Peeking via MSC2753 (#1370)

Initial implementation of MSC2753, as tested by https://github.com/matrix-org/sytest/pull/944.
Doesn't yet handle unpeeks, peeked EDUs, or history viz changing during a peek - these will follow.
https://github.com/matrix-org/dendrite/pull/1370 has full details.

10 files changed, 783 insertions, 52 deletions

diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index 67e656c9..b6ab9bd5 100644
--- a/syncapi/consumers/roomserver.go
+++ b/syncapi/consumers/roomserver.go
@@ -17,6 +17,7 @@ package consumers
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 
 	"github.com/Shopify/sarama"
 	"github.com/matrix-org/dendrite/internal"
@@ -26,11 +27,13 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/sync"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/sirupsen/logrus"
 	log "github.com/sirupsen/logrus"
 )
 
 // OutputRoomEventConsumer consumes events that originated in the room server.
 type OutputRoomEventConsumer struct {
+	cfg        *config.SyncAPI
 	rsAPI      api.RoomserverInternalAPI
 	rsConsumer *internal.ContinualConsumer
 	db         storage.Database
@@ -55,6 +58,7 @@ func NewOutputRoomEventConsumer(
 		PartitionStore: store,
 	}
 	s := &OutputRoomEventConsumer{
+		cfg:        cfg,
 		rsConsumer: &consumer,
 		db:         store,
 		notifier:   n,
@@ -100,6 +104,8 @@ func (s *OutputRoomEventConsumer) onMessage(msg *sarama.ConsumerMessage) error {
 		return s.onNewInviteEvent(context.TODO(), *output.NewInviteEvent)
 	case api.OutputTypeRetireInviteEvent:
 		return s.onRetireInviteEvent(context.TODO(), *output.RetireInviteEvent)
+	case api.OutputTypeNewPeek:
+		return s.onNewPeek(context.TODO(), *output.NewPeek)
 	case api.OutputTypeRedactedEvent:
 		return s.onRedactEvent(context.TODO(), *output.RedactedEvent)
 	default:
@@ -162,6 +168,12 @@ func (s *OutputRoomEventConsumer) onNewRoomEvent(
 		}).Panicf("roomserver output log: write event failure")
 		return nil
 	}
+
+	if pduPos, err = s.notifyJoinedPeeks(ctx, &ev, pduPos); err != nil {
+		logrus.WithError(err).Errorf("Failed to notifyJoinedPeeks for PDU pos %d", pduPos)
+		return err
+	}
+
 	s.notifier.OnNewEvent(&ev, "", nil, types.NewStreamToken(pduPos, 0, nil))
 
 	s.notifyKeyChanges(&ev)
@@ -184,6 +196,37 @@ func (s *OutputRoomEventConsumer) notifyKeyChanges(ev *gomatrixserverlib.Headere
 	}
 }
 
+func (s *OutputRoomEventConsumer) notifyJoinedPeeks(ctx context.Context, ev *gomatrixserverlib.HeaderedEvent, sp types.StreamPosition) (types.StreamPosition, error) {
+	if ev.Type() != gomatrixserverlib.MRoomMember {
+		return sp, nil
+	}
+	membership, err := ev.Membership()
+	if err != nil {
+		return sp, fmt.Errorf("ev.Membership: %w", err)
+	}
+	// TODO: check that it's a join and not a profile change (means unmarshalling prev_content)
+	if membership == gomatrixserverlib.Join {
+		// check it's a local join
+		_, domain, err := gomatrixserverlib.SplitID('@', *ev.StateKey())
+		if err != nil {
+			return sp, fmt.Errorf("gomatrixserverlib.SplitID: %w", err)
+		}
+		if domain != s.cfg.Matrix.ServerName {
+			return sp, nil
+		}
+
+		// cancel any peeks for it
+		peekSP, peekErr := s.db.DeletePeeks(ctx, ev.RoomID(), *ev.StateKey())
+		if peekErr != nil {
+			return sp, fmt.Errorf("s.db.DeletePeeks: %w", peekErr)
+		}
+		if peekSP > 0 {
+			sp = peekSP
+		}
+	}
+	return sp, nil
+}
+
 func (s *OutputRoomEventConsumer) onNewInviteEvent(
 	ctx context.Context, msg api.OutputNewInviteEvent,
 ) error {
@@ -219,6 +262,26 @@ func (s *OutputRoomEventConsumer) onRetireInviteEvent(
 	return nil
 }
 
