Add stub scheduler module for txn reliability

synapse/appservice/scheduler.py

+# -*- 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:
+                    ___________
+  \O/ --- event -->|           |                           +--------------+
+   |  - event ---->| EventPool |<-- poll 1/s for events ---|  EventSorter |
+  / \ ---- event ->|___________|                           +--------------+
+ USERS                                 ____________________________|
+                                      |        |       |
+                                      V        V       V
+                                     ASa       ASb     ASc
+                                    [e,e]      [e]   [e,e,e]
+                                      |
+                                      V
+      -````````-            +------------+
+      |````````|<--StoreTxn-|Transaction |
+      |Database|            |    Maker   |---> 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.
+"""
+
+
+class EventPool(object):
+    pass
+
+
+class EventSorter(object):
+    pass
+
+
+class TransactionMaker(object):
+    pass
+
+
+class Recoverer(object):
+    pass