Add tables for tracking the state of invites to the room server. (#165)

* Storage functions for invite events

* Add table for tracking membership state

* More stuff

* More stuff

* Use utility methods from gomatrixserverlib, rather than reimplementing them

* More stuff

* Return string rather than pointer to string

* Update gomatrixserverlib

* Use HTTP API for roomserver input.

* Use synchronous HTTP API for writing events to the roomserver

* Remove unused config for kafka topic

* Add new output types to roomserver for invites

* Write membership updates

* Separate filtering from pairing up changes in membershipChanges

* Fix SQL

* Fix SQL

* Namespace the tables

* Fix SQL

* Use clearer names for some of the variables

* Rename senderID for consistency

* Restructure update membership

* Comments

* More comment

* Fix SQL

* More comments

* Assign state keys inside the transaction

* Comment on the purpose of the latestEventsUpdater

* Comment on the purpose of updateMembership

* Remove duplicate fields from stateChange

* Attempt to rewrite comment in 'english'

* More comments

* Fix comment

* Comment

* more comments

src/github.com/matrix-org/dendrite/roomserver/api/output.go

@@ -21,8 +21,14 @@ import (
 // An OutputType is a type of roomserver output.
 type OutputType string
 
-// OutputTypeNewRoomEvent indicates that the event is an OutputNewRoomEvent
+const (
-const OutputTypeNewRoomEvent OutputType = "new_room_event"
+	// OutputTypeNewRoomEvent indicates that the event is an OutputNewRoomEvent
+	OutputTypeNewRoomEvent OutputType = "new_room_event"
+	// OutputTypeNewInviteEvent indicates that the event is an OutputNewInviteEvent
+	OutputTypeNewInviteEvent OutputType = "new_invite_event"
+	// OutputTypeRetireInviteEvent indicates that the event is an OutputRetireInviteEvent
+	OutputTypeRetireInviteEvent OutputType = "retire_invite_event"
+)
 
 // An OutputEvent is an entry in the roomserver output kafka log.
 // Consumers should check the type field when consuming this event.
@@ -31,6 +37,10 @@ type OutputEvent struct {
 	Type OutputType `json:"type"`
 	// The content of event with type OutputTypeNewRoomEvent
 	NewRoomEvent *OutputNewRoomEvent `json:"new_room_event,omitempty"`
+	// The content of event with type OutputTypeNewInviteEvent
+	NewInviteEvent *OutputNewInviteEvent `json:"new_invite_event,omitempty"`
+	// The content of event with type OutputTypeRetireInviteEvent
+	RetireInviteEvent *OutputRetireInviteEvent `json:"retire_invite_event,omitempty"`
 }
 
 // An OutputNewRoomEvent is written when the roomserver receives a new event.
@@ -98,3 +108,26 @@ type OutputNewRoomEvent struct {
 	// future proof the API for virtual hosting.
 	SendAsServer string `json:"send_as_server"`
 }
+
+// An OutputNewInviteEvent is written whenever an invite becomes active.
+// Invite events can be received outside of an existing room so have to be
+// tracked separately from the room events themselves.
+type OutputNewInviteEvent struct {
+	// The "m.room.member" invite event.
+	Event gomatrixserverlib.Event `json:"event"`
+}
+
+// An OutputRetireInviteEvent is written whenever an existing invite is no longer
+// active. An invite stops being active if the user joins the room or if the
+// invite is rejected by the user.
+type OutputRetireInviteEvent struct {
+	// The ID of the "m.room.member" invite event.
+	EventID string
+	// Optional event ID of the event that replaced the invite.
+	// This can be empty if the invite was rejected locally and we were unable
+	// to reach the server that originally sent the invite.
+	RetiredByEventID string
+	// The "membership" of the user after retiring the invite. One of "join"
+	// "leave" or "ban".
+	Membership string
+}

src/github.com/matrix-org/dendrite/roomserver/input/events.go

@@ -43,8 +43,8 @@ type RoomEventDatabase interface {
 
 // OutputRoomEventWriter has the APIs needed to write an event to the output logs.
 type OutputRoomEventWriter interface {
-	// Write an event.
+	// Write a list of events for a room
-	WriteOutputRoomEvent(output api.OutputNewRoomEvent) error
+	WriteOutputEvents(roomID string, updates []api.OutputEvent) error
 }
 
 func processRoomEvent(db RoomEventDatabase, ow OutputRoomEventWriter, input api.InputRoomEvent) error {

src/github.com/matrix-org/dendrite/roomserver/input/input.go

@@ -46,22 +46,21 @@ type RoomserverInputAPI struct {
 	processed int64
 }
 
-// WriteOutputRoomEvent implements OutputRoomEventWriter
+// WriteOutputEvents implements OutputRoomEventWriter
-func (r *RoomserverInputAPI) WriteOutputRoomEvent(output api.OutputNewRoomEvent) error {
+func (r *RoomserverInputAPI) WriteOutputEvents(roomID string, updates []api.OutputEvent) error {
-	var m sarama.ProducerMessage
+	messages := make([]*sarama.ProducerMessage, len(updates))
-	oe := api.OutputEvent{
+	for i := range updates {
-		Type:         api.OutputTypeNewRoomEvent,
+		value, err := json.Marshal(updates[i])
-		NewRoomEvent: &output,
-	}
-	value, err := json.Marshal(oe)
 		if err != nil {
 			return err
 		}
-	m.Topic = r.OutputRoomEventTopic
+		messages[i] = &sarama.ProducerMessage{
-	m.Key = sarama.StringEncoder("")
+			Topic: r.OutputRoomEventTopic,
-	m.Value = sarama.ByteEncoder(value)
+			Key:   sarama.StringEncoder(roomID),
-	_, _, err = r.Producer.SendMessage(&m)
+			Value: sarama.ByteEncoder(value),
-	return err
+		}
+	}
+	return r.Producer.SendMessages(messages)
 }
 
 // InputRoomEvents implements api.RoomserverInputAPI

src/github.com/matrix-org/dendrite/roomserver/input/latest_events.go

@@ -66,69 +66,88 @@ func updateLatestEvents(
 		}
 	}()
 
