How to implement “autoincrement” on Google AppEngine

Question

I have to label something in a \"strong monotone increasing\" fashion. Be it Invoice Numbers, shipping label numbers or the like.

1. A number MUST NOT BE used twi

Answer 1

I'm thinking in using the following solution: use CloudSQL (MySQL) to insert the records and assign the sequential ID (maybe with a Task Queue), later (using a Cron Task) move the records from CloudSQL back to the Datastore.

The entities also can have a UUID, so we can map the entities from the Datastore in CloudSQL, and also have the sequential ID (for legal reasons).

Answer 2

The gaetk - Google AppEngine Toolkit now comes with a simple library function to get a number in a sequence. It is based on Nick Johnson's transactional approach and can be used quite easily as a foundation for Martin von Löwis' sharding approach:

>>> from gaeth.sequences import * 
>>> init_sequence('invoce_number', start=1, end=0xffffffff)
>>> get_numbers('invoce_number', 2)
[1, 2]

The functionality is basically implemented like this:

def _get_numbers_helper(keys, needed):
  results = []

  for key in keys:
    seq = db.get(key)
    start = seq.current or seq.start
    end = seq.end
    avail = end - start
    consumed = needed
    if avail <= needed:
      seq.active = False
      consumed = avail
    seq.current = start + consumed
    seq.put()
    results += range(start, start + consumed)
    needed -= consumed
    if needed == 0:
      return results
  raise RuntimeError('Not enough sequence space to allocate %d numbers.' % needed)

def get_numbers(needed):
  query = gaetkSequence.all(keys_only=True).filter('active = ', True)
  return db.run_in_transaction(_get_numbers_helper, query.fetch(5), needed)

Answer 3

Take a look at how the sharded counters are made. It may help you. Also do you really need them to be numeric. If unique is satisfying just use the entity keys.