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#!/usr/bin/env python
"""Google App Engine sequential number generator.
Generates sequences of numbers from a database of sequences, invaliding
sequences as they are allocated. The implementation guarantees there will be
no gap in the numbers returned if the requested amount of numbers fits into the
current sequence.
Integrity/consistency is carried out by a transaction. Due to App Engines
architecture, and the fact that only one sequence is allocated from at a time,
all allocations of numbers from the database will synchronise on the same
resource and the performance will be low - around 5-10 queries per second.
init_sequence('test', start=1000, end=1003)
init_sequence('other', start=1005, end=1010)
init_sequence('test', start=10100, end=10250)
get_numbers('test', 5)
[1000, 1001, 1002, 10100, 10101]
# Performance could be achieved by a smart sharing setup. Rather than allocating
# from a single Sequence at a time, there could be several shards which have
# pre-allocated some amount of the Sequence (e.g. 100 of the sequence), and these
# could be allocated from on an e.g. random basis, performing a sort of load
# balancing of the transactions. This wouldn't necessarily guarantee a lack of
# gaps, but as long as enough small queries are received (e.g. for 1 numbers)
# over time the gaps will be filled.
# To ensure correct allocation from one sequence to the next, sequences must be
# in the same entity group. Practically, this means all sequences need to have
# "parent" set. init_sequence() ensures this.
# See for a general discussion of the
# problem space.
# Created 2010-11 by Sam Jansen for HUDORA
from google.appengine.ext import db
class gaetkSequence(db.Model):
"""Sequence of numbers, as contained in the spec."""
type = db.StringProperty() # to differentiate betwwen dirrerent types (invoices, consignments, ...)
start = db.IntegerProperty() # fist number to be allocated
end = db.IntegerProperty() # first number to be ot allocated -> [start ; end [
current = db.IntegerProperty() # this is currentliy selected for allocation
active = db.BooleanProperty(default=False) # this can be selected for allocation
created_at = db.DateTimeProperty(auto_now_add=True)
def __repr__(self):
return ('<gaetkSequence: type=%s, start=%d, end=%d, current=%s, active=%s, '
'created_at=%s>') % (self.type, self.start, self.end, self.current,, self.created_at)
def _init_sequence_helper(typ, start, end, root):
"""Transaction for `init_sequence()`."""
# ensure there are no overlapping ranges
query1 = gaetkSequence.all().ancestor(root).filter('type = ', typ).filter(
'start >= ', start).filter('start <', end).fetch(1)
query2 = gaetkSequence.all().ancestor(root).filter('type = ', typ).filter(
'end >= ', start).filter('end <', end).fetch(1)
if query1 or query2:
raise ValueError('%d:%d overlaps with %s/%s' % (start, end, query1, query2))
seq = gaetkSequence(type=typ, parent=root, start=start, end=end, active=True)
return seq
def init_sequence(typ, start, end):
"""Generate a Sequence for the first time. Idempotent."""
assert start < end
root = gaetkSequence.get_by_key_name('_%s_root' % typ)
if not root:
root = gaetkSequence.get_or_insert('_%s_root' % typ, type='_root', start=0, end=1, active=False)
return db.run_in_transaction(_init_sequence_helper, typ, start, end, root.key())
def _get_numbers_helper(keys, needed):
"""Transaction to allocate numbers from a sequence."""
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: = False
consumed = avail
seq.current = start + consumed
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(typ, needed):
"""Returns a list of sequential numbers from the database."""
query = gaetkSequence.all(keys_only=True).filter('type = ', typ).filter('active = ', True).order('start')
rows = query.fetch(5)
if rows:
return db.run_in_transaction(_get_numbers_helper, rows, needed)
raise Exception('No active sequences in database.')