Merge pull request #106 from matrix-org/application-services-txn-reliability

Application services transaction reliability (PR #106)

8 files changed, 606 insertions, 38 deletions

diff --git a/synapse/appservice/__init__.py b/synapse/appservice/__init__.py
index a268a6bcc4..c60db16b74 100644
--- a/synapse/appservice/__init__.py
+++ b/synapse/appservice/__init__.py
@@ -20,6 +20,50 @@ import re
 logger = logging.getLogger(__name__)
 
 
+class ApplicationServiceState(object):
+    DOWN = "down"
+    UP = "up"
+
+
+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):
+        """Sends this transaction using the provided AS API interface.
+
+        Args:
+            as_api(ApplicationServiceApi): The API to use to send.
+        Returns:
+            A Deferred which resolves to True if the transaction was sent.
+        """
+        return as_api.push_bulk(
+            service=self.service,
+            events=self.events,
+            txn_id=self.id
+        )
+
+    def complete(self, store):
+        """Completes this transaction as successful.
+
+        Marks this transaction ID on the application service and removes the
+        transaction contents from the database.
+
+        Args:
+            store: The database store to operate on.
+        Returns:
+            A Deferred which resolves to True if the transaction was completed.
+        """
+        return store.complete_appservice_txn(
+            service=self.service,
+            txn_id=self.id
+        )
+
+
 class ApplicationService(object):
     """Defines an application service. This definition is mostly what is
     provided to the /register AS API.
@@ -35,13 +79,13 @@ class ApplicationService(object):
     NS_LIST = [NS_USERS, NS_ALIASES, NS_ROOMS]
 
     def __init__(self, token, url=None, namespaces=None, hs_token=None,
-                 sender=None, txn_id=None):
+                 sender=None, id=None):
         self.token = token
         self.url = url
         self.hs_token = hs_token
         self.sender = sender
         self.namespaces = self._check_namespaces(namespaces)
-        self.txn_id = txn_id
+        self.id = id
 
     def _check_namespaces(self, namespaces):
         # Sanity check that it is of the form:
diff --git a/synapse/appservice/api.py b/synapse/appservice/api.py
index c2179f8d55..2a9becccb3 100644
--- a/synapse/appservice/api.py
+++ b/synapse/appservice/api.py
@@ -72,14 +72,19 @@ class ApplicationServiceApi(SimpleHttpClient):
         defer.returnValue(False)
 
     @defer.inlineCallbacks
-    def push_bulk(self, service, events):
+    def push_bulk(self, service, events, txn_id=None):
         events = self._serialize(events)
 
+        if txn_id is None:
+            logger.warning("push_bulk: Missing txn ID sending events to %s",
+                           service.url)
+            txn_id = str(0)
+        txn_id = str(txn_id)
+
         uri = service.url + ("/transactions/%s" %
-                             urllib.quote(str(0)))  # TODO txn_ids
-        response = None
+                             urllib.quote(txn_id))
         try:
-            response = yield self.put_json(
+            yield self.put_json(
                 uri=uri,
                 json_body={
                     "events": events
@@ -87,9 +92,8 @@ class ApplicationServiceApi(SimpleHttpClient):
                 args={
                     "access_token": service.hs_token
                 })
-            if response:  # just an empty json object
-                # TODO: Mark txn as sent successfully
-                defer.returnValue(True)
+            defer.returnValue(True)
+            return
         except CodeMessageException as e:
             logger.warning("push_bulk to %s received %s", uri, e.code)
         except Exception as ex:
@@ -97,8 +101,8 @@ class ApplicationServiceApi(SimpleHttpClient):
         defer.returnValue(False)
 
