use go module for dependencies (#594)

11 files changed, 1506 insertions, 0 deletions

diff --git a/appservice/README.md b/appservice/README.md
new file mode 100644
index 00000000..d7555744
--- /dev/null
+++ b/appservice/README.md
@@ -0,0 +1,10 @@
+# Application Service
+
+This component interfaces with external [Application
+Services](https://matrix.org/docs/spec/application_service/unstable.html).
+This includes any HTTP endpoints that application services call, as well as talking
+to any HTTP endpoints that application services provide themselves.
+
+## Consumers
+
+This component consumes and filters events from the Roomserver Kafka stream, passing on any necessary events to subscribing application services.
\ No newline at end of file
diff --git a/appservice/api/query.go b/appservice/api/query.go
new file mode 100644
index 00000000..9ec21448
--- /dev/null
+++ b/appservice/api/query.go
@@ -0,0 +1,178 @@
+// Copyright 2018 New Vector 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.
+
+// Package api contains methods used by dendrite components in multi-process
+// mode to send requests to the appservice component, typically in order to ask
+// an application service for some information.
+package api
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"net/http"
+
+	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
+	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
+	"github.com/matrix-org/gomatrixserverlib"
+
+	commonHTTP "github.com/matrix-org/dendrite/common/http"
+	opentracing "github.com/opentracing/opentracing-go"
+)
+
+// RoomAliasExistsRequest is a request to an application service
+// about whether a room alias exists
+type RoomAliasExistsRequest struct {
+	// Alias we want to lookup
+	Alias string `json:"alias"`
+}
+
+// RoomAliasExistsResponse is a response from an application service
+// about whether a room alias exists
+type RoomAliasExistsResponse struct {
+	AliasExists bool `json:"exists"`
+}
+
+// UserIDExistsRequest is a request to an application service about whether a
+// user ID exists
+type UserIDExistsRequest struct {
+	// UserID we want to lookup
+	UserID string `json:"user_id"`
+}
+
+// UserIDExistsRequestAccessToken is a request to an application service
+// about whether a user ID exists. Includes an access token
+type UserIDExistsRequestAccessToken struct {
+	// UserID we want to lookup
+	UserID      string `json:"user_id"`
+	AccessToken string `json:"access_token"`
+}
+
+// UserIDExistsResponse is a response from an application service about
+// whether a user ID exists
+type UserIDExistsResponse struct {
+	UserIDExists bool `json:"exists"`
+}
+
+// AppServiceQueryAPI is used to query user and room alias data from application
+// services
+type AppServiceQueryAPI interface {
+	// Check whether a room alias exists within any application service namespaces
+	RoomAliasExists(
+		ctx context.Context,
+		req *RoomAliasExistsRequest,
+		resp *RoomAliasExistsResponse,
+	) error
+	// Check whether a user ID exists within any application service namespaces
+	UserIDExists(
+		ctx context.Context,
+		req *UserIDExistsRequest,
+		resp *UserIDExistsResponse,
+	) error
+}
+
+// AppServiceRoomAliasExistsPath is the HTTP path for the RoomAliasExists API
+const AppServiceRoomAliasExistsPath = "/api/appservice/RoomAliasExists"
+
+// AppServiceUserIDExistsPath is the HTTP path for the UserIDExists API
+const AppServiceUserIDExistsPath = "/api/appservice/UserIDExists"
+
+// httpAppServiceQueryAPI contains the URL to an appservice query API and a
+// reference to a httpClient used to reach it
+type httpAppServiceQueryAPI struct {
+	appserviceURL string
+	httpClient    *http.Client
+}
+
+// NewAppServiceQueryAPIHTTP creates a AppServiceQueryAPI implemented by talking
+// to a HTTP POST API.
+// If httpClient is nil then it uses http.DefaultClient
+func NewAppServiceQueryAPIHTTP(
+	appserviceURL string,
+	httpClient *http.Client,
+) AppServiceQueryAPI {
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+	return &httpAppServiceQueryAPI{appserviceURL, httpClient}
+}
+
+// RoomAliasExists implements AppServiceQueryAPI
+func (h *httpAppServiceQueryAPI) RoomAliasExists(
+	ctx context.Context,
+	request *RoomAliasExistsRequest,
+	response *RoomAliasExistsResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "appserviceRoomAliasExists")
+	defer span.Finish()
+
+	apiURL := h.appserviceURL + AppServiceRoomAliasExistsPath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
+
+// UserIDExists implements AppServiceQueryAPI
+func (h *httpAppServiceQueryAPI) UserIDExists(
+	ctx context.Context,
+	request *UserIDExistsRequest,
+	response *UserIDExistsResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "appserviceUserIDExists")
+	defer span.Finish()
+
+	apiURL := h.appserviceURL + AppServiceUserIDExistsPath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
+
+// RetreiveUserProfile is a wrapper that queries both the local database and
+// application services for a given user's profile
+func RetreiveUserProfile(
+	ctx context.Context,
+	userID string,
+	asAPI AppServiceQueryAPI,
+	accountDB *accounts.Database,
+) (*authtypes.Profile, error) {
+	localpart, _, err := gomatrixserverlib.SplitID('@', userID)
+	if err != nil {
+		return nil, err
+	}
+
+	// Try to query the user from the local database
+	profile, err := accountDB.GetProfileByLocalpart(ctx, localpart)
+	if err != nil && err != sql.ErrNoRows {
+		return nil, err
+	} else if profile != nil {
+		return profile, nil
+	}
+
+	// Query the appservice component for the existence of an AS user
+	userReq := UserIDExistsRequest{UserID: userID}
+	var userResp UserIDExistsResponse
+	if err = asAPI.UserIDExists(ctx, &userReq, &userResp); err != nil {
+		return nil, err
+	}
+
+	// If no user exists, return
+	if !userResp.UserIDExists {
+		return nil, errors.New("no known profile for given user ID")
+	}
+
+	// Try to query the user from the local database again
+	profile, err = accountDB.GetProfileByLocalpart(ctx, localpart)
+	if err != nil {
+		return nil, err
+	}
+
+	// profile should not be nil at this point
+	return profile, nil
+}
diff --git a/appservice/appservice.go b/appservice/appservice.go
new file mode 100644
index 00000000..8703959f
--- /dev/null
+++ b/appservice/appservice.go
@@ -0,0 +1,132 @@
+// Copyright 2018 Vector Creations 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.
