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# -*- coding: utf-8 -*-
# Copyright 2015 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
This module controls the reliability for application service transactions.
The nominal flow through this module looks like:
_________
---ASa[e]-->| Event |
----ASb[e]->| Grouper |<-poll 1/s--+
--ASa[e]--->|_________| | ASa[e,e] ASb[e]
V
-````````- +------------+
|````````|<--StoreTxn-|Transaction |
|Database| | Controller |---> SEND TO AS
`--------` +------------+
What happens on SEND TO AS depends on the state of the Application Service:
- If the AS is marked as DOWN, do nothing.
- If the AS is marked as UP, send the transaction.
* SUCCESS : Increment where the AS is up to txn-wise and nuke the txn
contents from the db.
* FAILURE : Marked AS as DOWN and start Recoverer.
Recoverer attempts to recover ASes who have died. The flow for this looks like:
,--------------------- backoff++ --------------.
V |
START ---> Wait exp ------> Get oldest txn ID from ----> FAILURE
backoff DB and try to send it
^ |___________
Mark AS as | V
UP & quit +---------- YES SUCCESS
| | |
NO <--- Have more txns? <------ Mark txn success & nuke <-+
from db; incr AS pos.
Reset backoff.
This is all tied together by the AppServiceScheduler which DIs the required
components.
"""
class AppServiceScheduler(object):
""" Public facing API for this module. Does the required DI to tie the
components together. This also serves as the "event_pool", which in this
case is a simple array.
"""
def __init__(self, clock, store, as_api):
self.clock = clock
self.store = store
self.as_api = as_api
self.event_grouper = _EventGrouper()
def create_recoverer(service, callback):
return _Recoverer(clock, store, as_api, service, callback)
self.txn_ctrl = _TransactionController(
clock, store, as_api, self.event_grouper, create_recoverer
)
def start(self):
# check for any DOWN ASes and start recoverers for them.
_Recoverer.start(
self.clock, self.store, self.as_api, self.txn_ctrl.on_recovered
)
self.txn_ctrl.start_polling()
def submit_event_for_as(self, service, event):
self.event_grouper.on_receive(service, event)
class AppServiceTransaction(object):
"""Represents an application service transaction."""
def __init__(self, service, id, events):
self.service = service
self.id = id
self.events = events
def send(self, as_api):
# TODO sends this transaction using this as_api
pass
def complete(self, store):
# TODO increment txn id on AS and nuke txn contents from db
pass
class _EventGrouper(object):
"""Groups events for the same application service together.
"""
def __init__(self):
self.groups = {} # dict of {service: [events]}
def on_receive(self, service, event):
# TODO group this
pass
def drain_groups(self):
return self.groups
class _TransactionController(object):
def __init__(self, clock, store, as_api, event_grouper, recoverer_fn):
self.clock = clock
self.store = store
self.as_api = as_api
self.event_grouper = event_grouper
self.recoverer_fn = recoverer_fn
def start_polling(self):
groups = self.event_grouper.drain_groups()
for service in groups:
txn_id = self._get_next_txn_id(service)
txn = AppServiceTransaction(service, txn_id, groups[service])
self._store_txn(txn)
if self._is_service_up(service):
if txn.send(self.as_api):
txn.complete(self.store)
else:
# TODO mark AS as down
self._start_recoverer(service)
self.clock.call_later(1000, self.start_polling)
def on_recovered(self, service):
# TODO mark AS as UP
pass
def _start_recoverer(self, service):
recoverer = self.recoverer_fn(service, self.on_recovered)
recoverer.recover()
def _is_service_up(self, service):
pass
def _get_next_txn_id(self, service):
pass # TODO work out the next txn_id for this service
def _store_txn(self, txn):
pass
class _Recoverer(object):
@staticmethod
def start(clock, store, as_api, callback):
# TODO check for DOWN ASes and init recoverers
pass
def __init__(self, clock, store, as_api, service, callback):
self.clock = clock
self.store = store
self.as_api = as_api
self.service = service
self.callback = callback
self.backoff_counter = 1
def recover(self):
self.clock.call_later(2000 ** self.backoff_counter, self.retry)
def retry(self):
txn = self._get_oldest_txn()
if txn:
if txn.send(self.as_api):
txn.complete(self.store)
# reset the backoff counter and retry immediately
self.backoff_counter = 1
self.retry()
return
else:
self.backoff_counter += 1
self.recover()
return
else:
self._set_service_recovered()
def _set_service_recovered(self):
self.callback(self.service)
def _get_oldest_txn(self):
pass # returns AppServiceTransaction
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