+func (s *OutputRoomEventConsumer) onNewPeek(
+	ctx context.Context, msg api.OutputNewPeek,
+) error {
+	sp, err := s.db.AddPeek(ctx, msg.RoomID, msg.UserID, msg.DeviceID)
+	if err != nil {
+		// panic rather than continue with an inconsistent database
+		log.WithFields(log.Fields{
+			log.ErrorKey: err,
+		}).Panicf("roomserver output log: write peek failure")
+		return nil
+	}
+	// tell the notifier about the new peek so it knows to wake up new devices
+	s.notifier.OnNewPeek(msg.RoomID, msg.UserID, msg.DeviceID)
+
+	// we need to wake up the users who might need to now be peeking into this room,
+	// so we send in a dummy event to trigger a wakeup
+	s.notifier.OnNewEvent(nil, msg.RoomID, nil, types.NewStreamToken(sp, 0, nil))
+	return nil
+}
+
 func (s *OutputRoomEventConsumer) updateStateEvent(event gomatrixserverlib.HeaderedEvent) (gomatrixserverlib.HeaderedEvent, error) {
 	if event.StateKey() == nil {
 		return event, nil
diff --git a/syncapi/storage/interface.go b/syncapi/storage/interface.go
index 838fd547..807c7f5e 100644
--- a/syncapi/storage/interface.go
+++ b/syncapi/storage/interface.go
@@ -30,6 +30,8 @@ type Database interface {
 	internal.PartitionStorer
 	// AllJoinedUsersInRooms returns a map of room ID to a list of all joined user IDs.
 	AllJoinedUsersInRooms(ctx context.Context) (map[string][]string, error)
+	// AllPeekingDevicesInRooms returns a map of room ID to a list of all peeking devices.
+	AllPeekingDevicesInRooms(ctx context.Context) (map[string][]types.PeekingDevice, error)
 	// Events lookups a list of event by their event ID.
 	// Returns a list of events matching the requested IDs found in the database.
 	// If an event is not found in the database then it will be omitted from the list.
@@ -81,6 +83,12 @@ type Database interface {
 	// RetireInviteEvent removes an old invite event from the database. Returns the new position of the retired invite.
 	// Returns an error if there was a problem communicating with the database.
 	RetireInviteEvent(ctx context.Context, inviteEventID string) (types.StreamPosition, error)
+	// AddPeek adds a new peek to our DB for a given room by a given user's device.
+	// Returns an error if there was a problem communicating with the database.
+	AddPeek(ctx context.Context, RoomID, UserID, DeviceID string) (types.StreamPosition, error)
+	// DeletePeek deletes all peeks for a given room by a given user
+	// Returns an error if there was a problem communicating with the database.
+	DeletePeeks(ctx context.Context, RoomID, UserID string) (types.StreamPosition, error)
 	// SetTypingTimeoutCallback sets a callback function that is called right after
 	// a user is removed from the typing user list due to timeout.
 	SetTypingTimeoutCallback(fn cache.TimeoutCallbackFn)
diff --git a/syncapi/storage/postgres/peeks_table.go b/syncapi/storage/postgres/peeks_table.go
new file mode 100644
index 00000000..75eeac98
--- /dev/null
+++ b/syncapi/storage/postgres/peeks_table.go
@@ -0,0 +1,186 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// 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 postgres
+
+import (
+	"context"
+	"database/sql"
+	"time"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+const peeksSchema = `
+CREATE TABLE IF NOT EXISTS syncapi_peeks (
+	id BIGINT DEFAULT nextval('syncapi_stream_id'),
+	room_id TEXT NOT NULL,
+	user_id TEXT NOT NULL,
+	device_id TEXT NOT NULL,
+	deleted BOOL NOT NULL DEFAULT false,
+    -- When the peek was created in UNIX epoch ms.
+    creation_ts BIGINT NOT NULL,
+    UNIQUE(room_id, user_id, device_id)
+);
+
+CREATE INDEX IF NOT EXISTS syncapi_peeks_room_id_idx ON syncapi_peeks(room_id);
+CREATE INDEX IF NOT EXISTS syncapi_peeks_user_id_device_id_idx ON syncapi_peeks(user_id, device_id);
+`
+
+const insertPeekSQL = "" +
+	"INSERT INTO syncapi_peeks" +
+	" (room_id, user_id, device_id, creation_ts)" +
+	" VALUES ($1, $2, $3, $4)" +
+	" ON CONFLICT (room_id, user_id, device_id) DO UPDATE SET deleted=false, creation_ts=$4" +
+	" RETURNING id"
+
+const deletePeekSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=nextval('syncapi_stream_id') WHERE room_id = $1 AND user_id = $2 AND device_id = $3 RETURNING id"
+
+const deletePeeksSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=nextval('syncapi_stream_id') WHERE room_id = $1 AND user_id = $2 RETURNING id"
+
+// we care about all the peeks which were created in this range, deleted in this range,
+// or were created before this range but haven't been deleted yet.
+const selectPeeksInRangeSQL = "" +
+	"SELECT room_id, deleted, (id > $3 AND id <= $4) AS changed FROM syncapi_peeks WHERE user_id = $1 AND device_id = $2 AND ((id <= $3 AND NOT deleted) OR (id > $3 AND id <= $4))"
+
+const selectPeekingDevicesSQL = "" +
+	"SELECT room_id, user_id, device_id FROM syncapi_peeks WHERE deleted=false"
