<commit_before>"""This is Gittip's payday algorithm. I would appreciate feedback on it.

The payday algorithm is designed to be crash-resistant and parallelizable, but
it's not eventually consistent in the strict sense (iinm) because consistency
is always apodeictically knowable.

Exchanges (moving money between Gittip and the outside world) and transfers
(moving money amongst Gittip users) happen within an isolated event called
payday. This event has duration (it's not punctiliar). It is started
transactionally, and it ends transactionally, and inside of it, exchanges and
transfers happen transactionally (though the link between our db and our
processor's db could be tightened up; see #213). Exchanges immediately affect
the participant's balance, but transfers accrue against a "pending" column in
the database. Once the payday event has completed successfully, it ends with
the pending column being applied to the balance column and reset to NULL in a
single transaction.

"""
from __future__ import unicode_literals

from decimal import Decimal

import aspen.utils
from aspen import log
from aspen.utils import typecheck
from gittip.billing.exchanges import ach_credit, charge
from gittip.exceptions import NegativeBalance, NoBalancedCustomerHref, NotWhitelisted
from psycopg2 import IntegrityError


class NoPayday(Exception):
    def __str__(self):
        return "No payday found where one was expected."


class Payday(object):
    """Represent an abstract event during which money is moved.

    On Payday, we want to use a participant's Gittip balance to settle their
    tips due (pulling in more money via credit card as needed), but we only
    want to use their balance at the start of Payday. Balance changes should be
    atomic globally per-Payday.<commit_msg>update comments<commit_after>"""This is Gittip's payday algorithm.

Exchanges (moving money between Gittip and the outside world) and transfers
(moving money amongst Gittip users) happen within an isolated event called
payday. This event has duration (it's not punctiliar).

Payday is designed to be crash-resistant. Everything that can be rolled back
happens inside a single DB transaction. Exchanges cannot be rolled back, so they
immediately affect the participant's balance.

"""
from __future__ import unicode_literals

from decimal import Decimal

import aspen.utils
from aspen import log
from aspen.utils import typecheck
from gittip.billing.exchanges import ach_credit, charge
from gittip.exceptions import NegativeBalance, NoBalancedCustomerHref, NotWhitelisted
from psycopg2 import IntegrityError


class NoPayday(Exception):
    def __str__(self):
        return "No payday found where one was expected."


class Payday(object):
    """Represent an abstract event during which money is moved.

    On Payday, we want to use a participant's Gittip balance to settle their
    tips due (pulling in more money via credit card as needed), but we only
    want to use their balance at the start of Payday. Balance changes should be
    atomic globally per-Payday.
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