     @defer.inlineCallbacks
-    def push(self, service, event):
-        response = yield self.push_bulk(service, [event])
+    def push(self, service, event, txn_id=None):
+        response = yield self.push_bulk(service, [event], txn_id)
         defer.returnValue(response)
 
     def _serialize(self, events):
diff --git a/synapse/appservice/scheduler.py b/synapse/appservice/scheduler.py
new file mode 100644
index 0000000000..59b0b1f4ac
--- /dev/null
+++ b/synapse/appservice/scheduler.py
@@ -0,0 +1,254 @@
+# -*- 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:
+              __________
+1---ASa[e]-->|  Service |--> Queue ASa[f]
+2----ASb[e]->|  Queuer  |
+3--ASa[f]--->|__________|-----------+ ASa[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.
+"""
+
+from synapse.appservice import ApplicationServiceState
+from twisted.internet import defer
+import logging
+
+logger = logging.getLogger(__name__)
+
+
+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
+
+        def create_recoverer(service, callback):
+            return _Recoverer(clock, store, as_api, service, callback)
+
+        self.txn_ctrl = _TransactionController(
+            clock, store, as_api, create_recoverer
+        )
+        self.queuer = _ServiceQueuer(self.txn_ctrl)
+
+    @defer.inlineCallbacks
+    def start(self):
+        logger.info("Starting appservice scheduler")
+        # check for any DOWN ASes and start recoverers for them.
+        recoverers = yield _Recoverer.start(
+            self.clock, self.store, self.as_api, self.txn_ctrl.on_recovered
+        )
+        self.txn_ctrl.add_recoverers(recoverers)
+
+    def submit_event_for_as(self, service, event):
+        self.queuer.enqueue(service, event)
+
+
+class _ServiceQueuer(object):
+    """Queues events for the same application service together, sending
+    transactions as soon as possible. Once a transaction is sent successfully,
+    this schedules any other events in the queue to run.
+    """
+
+    def __init__(self, txn_ctrl):
+        self.queued_events = {}  # dict of {service_id: [events]}
+        self.pending_requests = {}  # dict of {service_id: Deferred}
+        self.txn_ctrl = txn_ctrl
+
+    def enqueue(self, service, event):
+        # if this service isn't being sent something
+        if not self.pending_requests.get(service.id):
+            self._send_request(service, [event])
+        else:
+            # add to queue for this service
+            if service.id not in self.queued_events:
+                self.queued_events[service.id] = []
+            self.queued_events[service.id].append(event)
+
+    def _send_request(self, service, events):
+        # send request and add callbacks
+        d = self.txn_ctrl.send(service, events)
+        d.addBoth(self._on_request_finish)
+        d.addErrback(self._on_request_fail)
+        self.pending_requests[service.id] = d
+
+    def _on_request_finish(self, service):
+        self.pending_requests[service.id] = None
+        # if there are queued events, then send them.
+        if (service.id in self.queued_events
+                and len(self.queued_events[service.id]) > 0):
+            self._send_request(service, self.queued_events[service.id])
+            self.queued_events[service.id] = []
+
+    def _on_request_fail(self, err):
+        logger.error("AS request failed: %s", err)
+
+
+class _TransactionController(object):
+
+    def __init__(self, clock, store, as_api, recoverer_fn):
+        self.clock = clock
+        self.store = store
+        self.as_api = as_api
+        self.recoverer_fn = recoverer_fn
+        # keep track of how many recoverers there are
+        self.recoverers = []
+
+    @defer.inlineCallbacks
+    def send(self, service, events):
+        try:
+            txn = yield self.store.create_appservice_txn(
+                service=service,
+                events=events
+            )
+            service_is_up = yield self._is_service_up(service)
+            if service_is_up:
+                sent = yield txn.send(self.as_api)
+                if sent:
+                    txn.complete(self.store)
+                else:
+                    self._start_recoverer(service)
+        except Exception as e:
+            logger.exception(e)
+            self._start_recoverer(service)
+        # request has finished
+        defer.returnValue(service)