-	err = doUpdateLatestEvents(db, updater, ow, roomNID, stateAtEvent, event, sendAsServer)
+	u := latestEventsUpdater{
-	return
+		db: db, updater: updater, ow: ow, roomNID: roomNID,
+		stateAtEvent: stateAtEvent, event: event, sendAsServer: sendAsServer,
+	}
+	return u.doUpdateLatestEvents()
 }
 
-func doUpdateLatestEvents(
+// latestEventsUpdater tracks the state used to update the latest events in the
-	db RoomEventDatabase,
+// room. It mostly just ferries state between the various function calls.
-	updater types.RoomRecentEventsUpdater,
+// The state could be passed using function arguments, but it becomes impractical
-	ow OutputRoomEventWriter,
+// when there are so many variables to pass around.
-	roomNID types.RoomNID,
+type latestEventsUpdater struct {
-	stateAtEvent types.StateAtEvent,
+	db           RoomEventDatabase
-	event gomatrixserverlib.Event,
+	updater      types.RoomRecentEventsUpdater
-	sendAsServer string,
+	ow           OutputRoomEventWriter
-) error {
+	roomNID      types.RoomNID
+	stateAtEvent types.StateAtEvent
+	event        gomatrixserverlib.Event
+	// Which server to send this event as.
+	sendAsServer string
+	// The eventID of the event that was processed before this one.
+	lastEventIDSent string
+	// The latest events in the room after processing this event.
+	latest []types.StateAtEventAndReference
+	// The state entries removed from and added to the current state of the
+	// room as a result of processing this event. They are sorted lists.
+	removed []types.StateEntry
+	added   []types.StateEntry
+	// The state entries that are removed and added to recover the state before
+	// the event being processed. They are sorted lists.
+	stateBeforeEventRemoves []types.StateEntry
+	stateBeforeEventAdds    []types.StateEntry
+	// The snapshots of current state before and after processing this event
+	oldStateNID types.StateSnapshotNID
+	newStateNID types.StateSnapshotNID
+}
+
+func (u *latestEventsUpdater) doUpdateLatestEvents() error {
 	var err error
 	var prevEvents []gomatrixserverlib.EventReference
-	prevEvents = event.PrevEvents()
+	prevEvents = u.event.PrevEvents()
-	oldLatest := updater.LatestEvents()
+	oldLatest := u.updater.LatestEvents()
-	lastEventIDSent := updater.LastEventIDSent()
+	u.lastEventIDSent = u.updater.LastEventIDSent()
-	oldStateNID := updater.CurrentStateSnapshotNID()
+	u.oldStateNID = u.updater.CurrentStateSnapshotNID()
 
-	if hasBeenSent, err := updater.HasEventBeenSent(stateAtEvent.EventNID); err != nil {
+	if hasBeenSent, err := u.updater.HasEventBeenSent(u.stateAtEvent.EventNID); err != nil {
 		return err
 	} else if hasBeenSent {
 		// Already sent this event so we can stop processing
 		return nil
 	}
 
-	if err = updater.StorePreviousEvents(stateAtEvent.EventNID, prevEvents); err != nil {
+	if err = u.updater.StorePreviousEvents(u.stateAtEvent.EventNID, prevEvents); err != nil {
 		return err
 	}
 
-	eventReference := event.EventReference()
+	eventReference := u.event.EventReference()
 	// Check if this event is already referenced by another event in the room.
 	var alreadyReferenced bool
-	if alreadyReferenced, err = updater.IsReferenced(eventReference); err != nil {
+	if alreadyReferenced, err = u.updater.IsReferenced(eventReference); err != nil {
 		return err
 	}
 
-	newLatest := calculateLatest(oldLatest, alreadyReferenced, prevEvents, types.StateAtEventAndReference{
+	u.latest = calculateLatest(oldLatest, alreadyReferenced, prevEvents, types.StateAtEventAndReference{
 		EventReference: eventReference,
-		StateAtEvent:   stateAtEvent,
+		StateAtEvent:   u.stateAtEvent,
 	})
 
-	latestStateAtEvents := make([]types.StateAtEvent, len(newLatest))
+	if err = u.latestState(); err != nil {
-	for i := range newLatest {
+		return err
-		latestStateAtEvents[i] = newLatest[i].StateAtEvent
 	}
-	newStateNID, err := state.CalculateAndStoreStateAfterEvents(db, roomNID, latestStateAtEvents)
+
+	updates, err := updateMemberships(u.db, u.updater, u.removed, u.added)
 	if err != nil {
 		return err
 	}
 
-	removed, added, err := state.DifferenceBetweeenStateSnapshots(db, oldStateNID, newStateNID)
+	update, err := u.makeOutputNewRoomEvent()
-	if err != nil {
-		return err
-	}
-
-	stateBeforeEventRemoves, stateBeforeEventAdds, err := state.DifferenceBetweeenStateSnapshots(
-		db, newStateNID, stateAtEvent.BeforeStateSnapshotNID,
-	)
 	if err != nil {
 		return err
 	}
+	updates = append(updates, *update)
 
 	// Send the event to the output logs.
 	// We do this inside the database transaction to ensure that we only mark an event as sent if we sent it.
@@ -138,24 +157,47 @@ func doUpdateLatestEvents(
 	// send the event asynchronously but we would need to ensure that 1) the events are written to the log in
 	// the correct order, 2) that pending writes are resent across restarts. In order to avoid writing all the
 	// necessary bookkeeping we'll keep the event sending synchronous for now.
-	if err = writeEvent(
+	if err = u.ow.WriteOutputEvents(u.event.RoomID(), updates); err != nil {
-		db, ow, lastEventIDSent, event, newLatest, removed, added,
-		stateBeforeEventRemoves, stateBeforeEventAdds, sendAsServer,
-	); err != nil {
 		return err
 	}
 
-	if err = updater.SetLatestEvents(roomNID, newLatest, stateAtEvent.EventNID, newStateNID); err != nil {
+	if err = u.updater.SetLatestEvents(u.roomNID, u.latest, u.stateAtEvent.EventNID, u.newStateNID); err != nil {
 		return err
 	}
 