+
+const selectMaxPeekIDSQL = "" +
+	"SELECT MAX(id) FROM syncapi_peeks"
+
+type peekStatements struct {
+	db                       *sql.DB
+	insertPeekStmt           *sql.Stmt
+	deletePeekStmt           *sql.Stmt
+	deletePeeksStmt          *sql.Stmt
+	selectPeeksInRangeStmt   *sql.Stmt
+	selectPeekingDevicesStmt *sql.Stmt
+	selectMaxPeekIDStmt      *sql.Stmt
+}
+
+func NewPostgresPeeksTable(db *sql.DB) (tables.Peeks, error) {
+	_, err := db.Exec(peeksSchema)
+	if err != nil {
+		return nil, err
+	}
+	s := &peekStatements{
+		db: db,
+	}
+	if s.insertPeekStmt, err = db.Prepare(insertPeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeekStmt, err = db.Prepare(deletePeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeeksStmt, err = db.Prepare(deletePeeksSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeeksInRangeStmt, err = db.Prepare(selectPeeksInRangeSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeekingDevicesStmt, err = db.Prepare(selectPeekingDevicesSQL); err != nil {
+		return nil, err
+	}
+	if s.selectMaxPeekIDStmt, err = db.Prepare(selectMaxPeekIDSQL); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *peekStatements) InsertPeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	nowMilli := time.Now().UnixNano() / int64(time.Millisecond)
+	stmt := sqlutil.TxStmt(txn, s.insertPeekStmt)
+	err = stmt.QueryRowContext(ctx, roomID, userID, deviceID, nowMilli).Scan(&streamPos)
+	return
+}
+
+func (s *peekStatements) DeletePeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	stmt := sqlutil.TxStmt(txn, s.deletePeekStmt)
+	err = stmt.QueryRowContext(ctx, roomID, userID, deviceID).Scan(&streamPos)
+	return
+}
+
+func (s *peekStatements) DeletePeeks(
+	ctx context.Context, txn *sql.Tx, roomID, userID string,
+) (streamPos types.StreamPosition, err error) {
+	stmt := sqlutil.TxStmt(txn, s.deletePeeksStmt)
+	err = stmt.QueryRowContext(ctx, roomID, userID).Scan(&streamPos)
+	return
+}
+
+func (s *peekStatements) SelectPeeksInRange(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string, r types.Range,
+) (peeks []types.Peek, err error) {
+	rows, err := sqlutil.TxStmt(txn, s.selectPeeksInRangeStmt).QueryContext(ctx, userID, deviceID, r.Low(), r.High())
+	if err != nil {
+		return
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeeksInRange: rows.close() failed")
+
+	for rows.Next() {
+		peek := types.Peek{}
+		var changed bool
+		if err = rows.Scan(&peek.RoomID, &peek.Deleted, &changed); err != nil {
+			return
+		}
+		peek.New = changed && !peek.Deleted
+		peeks = append(peeks, peek)
+	}
+
+	return peeks, rows.Err()
+}
+
+func (s *peekStatements) SelectPeekingDevices(
+	ctx context.Context,
+) (peekingDevices map[string][]types.PeekingDevice, err error) {
+	rows, err := s.selectPeekingDevicesStmt.QueryContext(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeekingDevices: rows.close() failed")
+
+	result := make(map[string][]types.PeekingDevice)
+	for rows.Next() {
+		var roomID, userID, deviceID string
+		if err := rows.Scan(&roomID, &userID, &deviceID); err != nil {
+			return nil, err
+		}
+		devices := result[roomID]
+		devices = append(devices, types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+		result[roomID] = devices
+	}
+	return result, nil
+}
+
+func (s *peekStatements) SelectMaxPeekID(
+	ctx context.Context, txn *sql.Tx,
+) (id int64, err error) {
+	var nullableID sql.NullInt64
+	stmt := sqlutil.TxStmt(txn, s.selectMaxPeekIDStmt)
+	err = stmt.QueryRowContext(ctx).Scan(&nullableID)
+	if nullableID.Valid {
+		id = nullableID.Int64
+	}
+	return
+}
diff --git a/syncapi/storage/postgres/syncserver.go b/syncapi/storage/postgres/syncserver.go
index e7f2c944..7f19722a 100644
--- a/syncapi/storage/postgres/syncserver.go
+++ b/syncapi/storage/postgres/syncserver.go
@@ -62,6 +62,10 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*SyncServerDatasource, e
 	if err != nil {
 		return nil, err
 	}
+	peeks, err := NewPostgresPeeksTable(d.db)
+	if err != nil {
+		return nil, err
+	}
 	topology, err := NewPostgresTopologyTable(d.db)
 	if err != nil {
 		return nil, err
@@ -82,6 +86,7 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*SyncServerDatasource, e
 		DB:                  d.db,
 		Writer:              d.writer,
 		Invites:             invites,
+		Peeks:               peeks,
 		AccountData:         accountData,
 		OutputEvents:        events,
 		Topology:            topology,
diff --git a/syncapi/storage/shared/syncserver.go b/syncapi/storage/shared/syncserver.go
index 6267dd3a..94580adb 100644
--- a/syncapi/storage/shared/syncserver.go
+++ b/syncapi/storage/shared/syncserver.go
@@ -30,7 +30,7 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/storage/tables"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
-	"github.com/sirupsen/logrus"
+	log "github.com/sirupsen/logrus"
 )
 