+
+package appservice
+
+import (
+	"context"
+	"net/http"
+	"sync"
+	"time"
+
+	appserviceAPI "github.com/matrix-org/dendrite/appservice/api"
+	"github.com/matrix-org/dendrite/appservice/consumers"
+	"github.com/matrix-org/dendrite/appservice/query"
+	"github.com/matrix-org/dendrite/appservice/routing"
+	"github.com/matrix-org/dendrite/appservice/storage"
+	"github.com/matrix-org/dendrite/appservice/types"
+	"github.com/matrix-org/dendrite/appservice/workers"
+	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
+	"github.com/matrix-org/dendrite/clientapi/auth/storage/devices"
+	"github.com/matrix-org/dendrite/common/basecomponent"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/common/transactions"
+	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/sirupsen/logrus"
+)
+
+// SetupAppServiceAPIComponent sets up and registers HTTP handlers for the AppServices
+// component.
+func SetupAppServiceAPIComponent(
+	base *basecomponent.BaseDendrite,
+	accountsDB *accounts.Database,
+	deviceDB *devices.Database,
+	federation *gomatrixserverlib.FederationClient,
+	roomserverAliasAPI roomserverAPI.RoomserverAliasAPI,
+	roomserverQueryAPI roomserverAPI.RoomserverQueryAPI,
+	transactionsCache *transactions.Cache,
+) appserviceAPI.AppServiceQueryAPI {
+	// Create a connection to the appservice postgres DB
+	appserviceDB, err := storage.NewDatabase(string(base.Cfg.Database.AppService))
+	if err != nil {
+		logrus.WithError(err).Panicf("failed to connect to appservice db")
+	}
+
+	// Wrap application services in a type that relates the application service and
+	// a sync.Cond object that can be used to notify workers when there are new
+	// events to be sent out.
+	workerStates := make([]types.ApplicationServiceWorkerState, len(base.Cfg.Derived.ApplicationServices))
+	for i, appservice := range base.Cfg.Derived.ApplicationServices {
+		m := sync.Mutex{}
+		ws := types.ApplicationServiceWorkerState{
+			AppService: appservice,
+			Cond:       sync.NewCond(&m),
+		}
+		workerStates[i] = ws
+
+		// Create bot account for this AS if it doesn't already exist
+		if err = generateAppServiceAccount(accountsDB, deviceDB, appservice); err != nil {
+			logrus.WithFields(logrus.Fields{
+				"appservice": appservice.ID,
+			}).WithError(err).Panicf("failed to generate bot account for appservice")
+		}
+	}
+
+	// Create appserivce query API with an HTTP client that will be used for all
+	// outbound and inbound requests (inbound only for the internal API)
+	appserviceQueryAPI := query.AppServiceQueryAPI{
+		HTTPClient: &http.Client{
+			Timeout: time.Second * 30,
+		},
+		Cfg: base.Cfg,
+	}
+
+	appserviceQueryAPI.SetupHTTP(http.DefaultServeMux)
+
+	consumer := consumers.NewOutputRoomEventConsumer(
+		base.Cfg, base.KafkaConsumer, accountsDB, appserviceDB,
+		roomserverQueryAPI, roomserverAliasAPI, workerStates,
+	)
+	if err := consumer.Start(); err != nil {
+		logrus.WithError(err).Panicf("failed to start appservice roomserver consumer")
+	}
+
+	// Create application service transaction workers
+	if err := workers.SetupTransactionWorkers(appserviceDB, workerStates); err != nil {
+		logrus.WithError(err).Panicf("failed to start app service transaction workers")
+	}
+
+	// Set up HTTP Endpoints
+	routing.Setup(
+		base.APIMux, *base.Cfg, roomserverQueryAPI, roomserverAliasAPI,
+		accountsDB, federation, transactionsCache,
+	)
+
+	return &appserviceQueryAPI
+}
+
+// generateAppServiceAccounts creates a dummy account based off the
+// `sender_localpart` field of each application service if it doesn't
+// exist already
+func generateAppServiceAccount(
+	accountsDB *accounts.Database,
+	deviceDB *devices.Database,
+	as config.ApplicationService,
+) error {
+	ctx := context.Background()
+
+	// Create an account for the application service
+	acc, err := accountsDB.CreateAccount(ctx, as.SenderLocalpart, "", as.ID)
+	if err != nil {
+		return err
+	} else if acc == nil {
+		// This account already exists
+		return nil
+	}
+
+	// Create a dummy device with a dummy token for the application service
+	_, err = deviceDB.CreateDevice(ctx, as.SenderLocalpart, nil, as.ASToken, &as.SenderLocalpart)
+	return err
+}
diff --git a/appservice/consumers/roomserver.go b/appservice/consumers/roomserver.go
new file mode 100644
index 00000000..dbdae532
--- /dev/null
+++ b/appservice/consumers/roomserver.go
@@ -0,0 +1,210 @@
+// Copyright 2018 Vector Creations 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.
+
+package consumers
+
+import (
+	"context"
+	"encoding/json"
+
+	"github.com/matrix-org/dendrite/appservice/storage"
+	"github.com/matrix-org/dendrite/appservice/types"
+	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
+
+	log "github.com/sirupsen/logrus"
+	sarama "gopkg.in/Shopify/sarama.v1"
+)
+
+// OutputRoomEventConsumer consumes events that originated in the room server.
+type OutputRoomEventConsumer struct {
+	roomServerConsumer *common.ContinualConsumer
+	db                 *accounts.Database
+	asDB               *storage.Database
+	query              api.RoomserverQueryAPI
+	alias              api.RoomserverAliasAPI
+	serverName         string
+	workerStates       []types.ApplicationServiceWorkerState
+}
+
+// NewOutputRoomEventConsumer creates a new OutputRoomEventConsumer. Call
+// Start() to begin consuming from room servers.
