20:
            deferred.defer(some_task, trans,  _countdown = 500)                
            trans =[]            
    if time.time() > end_time:
        logging.info(count)            
        if len(trans)>0:                
           deferred.defer(some_task, trans, _countdown = 500)
           trans =[]
        logging.info('time limit exceeded setting next call to queue')
        cursor = transacts.cursor()
        deferred.defer(test_cursor_task, cursor)
        logging.info('returning false')
        return False


    return True


  return HttpResponse('')

Hope this helps someone.

Thanks Bert