 // Database is a temporary struct until we have made syncserver.go the same for both pq/sqlite
@@ -39,6 +39,7 @@ type Database struct {
 	DB                  *sql.DB
 	Writer              sqlutil.Writer
 	Invites             tables.Invites
+	Peeks               tables.Peeks
 	AccountData         tables.AccountData
 	OutputEvents        tables.Events
 	Topology            tables.Topology
@@ -120,6 +121,10 @@ func (d *Database) AllJoinedUsersInRooms(ctx context.Context) (map[string][]stri
 	return d.CurrentRoomState.SelectJoinedUsers(ctx)
 }
 
+func (d *Database) AllPeekingDevicesInRooms(ctx context.Context) (map[string][]types.PeekingDevice, error) {
+	return d.Peeks.SelectPeekingDevices(ctx)
+}
+
 func (d *Database) GetStateEvent(
 	ctx context.Context, roomID, evType, stateKey string,
 ) (*gomatrixserverlib.HeaderedEvent, error) {
@@ -141,7 +146,7 @@ func (d *Database) AddInviteEvent(
 ) (sp types.StreamPosition, err error) {
 	_ = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
 		sp, err = d.Invites.InsertInviteEvent(ctx, txn, inviteEvent)
-		return nil
+		return err
 	})
 	return
 }
@@ -153,11 +158,41 @@ func (d *Database) RetireInviteEvent(
 ) (sp types.StreamPosition, err error) {
 	_ = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
 		sp, err = d.Invites.DeleteInviteEvent(ctx, txn, inviteEventID)
-		return nil
+		return err
 	})
 	return
 }
 