+func NewOutputRoomEventConsumer(
+	cfg *config.Dendrite,
+	kafkaConsumer sarama.Consumer,
+	store *accounts.Database,
+	appserviceDB *storage.Database,
+	queryAPI api.RoomserverQueryAPI,
+	aliasAPI api.RoomserverAliasAPI,
+	workerStates []types.ApplicationServiceWorkerState,
+) *OutputRoomEventConsumer {
+	consumer := common.ContinualConsumer{
+		Topic:          string(cfg.Kafka.Topics.OutputRoomEvent),
+		Consumer:       kafkaConsumer,
+		PartitionStore: store,
+	}
+	s := &OutputRoomEventConsumer{
+		roomServerConsumer: &consumer,
+		db:                 store,
+		asDB:               appserviceDB,
+		query:              queryAPI,
+		alias:              aliasAPI,
+		serverName:         string(cfg.Matrix.ServerName),
+		workerStates:       workerStates,
+	}
+	consumer.ProcessMessage = s.onMessage
+
+	return s
+}
+
+// Start consuming from room servers
+func (s *OutputRoomEventConsumer) Start() error {
+	return s.roomServerConsumer.Start()
+}
+
+// onMessage is called when the appservice component receives a new event from
+// the room server output log.
+func (s *OutputRoomEventConsumer) onMessage(msg *sarama.ConsumerMessage) error {
+	// Parse out the event JSON
+	var output api.OutputEvent
+	if err := json.Unmarshal(msg.Value, &output); err != nil {
+		// If the message was invalid, log it and move on to the next message in the stream
+		log.WithError(err).Errorf("roomserver output log: message parse failure")
+		return nil
+	}
+
+	if output.Type != api.OutputTypeNewRoomEvent {
+		log.WithField("type", output.Type).Debug(
+			"roomserver output log: ignoring unknown output type",
+		)
+		return nil
+	}
+
+	ev := output.NewRoomEvent.Event
+	log.WithFields(log.Fields{
+		"event_id": ev.EventID(),
+		"room_id":  ev.RoomID(),
+		"type":     ev.Type(),
+	}).Info("appservice received an event from roomserver")
+
+	missingEvents, err := s.lookupMissingStateEvents(output.NewRoomEvent.AddsStateEventIDs, ev)
+	if err != nil {
+		return err
+	}
+	events := append(missingEvents, ev)
+
+	// Send event to any relevant application services
+	return s.filterRoomserverEvents(context.TODO(), events)
+}
+
+// lookupMissingStateEvents looks up the state events that are added by a new event,
+// and returns any not already present.
+func (s *OutputRoomEventConsumer) lookupMissingStateEvents(
+	addsStateEventIDs []string, event gomatrixserverlib.Event,
+) ([]gomatrixserverlib.Event, error) {
+	// Fast path if there aren't any new state events.
+	if len(addsStateEventIDs) == 0 {
+		return []gomatrixserverlib.Event{}, nil
+	}
+
+	// Fast path if the only state event added is the event itself.
+	if len(addsStateEventIDs) == 1 && addsStateEventIDs[0] == event.EventID() {
+		return []gomatrixserverlib.Event{}, nil
+	}
+
+	result := []gomatrixserverlib.Event{}
+	missing := []string{}
+	for _, id := range addsStateEventIDs {
+		if id != event.EventID() {
+			// If the event isn't the current one, add it to the list of events
+			// to retrieve from the roomserver
+			missing = append(missing, id)
+		}
+	}
+
+	// Request the missing events from the roomserver
+	eventReq := api.QueryEventsByIDRequest{EventIDs: missing}
+	var eventResp api.QueryEventsByIDResponse
+	if err := s.query.QueryEventsByID(context.TODO(), &eventReq, &eventResp); err != nil {
+		return nil, err
+	}
+
+	result = append(result, eventResp.Events...)
+
+	return result, nil
+}
+
+// filterRoomserverEvents takes in events and decides whether any of them need
+// to be passed on to an external application service. It does this by checking
+// each namespace of each registered application service, and if there is a
+// match, adds the event to the queue for events to be sent to a particular
+// application service.
+func (s *OutputRoomEventConsumer) filterRoomserverEvents(
+	ctx context.Context,
+	events []gomatrixserverlib.Event,
+) error {
+	for _, ws := range s.workerStates {
+		for _, event := range events {
+			// Check if this event is interesting to this application service
+			if s.appserviceIsInterestedInEvent(ctx, event, ws.AppService) {
+				// Queue this event to be sent off to the application service
+				if err := s.asDB.StoreEvent(ctx, ws.AppService.ID, &event); err != nil {
+					log.WithError(err).Warn("failed to insert incoming event into appservices database")
+				} else {
+					// Tell our worker to send out new messages by updating remaining message
+					// count and waking them up with a broadcast
+					ws.NotifyNewEvents()
+				}
+			}
+		}
+	}
+
+	return nil
+}
+
+// appserviceIsInterestedInEvent returns a boolean depending on whether a given
+// event falls within one of a given application service's namespaces.
+func (s *OutputRoomEventConsumer) appserviceIsInterestedInEvent(ctx context.Context, event gomatrixserverlib.Event, appservice config.ApplicationService) bool {
+	// No reason to queue events if they'll never be sent to the application
+	// service
+	if appservice.URL == "" {
+		return false
+	}
+
+	// Check Room ID and Sender of the event
+	if appservice.IsInterestedInUserID(event.Sender()) ||
+		appservice.IsInterestedInRoomID(event.RoomID()) {
+		return true
+	}
+
+	// Check all known room aliases of the room the event came from
+	queryReq := api.GetAliasesForRoomIDRequest{RoomID: event.RoomID()}
+	var queryRes api.GetAliasesForRoomIDResponse
+	if err := s.alias.GetAliasesForRoomID(ctx, &queryReq, &queryRes); err == nil {
+		for _, alias := range queryRes.Aliases {
+			if appservice.IsInterestedInRoomAlias(alias) {
+				return true
+			}
+		}
+	} else {
+		log.WithFields(log.Fields{
+			"room_id": event.RoomID(),
+		}).WithError(err).Errorf("Unable to get aliases for room")
+	}
+
+	return false
+}
diff --git a/appservice/query/query.go b/appservice/query/query.go
new file mode 100644
index 00000000..fde3ab09
--- /dev/null
+++ b/appservice/query/query.go
@@ -0,0 +1,214 @@
+// Copyright 2018 New Vector 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.
