Add a component for sending event to remote matrix servers using fede… (#148)

* Add a component for sending event to remote matrix servers using federation

* Use named constants

* Comment on why we combine the deltas

* Comments

* Copyright headers!

* Fix the copy+Pasted comment

* Add tests for combineDeltas

* Review comments

* More review comments

* More comments

* Even more comments

* Name the mutexes

* Document what the mutexes are protecting

* Simplify the SQL, and more comments

src/github.com/matrix-org/dendrite/cmd/dendrite-federation-sender-server/main.go

@@ -0,0 +1,74 @@
+// Copyright 2017 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 main
+
+import (
+	"flag"
+	"net/http"
+	"os"
+
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/federationsender/consumers"
+	"github.com/matrix-org/dendrite/federationsender/queue"
+	"github.com/matrix-org/dendrite/federationsender/storage"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/prometheus/client_golang/prometheus"
+
+	log "github.com/Sirupsen/logrus"
+)
+
+var configPath = flag.String("config", "dendrite.yaml", "The path to the config file. For more information, see the config file in this repository.")
+
+func main() {
+	common.SetupLogging(os.Getenv("LOG_DIR"))
+
+	flag.Parse()
+
+	if *configPath == "" {
+		log.Fatal("--config must be supplied")
+	}
+	cfg, err := config.Load(*configPath)
+	if err != nil {
+		log.Fatalf("Invalid config file: %s", err)
+	}
+
+	log.Info("config: ", cfg)
+
+	db, err := storage.NewDatabase(string(cfg.Database.FederationSender))
+	if err != nil {
+		log.Panicf("startup: failed to create federation sender database with data source %s : %s", cfg.Database.FederationSender, err)
+	}
+
+	federation := gomatrixserverlib.NewFederationClient(
+		cfg.Matrix.ServerName, cfg.Matrix.KeyID, cfg.Matrix.PrivateKey,
+	)
+
+	queues := queue.NewOutgoingQueues(cfg.Matrix.ServerName, federation)
+
+	consumer, err := consumers.NewOutputRoomEvent(cfg, queues, db)
+	if err != nil {
+		log.WithError(err).Panicf("startup: failed to create room server consumer")
+	}
+	if err = consumer.Start(); err != nil {
+		log.WithError(err).Panicf("startup: failed to start room server consumer")
+	}
+
+	http.DefaultServeMux.Handle("/metrics", prometheus.Handler())
+
+	if err := http.ListenAndServe(string(cfg.Listen.FederationSender), nil); err != nil {
+		panic(err)
+	}
+}

src/github.com/matrix-org/dendrite/common/config/config.go

@@ -122,16 +122,20 @@ type Dendrite struct {
 		// The RoomServer database stores information about matrix rooms.
 		// It is only accessed by the RoomServer.
 		RoomServer DataSource `yaml:"room_server"`
+		// The FederationSender database stores information used by the FederationSender
+		// It is only accessed by the FederationSender.
+		FederationSender DataSource `yaml:"federation_sender"`
 	} `yaml:"database"`
 
 	// The internal addresses the components will listen on.
 	// These should not be exposed externally as they expose metrics and debugging APIs.
 	Listen struct {
-		MediaAPI      Address `yaml:"media_api"`
+		MediaAPI         Address `yaml:"media_api"`
-		ClientAPI     Address `yaml:"client_api"`
+		ClientAPI        Address `yaml:"client_api"`
-		FederationAPI Address `yaml:"federation_api"`
+		FederationAPI    Address `yaml:"federation_api"`
-		SyncAPI       Address `yaml:"sync_api"`
+		SyncAPI          Address `yaml:"sync_api"`
-		RoomServer    Address `yaml:"room_server"`
+		RoomServer       Address `yaml:"room_server"`
+		FederationSender Address `yaml:"federation_sender"`
 	} `yaml:"listen"`
 }
 