+// AddPeek tracks the fact that a user has started peeking.
+// If the peek was successfully stored this returns the stream ID it was stored at.
+// Returns an error if there was a problem communicating with the database.
+func (d *Database) AddPeek(
+	ctx context.Context, roomID, userID, deviceID string,
+) (sp types.StreamPosition, err error) {
+	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
+		sp, err = d.Peeks.InsertPeek(ctx, txn, roomID, userID, deviceID)
+		return err
+	})
+	return
+}
+
+// DeletePeeks tracks the fact that a user has stopped peeking from all devices
+// If the peeks was successfully deleted this returns the stream ID it was stored at.
+// Returns an error if there was a problem communicating with the database.
+func (d *Database) DeletePeeks(
+	ctx context.Context, roomID, userID string,
+) (sp types.StreamPosition, err error) {
+	err = d.Writer.Do(d.DB, nil, func(txn *sql.Tx) error {
+		sp, err = d.Peeks.DeletePeeks(ctx, txn, roomID, userID)
+		return err
+	})
+	if err == sql.ErrNoRows {
+		sp = 0
+		err = nil
+	}
+	return
+}
+
 // GetAccountDataInRange returns all account data for a given user inserted or
 // updated between two given positions
 // Returns a map following the format data[roomID] = []dataTypes
@@ -196,7 +231,7 @@ func (d *Database) StreamEventsToEvents(device *userapi.Device, in []types.Strea
 					"transaction_id", in[i].TransactionID.TransactionID,
 				)
 				if err != nil {
-					logrus.WithFields(logrus.Fields{
+					log.WithFields(log.Fields{
 						"event_id": out[i].EventID(),
 					}).WithError(err).Warnf("Failed to add transaction ID to event")
 				}
@@ -389,7 +424,6 @@ func (d *Database) EventPositionInTopology(
 func (d *Database) syncPositionTx(
 	ctx context.Context, txn *sql.Tx,
 ) (sp types.StreamingToken, err error) {
-
 	maxEventID, err := d.OutputEvents.SelectMaxEventID(ctx, txn)
 	if err != nil {
 		return sp, err
@@ -408,6 +442,13 @@ func (d *Database) syncPositionTx(
 	if maxInviteID > maxEventID {
 		maxEventID = maxInviteID
 	}
+	maxPeekID, err := d.Peeks.SelectMaxPeekID(ctx, txn)
+	if err != nil {
+		return sp, err
+	}
+	if maxPeekID > maxEventID {
+		maxEventID = maxPeekID
+	}
 	sp = types.NewStreamToken(types.StreamPosition(maxEventID), types.StreamPosition(d.EDUCache.GetLatestSyncPosition()), nil)
 	return
 }
@@ -566,6 +607,8 @@ func (d *Database) IncrementalSync(
 		}
 	}
 
+	// TODO: handle EDUs in peeked rooms
+
 	err = d.addEDUDeltaToResponse(
 		fromPos, toPos, joinedRoomIDs, res,
 	)
@@ -582,7 +625,7 @@ func (d *Database) RedactEvent(ctx context.Context, redactedEventID string, reda
 		return err
 	}
 	if len(redactedEvents) == 0 {
-		logrus.WithField("event_id", redactedEventID).WithField("redaction_event", redactedBecause.EventID()).Warnf("missing redacted event for redaction")
+		log.WithField("event_id", redactedEventID).WithField("redaction_event", redactedBecause.EventID()).Warnf("missing redacted event for redaction")
 		return nil
 	}
 	eventToRedact := redactedEvents[0].Unwrap()
@@ -604,7 +647,7 @@ func (d *Database) RedactEvent(ctx context.Context, redactedEventID string, reda
 // nolint:nakedret
 func (d *Database) getResponseWithPDUsForCompleteSync(
 	ctx context.Context, res *types.Response,
-	userID string,
+	userID string, deviceID string,
 	numRecentEventsPerRoom int,
 ) (
 	toPos types.StreamingToken,
@@ -644,46 +687,32 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 
 	// Build up a /sync response. Add joined rooms.
 	for _, roomID := range joinedRoomIDs {
-		var stateEvents []gomatrixserverlib.HeaderedEvent
-		stateEvents, err = d.CurrentRoomState.SelectCurrentState(ctx, txn, roomID, &stateFilter)
-		if err != nil {
-			return
-		}
-		// TODO: When filters are added, we may need to call this multiple times to get enough events.
-		//       See: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L316
-		var recentStreamEvents []types.StreamEvent
-		var limited bool
-		recentStreamEvents, limited, err = d.OutputEvents.SelectRecentEvents(
-			ctx, txn, roomID, r, numRecentEventsPerRoom, true, true,
+		var jr *types.JoinResponse
+		jr, err = d.getJoinResponseForCompleteSync(
+			ctx, txn, roomID, r, &stateFilter, numRecentEventsPerRoom,
 		)
 		if err != nil {
 			return
 		}
+		res.Rooms.Join[roomID] = *jr
+	}
 