+
+// Package query handles requests from other internal dendrite components when
+// they interact with the AppServiceQueryAPI.
+package query
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/url"
+	"time"
+
+	"github.com/matrix-org/dendrite/appservice/api"
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/util"
+	opentracing "github.com/opentracing/opentracing-go"
+	log "github.com/sirupsen/logrus"
+)
+
+const roomAliasExistsPath = "/rooms/"
+const userIDExistsPath = "/users/"
+
+// AppServiceQueryAPI is an implementation of api.AppServiceQueryAPI
+type AppServiceQueryAPI struct {
+	HTTPClient *http.Client
+	Cfg        *config.Dendrite
+}
+
+// RoomAliasExists performs a request to '/room/{roomAlias}' on all known
+// handling application services until one admits to owning the room
+func (a *AppServiceQueryAPI) RoomAliasExists(
+	ctx context.Context,
+	request *api.RoomAliasExistsRequest,
+	response *api.RoomAliasExistsResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "ApplicationServiceRoomAlias")
+	defer span.Finish()
+
+	// Create an HTTP client if one does not already exist
+	if a.HTTPClient == nil {
+		a.HTTPClient = makeHTTPClient()
+	}
+
+	// Determine which application service should handle this request
+	for _, appservice := range a.Cfg.Derived.ApplicationServices {
+		if appservice.URL != "" && appservice.IsInterestedInRoomAlias(request.Alias) {
+			// The full path to the rooms API, includes hs token
+			URL, err := url.Parse(appservice.URL + roomAliasExistsPath)
+			URL.Path += request.Alias
+			apiURL := URL.String() + "?access_token=" + appservice.HSToken
+
+			// Send a request to each application service. If one responds that it has
+			// created the room, immediately return.
+			req, err := http.NewRequest(http.MethodGet, apiURL, nil)
+			if err != nil {
+				return err
+			}
+			req = req.WithContext(ctx)
+
+			resp, err := a.HTTPClient.Do(req)
+			if resp != nil {
+				defer func() {
+					err = resp.Body.Close()
+					if err != nil {
+						log.WithFields(log.Fields{
+							"appservice_id": appservice.ID,
+							"status_code":   resp.StatusCode,
+						}).WithError(err).Error("Unable to close application service response body")
+					}
+				}()
+			}
+			if err != nil {
+				log.WithError(err).Errorf("Issue querying room alias on application service %s", appservice.ID)
+				return err
+			}
+			switch resp.StatusCode {
+			case http.StatusOK:
+				// OK received from appservice. Room exists
+				response.AliasExists = true
+				return nil
+			case http.StatusNotFound:
+				// Room does not exist
+			default:
+				// Application service reported an error. Warn
+				log.WithFields(log.Fields{
+					"appservice_id": appservice.ID,
+					"status_code":   resp.StatusCode,
+				}).Warn("Application service responded with non-OK status code")
+			}
+		}
+	}
+
+	response.AliasExists = false
+	return nil
+}
+
+// UserIDExists performs a request to '/users/{userID}' on all known
+// handling application services until one admits to owning the user ID
+func (a *AppServiceQueryAPI) UserIDExists(
+	ctx context.Context,
+	request *api.UserIDExistsRequest,
+	response *api.UserIDExistsResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "ApplicationServiceUserID")
+	defer span.Finish()
+
+	// Create an HTTP client if one does not already exist
+	if a.HTTPClient == nil {
+		a.HTTPClient = makeHTTPClient()
+	}
+
+	// Determine which application service should handle this request
+	for _, appservice := range a.Cfg.Derived.ApplicationServices {
+		if appservice.URL != "" && appservice.IsInterestedInUserID(request.UserID) {
+			// The full path to the rooms API, includes hs token
+			URL, err := url.Parse(appservice.URL + userIDExistsPath)
+			URL.Path += request.UserID
+			apiURL := URL.String() + "?access_token=" + appservice.HSToken
+
+			// Send a request to each application service. If one responds that it has
+			// created the user, immediately return.
+			req, err := http.NewRequest(http.MethodGet, apiURL, nil)
+			if err != nil {
+				return err
+			}
+			resp, err := a.HTTPClient.Do(req.WithContext(ctx))
+			if resp != nil {
+				defer func() {
+					err = resp.Body.Close()
+					if err != nil {
+						log.WithFields(log.Fields{
+							"appservice_id": appservice.ID,
+							"status_code":   resp.StatusCode,
+						}).Error("Unable to close application service response body")
+					}
+				}()
+			}
+			if err != nil {
+				log.WithFields(log.Fields{
+					"appservice_id": appservice.ID,
+				}).WithError(err).Error("issue querying user ID on application service")
+				return err
+			}
+			if resp.StatusCode == http.StatusOK {
+				// StatusOK received from appservice. User ID exists
+				response.UserIDExists = true
+				return nil
+			}
+
+			// Log non OK
+			log.WithFields(log.Fields{
+				"appservice_id": appservice.ID,
+				"status_code":   resp.StatusCode,
+			}).Warn("application service responded with non-OK status code")
+		}
+	}
+
+	response.UserIDExists = false
+	return nil
+}
+
+// makeHTTPClient creates an HTTP client with certain options that will be used for all query requests to application services
+func makeHTTPClient() *http.Client {
+	return &http.Client{
+		Timeout: time.Second * 30,
+	}
+}
+
+// SetupHTTP adds the AppServiceQueryPAI handlers to the http.ServeMux. This
+// handles and muxes incoming api requests the to internal AppServiceQueryAPI.