src/github.com/matrix-org/dendrite/federationsender/consumers/roomserver.go

@@ -0,0 +1,361 @@
+// Copyright 2017 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 (
+	"encoding/json"
+	"fmt"
+	"strings"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/federationsender/queue"
+	"github.com/matrix-org/dendrite/federationsender/storage"
+	"github.com/matrix-org/dendrite/federationsender/types"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
+	sarama "gopkg.in/Shopify/sarama.v1"
+)
+
+// OutputRoomEvent consumes events that originated in the room server.
+type OutputRoomEvent struct {
+	roomServerConsumer *common.ContinualConsumer
+	db                 *storage.Database
+	queues             *queue.OutgoingQueues
+	query              api.RoomserverQueryAPI
+}
+
+// NewOutputRoomEvent creates a new OutputRoomEvent consumer. Call Start() to begin consuming from room servers.
+func NewOutputRoomEvent(cfg *config.Dendrite, queues *queue.OutgoingQueues, store *storage.Database) (*OutputRoomEvent, error) {
+	kafkaConsumer, err := sarama.NewConsumer(cfg.Kafka.Addresses, nil)
+	if err != nil {
+		return nil, err
+	}
+	roomServerURL := cfg.RoomServerURL()
+
+	consumer := common.ContinualConsumer{
+		Topic:          string(cfg.Kafka.Topics.OutputRoomEvent),
+		Consumer:       kafkaConsumer,
+		PartitionStore: store,
+	}
+	s := &OutputRoomEvent{
+		roomServerConsumer: &consumer,
+		db:                 store,
+		queues:             queues,
+		query:              api.NewRoomserverQueryAPIHTTP(roomServerURL, nil),
+	}
+	consumer.ProcessMessage = s.onMessage
+
+	return s, nil
+}
+
+// Start consuming from room servers
+func (s *OutputRoomEvent) Start() error {
+	return s.roomServerConsumer.Start()
+}
+
+// onMessage is called when the federation server receives a new event from the room server output log.
+// It is unsafe to call this with messages for the same room in multiple gorountines
+// because updates it will likely fail with a types.EventIDMismatchError when it
+// realises that it cannot update the room state using the deltas.
+func (s *OutputRoomEvent) onMessage(msg *sarama.ConsumerMessage) error {
+	// Parse out the event JSON
+	var output api.OutputRoomEvent
+	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
+	}
+
+	ev, err := gomatrixserverlib.NewEventFromTrustedJSON(output.Event, false)
+	if err != nil {
+		log.WithError(err).Errorf("roomserver output log: event parse failure")
+		return nil
+	}
+	log.WithFields(log.Fields{
+		"event_id":       ev.EventID(),
+		"room_id":        ev.RoomID(),
+		"send_as_server": output.SendAsServer,
+	}).Info("received event from roomserver")
+
+	if err = s.processMessage(output, ev); err != nil {
+		// panic rather than continue with an inconsistent database
+		log.WithFields(log.Fields{
+			"event":      string(ev.JSON()),
+			log.ErrorKey: err,
+			"add":        output.AddsStateEventIDs,
+			"del":        output.RemovesStateEventIDs,
+		}).Panicf("roomserver output log: write event failure")
+		return nil
+	}
+
+	return nil
+}
+
+// processMessage updates the list of currently joined hosts in the room
+// and then sends the event to the hosts that were joined before the event.
+func (s *OutputRoomEvent) processMessage(ore api.OutputRoomEvent, ev gomatrixserverlib.Event) error {
+	addsStateEvents, err := s.lookupStateEvents(ore.AddsStateEventIDs, ev)
+	if err != nil {
+		return err
+	}
+	addsJoinedHosts, err := joinedHostsFromEvents(addsStateEvents)
+	if err != nil {
+		return err
+	}
+	// Update our copy of the current state.
+	// We keep a copy of the current state because the state at each event is
+	// expressed as a delta against the current state.
+	// TODO: handle EventIDMismatchError and recover the current state by talking
+	// to the roomserver