-		// Retrieve the backward topology position, i.e. the position of the
-		// oldest event in the room's topology.
-		var prevBatchStr string
-		if len(recentStreamEvents) > 0 {
-			var backwardTopologyPos, backwardStreamPos types.StreamPosition
-			backwardTopologyPos, backwardStreamPos, err = d.Topology.SelectPositionInTopology(ctx, txn, recentStreamEvents[0].EventID())
+	// Add peeked rooms.
+	peeks, err := d.Peeks.SelectPeeksInRange(ctx, txn, userID, deviceID, r)
+	if err != nil {
+		return
+	}
+	for _, peek := range peeks {
+		if !peek.Deleted {
+			var jr *types.JoinResponse
+			jr, err = d.getJoinResponseForCompleteSync(
+				ctx, txn, peek.RoomID, r, &stateFilter, numRecentEventsPerRoom,
+			)
 			if err != nil {
 				return
 			}
-			prevBatch := types.NewTopologyToken(backwardTopologyPos, backwardStreamPos)
-			prevBatch.Decrement()
-			prevBatchStr = prevBatch.String()
+			res.Rooms.Peek[peek.RoomID] = *jr
 		}
-
-		// We don't include a device here as we don't need to send down
-		// transaction IDs for complete syncs
-		recentEvents := d.StreamEventsToEvents(nil, recentStreamEvents)
-		stateEvents = removeDuplicates(stateEvents, recentEvents)
-		jr := types.NewJoinResponse()
-		jr.Timeline.PrevBatch = prevBatchStr
-		jr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
-		jr.Timeline.Limited = limited
-		jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(stateEvents, gomatrixserverlib.FormatSync)
-		res.Rooms.Join[roomID] = *jr
 	}
 
 	if err = d.addInvitesToResponse(ctx, txn, userID, r, res); err != nil {
@@ -694,17 +723,68 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 	return //res, toPos, joinedRoomIDs, err
 }
 
+func (d *Database) getJoinResponseForCompleteSync(
+	ctx context.Context, txn *sql.Tx,
+	roomID string,
+	r types.Range,
+	stateFilter *gomatrixserverlib.StateFilter,
+	numRecentEventsPerRoom int,
+) (jr *types.JoinResponse, err error) {
+	var stateEvents []gomatrixserverlib.HeaderedEvent
+	stateEvents, err = d.CurrentRoomState.SelectCurrentState(ctx, txn, roomID, stateFilter)
+	if err != nil {
+		return
+	}
+	// TODO: When filters are added, we may need to call this multiple times to get enough events.
+	//       See: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L316
+	var recentStreamEvents []types.StreamEvent
+	var limited bool
+	recentStreamEvents, limited, err = d.OutputEvents.SelectRecentEvents(
+		ctx, txn, roomID, r, numRecentEventsPerRoom, true, true,
+	)
+	if err != nil {
+		return
+	}
+
+	// Retrieve the backward topology position, i.e. the position of the
+	// oldest event in the room's topology.
+	var prevBatchStr string
+	if len(recentStreamEvents) > 0 {
+		var backwardTopologyPos, backwardStreamPos types.StreamPosition
+		backwardTopologyPos, backwardStreamPos, err = d.Topology.SelectPositionInTopology(ctx, txn, recentStreamEvents[0].EventID())
+		if err != nil {
+			return
+		}
+		prevBatch := types.NewTopologyToken(backwardTopologyPos, backwardStreamPos)
+		prevBatch.Decrement()
+		prevBatchStr = prevBatch.String()
+	}
+
+	// We don't include a device here as we don't need to send down
+	// transaction IDs for complete syncs
+	recentEvents := d.StreamEventsToEvents(nil, recentStreamEvents)
+	stateEvents = removeDuplicates(stateEvents, recentEvents)
+	jr = types.NewJoinResponse()
+	jr.Timeline.PrevBatch = prevBatchStr
+	jr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
+	jr.Timeline.Limited = limited
+	jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(stateEvents, gomatrixserverlib.FormatSync)
+	return jr, nil
+}
+
 func (d *Database) CompleteSync(
 	ctx context.Context, res *types.Response,
 	device userapi.Device, numRecentEventsPerRoom int,
 ) (*types.Response, error) {
 	toPos, joinedRoomIDs, err := d.getResponseWithPDUsForCompleteSync(
-		ctx, res, device.UserID, numRecentEventsPerRoom,
+		ctx, res, device.UserID, device.ID, numRecentEventsPerRoom,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("d.getResponseWithPDUsForCompleteSync: %w", err)
 	}
 