-	if err = updater.MarkEventAsSent(stateAtEvent.EventNID); err != nil {
+	if err = u.updater.MarkEventAsSent(u.stateAtEvent.EventNID); err != nil {
 		return err
 	}
 
 	return nil
 }
 
+func (u *latestEventsUpdater) latestState() error {
+	var err error
+
+	latestStateAtEvents := make([]types.StateAtEvent, len(u.latest))
+	for i := range u.latest {
+		latestStateAtEvents[i] = u.latest[i].StateAtEvent
+	}
+	u.newStateNID, err = state.CalculateAndStoreStateAfterEvents(u.db, u.roomNID, latestStateAtEvents)
+	if err != nil {
+		return err
+	}
+
+	u.removed, u.added, err = state.DifferenceBetweeenStateSnapshots(u.db, u.oldStateNID, u.newStateNID)
+	if err != nil {
+		return err
+	}
+
+	u.stateBeforeEventRemoves, u.stateBeforeEventAdds, err = state.DifferenceBetweeenStateSnapshots(
+		u.db, u.newStateNID, u.stateAtEvent.BeforeStateSnapshotNID,
+	)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
 func calculateLatest(oldLatest []types.StateAtEventAndReference, alreadyReferenced bool, prevEvents []gomatrixserverlib.EventReference, newEvent types.StateAtEventAndReference) []types.StateAtEventAndReference {
 	var alreadyInLatest bool
 	var newLatest []types.StateAtEventAndReference
@@ -189,57 +231,55 @@ func calculateLatest(oldLatest []types.StateAtEventAndReference, alreadyReferenc
 	return newLatest
 }
 
-func writeEvent(
+func (u *latestEventsUpdater) makeOutputNewRoomEvent() (*api.OutputEvent, error) {
-	db RoomEventDatabase, ow OutputRoomEventWriter, lastEventIDSent string,
-	event gomatrixserverlib.Event, latest []types.StateAtEventAndReference,
-	removed, added []types.StateEntry,
-	stateBeforeEventRemoves, stateBeforeEventAdds []types.StateEntry,
-	sendAsServer string,
-) error {
 
-	latestEventIDs := make([]string, len(latest))
+	latestEventIDs := make([]string, len(u.latest))
-	for i := range latest {
+	for i := range u.latest {
-		latestEventIDs[i] = latest[i].EventID
+		latestEventIDs[i] = u.latest[i].EventID
 	}
 
 	ore := api.OutputNewRoomEvent{
-		Event:           event,
+		Event:           u.event,
-		LastSentEventID: lastEventIDSent,
+		LastSentEventID: u.lastEventIDSent,
 		LatestEventIDs:  latestEventIDs,
 	}
 
 	var stateEventNIDs []types.EventNID
-	for _, entry := range added {
+	for _, entry := range u.added {
 		stateEventNIDs = append(stateEventNIDs, entry.EventNID)
 	}
-	for _, entry := range removed {
+	for _, entry := range u.removed {
 		stateEventNIDs = append(stateEventNIDs, entry.EventNID)
 	}
-	for _, entry := range stateBeforeEventRemoves {
+	for _, entry := range u.stateBeforeEventRemoves {
 		stateEventNIDs = append(stateEventNIDs, entry.EventNID)
 	}
-	for _, entry := range stateBeforeEventAdds {
+	for _, entry := range u.stateBeforeEventAdds {
 		stateEventNIDs = append(stateEventNIDs, entry.EventNID)
 	}
 	stateEventNIDs = stateEventNIDs[:util.SortAndUnique(eventNIDSorter(stateEventNIDs))]
-	eventIDMap, err := db.EventIDs(stateEventNIDs)
+	eventIDMap, err := u.db.EventIDs(stateEventNIDs)
 	if err != nil {
-		return err
+		return nil, err
 	}
-	for _, entry := range added {
+	for _, entry := range u.added {
 		ore.AddsStateEventIDs = append(ore.AddsStateEventIDs, eventIDMap[entry.EventNID])
 	}
-	for _, entry := range removed {
+	for _, entry := range u.removed {
 		ore.RemovesStateEventIDs = append(ore.RemovesStateEventIDs, eventIDMap[entry.EventNID])
 	}
-	for _, entry := range stateBeforeEventRemoves {
+	for _, entry := range u.stateBeforeEventRemoves {
 		ore.StateBeforeRemovesEventIDs = append(ore.StateBeforeRemovesEventIDs, eventIDMap[entry.EventNID])
 	}
-	for _, entry := range stateBeforeEventAdds {
+	for _, entry := range u.stateBeforeEventAdds {
 		ore.StateBeforeAddsEventIDs = append(ore.StateBeforeAddsEventIDs, eventIDMap[entry.EventNID])
 	}
-	ore.SendAsServer = sendAsServer
+	ore.SendAsServer = u.sendAsServer
-	return ow.WriteOutputRoomEvent(ore)
+
+	return &api.OutputEvent{
+		Type:         api.OutputTypeNewRoomEvent,
+		NewRoomEvent: &ore,
+	}, nil
 }
 