+	oldJoinedHosts, err := s.db.UpdateRoom(
+		ev.RoomID(), ore.LastSentEventID, ev.EventID(),
+		addsJoinedHosts, ore.RemovesStateEventIDs,
+	)
+	if err != nil {
+		return err
+	}
+
+	if ore.SendAsServer == api.DoNotSendToOtherServers {
+		// Ignore event that we don't need to send anywhere.
+		return nil
+	}
+
+	// Work out which hosts were joined at the event itself.
+	joinedHostsAtEvent, err := s.joinedHostsAtEvent(ore, ev, oldJoinedHosts)
+	if err != nil {
+		return err
+	}
+
+	// Send the event.
+	if err = s.queues.SendEvent(
+		&ev, gomatrixserverlib.ServerName(ore.SendAsServer), joinedHostsAtEvent,
+	); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// joinedHostsAtEvent works out a list of matrix servers that were joined to
+// the room at the event.
+// It is important to use the state at the event for sending messages because:
+//   1) We shouldn't send messages to servers that weren't in the room.
+//   2) If a server is kicked from the rooms it should still be told about the
+//      kick event,
+// Usually the list can be calculated locally, but sometimes it will need fetch
+// events from the room server.
+// Returns an error if there was a problem talking to the room server.
+func (s *OutputRoomEvent) joinedHostsAtEvent(
+	ore api.OutputRoomEvent, ev gomatrixserverlib.Event, oldJoinedHosts []types.JoinedHost,
+) ([]gomatrixserverlib.ServerName, error) {
+	// Combine the delta into a single delta so that the adds and removes can
+	// cancel each other out. This should reduce the number of times we need
+	// to fetch a state event from the room server.
+	combinedAdds, combinedRemoves := combineDeltas(
+		ore.AddsStateEventIDs, ore.RemovesStateEventIDs,
+		ore.StateBeforeAddsEventIDs, ore.StateBeforeRemovesEventIDs,
+	)
+	combinedAddsEvents, err := s.lookupStateEvents(combinedAdds, ev)
+	if err != nil {
+		return nil, err
+	}
+
+	combinedAddsJoinedHosts, err := joinedHostsFromEvents(combinedAddsEvents)
+	if err != nil {
+		return nil, err
+	}
+
+	removed := map[string]bool{}
+	for _, eventID := range combinedRemoves {
+		removed[eventID] = true
+	}
+
+	joined := map[gomatrixserverlib.ServerName]bool{}
+	for _, joinedHost := range oldJoinedHosts {
+		if removed[joinedHost.MemberEventID] {
+			// This m.room.member event is part of the current state of the
+			// room, but not part of the state at the event we are processing
+			// Therefore we can't use it to tell whether the server was in
+			// the room at the event.
+			continue
+		}
+		joined[joinedHost.ServerName] = true
+	}
+
+	for _, joinedHost := range combinedAddsJoinedHosts {
+		// This m.room.member event was part of the state of the room at the
+		// event, but isn't part of the current state of the room now.
+		joined[joinedHost.ServerName] = true
+	}
+
+	var result []gomatrixserverlib.ServerName
+	for serverName, include := range joined {
+		if include {
+			result = append(result, serverName)
+		}
+	}
+	return result, nil
+}
+
+// joinedHostsFromEvents turns a list of state events into a list of joined hosts.
+// This errors if one of the events was invalid.
+// It should be impossible for an invalid event to get this far in the pipeline.
+func joinedHostsFromEvents(evs []gomatrixserverlib.Event) ([]types.JoinedHost, error) {
+	var joinedHosts []types.JoinedHost
+	for _, ev := range evs {
+		if ev.Type() != "m.room.member" || ev.StateKey() == nil {
+			continue
+		}
+		var content struct {
+			Membership string `json:"membership"`
+		}
+		if err := json.Unmarshal(ev.Content(), &content); err != nil {
+			return nil, err
+		}
+		if content.Membership != "join" {
+			continue
+		}
+		serverName, err := domainFromID(*ev.StateKey())
+		if err != nil {
+			return nil, err
+		}
+		joinedHosts = append(joinedHosts, types.JoinedHost{