+	// TODO: handle EDUs in peeked rooms
+
 	// Use a zero value SyncPosition for fromPos so all EDU states are added.
 	err = d.addEDUDeltaToResponse(
 		types.NewStreamToken(0, 0, nil), toPos, joinedRoomIDs, res,
@@ -803,6 +883,12 @@ func (d *Database) addRoomDeltaToResponse(
 		return err
 	}
 
+	// XXX: should we ever get this far if we have no recent events or state in this room?
+	// in practice we do for peeks, but possibly not joins?
+	if len(recentEvents) == 0 && len(delta.stateEvents) == 0 {
+		return nil
+	}
+
 	switch delta.membership {
 	case gomatrixserverlib.Join:
 		jr := types.NewJoinResponse()
@@ -812,6 +898,14 @@ func (d *Database) addRoomDeltaToResponse(
 		jr.Timeline.Limited = limited
 		jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
 		res.Rooms.Join[delta.roomID] = *jr
+	case gomatrixserverlib.Peek:
+		jr := types.NewJoinResponse()
+
+		jr.Timeline.PrevBatch = prevBatch.String()
+		jr.Timeline.Events = gomatrixserverlib.HeaderedToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
+		jr.Timeline.Limited = limited
+		jr.State.Events = gomatrixserverlib.HeaderedToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
+		res.Rooms.Peek[delta.roomID] = *jr
 	case gomatrixserverlib.Leave:
 		fallthrough // transitions to leave are the same as ban
 	case gomatrixserverlib.Ban:
@@ -918,6 +1012,7 @@ func (d *Database) fetchMissingStateEvents(
 // exclusive of oldPos, inclusive of newPos, for the rooms in which
 // the user has new membership events.
 // A list of joined room IDs is also returned in case the caller needs it.
+// nolint:gocyclo
 func (d *Database) getStateDeltas(
 	ctx context.Context, device *userapi.Device, txn *sql.Tx,
 	r types.Range, userID string,
@@ -933,7 +1028,7 @@ func (d *Database) getStateDeltas(
 	// - Get all CURRENTLY joined rooms, and add them to 'joined' block.
 	var deltas []stateDelta
 
-	// get all the state events ever between these two positions
+	// get all the state events ever (i.e. for all available rooms) between these two positions
 	stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, r, stateFilter)
 	if err != nil {
 		return nil, nil, err
@@ -943,6 +1038,34 @@ func (d *Database) getStateDeltas(
 		return nil, nil, err
 	}
 
+	// find out which rooms this user is peeking, if any.
+	// We do this before joins so any peeks get overwritten
+	peeks, err := d.Peeks.SelectPeeksInRange(ctx, txn, userID, device.ID, r)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// add peek blocks
+	for _, peek := range peeks {
+		if peek.New {
+			// send full room state down instead of a delta
+			var s []types.StreamEvent
+			s, err = d.currentStateStreamEventsForRoom(ctx, txn, peek.RoomID, stateFilter)
+			if err != nil {
+				return nil, nil, err
+			}
+			state[peek.RoomID] = s
+		}
+		if !peek.Deleted {
+			deltas = append(deltas, stateDelta{
+				membership:  gomatrixserverlib.Peek,
+				stateEvents: d.StreamEventsToEvents(device, state[peek.RoomID]),
+				roomID:      peek.RoomID,
+			})
+		}
+	}
+
+	// handle newly joined rooms and non-joined rooms
 	for roomID, stateStreamEvents := range state {
 		for _, ev := range stateStreamEvents {
 			// TODO: Currently this will incorrectly add rooms which were ALREADY joined but they sent another no-op join event.
@@ -993,6 +1116,7 @@ func (d *Database) getStateDeltas(
 // requests with full_state=true.
 // Fetches full state for all joined rooms and uses selectStateInRange to get
 // updates for other rooms.
+// nolint:gocyclo
 func (d *Database) getStateDeltasForFullStateSync(
 	ctx context.Context, device *userapi.Device, txn *sql.Tx,
 	r types.Range, userID string,
@@ -1001,6 +1125,26 @@ func (d *Database) getStateDeltasForFullStateSync(
 	// Use a reasonable initial capacity
 	deltas := make(map[string]stateDelta)
 