+func (a *AppServiceQueryAPI) SetupHTTP(servMux *http.ServeMux) {
+	servMux.Handle(
+		api.AppServiceRoomAliasExistsPath,
+		common.MakeInternalAPI("appserviceRoomAliasExists", func(req *http.Request) util.JSONResponse {
+			var request api.RoomAliasExistsRequest
+			var response api.RoomAliasExistsResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.ErrorResponse(err)
+			}
+			if err := a.RoomAliasExists(req.Context(), &request, &response); err != nil {
+				return util.ErrorResponse(err)
+			}
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
+	servMux.Handle(
+		api.AppServiceUserIDExistsPath,
+		common.MakeInternalAPI("appserviceUserIDExists", func(req *http.Request) util.JSONResponse {
+			var request api.UserIDExistsRequest
+			var response api.UserIDExistsResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.ErrorResponse(err)
+			}
+			if err := a.UserIDExists(req.Context(), &request, &response); err != nil {
+				return util.ErrorResponse(err)
+			}
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
+}
diff --git a/appservice/routing/routing.go b/appservice/routing/routing.go
new file mode 100644
index 00000000..f0b8461d
--- /dev/null
+++ b/appservice/routing/routing.go
@@ -0,0 +1,61 @@
+// Copyright 2018 Vector Creations 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.
+
+package routing
+
+import (
+	"net/http"
+
+	"github.com/gorilla/mux"
+	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/common/transactions"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+const pathPrefixApp = "/_matrix/app/r0"
+
+// Setup registers HTTP handlers with the given ServeMux. It also supplies the given http.Client
+// to clients which need to make outbound HTTP requests.
+func Setup(
+	apiMux *mux.Router, cfg config.Dendrite, // nolint: unparam
+	queryAPI api.RoomserverQueryAPI, aliasAPI api.RoomserverAliasAPI, // nolint: unparam
+	accountDB *accounts.Database, // nolint: unparam
+	federation *gomatrixserverlib.FederationClient, // nolint: unparam
+	transactionsCache *transactions.Cache, // nolint: unparam
+) {
+	appMux := apiMux.PathPrefix(pathPrefixApp).Subrouter()
+
+	appMux.Handle("/alias",
+		common.MakeExternalAPI("alias", func(req *http.Request) util.JSONResponse {
+			// TODO: Implement
+			return util.JSONResponse{
+				Code: http.StatusOK,
+				JSON: nil,
+			}
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+	appMux.Handle("/user",
+		common.MakeExternalAPI("user", func(req *http.Request) util.JSONResponse {
+			// TODO: Implement
+			return util.JSONResponse{
+				Code: http.StatusOK,
+				JSON: nil,
+			}
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+}
diff --git a/appservice/storage/appservice_events_table.go b/appservice/storage/appservice_events_table.go
new file mode 100644
index 00000000..285bbf48
--- /dev/null
+++ b/appservice/storage/appservice_events_table.go
@@ -0,0 +1,248 @@
+// Copyright 2018 New Vector 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.
+
+package storage
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"time"
+
+	"github.com/matrix-org/gomatrixserverlib"
+	log "github.com/sirupsen/logrus"
+)
+
+const appserviceEventsSchema = `
+-- Stores events to be sent to application services
+CREATE TABLE IF NOT EXISTS appservice_events (
+	-- An auto-incrementing id unique to each event in the table
+	id BIGSERIAL NOT NULL PRIMARY KEY,
+	-- The ID of the application service the event will be sent to
+	as_id TEXT NOT NULL,
+	-- JSON representation of the event
+	event_json TEXT NOT NULL,
+	-- The ID of the transaction that this event is a part of
+	txn_id BIGINT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS appservice_events_as_id ON appservice_events(as_id);
+`
+
+const selectEventsByApplicationServiceIDSQL = "" +
+	"SELECT id, event_json, txn_id " +
+	"FROM appservice_events WHERE as_id = $1 ORDER BY txn_id DESC, id ASC"
+
+const countEventsByApplicationServiceIDSQL = "" +
+	"SELECT COUNT(id) FROM appservice_events WHERE as_id = $1"
+
+const insertEventSQL = "" +
+	"INSERT INTO appservice_events(as_id, event_json, txn_id) " +
+	"VALUES ($1, $2, $3)"
+
+const updateTxnIDForEventsSQL = "" +
+	"UPDATE appservice_events SET txn_id = $1 WHERE as_id = $2 AND id <= $3"
+
+const deleteEventsBeforeAndIncludingIDSQL = "" +
+	"DELETE FROM appservice_events WHERE as_id = $1 AND id <= $2"
+
+const (
+	// A transaction ID number that no transaction should ever have. Used for
+	// checking again the default value.
+	invalidTxnID = -2
+)
+
+type eventsStatements struct {
+	selectEventsByApplicationServiceIDStmt *sql.Stmt
+	countEventsByApplicationServiceIDStmt  *sql.Stmt
+	insertEventStmt                        *sql.Stmt
+	updateTxnIDForEventsStmt               *sql.Stmt
+	deleteEventsBeforeAndIncludingIDStmt   *sql.Stmt
+}
+
+func (s *eventsStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(appserviceEventsSchema)
+	if err != nil {
+		return
+	}
+
+	if s.selectEventsByApplicationServiceIDStmt, err = db.Prepare(selectEventsByApplicationServiceIDSQL); err != nil {
+		return
+	}
+	if s.countEventsByApplicationServiceIDStmt, err = db.Prepare(countEventsByApplicationServiceIDSQL); err != nil {
+		return
+	}
+	if s.insertEventStmt, err = db.Prepare(insertEventSQL); err != nil {
+		return
+	}
+	if s.updateTxnIDForEventsStmt, err = db.Prepare(updateTxnIDForEventsSQL); err != nil {
+		return
+	}
+	if s.deleteEventsBeforeAndIncludingIDStmt, err = db.Prepare(deleteEventsBeforeAndIncludingIDSQL); err != nil {
+		return
+	}
+
+	return
+}
+
+// selectEventsByApplicationServiceID takes in an application service ID and
+// returns a slice of events that need to be sent to that application service,
+// as well as an int later used to remove these same events from the database
+// once successfully sent to an application service.