+
+    @defer.inlineCallbacks
+    def on_recovered(self, recoverer):
+        self.recoverers.remove(recoverer)
+        logger.info("Successfully recovered application service AS ID %s",
+                    recoverer.service.id)
+        logger.info("Remaining active recoverers: %s", len(self.recoverers))
+        yield self.store.set_appservice_state(
+            recoverer.service,
+            ApplicationServiceState.UP
+        )
+
+    def add_recoverers(self, recoverers):
+        for r in recoverers:
+            self.recoverers.append(r)
+        if len(recoverers) > 0:
+            logger.info("New active recoverers: %s", len(self.recoverers))
+
+    @defer.inlineCallbacks
+    def _start_recoverer(self, service):
+        yield self.store.set_appservice_state(
+            service,
+            ApplicationServiceState.DOWN
+        )
+        logger.info(
+            "Application service falling behind. Starting recoverer. AS ID %s",
+            service.id
+        )
+        recoverer = self.recoverer_fn(service, self.on_recovered)
+        self.add_recoverers([recoverer])
+        recoverer.recover()
+
+    @defer.inlineCallbacks
+    def _is_service_up(self, service):
+        state = yield self.store.get_appservice_state(service)
+        defer.returnValue(state == ApplicationServiceState.UP or state is None)
+
+
+class _Recoverer(object):
+
+    @staticmethod
+    @defer.inlineCallbacks
+    def start(clock, store, as_api, callback):
+        services = yield store.get_appservices_by_state(
+            ApplicationServiceState.DOWN
+        )
+        recoverers = [
+            _Recoverer(clock, store, as_api, s, callback) for s in services
+        ]
+        for r in recoverers:
+            logger.info("Starting recoverer for AS ID %s which was marked as "
+                        "DOWN", r.service.id)
+            r.recover()
+        defer.returnValue(recoverers)
+
+    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((2 ** self.backoff_counter), self.retry)
+
+    def _backoff(self):
+        # cap the backoff to be around 18h => (2^16) = 65536 secs
+        if self.backoff_counter < 16:
+            self.backoff_counter += 1
+        self.recover()
+
+    @defer.inlineCallbacks
+    def retry(self):
+        try:
+            txn = yield self.store.get_oldest_unsent_txn(self.service)
+            if txn:
+                logger.info("Retrying transaction %s for AS ID %s",
+                            txn.id, txn.service.id)
+                sent = yield txn.send(self.as_api)
+                if sent:
+                    yield txn.complete(self.store)
+                    # reset the backoff counter and retry immediately
+                    self.backoff_counter = 1
+                    yield self.retry()
+                else:
+                    self._backoff()
+            else:
+                self._set_service_recovered()
+        except Exception as e:
+            logger.exception(e)
+            self._backoff()
+
+    def _set_service_recovered(self):
+        self.callback(self)
diff --git a/synapse/handlers/__init__.py b/synapse/handlers/__init__.py
index 8d345bf936..0c51d615ec 100644
--- a/synapse/handlers/__init__.py
+++ b/synapse/handlers/__init__.py
@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from synapse.appservice.scheduler import AppServiceScheduler
 from synapse.appservice.api import ApplicationServiceApi
 from .register import RegistrationHandler
 from .room import (
@@ -54,7 +55,12 @@ class Handlers(object):
         self.directory_handler = DirectoryHandler(hs)
         self.typing_notification_handler = TypingNotificationHandler(hs)
         self.admin_handler = AdminHandler(hs)
+        asapi = ApplicationServiceApi(hs)
         self.appservice_handler = ApplicationServicesHandler(
-            hs, ApplicationServiceApi(hs)
+            hs, asapi, AppServiceScheduler(
+                clock=hs.get_clock(),
+                store=hs.get_datastore(),
+                as_api=asapi
+            )
         )
         self.sync_handler = SyncHandler(hs)
diff --git a/synapse/handlers/appservice.py b/synapse/handlers/appservice.py
index 2c488a46f6..58b5b60bb7 100644
--- a/synapse/handlers/appservice.py
+++ b/synapse/handlers/appservice.py
@@ -26,15 +26,28 @@ import logging
 logger = logging.getLogger(__name__)
 