 type eventNIDSorter []types.EventNID

src/github.com/matrix-org/dendrite/roomserver/input/membership.go

@@ -0,0 +1,297 @@
+// 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 input
+
+import (
+	"fmt"
+
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/roomserver/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// updateMembership updates the current membership and the invites for each
+// user affected by a change in the current state of the room.
+// Returns a list of output events to write to the kafka log to inform the
+// consumers about the invites added or retired by the change in current state.
+func updateMemberships(
+	db RoomEventDatabase, updater types.RoomRecentEventsUpdater, removed, added []types.StateEntry,
+) ([]api.OutputEvent, error) {
+	changes := membershipChanges(removed, added)
+	var eventNIDs []types.EventNID
+	for _, change := range changes {
+		if change.addedEventNID != 0 {
+			eventNIDs = append(eventNIDs, change.addedEventNID)
+		}
+		if change.removedEventNID != 0 {
+			eventNIDs = append(eventNIDs, change.removedEventNID)
+		}
+	}
+
+	// Load the event JSON so we can look up the "membership" key.
+	// TODO: Maybe add a membership key to the events table so we can load that
+	// key without having to load the entire event JSON?
+	events, err := db.Events(eventNIDs)
+	if err != nil {
+		return nil, err
+	}
+
+	var updates []api.OutputEvent
+
+	for _, change := range changes {
+		var ae *gomatrixserverlib.Event
+		var re *gomatrixserverlib.Event
+		targetUserNID := change.EventStateKeyNID
+		if change.removedEventNID != 0 {
+			ev, _ := eventMap(events).lookup(change.removedEventNID)
+			if ev != nil {
+				re = &ev.Event
+			}
+		}
+		if change.addedEventNID != 0 {
+			ev, _ := eventMap(events).lookup(change.addedEventNID)
+			if ev != nil {
+				ae = &ev.Event
+			}
+		}
+		if updates, err = updateMembership(updater, targetUserNID, re, ae, updates); err != nil {
+			return nil, err
+		}
+	}
+	return nil, nil
+}
+
+func updateMembership(
+	updater types.RoomRecentEventsUpdater, targetUserNID types.EventStateKeyNID,
+	remove, add *gomatrixserverlib.Event,
+	updates []api.OutputEvent,
+) ([]api.OutputEvent, error) {
+	var err error
+	// Default the membership to "leave" if no event was added or removed.
+	old := "leave"
+	new := "leave"
+
+	if remove != nil {
+		old, err = remove.Membership()
+		if err != nil {
+			return nil, err
+		}
+	}
+	if add != nil {
+		new, err = add.Membership()
+		if err != nil {
+			return nil, err
+		}
+	}
+	if old == new {
+		// If the membership is the same then nothing changed and we can return
+		// immediately. This should help speed up processing for display name
+		// changes where the membership is "join" both before and after.
+		return updates, nil
+	}
+
+	mu, err := updater.MembershipUpdater(targetUserNID)
+	if err != nil {
+		return nil, err
+	}
+
+	switch new {
+	case "invite":
+		return updateToInviteMembership(mu, add, updates)
+	case "join":
+		return updateToJoinMembership(mu, add, updates)
+	case "leave", "ban":
+		return updateToLeaveMembership(mu, add, new, updates)
+	default:
+		panic(fmt.Errorf(
+			"input: membership %q is not one of the allowed values", new,
+		))
+	}
+}
+
+func updateToInviteMembership(
+	mu types.MembershipUpdater, add *gomatrixserverlib.Event, updates []api.OutputEvent,
+) ([]api.OutputEvent, error) {
+	// We may have already sent the invite to the user, either because we are
+	// reprocessing this event, or because the we received this invite from a
+	// remote server via the federation invite API. In those cases we don't need
+	// to send the event.
+	needsSending, err := mu.SetToInvite(*add)
+	if err != nil {
+		return nil, err
+	}
+	if needsSending {
+		// We notify the consumers using a special event even though we will
+		// notify them about the change in current state as part of the normal
+		// room event stream. This ensures that the consumers only have to
+		// consider a single stream of events when determining whether a user
+		// is invited, rather than having to combine multiple streams themselves.
+		onie := api.OutputNewInviteEvent{
+			Event: *add,
+		}
+		updates = append(updates, api.OutputEvent{
+			Type:           api.OutputTypeNewInviteEvent,
+			NewInviteEvent: &onie,
+		})
+	}
+	return updates, nil
+}
+
+func updateToJoinMembership(
+	mu types.MembershipUpdater, add *gomatrixserverlib.Event, updates []api.OutputEvent,
+) ([]api.OutputEvent, error) {
+	// If the user is already marked as being joined then we can return immediately.
+	// TODO: Is this code reachable given the "old != new" guard in updateMembership?
+	if mu.IsJoin() {
+		return updates, nil
+	}
+	// When we mark a user as being joined we will invalidate any invites that
+	// are active for that user. We notify the consumers that the invites have
+	// been retired using a special event, even though they could infer this
+	// by studying the state changes in the room event stream.
+	retired, err := mu.SetToJoin(add.Sender())
+	if err != nil {
+		return nil, err
+	}
+	for _, eventID := range retired {
+		orie := api.OutputRetireInviteEvent{
+			EventID:    eventID,
+			Membership: "join",
+		}
+		if add != nil {
+			orie.RetiredByEventID = add.EventID()
+		}
+		updates = append(updates, api.OutputEvent{
+			Type:              api.OutputTypeRetireInviteEvent,
+			RetireInviteEvent: &orie,
+		})
+	}
+	return updates, nil
+}
+
+func updateToLeaveMembership(
+	mu types.MembershipUpdater, add *gomatrixserverlib.Event,
+	newMembership string, updates []api.OutputEvent,
+) ([]api.OutputEvent, error) {
+	// If the user is already neither joined, nor invited to the room then we
+	// can return immediately.
+	if mu.IsLeave() {
+		return updates, nil
+	}
+	// When we mark a user as having left we will invalidate any invites that
+	// are active for that user. We notify the consumers that the invites have
+	// been retired using a special event, even though they could infer this
+	// by studying the state changes in the room event stream.
+	retired, err := mu.SetToLeave(add.Sender())
+	if err != nil {
+		return nil, err
+	}
+	for _, eventID := range retired {
+		orie := api.OutputRetireInviteEvent{
+			EventID:    eventID,
+			Membership: newMembership,
+		}
+		if add != nil {
+			orie.RetiredByEventID = add.EventID()
+		}
+		updates = append(updates, api.OutputEvent{
+			Type:              api.OutputTypeRetireInviteEvent,
+			RetireInviteEvent: &orie,
+		})
+	}
+	return updates, nil
+}
+
+// membershipChanges pairs up the membership state changes from a sorted list
+// of state removed and a sorted list of state added.
+func membershipChanges(removed, added []types.StateEntry) []stateChange {
+	changes := pairUpChanges(removed, added)
+	var result []stateChange
+	for _, c := range changes {
+		if c.EventTypeNID == types.MRoomMemberNID {
+			result = append(result, c)
+		}
+	}
+	return result
+}
+
+type stateChange struct {
+	types.StateKeyTuple
+	removedEventNID types.EventNID
+	addedEventNID   types.EventNID
+}
+
+// pairUpChanges pairs up the state events added and removed for each type,
+// state key tuple. Assumes that removed and added are sorted.
+func pairUpChanges(removed, added []types.StateEntry) []stateChange {
+	var ai int
+	var ri int
+	var result []stateChange
+	for {
+		switch {
+		case ai == len(added):
+			// We've reached the end of the added entries.
+			// The rest of the removed list are events that were removed without
+			// an event with the same state key being added.
+			for _, s := range removed[ri:] {
+				result = append(result, stateChange{
+					StateKeyTuple:   s.StateKeyTuple,
+					removedEventNID: s.EventNID,
+				})
+			}
+			return result
+		case ri == len(removed):
+			// We've reached the end of the removed entries.
+			// The rest of the added list are events that were added without
+			// an event with the same state key being removed.
+			for _, s := range added[ai:] {
+				result = append(result, stateChange{
+					StateKeyTuple: s.StateKeyTuple,
+					addedEventNID: s.EventNID,
+				})
+			}
+			return result
+		case added[ai].StateKeyTuple == removed[ri].StateKeyTuple:
+			// The tuple is in both lists so an event with that key is being
+			// removed and another event with the same key is being added.
+			result = append(result, stateChange{
+				StateKeyTuple:   added[ai].StateKeyTuple,
+				removedEventNID: removed[ri].EventNID,
+				addedEventNID:   added[ai].EventNID,
+			})
+			ai++
+			ri++
+		case added[ai].StateKeyTuple.LessThan(removed[ri].StateKeyTuple):
+			// The lists are sorted so the added entry being less than the
+			// removed entry means that the added event was added without an
+			// event with the same key being removed.
+			result = append(result, stateChange{
+				StateKeyTuple: added[ai].StateKeyTuple,
+				addedEventNID: added[ai].EventNID,
+			})
+			ai++
+		default:
+			// Reaching the default case implies that the removed entry is less
+			// than the added entry. Since the lists are sorted this means that
+			// the removed event was removed without an event with the same
+			// key being added.
+			result = append(result, stateChange{
+				StateKeyTuple:   removed[ai].StateKeyTuple,
+				removedEventNID: removed[ri].EventNID,
+			})
+			ri++
+		}
+	}
+}