+func (s *eventsStatements) selectEventsByApplicationServiceID(
+	ctx context.Context,
+	applicationServiceID string,
+	limit int,
+) (
+	txnID, maxID int,
+	events []gomatrixserverlib.Event,
+	eventsRemaining bool,
+	err error,
+) {
+	// Retrieve events from the database. Unsuccessfully sent events first
+	eventRows, err := s.selectEventsByApplicationServiceIDStmt.QueryContext(ctx, applicationServiceID)
+	if err != nil {
+		return
+	}
+	defer func() {
+		err = eventRows.Close()
+		if err != nil {
+			log.WithFields(log.Fields{
+				"appservice": applicationServiceID,
+			}).WithError(err).Fatalf("appservice unable to select new events to send")
+		}
+	}()
+	events, maxID, txnID, eventsRemaining, err = retrieveEvents(eventRows, limit)
+	if err != nil {
+		return
+	}
+
+	return
+}
+
+func retrieveEvents(eventRows *sql.Rows, limit int) (events []gomatrixserverlib.Event, maxID, txnID int, eventsRemaining bool, err error) {
+	// Get current time for use in calculating event age
+	nowMilli := time.Now().UnixNano() / int64(time.Millisecond)
+
+	// Iterate through each row and store event contents
+	// If txn_id changes dramatically, we've switched from collecting old events to
+	// new ones. Send back those events first.
+	lastTxnID := invalidTxnID
+	for eventsProcessed := 0; eventRows.Next(); {
+		var event gomatrixserverlib.Event
+		var eventJSON []byte
+		var id int
+		err = eventRows.Scan(
+			&id,
+			&eventJSON,
+			&txnID,
+		)
+		if err != nil {
+			return nil, 0, 0, false, err
+		}
+
+		// Unmarshal eventJSON
+		if err = json.Unmarshal(eventJSON, &event); err != nil {
+			return nil, 0, 0, false, err
+		}
+
+		// If txnID has changed on this event from the previous event, then we've
+		// reached the end of a transaction's events. Return only those events.
+		if lastTxnID > invalidTxnID && lastTxnID != txnID {
+			return events, maxID, lastTxnID, true, nil
+		}
+		lastTxnID = txnID
+
+		// Limit events that aren't part of an old transaction
+		if txnID == -1 {
+			// Return if we've hit the limit
+			if eventsProcessed++; eventsProcessed > limit {
+				return events, maxID, lastTxnID, true, nil
+			}
+		}
+
+		if id > maxID {
+			maxID = id
+		}
+
+		// Portion of the event that is unsigned due to rapid change
+		// TODO: Consider removing age as not many app services use it
+		if err = event.SetUnsignedField("age", nowMilli-int64(event.OriginServerTS())); err != nil {
+			return nil, 0, 0, false, err
+		}
+
+		events = append(events, event)
+	}
+
+	return
+}
+
+// countEventsByApplicationServiceID inserts an event mapped to its corresponding application service
+// IDs into the db.
+func (s *eventsStatements) countEventsByApplicationServiceID(
+	ctx context.Context,
+	appServiceID string,
+) (int, error) {
+	var count int
+	err := s.countEventsByApplicationServiceIDStmt.QueryRowContext(ctx, appServiceID).Scan(&count)
+	if err != nil && err != sql.ErrNoRows {
+		return 0, err
+	}
+
+	return count, nil
+}
+
+// insertEvent inserts an event mapped to its corresponding application service
+// IDs into the db.
+func (s *eventsStatements) insertEvent(
+	ctx context.Context,
+	appServiceID string,
+	event *gomatrixserverlib.Event,
+) (err error) {
+	// Convert event to JSON before inserting
+	eventJSON, err := json.Marshal(event)
+	if err != nil {
+		return err
+	}
+
+	_, err = s.insertEventStmt.ExecContext(
+		ctx,
+		appServiceID,
+		eventJSON,
+		-1, // No transaction ID yet
+	)
+	return
+}
+
+// updateTxnIDForEvents sets the transactionID for a collection of events. Done
+// before sending them to an AppService. Referenced before sending to make sure
+// we aren't constructing multiple transactions with the same events.
+func (s *eventsStatements) updateTxnIDForEvents(
+	ctx context.Context,
+	appserviceID string,
+	maxID, txnID int,
+) (err error) {
+	_, err = s.updateTxnIDForEventsStmt.ExecContext(ctx, txnID, appserviceID, maxID)
+	return
+}
+
+// deleteEventsBeforeAndIncludingID removes events matching given IDs from the database.
+func (s *eventsStatements) deleteEventsBeforeAndIncludingID(
+	ctx context.Context,
+	appserviceID string,
+	eventTableID int,
+) (err error) {
+	_, err = s.deleteEventsBeforeAndIncludingIDStmt.ExecContext(ctx, appserviceID, eventTableID)
+	return
+}
diff --git a/appservice/storage/storage.go b/appservice/storage/storage.go
new file mode 100644
index 00000000..b68989fb
--- /dev/null
+++ b/appservice/storage/storage.go
@@ -0,0 +1,110 @@
+// Copyright 2018 New Vector 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.
+
+package storage
+
+import (
+	"context"
+	"database/sql"
+
+	// Import postgres database driver
+	_ "github.com/lib/pq"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// Database stores events intended to be later sent to application services
+type Database struct {
+	events eventsStatements
+	txnID  txnStatements
+	db     *sql.DB
+}
+
+// NewDatabase opens a new database
+func NewDatabase(dataSourceName string) (*Database, error) {
+	var result Database
+	var err error
+	if result.db, err = sql.Open("postgres", dataSourceName); err != nil {
+		return nil, err
+	}
+	if err = result.prepare(); err != nil {
+		return nil, err
+	}
+	return &result, nil
+}
+
+func (d *Database) prepare() error {
+	if err := d.events.prepare(d.db); err != nil {
+		return err
+	}
+
+	return d.txnID.prepare(d.db)
+}
+
+// StoreEvent takes in a gomatrixserverlib.Event and stores it in the database
+// for a transaction worker to pull and later send to an application service.
+func (d *Database) StoreEvent(
+	ctx context.Context,
+	appServiceID string,
+	event *gomatrixserverlib.Event,
+) error {
+	return d.events.insertEvent(ctx, appServiceID, event)
+}
+
+// GetEventsWithAppServiceID returns a slice of events and their IDs intended to
+// be sent to an application service given its ID.