 
+def log_failure(failure):
+    logger.error(
+        "Application Services Failure",
+        exc_info=(
+            failure.type,
+            failure.value,
+            failure.getTracebackObject()
+        )
+    )
+
+
 # NB: Purposefully not inheriting BaseHandler since that contains way too much
 # setup code which this handler does not need or use. This makes testing a lot
 # easier.
 class ApplicationServicesHandler(object):
 
-    def __init__(self, hs, appservice_api):
+    def __init__(self, hs, appservice_api, appservice_scheduler):
         self.store = hs.get_datastore()
         self.hs = hs
         self.appservice_api = appservice_api
+        self.scheduler = appservice_scheduler
+        self.started_scheduler = False
 
     @defer.inlineCallbacks
     def register(self, app_service):
@@ -46,13 +59,13 @@ class ApplicationServicesHandler(object):
             )
             if not stored_service:
                 raise StoreError(404, "Application service not found")
+            app_service.id = stored_service.id
         except StoreError:
             raise SynapseError(
                 403, "Unrecognised application services token. "
                 "Consult the home server admin.",
                 errcode=Codes.FORBIDDEN
             )
-
         app_service.hs_token = self._generate_hs_token()
 
         # create a sender for this application service which is used when
@@ -90,9 +103,13 @@ class ApplicationServicesHandler(object):
         if event.type == EventTypes.Member:
             yield self._check_user_exists(event.state_key)
 
-        # Fork off pushes to these services - XXX First cut, best effort
+        if not self.started_scheduler:
+            self.scheduler.start().addErrback(log_failure)
+            self.started_scheduler = True
+
+        # Fork off pushes to these services
         for service in services:
-            self.appservice_api.push(service, event)
+            self.scheduler.submit_event_for_as(service, event)
 
     @defer.inlineCallbacks
     def query_user_exists(self, user_id):
diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index c69d11261c..f4dec70393 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -14,9 +14,10 @@
 # limitations under the License.
 
 from twisted.internet import defer
-
+from .appservice import (
+    ApplicationServiceStore, ApplicationServiceTransactionStore
+)
 from ._base import Cache
-from .appservice import ApplicationServiceStore
 from .directory import DirectoryStore
 from .events import EventsStore
 from .presence import PresenceStore
@@ -50,7 +51,7 @@ logger = logging.getLogger(__name__)
 
 # Remember to update this number every time a change is made to database
 # schema files, so the users will be informed on server restarts.
-SCHEMA_VERSION = 14
+SCHEMA_VERSION = 15
 
 dir_path = os.path.abspath(os.path.dirname(__file__))
 
@@ -71,6 +72,7 @@ class DataStore(RoomMemberStore, RoomStore,
                 FilteringStore,
                 PusherStore,
                 PushRuleStore,
+                ApplicationServiceTransactionStore,
                 EventsStore,
                 ):
 
diff --git a/synapse/storage/appservice.py b/synapse/storage/appservice.py
index 850676ce6c..93304a745f 100644
--- a/synapse/storage/appservice.py
+++ b/synapse/storage/appservice.py
@@ -13,13 +13,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-import simplejson
 from simplejson import JSONDecodeError
+import simplejson as json
 from twisted.internet import defer
 
 from synapse.api.constants import Membership
 from synapse.api.errors import StoreError
-from synapse.appservice import ApplicationService
+from synapse.appservice import ApplicationService, AppServiceTransaction
 from synapse.storage.roommember import RoomsForUser
 from ._base import SQLBaseStore
 
@@ -37,7 +37,7 @@ class ApplicationServiceStore(SQLBaseStore):
     def __init__(self, hs):
         super(ApplicationServiceStore, self).__init__(hs)
         self.services_cache = []
-        self.cache_defer = self._populate_cache()
+        self.cache_defer = self._populate_appservice_cache()
         self.cache_defer.addErrback(log_failure)
 