+			MemberEventID: ev.EventID(), ServerName: serverName,
+		})
+	}
+	return joinedHosts, nil
+}
+
+// combineDeltas combines two deltas into a single delta.
+// Assumes that the order of operations is add(1), remove(1), add(2), remove(2).
+// Removes duplicate entries and redundant operations from each delta.
+func combineDeltas(adds1, removes1, adds2, removes2 []string) (adds, removes []string) {
+	addSet := map[string]bool{}
+	removeSet := map[string]bool{}
+
+	// combine processes each unique value in a list.
+	// If the value is in the removeFrom set then it is removed from that set.
+	// Otherwise it is added to the addTo set.
+	combine := func(values []string, removeFrom, addTo map[string]bool) {
+		processed := map[string]bool{}
+		for _, value := range values {
+			if processed[value] {
+				continue
+			}
+			processed[value] = true
+			if removeFrom[value] {
+				delete(removeFrom, value)
+			} else {
+				addTo[value] = true
+			}
+		}
+	}
+
+	combine(adds1, nil, addSet)
+	combine(removes1, addSet, removeSet)
+	combine(adds2, removeSet, addSet)
+	combine(removes2, addSet, removeSet)
+
+	for value := range addSet {
+		adds = append(adds, value)
+	}
+	for value := range removeSet {
+		removes = append(removes, value)
+	}
+	return
+}
+
+// lookupStateEvents looks up the state events that are added by a new event.
+func (s *OutputRoomEvent) lookupStateEvents(
+	addsStateEventIDs []string, event gomatrixserverlib.Event,
+) ([]gomatrixserverlib.Event, error) {
+	// Fast path if there aren't any new state events.
+	if len(addsStateEventIDs) == 0 {
+		return nil, nil
+	}
+
+	// Fast path if the only state event added is the event itself.
+	if len(addsStateEventIDs) == 1 && addsStateEventIDs[0] == event.EventID() {
+		return []gomatrixserverlib.Event{event}, nil
+	}
+
+	missing := addsStateEventIDs
+	var result []gomatrixserverlib.Event
+
+	// Check if event itself is being added.
+	for _, eventID := range missing {
+		if eventID == event.EventID() {
+			result = append(result, event)
+			break
+		}
+	}
+	missing = missingEventsFrom(result, addsStateEventIDs)
+
+	if len(missing) == 0 {
+		return result, nil
+	}
+
+	// At this point the missing events are neither the event itself nor are
+	// they present in our local database. Our only option is to fetch them
+	// from the roomserver using the query API.
+	eventReq := api.QueryEventsByIDRequest{EventIDs: missing}
+	var eventResp api.QueryEventsByIDResponse
+	if err := s.query.QueryEventsByID(&eventReq, &eventResp); err != nil {
+		return nil, err
+	}
+
+	result = append(result, eventResp.Events...)
+	missing = missingEventsFrom(result, addsStateEventIDs)
+
+	if len(missing) != 0 {
+		return nil, fmt.Errorf(
+			"missing %d state events IDs at event %q", len(missing), event.EventID(),
+		)
+	}
+
+	return result, nil
+}
+
+func missingEventsFrom(events []gomatrixserverlib.Event, required []string) []string {
+	have := map[string]bool{}
+	for _, event := range events {
+		have[event.EventID()] = true
+	}
+	var missing []string
+	for _, eventID := range required {
+		if !have[eventID] {
+			missing = append(missing, eventID)
+		}
+	}
+	return missing
+}
+
+// domainFromID returns everything after the first ":" character to extract
+// the domain part of a matrix ID.
+// TODO: duplicated from gomatrixserverlib.
+func domainFromID(id string) (gomatrixserverlib.ServerName, error) {
+	// IDs have the format: SIGIL LOCALPART ":" DOMAIN
+	// Split on the first ":" character since the domain can contain ":"
+	// characters.
+	parts := strings.SplitN(id, ":", 2)
+	if len(parts) != 2 {
+		// The ID must have a ":" character.
+		return "", fmt.Errorf("invalid ID: %q", id)
+	}
+	// Return everything after the first ":" character.
+	return gomatrixserverlib.ServerName(parts[1]), nil
+}