+func (d *Database) GetEventsWithAppServiceID(
+	ctx context.Context,
+	appServiceID string,
+	limit int,
+) (int, int, []gomatrixserverlib.Event, bool, error) {
+	return d.events.selectEventsByApplicationServiceID(ctx, appServiceID, limit)
+}
+
+// CountEventsWithAppServiceID returns the number of events destined for an
+// application service given its ID.
+func (d *Database) CountEventsWithAppServiceID(
+	ctx context.Context,
+	appServiceID string,
+) (int, error) {
+	return d.events.countEventsByApplicationServiceID(ctx, appServiceID)
+}
+
+// UpdateTxnIDForEvents takes in an application service ID and a
+// and stores them in the DB, unless the pair already exists, in
+// which case it updates them.
+func (d *Database) UpdateTxnIDForEvents(
+	ctx context.Context,
+	appserviceID string,
+	maxID, txnID int,
+) error {
+	return d.events.updateTxnIDForEvents(ctx, appserviceID, maxID, txnID)
+}
+
+// RemoveEventsBeforeAndIncludingID removes all events from the database that
+// are less than or equal to a given maximum ID. IDs here are implemented as a
+// serial, thus this should always delete events in chronological order.
+func (d *Database) RemoveEventsBeforeAndIncludingID(
+	ctx context.Context,
+	appserviceID string,
+	eventTableID int,
+) error {
+	return d.events.deleteEventsBeforeAndIncludingID(ctx, appserviceID, eventTableID)
+}
+
+// GetLatestTxnID returns the latest available transaction id
+func (d *Database) GetLatestTxnID(
+	ctx context.Context,
+) (int, error) {
+	return d.txnID.selectTxnID(ctx)
+}
diff --git a/appservice/storage/txn_id_counter_table.go b/appservice/storage/txn_id_counter_table.go
new file mode 100644
index 00000000..7b0fa378
--- /dev/null
+++ b/appservice/storage/txn_id_counter_table.go
@@ -0,0 +1,52 @@
+// Copyright 2018 New Vector 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.
+
+package storage
+
+import (
+	"context"
+	"database/sql"
+)
+
+const txnIDSchema = `
+-- Keeps a count of the current transaction ID
+CREATE SEQUENCE IF NOT EXISTS txn_id_counter START 1;
+`
+
+const selectTxnIDSQL = "SELECT nextval('txn_id_counter')"
+
+type txnStatements struct {
+	selectTxnIDStmt *sql.Stmt
+}
+
+func (s *txnStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(txnIDSchema)
+	if err != nil {
+		return
+	}
+
+	if s.selectTxnIDStmt, err = db.Prepare(selectTxnIDSQL); err != nil {
+		return
+	}
+
+	return
+}
+
+// selectTxnID selects the latest ascending transaction ID
+func (s *txnStatements) selectTxnID(
+	ctx context.Context,
+) (txnID int, err error) {
+	err = s.selectTxnIDStmt.QueryRowContext(ctx).Scan(&txnID)
+	return
+}
diff --git a/appservice/types/types.go b/appservice/types/types.go
new file mode 100644
index 00000000..aac73155
--- /dev/null
+++ b/appservice/types/types.go
@@ -0,0 +1,64 @@
+// 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.
+
+package types
+
+import (
+	"sync"
+
+	"github.com/matrix-org/dendrite/common/config"
+)
+
+const (
+	// AppServiceDeviceID is the AS dummy device ID
+	AppServiceDeviceID = "AS_Device"
+)
+
+// ApplicationServiceWorkerState is a type that couples an application service,
+// a lockable condition as well as some other state variables, allowing the
+// roomserver to notify appservice workers when there are events ready to send
+// externally to application services.
+type ApplicationServiceWorkerState struct {
+	AppService config.ApplicationService
+	Cond       *sync.Cond
+	// Events ready to be sent
+	EventsReady bool
+	// Backoff exponent (2^x secs). Max 6, aka 64s.
+	Backoff int
+}
+
+// NotifyNewEvents wakes up all waiting goroutines, notifying that events remain
+// in the event queue for this application service worker.
+func (a *ApplicationServiceWorkerState) NotifyNewEvents() {
+	a.Cond.L.Lock()
+	a.EventsReady = true
+	a.Cond.Broadcast()
+	a.Cond.L.Unlock()
+}
+
+// FinishEventProcessing marks all events of this worker as being sent to the
+// application service.
+func (a *ApplicationServiceWorkerState) FinishEventProcessing() {
+	a.Cond.L.Lock()
+	a.EventsReady = false
+	a.Cond.L.Unlock()
+}
+
+// WaitForNewEvents causes the calling goroutine to wait on the worker state's
+// condition for a broadcast or similar wakeup, if there are no events ready.
+func (a *ApplicationServiceWorkerState) WaitForNewEvents() {
+	a.Cond.L.Lock()
+	if !a.EventsReady {
+		a.Cond.Wait()
+	}
+	a.Cond.L.Unlock()
+}
diff --git a/appservice/workers/transaction_scheduler.go b/appservice/workers/transaction_scheduler.go
new file mode 100644
index 00000000..0330eb9e
--- /dev/null
+++ b/appservice/workers/transaction_scheduler.go
@@ -0,0 +1,227 @@
+// Copyright 2018 Vector Creations 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.
+
+package workers
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"math"
+	"net/http"
+	"time"
+
+	"github.com/matrix-org/dendrite/appservice/storage"
+	"github.com/matrix-org/dendrite/appservice/types"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/gomatrixserverlib"
+	log "github.com/sirupsen/logrus"
+)
+
+var (
+	// Maximum size of events sent in each transaction.
+	transactionBatchSize = 50
+	// Timeout for sending a single transaction to an application service.
+	transactionTimeout = time.Second * 60
+)
+
+// SetupTransactionWorkers spawns a separate goroutine for each application
+// service. Each of these "workers" handle taking all events intended for their
+// app service, batch them up into a single transaction (up to a max transaction
+// size), then send that off to the AS's /transactions/{txnID} endpoint. It also
+// handles exponentially backing off in case the AS isn't currently available.