     @defer.inlineCallbacks
@@ -101,11 +101,12 @@ class ApplicationServiceStore(SQLBaseStore):
         if not service.hs_token:
             raise StoreError(500, "No HS token")
 
-        yield self.runInteraction(
+        as_id = yield self.runInteraction(
             "update_app_service",
             self._update_app_service_txn,
             service
         )
+        service.id = as_id
 
         # update cache TODO: Should this be in the txn?
         for (index, cache_service) in enumerate(self.services_cache):
@@ -124,7 +125,7 @@ class ApplicationServiceStore(SQLBaseStore):
                 "update_app_service_txn: Failed to find as_id for token=",
                 service.token
             )
-            return False
+            return
 
         txn.execute(
             "UPDATE application_services SET url=?, hs_token=?, sender=? "
@@ -142,9 +143,9 @@ class ApplicationServiceStore(SQLBaseStore):
                     txn.execute(
                         "INSERT INTO application_services_regex("
                         "as_id, namespace, regex) values(?,?,?)",
-                        (as_id, ns_int, simplejson.dumps(regex_obj))
+                        (as_id, ns_int, json.dumps(regex_obj))
                     )
-        return True
+        return as_id
 
     def _get_as_id_txn(self, txn, token):
         cursor = txn.execute(
@@ -277,12 +278,7 @@ class ApplicationServiceStore(SQLBaseStore):
 
         return rooms_for_user_matching_user_id
 
-    @defer.inlineCallbacks
-    def _populate_cache(self):
-        """Populates the ApplicationServiceCache from the database."""
-        sql = ("SELECT * FROM application_services LEFT JOIN "
-               "application_services_regex ON application_services.id = "
-               "application_services_regex.as_id")
+    def _parse_services_dict(self, results):
         # SQL results in the form:
         # [
         #   {
@@ -296,12 +292,14 @@ class ApplicationServiceStore(SQLBaseStore):
         #   }
         # ]
         services = {}
-        results = yield self._execute_and_decode("_populate_cache", sql)
         for res in results:
             as_token = res["token"]
+            if as_token is None:
+                continue
             if as_token not in services:
                 # add the service
                 services[as_token] = {
+                    "id": res["id"],
                     "url": res["url"],
                     "token": as_token,
                     "hs_token": res["hs_token"],
@@ -319,20 +317,232 @@ class ApplicationServiceStore(SQLBaseStore):
             try:
                 services[as_token]["namespaces"][
                     ApplicationService.NS_LIST[ns_int]].append(
-                    simplejson.loads(res["regex"])
+                    json.loads(res["regex"])
                 )
             except IndexError:
                 logger.error("Bad namespace enum '%s'. %s", ns_int, res)
             except JSONDecodeError:
                 logger.error("Bad regex object '%s'", res["regex"])
 