src/github.com/matrix-org/dendrite/federationsender/consumers/roomserver_test.go

@@ -0,0 +1,53 @@
+// Copyright 2017 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 (
+	"testing"
+)
+
+func TestCombineNoOp(t *testing.T) {
+	inputAdd1 := []string{"a", "b", "c"}
+	inputDel1 := []string{"a", "b", "d"}
+	inputAdd2 := []string{"a", "d", "e"}
+	inputDel2 := []string{"a", "c", "e", "e"}
+
+	gotAdd, gotDel := combineDeltas(inputAdd1, inputDel1, inputAdd2, inputDel2)
+
+	if len(gotAdd) != 0 {
+		t.Errorf("wanted combined adds to be an empty list, got %#v", gotAdd)
+	}
+
+	if len(gotDel) != 0 {
+		t.Errorf("wanted combined removes to be an empty list, got %#v", gotDel)
+	}
+}
+
+func TestCombineDedup(t *testing.T) {
+	inputAdd1 := []string{"a", "a"}
+	inputDel1 := []string{"b", "b"}
+	inputAdd2 := []string{"a", "a"}
+	inputDel2 := []string{"b", "b"}
+
+	gotAdd, gotDel := combineDeltas(inputAdd1, inputDel1, inputAdd2, inputDel2)
+
+	if len(gotAdd) != 1 || gotAdd[0] != "a" {
+		t.Errorf("wanted combined adds to be %#v, got %#v", []string{"a"}, gotAdd)
+	}
+
+	if len(gotDel) != 1 || gotDel[0] != "b" {
+		t.Errorf("wanted combined removes to be %#v, got %#v", []string{"b"}, gotDel)
+	}
+}

src/github.com/matrix-org/dendrite/federationsender/queue/destinationqueue.go

@@ -0,0 +1,105 @@
+// Copyright 2017 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 queue
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// destinationQueue is a queue of events for a single destination.
+// It is responsible for sending the events to the destination and
+// ensures that only one request is in flight to a given destination
+// at a time.
+type destinationQueue struct {
+	client      *gomatrixserverlib.FederationClient
+	origin      gomatrixserverlib.ServerName
+	destination gomatrixserverlib.ServerName
+	// The running mutex protects running, sentCounter, lastTransactionIDs and
+	// pendingEvents.
+	runningMutex       sync.Mutex
+	running            bool
+	sentCounter        int
+	lastTransactionIDs []gomatrixserverlib.TransactionID
+	pendingEvents      []*gomatrixserverlib.Event
+}
+
+// Send event adds the event to the pending queue for the destination.
+// If the queue is empty then it starts a background goroutine to
+// start sending events to that destination.
+func (oq *destinationQueue) sendEvent(ev *gomatrixserverlib.Event) {
+	oq.runningMutex.Lock()
+	defer oq.runningMutex.Unlock()
+	oq.pendingEvents = append(oq.pendingEvents, ev)
+	if !oq.running {
+		oq.running = true
+		go oq.backgroundSend()
+	}
+}
+
+func (oq *destinationQueue) backgroundSend() {
+	for {
+		t := oq.next()
+		if t == nil {
+			// If the queue is empty then stop processing for this destination.
+			// TODO: Remove this destination from the queue map.
+			return
+		}
+
+		// TODO: handle retries.
+		// TODO: blacklist uncooperative servers.
+
+		_, err := oq.client.SendTransaction(*t)
+		if err != nil {
+			log.WithFields(log.Fields{
+				"destination": oq.destination,
+				log.ErrorKey:  err,
+			}).Info("problem sending transaction")
+		}
+	}
+}
+
+// next creates a new transaction from the pending event queue
+// and flushes the queue.
+// Returns nil if the queue was empty.
+func (oq *destinationQueue) next() *gomatrixserverlib.Transaction {
+	oq.runningMutex.Lock()
+	defer oq.runningMutex.Unlock()
+	if len(oq.pendingEvents) == 0 {
+		oq.running = false
+		return nil
+	}
+	var t gomatrixserverlib.Transaction
+	now := gomatrixserverlib.AsTimestamp(time.Now())
+	t.TransactionID = gomatrixserverlib.TransactionID(fmt.Sprintf("%d-%d", now, oq.sentCounter))
+	t.Origin = oq.origin
+	t.Destination = oq.destination
+	t.OriginServerTS = now
+	t.PreviousIDs = oq.lastTransactionIDs
+	if t.PreviousIDs == nil {
+		t.PreviousIDs = []gomatrixserverlib.TransactionID{}
+	}
+	oq.lastTransactionIDs = []gomatrixserverlib.TransactionID{t.TransactionID}
+	for _, pdu := range oq.pendingEvents {
+		t.PDUs = append(t.PDUs, *pdu)
+	}
+	oq.pendingEvents = nil
+	oq.sentCounter += len(t.PDUs)
+	return &t
+}