src/github.com/matrix-org/dendrite/roomserver/storage/event_state_keys_table.go

@@ -76,15 +76,23 @@ func (s *eventStateKeyStatements) prepare(db *sql.DB) (err error) {
 	}.prepare(db)
 }
 
-func (s *eventStateKeyStatements) insertEventStateKeyNID(eventStateKey string) (types.EventStateKeyNID, error) {
+func (s *eventStateKeyStatements) insertEventStateKeyNID(txn *sql.Tx, eventStateKey string) (types.EventStateKeyNID, error) {
 	var eventStateKeyNID int64
-	err := s.insertEventStateKeyNIDStmt.QueryRow(eventStateKey).Scan(&eventStateKeyNID)
+	stmt := s.insertEventStateKeyNIDStmt
+	if txn != nil {
+		stmt = txn.Stmt(stmt)
+	}
+	err := stmt.QueryRow(eventStateKey).Scan(&eventStateKeyNID)
 	return types.EventStateKeyNID(eventStateKeyNID), err
 }
 
-func (s *eventStateKeyStatements) selectEventStateKeyNID(eventStateKey string) (types.EventStateKeyNID, error) {
+func (s *eventStateKeyStatements) selectEventStateKeyNID(txn *sql.Tx, eventStateKey string) (types.EventStateKeyNID, error) {
 	var eventStateKeyNID int64
-	err := s.selectEventStateKeyNIDStmt.QueryRow(eventStateKey).Scan(&eventStateKeyNID)
+	stmt := s.selectEventStateKeyNIDStmt
+	if txn != nil {
+		stmt = txn.Stmt(stmt)
+	}
+	err := stmt.QueryRow(eventStateKey).Scan(&eventStateKeyNID)
 	return types.EventStateKeyNID(eventStateKeyNID), err
 }
 