-        # TODO get last successful txn id f.e. service
+        service_list = []
         for service in services.values():
-            logger.info("Found application service: %s", service)
-            self.services_cache.append(ApplicationService(
+            service_list.append(ApplicationService(
                 token=service["token"],
                 url=service["url"],
                 namespaces=service["namespaces"],
                 hs_token=service["hs_token"],
-                sender=service["sender"]
+                sender=service["sender"],
+                id=service["id"]
             ))
+        return service_list
+
+    @defer.inlineCallbacks
+    def _populate_appservice_cache(self):
+        """Populates the ApplicationServiceCache from the database."""
+        sql = ("SELECT r.*, a.* FROM application_services AS a LEFT JOIN "
+               "application_services_regex AS r ON a.id = r.as_id")
+
+        results = yield self._execute_and_decode("appservice_cache", sql)
+        services = self._parse_services_dict(results)
+
+        for service in services:
+            logger.info("Found application service: %s", service)
+            self.services_cache.append(service)
+
+
+class ApplicationServiceTransactionStore(SQLBaseStore):
+
+    def __init__(self, hs):
+        super(ApplicationServiceTransactionStore, self).__init__(hs)
+
+    @defer.inlineCallbacks
+    def get_appservices_by_state(self, state):
+        """Get a list of application services based on their state.
+
+        Args:
+            state(ApplicationServiceState): The state to filter on.
+        Returns:
+            A Deferred which resolves to a list of ApplicationServices, which
+            may be empty.
+        """
+        sql = (
+            "SELECT r.*, a.* FROM application_services_state AS s LEFT JOIN"
+            " application_services AS a ON a.id=s.as_id LEFT JOIN"
+            " application_services_regex AS r ON r.as_id=a.id WHERE state = ?"
+        )
+        results = yield self._execute_and_decode(
+            "get_appservices_by_state", sql, state
+        )
+        # NB: This assumes this class is linked with ApplicationServiceStore
+        defer.returnValue(self._parse_services_dict(results))
+
+    @defer.inlineCallbacks
+    def get_appservice_state(self, service):
+        """Get the application service state.
+
+        Args:
+            service(ApplicationService): The service whose state to set.
+        Returns:
+            A Deferred which resolves to ApplicationServiceState.
+        """
+        result = yield self._simple_select_one(
+            "application_services_state",
+            dict(as_id=service.id),
+            ["state"],
+            allow_none=True
+        )
+        if result:
+            defer.returnValue(result.get("state"))
+            return
+        defer.returnValue(None)
+
+    def set_appservice_state(self, service, state):
+        """Set the application service state.
+
+        Args:
+            service(ApplicationService): The service whose state to set.
+            state(ApplicationServiceState): The connectivity state to apply.
+        Returns:
+            A Deferred which resolves when the state was set successfully.
+        """
+        return self._simple_upsert(
+            "application_services_state",
+            dict(as_id=service.id),
+            dict(state=state)
+        )
+
+    def create_appservice_txn(self, service, events):
+        """Atomically creates a new transaction for this application service
+        with the given list of events.
+
+        Args:
+            service(ApplicationService): The service who the transaction is for.
+            events(list<Event>): A list of events to put in the transaction.
+        Returns:
+            AppServiceTransaction: A new transaction.
+        """
+        return self.runInteraction(
+            "create_appservice_txn",
+            self._create_appservice_txn,
+            service, events
+        )
+
+    def _create_appservice_txn(self, txn, service, events):
+        # work out new txn id (highest txn id for this service += 1)
+        # The highest id may be the last one sent (in which case it is last_txn)
+        # or it may be the highest in the txns list (which are waiting to be/are
+        # being sent)
+        last_txn_id = self._get_last_txn(txn, service.id)
+
+        result = txn.execute(
+            "SELECT MAX(txn_id) FROM application_services_txns WHERE as_id=?",
+            (service.id,)
+        )
+        highest_txn_id = result.fetchone()[0]
+        if highest_txn_id is None:
+            highest_txn_id = 0
+
+        new_txn_id = max(highest_txn_id, last_txn_id) + 1