+func SetupTransactionWorkers(
+	appserviceDB *storage.Database,
+	workerStates []types.ApplicationServiceWorkerState,
+) error {
+	// Create a worker that handles transmitting events to a single homeserver
+	for _, workerState := range workerStates {
+		// Don't create a worker if this AS doesn't want to receive events
+		if workerState.AppService.URL != "" {
+			go worker(appserviceDB, workerState)
+		}
+	}
+	return nil
+}
+
+// worker is a goroutine that sends any queued events to the application service
+// it is given.
+func worker(db *storage.Database, ws types.ApplicationServiceWorkerState) {
+	log.WithFields(log.Fields{
+		"appservice": ws.AppService.ID,
+	}).Info("starting application service")
+	ctx := context.Background()
+
+	// Create a HTTP client for sending requests to app services
+	client := &http.Client{
+		Timeout: transactionTimeout,
+	}
+
+	// Initial check for any leftover events to send from last time
+	eventCount, err := db.CountEventsWithAppServiceID(ctx, ws.AppService.ID)
+	if err != nil {
+		log.WithFields(log.Fields{
+			"appservice": ws.AppService.ID,
+		}).WithError(err).Fatal("appservice worker unable to read queued events from DB")
+		return
+	}
+	if eventCount > 0 {
+		ws.NotifyNewEvents()
+	}
+
+	// Loop forever and keep waiting for more events to send
+	for {
+		// Wait for more events if we've sent all the events in the database
+		ws.WaitForNewEvents()
+
+		// Batch events up into a transaction
+		transactionJSON, txnID, maxEventID, eventsRemaining, err := createTransaction(ctx, db, ws.AppService.ID)
+		if err != nil {
+			log.WithFields(log.Fields{
+				"appservice": ws.AppService.ID,
+			}).WithError(err).Fatal("appservice worker unable to create transaction")
+
+			return
+		}
+
+		// Send the events off to the application service
+		// Backoff if the application service does not respond
+		err = send(client, ws.AppService, txnID, transactionJSON)
+		if err != nil {
+			// Backoff
+			backoff(&ws, err)
+			continue
+		}
+
+		// We sent successfully, hooray!
+		ws.Backoff = 0
+
+		// Transactions have a maximum event size, so there may still be some events
+		// left over to send. Keep sending until none are left
+		if !eventsRemaining {
+			ws.FinishEventProcessing()
+		}
+
+		// Remove sent events from the DB
+		err = db.RemoveEventsBeforeAndIncludingID(ctx, ws.AppService.ID, maxEventID)
+		if err != nil {
+			log.WithFields(log.Fields{
+				"appservice": ws.AppService.ID,
+			}).WithError(err).Fatal("unable to remove appservice events from the database")
+			return
+		}
+	}
+}
+
+// backoff pauses the calling goroutine for a 2^some backoff exponent seconds
+func backoff(ws *types.ApplicationServiceWorkerState, err error) {
+	// Calculate how long to backoff for
+	backoffDuration := time.Duration(math.Pow(2, float64(ws.Backoff)))
+	backoffSeconds := time.Second * backoffDuration
+
+	log.WithFields(log.Fields{
+		"appservice": ws.AppService.ID,
+	}).WithError(err).Warnf("unable to send transactions successfully, backing off for %ds",
+		backoffDuration)
+
+	ws.Backoff++
+	if ws.Backoff > 6 {
+		ws.Backoff = 6
+	}
+
+	// Backoff
+	time.Sleep(backoffSeconds)
+}
+
+// createTransaction takes in a slice of AS events, stores them in an AS
+// transaction, and JSON-encodes the results.
+func createTransaction(
+	ctx context.Context,
+	db *storage.Database,
+	appserviceID string,
+) (
+	transactionJSON []byte,
+	txnID, maxID int,
+	eventsRemaining bool,
+	err error,
+) {
+	// Retrieve the latest events from the DB (will return old events if they weren't successfully sent)
+	txnID, maxID, events, eventsRemaining, err := db.GetEventsWithAppServiceID(ctx, appserviceID, transactionBatchSize)
+	if err != nil {
+		log.WithFields(log.Fields{
+			"appservice": appserviceID,
+		}).WithError(err).Fatalf("appservice worker unable to read queued events from DB")
+
+		return
+	}
+
+	// Check if these events do not already have a transaction ID
+	if txnID == -1 {
+		// If not, grab next available ID from the DB
+		txnID, err = db.GetLatestTxnID(ctx)
+		if err != nil {
+			return nil, 0, 0, false, err
+		}
+
+		// Mark new events with current transactionID
+		if err = db.UpdateTxnIDForEvents(ctx, appserviceID, maxID, txnID); err != nil {
+			return nil, 0, 0, false, err
+		}
+	}
+
+	// Create a transaction and store the events inside
+	transaction := gomatrixserverlib.ApplicationServiceTransaction{
+		Events: events,
+	}
+
+	transactionJSON, err = json.Marshal(transaction)
+	if err != nil {
+		return
+	}
+
+	return
+}
+
+// send sends events to an application service. Returns an error if an OK was not
+// received back from the application service or the request timed out.
+func send(
+	client *http.Client,
+	appservice config.ApplicationService,
+	txnID int,
+	transaction []byte,
+) error {
+	// POST a transaction to our AS
+	address := fmt.Sprintf("%s/transactions/%d", appservice.URL, txnID)
+	resp, err := client.Post(address, "application/json", bytes.NewBuffer(transaction))
+	if err != nil {
+		return err
+	}
+	defer func() {
+		err := resp.Body.Close()
+		if err != nil {
+			log.WithFields(log.Fields{
+				"appservice": appservice.ID,
+			}).WithError(err).Error("unable to close response body from application service")
+		}
+	}()
+
+	// Check the AS received the events correctly
+	if resp.StatusCode != http.StatusOK {
+		// TODO: Handle non-200 error codes from application services
+		return fmt.Errorf("non-OK status code %d returned from AS", resp.StatusCode)
+	}
+
+	return nil
+}