src/github.com/matrix-org/dendrite/roomserver/storage/invite_table.go

@@ -0,0 +1,149 @@
+// 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/roomserver/types"
+)
+
+const inviteSchema = `
+CREATE TABLE IF NOT EXISTS roomserver_invites (
+	-- The string ID of the invite event itself.
+	-- We can't use a numeric event ID here because we don't always have
+	-- enough information to store an invite in the event table.
+	-- In particular we don't always have a chain of auth_events for invites
+	-- received over federation.
+	invite_event_id TEXT PRIMARY KEY,
+	-- The numeric ID of the room the invite m.room.member event is in.
+	room_nid BIGINT NOT NULL,
+	-- The numeric ID for the state key of the invite m.room.member event.
+	-- This tells us who the invite is for.
+	-- This is used to query the active invites for a user.
+	target_nid BIGINT NOT NULL,
+	-- The numeric ID for the sender of the invite m.room.member event.
+	-- This tells us who sent the invite.
+	-- This is used to work out which matrix server we should talk to when
+	-- we try to join the room.
+	sender_nid BIGINT NOT NULL DEFAULT 0,
+	-- This is used to track whether the invite is still active.
+	-- This is set implicitly when processing new join and leave events and
+	-- explicitly when rejecting events over federation.
+	retired BOOLEAN NOT NULL DEFAULT FALSE,
+	-- The invite event JSON.
+	invite_event_json TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS roomserver_invites_active_idx ON roomserver_invites (target_nid, room_nid)
+	WHERE NOT retired;
+`
+const insertInviteEventSQL = "" +
+	"INSERT INTO roomserver_invites (invite_event_id, room_nid, target_nid," +
+	" sender_nid, invite_event_json) VALUES ($1, $2, $3, $4, $5)" +
+	" ON CONFLICT DO NOTHING"
+
+const selectInviteActiveForUserInRoomSQL = "" +
+	"SELECT sender_nid FROM roomserver_invites" +
+	" WHERE target_nid = $1 AND room_nid = $2" +
+	" AND NOT retired"
+
+// Retire every active invite for a user in a room.
+// Ideally we'd know which invite events were retired by a given update so we
+// wouldn't need to remove every active invite.
+// However the matrix protocol doesn't give us a way to reliably identify the
+// invites that were retired, so we are forced to retire all of them.
+const updateInviteRetiredSQL = "" +
+	"UPDATE roomserver_invites SET retired = TRUE" +
+	" WHERE room_nid = $1 AND target_nid = $2 AND NOT retired" +
+	" RETURNING invite_event_id"
+
+type inviteStatements struct {
+	insertInviteEventStmt               *sql.Stmt
+	selectInviteActiveForUserInRoomStmt *sql.Stmt
+	updateInviteRetiredStmt             *sql.Stmt
+}
+
+func (s *inviteStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(inviteSchema)
+	if err != nil {
+		return
+	}
+
+	return statementList{
+		{&s.insertInviteEventStmt, insertInviteEventSQL},
+		{&s.selectInviteActiveForUserInRoomStmt, selectInviteActiveForUserInRoomSQL},
+		{&s.updateInviteRetiredStmt, updateInviteRetiredSQL},
+	}.prepare(db)
+}
+
+func (s *inviteStatements) insertInviteEvent(
+	txn *sql.Tx, inviteEventID string, roomNID types.RoomNID,
+	targetUserNID, senderUserNID types.EventStateKeyNID,
+	inviteEventJSON []byte,
+) (bool, error) {
+	result, err := txn.Stmt(s.insertInviteEventStmt).Exec(
+		inviteEventID, roomNID, targetUserNID, senderUserNID, inviteEventJSON,
+	)
+	if err != nil {
+		return false, err
+	}
+	count, err := result.RowsAffected()
+	if err != nil {
+		return false, err
+	}
+	return count != 0, nil
+}
+
+func (s *inviteStatements) updateInviteRetired(
+	txn *sql.Tx, roomNID types.RoomNID, targetUserNID types.EventStateKeyNID,
+) ([]string, error) {
+	rows, err := txn.Stmt(s.updateInviteRetiredStmt).Query(roomNID, targetUserNID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var result []string
+	for rows.Next() {
+		var inviteEventID string
+		if err := rows.Scan(&inviteEventID); err != nil {
+			return nil, err
+		}
+		result = append(result, inviteEventID)
+	}
+	return result, nil
+}
+
+// selectInviteActiveForUserInRoom returns a list of sender state key NIDs
+func (s *inviteStatements) selectInviteActiveForUserInRoom(
+	targetUserNID types.EventStateKeyNID, roomNID types.RoomNID,
+) ([]types.EventStateKeyNID, error) {
+	rows, err := s.selectInviteActiveForUserInRoomStmt.Query(
+		targetUserNID, roomNID,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var result []types.EventStateKeyNID
+	for rows.Next() {
+		var senderUserNID int64
+		if err := rows.Scan(&senderUserNID); err != nil {
+			return nil, err
+		}
+		result = append(result, types.EventStateKeyNID(senderUserNID))
+	}
+	return result, nil
+}

src/github.com/matrix-org/dendrite/roomserver/storage/membership_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/matrix-org/dendrite/roomserver/types"
+)
+
+type membershipState int64
+
+const (
+	membershipStateLeaveOrBan membershipState = 1
+	membershipStateInvite     membershipState = 2
+	membershipStateJoin       membershipState = 3
+)
+
+const membershipSchema = `
+-- The membership table is used to coordinate updates between the invite table
+-- and the room state tables.
+-- This table is updated in one of 3 ways:
+--   1) The membership of a user changes within the current state of the room.
+--   2) An invite is received outside of a room over federation. 
+--   3) An invite is rejected outside of a room over federation.
+CREATE TABLE IF NOT EXISTS roomserver_membership (
+	room_nid BIGINT NOT NULL,
+	-- Numeric state key ID for the user ID this state is for.
+	target_nid BIGINT NOT NULL,
+	-- Numeric state key ID for the user ID who changed the state.
+	-- This may be 0 since it is not always possible to identify the user that
+	-- changed the state.
+	sender_nid BIGINT NOT NULL DEFAULT 0,
+	-- The state the user is in within this room.
+	-- Default value is "membershipStateLeaveOrBan"
+	membership_nid BIGINT NOT NULL DEFAULT 1,
+	UNIQUE (room_nid, target_nid)
+);
+`
+
+// Insert a row in to membership table so that it can be locked by the
+// SELECT FOR UPDATE
+const insertMembershipSQL = "" +
+	"INSERT INTO roomserver_membership (room_nid, target_nid)" +
+	" VALUES ($1, $2)" +
+	" ON CONFLICT DO NOTHING"
+
+const selectMembershipForUpdateSQL = "" +
+	"SELECT membership_nid FROM roomserver_membership" +
+	" WHERE room_nid = $1 AND target_nid = $2 FOR UPDATE"
+
+const updateMembershipSQL = "" +
+	"UPDATE roomserver_membership SET sender_nid = $3, membership_nid = $4" +
+	" WHERE room_nid = $1 AND target_nid = $2"
+
+type membershipStatements struct {
+	insertMembershipStmt          *sql.Stmt
+	selectMembershipForUpdateStmt *sql.Stmt
+	updateMembershipStmt          *sql.Stmt
+}
+
+func (s *membershipStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(membershipSchema)
+	if err != nil {
+		return
+	}
+
+	return statementList{
+		{&s.insertMembershipStmt, insertMembershipSQL},
+		{&s.selectMembershipForUpdateStmt, selectMembershipForUpdateSQL},
+		{&s.updateMembershipStmt, updateMembershipSQL},
+	}.prepare(db)
+}
+
+func (s *membershipStatements) insertMembership(
+	txn *sql.Tx, roomNID types.RoomNID, targetUserNID types.EventStateKeyNID,
+) error {
+	_, err := txn.Stmt(s.insertMembershipStmt).Exec(roomNID, targetUserNID)
+	return err
+}
+
+func (s *membershipStatements) selectMembershipForUpdate(
+	txn *sql.Tx, roomNID types.RoomNID, targetUserNID types.EventStateKeyNID,
+) (membership membershipState, err error) {
+	err = txn.Stmt(s.selectMembershipForUpdateStmt).QueryRow(
+		roomNID, targetUserNID,
+	).Scan(&membership)
+	return
+}
+
+func (s *membershipStatements) updateMembership(
+	txn *sql.Tx, roomNID types.RoomNID, targetUserNID types.EventStateKeyNID,
+	senderUserNID types.EventStateKeyNID, membership membershipState,
+) error {
+	_, err := txn.Stmt(s.updateMembershipStmt).Exec(
+		roomNID, targetUserNID, senderUserNID, membership,
+	)
+	return err
+}