+
+        # Insert new txn into txn table
+        event_ids = [e.event_id for e in events]
+        txn.execute(
+            "INSERT INTO application_services_txns(as_id, txn_id, event_ids) "
+            "VALUES(?,?,?)",
+            (service.id, new_txn_id, json.dumps(event_ids))
+        )
+        return AppServiceTransaction(
+            service=service, id=new_txn_id, events=events
+        )
+
+    def complete_appservice_txn(self, txn_id, service):
+        """Completes an application service transaction.
+
+        Args:
+            txn_id(str): The transaction ID being completed.
+            service(ApplicationService): The application service which was sent
+            this transaction.
+        Returns:
+            A Deferred which resolves if this transaction was stored
+            successfully.
+        """
+        return self.runInteraction(
+            "complete_appservice_txn",
+            self._complete_appservice_txn,
+            txn_id, service
+        )
+
+    def _complete_appservice_txn(self, txn, txn_id, service):
+        txn_id = int(txn_id)
+
+        # Debugging query: Make sure the txn being completed is EXACTLY +1 from
+        # what was there before. If it isn't, we've got problems (e.g. the AS
+        # has probably missed some events), so whine loudly but still continue,
+        # since it shouldn't fail completion of the transaction.
+        last_txn_id = self._get_last_txn(txn, service.id)
+        if (last_txn_id + 1) != txn_id:
+            logger.error(
+                "appservice: Completing a transaction which has an ID > 1 from "
+                "the last ID sent to this AS. We've either dropped events or "
+                "sent it to the AS out of order. FIX ME. last_txn=%s "
+                "completing_txn=%s service_id=%s", last_txn_id, txn_id,
+                service.id
+            )
+
+        # Set current txn_id for AS to 'txn_id'
+        self._simple_upsert_txn(
+            txn, "application_services_state", dict(as_id=service.id),
+            dict(last_txn=txn_id)
+        )
+
+        # Delete txn
+        self._simple_delete_txn(
+            txn, "application_services_txns",
+            dict(txn_id=txn_id, as_id=service.id)
+        )
+
+    def get_oldest_unsent_txn(self, service):
+        """Get the oldest transaction which has not been sent for this
+        service.
+
+        Args:
+            service(ApplicationService): The app service to get the oldest txn.
+        Returns:
+            A Deferred which resolves to an AppServiceTransaction or
+            None.
+        """
+        return self.runInteraction(
+            "get_oldest_unsent_appservice_txn",
+            self._get_oldest_unsent_txn,
+            service
+        )
+
+    def _get_oldest_unsent_txn(self, txn, service):
+        # Monotonically increasing txn ids, so just select the smallest
+        # one in the txns table (we delete them when they are sent)
+        result = txn.execute(
+            "SELECT MIN(txn_id), * FROM application_services_txns WHERE as_id=?",
+            (service.id,)
+        )
+        entry = self.cursor_to_dict(result)[0]
+        if not entry or entry["txn_id"] is None:
+            # the min(txn_id) part will force a row, so entry may not be None
+            return None
+
+        event_ids = json.loads(entry["event_ids"])
+        events = self._get_events_txn(txn, event_ids)
+
+        return AppServiceTransaction(
+            service=service, id=entry["txn_id"], events=events
+        )
+
+    def _get_last_txn(self, txn, service_id):
+        result = txn.execute(
+            "SELECT last_txn FROM application_services_state WHERE as_id=?",
+            (service_id,)
+        )
+        last_txn_id = result.fetchone()
+        if last_txn_id is None or last_txn_id[0] is None:  # no row exists
+            return 0
+        else:
+            return int(last_txn_id[0])  # select 'last_txn' col
diff --git a/synapse/storage/schema/delta/15/appservice_txns.sql b/synapse/storage/schema/delta/15/appservice_txns.sql
new file mode 100644
index 0000000000..13bbb2de2e
--- /dev/null
+++ b/synapse/storage/schema/delta/15/appservice_txns.sql
@@ -0,0 +1,31 @@
+/* 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.
+ */
+
+CREATE TABLE IF NOT EXISTS application_services_state(
+    as_id INTEGER PRIMARY KEY,
+    state TEXT,
+    last_txn TEXT,
+    FOREIGN KEY(as_id) REFERENCES application_services(id)
+);
+
+CREATE TABLE IF NOT EXISTS application_services_txns(
+    as_id INTEGER NOT NULL,
+    txn_id INTEGER NOT NULL,
+    event_ids TEXT NOT NULL,
+    UNIQUE(as_id, txn_id) ON CONFLICT ROLLBACK
+);
+
+
+