src/github.com/matrix-org/dendrite/federationsender/queue/queue.go

@@ -0,0 +1,95 @@
+// Copyright 2017 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 queue
+
+import (
+	"fmt"
+	"sync"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// OutgoingQueues is a collection of queues for sending transactions to other
+// matrix servers
+type OutgoingQueues struct {
+	origin gomatrixserverlib.ServerName
+	client *gomatrixserverlib.FederationClient
+	// The queuesMutex protects queues
+	queuesMutex sync.Mutex
+	queues      map[gomatrixserverlib.ServerName]*destinationQueue
+}
+
+// NewOutgoingQueues makes a new OutgoingQueues
+func NewOutgoingQueues(origin gomatrixserverlib.ServerName, client *gomatrixserverlib.FederationClient) *OutgoingQueues {
+	return &OutgoingQueues{
+		origin: origin,
+		client: client,
+		queues: map[gomatrixserverlib.ServerName]*destinationQueue{},
+	}
+}
+
+// SendEvent sends an event to the destinations
+func (oqs *OutgoingQueues) SendEvent(
+	ev *gomatrixserverlib.Event, origin gomatrixserverlib.ServerName,
+	destinations []gomatrixserverlib.ServerName,
+) error {
+	if origin != oqs.origin {
+		// TODO: Support virtual hosting by allowing us to send events using
+		// different origin server names.
+		// For now assume we are always asked to send as the single server configured
+		// in the dendrite config.
+		return fmt.Errorf(
+			"sendevent: unexpected server to send as: got %q expected %q",
+			origin, oqs.origin,
+		)
+	}
+
+	// Remove our own server from the list of destinations.
+	destinations = filterDestinations(oqs.origin, destinations)
+
+	log.WithFields(log.Fields{
+		"destinations": destinations, "event": ev.EventID(),
+	}).Info("Sending event")
+
+	oqs.queuesMutex.Lock()
+	defer oqs.queuesMutex.Unlock()
+	for _, destination := range destinations {
+		oq := oqs.queues[destination]
+		if oq == nil {
+			oq = &destinationQueue{
+				origin:      oqs.origin,
+				destination: destination,
+				client:      oqs.client,
+			}
+			oqs.queues[destination] = oq
+		}
+		oq.sendEvent(ev)
+	}
+	return nil
+}
+
+// filterDestinations removes our own server from the list of destinations.
+// Otherwise we could end up trying to talk to ourselves.
+func filterDestinations(origin gomatrixserverlib.ServerName, destinations []gomatrixserverlib.ServerName) []gomatrixserverlib.ServerName {
+	var result []gomatrixserverlib.ServerName
+	for _, destination := range destinations {
+		if destination == origin {
+			continue
+		}
+		result = append(result, destination)
+	}
+	return result
+}

src/github.com/matrix-org/dendrite/federationsender/storage/joined_hosts_table.go