src/github.com/matrix-org/dendrite/roomserver/storage/sql.go

@@ -28,6 +28,8 @@ type statements struct {
 	stateBlockStatements
 	previousEventStatements
 	roomAliasesStatements
+	inviteStatements
+	membershipStatements
 }
 
 func (s *statements) prepare(db *sql.DB) error {
@@ -43,6 +45,8 @@ func (s *statements) prepare(db *sql.DB) error {
 		s.stateBlockStatements.prepare,
 		s.previousEventStatements.prepare,
 		s.roomAliasesStatements.prepare,
+		s.inviteStatements.prepare,
+		s.membershipStatements.prepare,
 	} {
 		if err = prepare(db); err != nil {
 			return err

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

@@ -16,6 +16,7 @@ package storage
 
 import (
 	"database/sql"
+
 	// Import the postgres database driver.
 	_ "github.com/lib/pq"
 	"github.com/matrix-org/dendrite/roomserver/types"
@@ -64,7 +65,7 @@ func (d *Database) StoreEvent(event gomatrixserverlib.Event, authEventNIDs []typ
 	// Assigned a numeric ID for the state_key if there is one present.
 	// Otherwise set the numeric ID for the state_key to 0.
 	if eventStateKey != nil {
-		if eventStateKeyNID, err = d.assignStateKeyNID(*eventStateKey); err != nil {
+		if eventStateKeyNID, err = d.assignStateKeyNID(nil, *eventStateKey); err != nil {
 			return 0, types.StateAtEvent{}, err
 		}
 	}
@@ -131,15 +132,15 @@ func (d *Database) assignEventTypeNID(eventType string) (types.EventTypeNID, err
 	return eventTypeNID, err
 }
 
-func (d *Database) assignStateKeyNID(eventStateKey string) (types.EventStateKeyNID, error) {
+func (d *Database) assignStateKeyNID(txn *sql.Tx, eventStateKey string) (types.EventStateKeyNID, error) {
 	// Check if we already have a numeric ID in the database.
-	eventStateKeyNID, err := d.statements.selectEventStateKeyNID(eventStateKey)
+	eventStateKeyNID, err := d.statements.selectEventStateKeyNID(txn, eventStateKey)
 	if err == sql.ErrNoRows {
 		// We don't have a numeric ID so insert one into the database.
-		eventStateKeyNID, err = d.statements.insertEventStateKeyNID(eventStateKey)
+		eventStateKeyNID, err = d.statements.insertEventStateKeyNID(txn, eventStateKey)
 		if err == sql.ErrNoRows {
 			// We raced with another insert so run the select again.
-			eventStateKeyNID, err = d.statements.selectEventStateKeyNID(eventStateKey)
+			eventStateKeyNID, err = d.statements.selectEventStateKeyNID(txn, eventStateKey)
 		}
 	}
 	return eventStateKeyNID, err
@@ -249,12 +250,15 @@ func (d *Database) GetLatestEventsForUpdate(roomNID types.RoomNID) (types.RoomRe
 			return nil, err
 		}
 	}
-	return &roomRecentEventsUpdater{txn, d, stateAndRefs, lastEventIDSent, currentStateSnapshotNID}, nil
+	return &roomRecentEventsUpdater{
+		transaction{txn}, d, roomNID, stateAndRefs, lastEventIDSent, currentStateSnapshotNID,
+	}, nil
 }
 
 type roomRecentEventsUpdater struct {
-	txn                     *sql.Tx
+	transaction
 	d                       *Database
+	roomNID                 types.RoomNID
 	latestEvents            []types.StateAtEventAndReference
 	lastEventIDSent         string
 	currentStateSnapshotNID types.StateSnapshotNID
@@ -319,14 +323,8 @@ func (u *roomRecentEventsUpdater) MarkEventAsSent(eventNID types.EventNID) error
 	return u.d.statements.updateEventSentToOutput(u.txn, eventNID)
 }
 
-// Commit implements types.RoomRecentEventsUpdater
+func (u *roomRecentEventsUpdater) MembershipUpdater(targetUserNID types.EventStateKeyNID) (types.MembershipUpdater, error) {
-func (u *roomRecentEventsUpdater) Commit() error {
+	return u.d.membershipUpdaterTxn(u.txn, u.roomNID, targetUserNID)
-	return u.txn.Commit()
-}
-
-// Rollback implements types.RoomRecentEventsUpdater
-func (u *roomRecentEventsUpdater) Rollback() error {
-	return u.txn.Rollback()
 }
 