@@ -0,0 +1,111 @@
+// Copyright 2017 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 storage
+
+import (
+	"database/sql"
+
+	"github.com/lib/pq"
+	"github.com/matrix-org/dendrite/federationsender/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+const joinedHostsSchema = `
+-- The joined_hosts table stores a list of m.room.member event ids in the
+-- current state for each room where the membership is "join".
+-- There will be an entry for every user that is joined to the room.
+CREATE TABLE IF NOT EXISTS joined_hosts (
+    -- The string ID of the room.
+    room_id TEXT NOT NULL,
+    -- The event ID of the m.room.member join event.
+    event_id TEXT NOT NULL,
+    -- The domain part of the user ID the m.room.member event is for.
+    server_name TEXT NOT NULL
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS joined_hosts_event_id_idx
+    ON joined_hosts (event_id);
+
+CREATE INDEX IF NOT EXISTS joined_hosts_room_id_idx
+    ON joined_hosts (room_id)
+`
+
+const insertJoinedHostsSQL = "" +
+	"INSERT INTO joined_hosts (room_id, event_id, server_name)" +
+	" VALUES ($1, $2, $3)"
+
+const deleteJoinedHostsSQL = "" +
+	"DELETE FROM joined_hosts WHERE event_id = ANY($1)"
+
+const selectJoinedHostsSQL = "" +
+	"SELECT event_id, server_name FROM joined_hosts" +
+	" WHERE room_id = $1"
+
+type joinedHostsStatements struct {
+	insertJoinedHostsStmt *sql.Stmt
+	deleteJoinedHostsStmt *sql.Stmt
+	selectJoinedHostsStmt *sql.Stmt
+}
+
+func (s *joinedHostsStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(joinedHostsSchema)
+	if err != nil {
+		return
+	}
+	if s.insertJoinedHostsStmt, err = db.Prepare(insertJoinedHostsSQL); err != nil {
+		return
+	}
+	if s.deleteJoinedHostsStmt, err = db.Prepare(deleteJoinedHostsSQL); err != nil {
+		return
+	}
+	if s.selectJoinedHostsStmt, err = db.Prepare(selectJoinedHostsSQL); err != nil {
+		return
+	}
+	return
+}
+
+func (s *joinedHostsStatements) insertJoinedHosts(
+	txn *sql.Tx, roomID, eventID string, serverName gomatrixserverlib.ServerName,
+) error {
+	_, err := txn.Stmt(s.insertJoinedHostsStmt).Exec(roomID, eventID, serverName)
+	return err
+}
+
+func (s *joinedHostsStatements) deleteJoinedHosts(txn *sql.Tx, eventIDs []string) error {
+	_, err := txn.Stmt(s.deleteJoinedHostsStmt).Exec(pq.StringArray(eventIDs))
+	return err
+}
+
+func (s *joinedHostsStatements) selectJoinedHosts(txn *sql.Tx, roomID string,
+) ([]types.JoinedHost, error) {
+	rows, err := txn.Stmt(s.selectJoinedHostsStmt).Query(roomID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var result []types.JoinedHost
+	for rows.Next() {
+		var eventID, serverName string
+		if err = rows.Scan(&eventID, &serverName); err != nil {
+			return nil, err
+		}
+		result = append(result, types.JoinedHost{
+			MemberEventID: eventID,
+			ServerName:    gomatrixserverlib.ServerName(serverName),
+		})
+	}
+	return result, nil
+}

src/github.com/matrix-org/dendrite/federationsender/storage/room_table.go

@@ -0,0 +1,89 @@
+// Copyright 2017 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 storage
+
+import (
+	"database/sql"
+)
+
+const roomSchema = `
+CREATE TABLE IF NOT EXISTS rooms (
+    -- The string ID of the room
+    room_id TEXT PRIMARY KEY,
+    -- The most recent event state by the room server.
+    -- We can use this to tell if our view of the room state has become
+    -- desynchronised.
+    last_event_id TEXT NOT NULL
+);`
+
+const insertRoomSQL = "" +
+	"INSERT INTO rooms (room_id, last_event_id) VALUES ($1, '')" +
+	" ON CONFLICT DO NOTHING"
+
+const selectRoomForUpdateSQL = "" +
+	"SELECT last_event_id FROM rooms WHERE room_id = $1 FOR UPDATE"
+
+const updateRoomSQL = "" +
+	"UPDATE rooms SET last_event_id = $2 WHERE room_id = $1"
+
+type roomStatements struct {
+	insertRoomStmt          *sql.Stmt
+	selectRoomForUpdateStmt *sql.Stmt
+	updateRoomStmt          *sql.Stmt
+}
+
+func (s *roomStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(roomSchema)
+	if err != nil {
+		return
+	}
+
+	if s.insertRoomStmt, err = db.Prepare(insertRoomSQL); err != nil {
+		return
+	}
+	if s.selectRoomForUpdateStmt, err = db.Prepare(selectRoomForUpdateSQL); err != nil {
+		return
+	}
+	if s.updateRoomStmt, err = db.Prepare(updateRoomSQL); err != nil {
+		return
+	}
+	return
+}
+
+// insertRoom inserts the room if it didn't already exist.
+// If the room didn't exist then last_event_id is set to the empty string.
+func (s *roomStatements) insertRoom(txn *sql.Tx, roomID string) error {
+	_, err := txn.Stmt(s.insertRoomStmt).Exec(roomID)
+	return err
+}
+
+// selectRoomForUpdate locks the row for the room and returns the last_event_id.
+// The row must already exist in the table. Callers can ensure that the row
+// exists by calling insertRoom first.
+func (s *roomStatements) selectRoomForUpdate(txn *sql.Tx, roomID string) (string, error) {
+	var lastEventID string
+	err := txn.Stmt(s.selectRoomForUpdateStmt).QueryRow(roomID).Scan(&lastEventID)
+	if err != nil {
+		return "", err
+	}
+	return lastEventID, nil
+}
+
+// updateRoom updates the last_event_id for the room. selectRoomForUpdate should
+// have already been called earlier within the transaction.
+func (s *roomStatements) updateRoom(txn *sql.Tx, roomID, lastEventID string) error {
+	_, err := txn.Stmt(s.updateRoomStmt).Exec(roomID, lastEventID)
+	return err
+}