 // RoomNID implements query.RoomserverQueryAPIDB
@@ -381,3 +379,124 @@ func (d *Database) StateEntriesForTuples(
 ) ([]types.StateEntryList, error) {
 	return d.statements.bulkSelectFilteredStateBlockEntries(stateBlockNIDs, stateKeyTuples)
 }
+
+type membershipUpdater struct {
+	transaction
+	d             *Database
+	roomNID       types.RoomNID
+	targetUserNID types.EventStateKeyNID
+	membership    membershipState
+}
+
+func (d *Database) membershipUpdaterTxn(
+	txn *sql.Tx, roomNID types.RoomNID, targetUserNID types.EventStateKeyNID,
+) (types.MembershipUpdater, error) {
+
+	if err := d.statements.insertMembership(txn, roomNID, targetUserNID); err != nil {
+		return nil, err
+	}
+
+	membership, err := d.statements.selectMembershipForUpdate(txn, roomNID, targetUserNID)
+	if err != nil {
+		return nil, err
+	}
+
+	return &membershipUpdater{
+		transaction{txn}, d, roomNID, targetUserNID, membership,
+	}, nil
+}
+
+// IsInvite implements types.MembershipUpdater
+func (u *membershipUpdater) IsInvite() bool {
+	return u.membership == membershipStateInvite
+}
+
+// IsJoin implements types.MembershipUpdater
+func (u *membershipUpdater) IsJoin() bool {
+	return u.membership == membershipStateJoin
+}
+
+// IsLeave implements types.MembershipUpdater
+func (u *membershipUpdater) IsLeave() bool {
+	return u.membership == membershipStateLeaveOrBan
+}
+
+// SetToInvite implements types.MembershipUpdater
+func (u *membershipUpdater) SetToInvite(event gomatrixserverlib.Event) (bool, error) {
+	senderUserNID, err := u.d.assignStateKeyNID(u.txn, event.Sender())
+	if err != nil {
+		return false, err
+	}
+	inserted, err := u.d.statements.insertInviteEvent(
+		u.txn, event.EventID(), u.roomNID, u.targetUserNID, senderUserNID, event.JSON(),
+	)
+	if err != nil {
+		return false, err
+	}
+	if u.membership != membershipStateInvite {
+		if err = u.d.statements.updateMembership(
+			u.txn, u.roomNID, u.targetUserNID, senderUserNID, membershipStateInvite,
+		); err != nil {
+			return false, err
+		}
+	}
+	return inserted, nil
+}
+
+// SetToJoin implements types.MembershipUpdater
+func (u *membershipUpdater) SetToJoin(senderUserID string) ([]string, error) {
+	senderUserNID, err := u.d.assignStateKeyNID(u.txn, senderUserID)
+	if err != nil {
+		return nil, err
+	}
+	inviteEventIDs, err := u.d.statements.updateInviteRetired(
+		u.txn, u.roomNID, u.targetUserNID,
+	)
+	if err != nil {
+		return nil, err
+	}
+	if u.membership != membershipStateJoin {
+		if err = u.d.statements.updateMembership(
+			u.txn, u.roomNID, u.targetUserNID, senderUserNID, membershipStateJoin,
+		); err != nil {
+			return nil, err
+		}
+	}
+	return inviteEventIDs, nil
+}
+
+// SetToLeave implements types.MembershipUpdater
+func (u *membershipUpdater) SetToLeave(senderUserID string) ([]string, error) {
+	senderUserNID, err := u.d.assignStateKeyNID(u.txn, senderUserID)
+	if err != nil {
+		return nil, err
+	}
+	inviteEventIDs, err := u.d.statements.updateInviteRetired(
+		u.txn, u.roomNID, u.targetUserNID,
+	)
+	if err != nil {
+		return nil, err
+	}
+	if u.membership != membershipStateLeaveOrBan {
+		if err = u.d.statements.updateMembership(
+			u.txn, u.roomNID, u.targetUserNID, senderUserNID, membershipStateLeaveOrBan,
+		); err != nil {
+			return nil, err
+		}
+	}
+	return inviteEventIDs, nil
+}
+
+type transaction struct {
+	txn *sql.Tx
+}
+
+// Commit implements types.Transaction
+func (t *transaction) Commit() error {
+	return t.txn.Commit()
+}
+
+// Rollback implements types.Transaction
+func (t *transaction) Rollback() error {
+	return t.txn.Rollback()
+}

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

@@ -135,9 +135,17 @@ type StateEntryList struct {
 	StateEntries  []StateEntry
 }
 
+// A Transaction is something that can be committed or rolledback.
+type Transaction interface {
+	// Commit the transaction
+	Commit() error
+	// Rollback the transaction.
+	Rollback() error
+}
+
 // A RoomRecentEventsUpdater is used to update the recent events in a room.
 // (On postgresql this wraps a database transaction that holds a "FOR UPDATE"
-//  lock on the row holding the latest events for the room.)
+//  lock on the row in the rooms table holding the latest events for the room.)
 type RoomRecentEventsUpdater interface {
 	// The latest event IDs and state in the room.
 	LatestEvents() []StateAtEventAndReference
@@ -163,10 +171,36 @@ type RoomRecentEventsUpdater interface {
 	HasEventBeenSent(eventNID EventNID) (bool, error)
 	// Mark the event as having been sent to the output logs.
 	MarkEventAsSent(eventNID EventNID) error
-	// Commit the transaction
+	// Build a membership updater for the target user in this room.
-	Commit() error
+	// It will share the same transaction as this updater.
-	// Rollback the transaction.
+	MembershipUpdater(targetUserNID EventStateKeyNID) (MembershipUpdater, error)
-	Rollback() error
+	// Implements Transaction so it can be committed or rolledback
+	Transaction
+}
+
+// A MembershipUpdater is used to update the membership of a user in a room.
+// (On postgresql this wraps a database transaction that holds a "FOR UPDATE"
+//  lock on the row in the membership table for this user in the room)
+// The caller should call one of SetToInvite, SetToJoin or SetToLeave once to
+// make the update, or none of them if no update is required.
+type MembershipUpdater interface {
+	// True if the target user is invited to the room before updating.
+	IsInvite() bool
+	// True if the target user is joined to the room before updating.
+	IsJoin() bool
+	// True if the target user is not invited or joined to the room before updating.
+	IsLeave() bool
+	// Set the state to invite.
+	// Returns whether this invite needs to be sent
+	SetToInvite(event gomatrixserverlib.Event) (needsSending bool, err error)
+	// Set the state to join.
+	// Returns a list of invite event IDs that this state change retired.
+	SetToJoin(senderUserID string) (inviteEventIDs []string, err error)
+	// Set the state to leave.
+	// Returns a list of invite event IDs that this state change retired.
+	SetToLeave(senderUserID string) (inviteEventIDs []string, err error)
+	// Implements Transaction so it can be committed or rolledback.
+	Transaction
 }
 
 // A MissingEventError is an error that happened because the roomserver was