src/github.com/matrix-org/dendrite/federationsender/storage/storage.go

@@ -0,0 +1,126 @@
+// Copyright 2017 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 storage
+
+import (
+	"database/sql"
+
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/federationsender/types"
+)
+
+// Database stores information needed by the federation sender
+type Database struct {
+	joinedHostsStatements
+	roomStatements
+	common.PartitionOffsetStatements
+	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 {
+	var err error
+
+	if err = d.joinedHostsStatements.prepare(d.db); err != nil {
+		return err
+	}
+
+	if err = d.roomStatements.prepare(d.db); err != nil {
+		return err
+	}
+
+	if err = d.PartitionOffsetStatements.Prepare(d.db); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// PartitionOffsets implements common.PartitionStorer
+func (d *Database) PartitionOffsets(topic string) ([]common.PartitionOffset, error) {
+	return d.SelectPartitionOffsets(topic)
+}
+
+// SetPartitionOffset implements common.PartitionStorer
+func (d *Database) SetPartitionOffset(topic string, partition int32, offset int64) error {
+	return d.UpsertPartitionOffset(topic, partition, offset)
+}
+
+// UpdateRoom updates the joined hosts for a room and returns what the joined
+// hosts were before the update.
+func (d *Database) UpdateRoom(
+	roomID, oldEventID, newEventID string,
+	addHosts []types.JoinedHost,
+	removeHosts []string,
+) (joinedHosts []types.JoinedHost, err error) {
+	err = runTransaction(d.db, func(txn *sql.Tx) error {
+		if err = d.insertRoom(txn, roomID); err != nil {
+			return err
+		}
+		lastSentEventID, err := d.selectRoomForUpdate(txn, roomID)
+		if err != nil {
+			return err
+		}
+		if lastSentEventID != oldEventID {
+			return types.EventIDMismatchError{lastSentEventID, oldEventID}
+		}
+		joinedHosts, err = d.selectJoinedHosts(txn, roomID)
+		if err != nil {
+			return err
+		}
+		for _, add := range addHosts {
+			err = d.insertJoinedHosts(txn, roomID, add.MemberEventID, add.ServerName)
+			if err != nil {
+				return err
+			}
+		}
+		if err = d.deleteJoinedHosts(txn, removeHosts); err != nil {
+			return err
+		}
+		return d.updateRoom(txn, roomID, newEventID)
+	})
+	return
+}
+
+func runTransaction(db *sql.DB, fn func(txn *sql.Tx) error) (err error) {
+	txn, err := db.Begin()
+	if err != nil {
+		return
+	}
+	defer func() {
+		if r := recover(); r != nil {
+			txn.Rollback()
+			panic(r)
+		} else if err != nil {
+			txn.Rollback()
+		} else {
+			err = txn.Commit()
+		}
+	}()
+	err = fn(txn)
+	return
+}

src/github.com/matrix-org/dendrite/federationsender/types/types.go

@@ -0,0 +1,45 @@
+// Copyright 2017 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 types
+
+import (
+	"fmt"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// A JoinedHost is a server that is joined to a matrix room.
+type JoinedHost struct {
+	// The MemberEventID of a m.room.member join event.
+	MemberEventID string
+	// The domain part of the state key of the m.room.member join event
+	ServerName gomatrixserverlib.ServerName
+}
+
+// A EventIDMismatchError indicates that we have got out of sync with the
+// room server.
+type EventIDMismatchError struct {
+	// The event ID we have stored in our local database.
+	DatabaseID string
+	// The event ID received from the room server.
+	RoomServerID string
+}
+
+func (e EventIDMismatchError) Error() string {
+	return fmt.Sprintf(
+		"mismatched last sent event ID: had %q in database got %q from room server",
+		e.DatabaseID, e.RoomServerID,
+	)
+}