+	peeks, err := d.Peeks.SelectPeeksInRange(ctx, txn, userID, device.ID, r)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Add full states for all peeking rooms
+	for _, peek := range peeks {
+		if !peek.Deleted {
+			s, stateErr := d.currentStateStreamEventsForRoom(ctx, txn, peek.RoomID, stateFilter)
+			if stateErr != nil {
+				return nil, nil, stateErr
+			}
+			deltas[peek.RoomID] = stateDelta{
+				membership:  gomatrixserverlib.Peek,
+				stateEvents: d.StreamEventsToEvents(device, s),
+				roomID:      peek.RoomID,
+			}
+		}
+	}
+
 	// Get all the state events ever between these two positions
 	stateNeeded, eventMap, err := d.OutputEvents.SelectStateInRange(ctx, txn, r, stateFilter)
 	if err != nil {
diff --git a/syncapi/storage/sqlite3/peeks_table.go b/syncapi/storage/sqlite3/peeks_table.go
new file mode 100644
index 00000000..d755e28c
--- /dev/null
+++ b/syncapi/storage/sqlite3/peeks_table.go
@@ -0,0 +1,206 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// 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 sqlite3
+
+import (
+	"context"
+	"database/sql"
+	"time"
+
+	"github.com/matrix-org/dendrite/internal"
+	"github.com/matrix-org/dendrite/internal/sqlutil"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+const peeksSchema = `
+CREATE TABLE IF NOT EXISTS syncapi_peeks (
+	id INTEGER,
+	room_id TEXT NOT NULL,
+	user_id TEXT NOT NULL,
+	device_id TEXT NOT NULL,
+	deleted BOOL NOT NULL DEFAULT false,
+    -- When the peek was created in UNIX epoch ms.
+    creation_ts INTEGER NOT NULL,
+    UNIQUE(room_id, user_id, device_id)
+);
+
+CREATE INDEX IF NOT EXISTS syncapi_peeks_room_id_idx ON syncapi_peeks(room_id);
+CREATE INDEX IF NOT EXISTS syncapi_peeks_user_id_device_id_idx ON syncapi_peeks(user_id, device_id);
+`
+
+const insertPeekSQL = "" +
+	"INSERT OR REPLACE INTO syncapi_peeks" +
+	" (id, room_id, user_id, device_id, creation_ts, deleted)" +
+	" VALUES ($1, $2, $3, $4, $5, false)"
+
+const deletePeekSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=$1 WHERE room_id = $2 AND user_id = $3 AND device_id = $4"
+
+const deletePeeksSQL = "" +
+	"UPDATE syncapi_peeks SET deleted=true, id=$1 WHERE room_id = $2 AND user_id = $3"
+
+// we care about all the peeks which were created in this range, deleted in this range,
+// or were created before this range but haven't been deleted yet.
+// BEWARE: sqlite chokes on out of order substitution strings.
+const selectPeeksInRangeSQL = "" +
+	"SELECT id, room_id, deleted FROM syncapi_peeks WHERE user_id = $1 AND device_id = $2 AND ((id <= $3 AND NOT deleted=true) OR (id > $3 AND id <= $4))"
+
+const selectPeekingDevicesSQL = "" +
+	"SELECT room_id, user_id, device_id FROM syncapi_peeks WHERE deleted=false"
+
+const selectMaxPeekIDSQL = "" +
+	"SELECT MAX(id) FROM syncapi_peeks"
+
+type peekStatements struct {
+	db                       *sql.DB
+	streamIDStatements       *streamIDStatements
+	insertPeekStmt           *sql.Stmt
+	deletePeekStmt           *sql.Stmt
+	deletePeeksStmt          *sql.Stmt
+	selectPeeksInRangeStmt   *sql.Stmt
+	selectPeekingDevicesStmt *sql.Stmt
+	selectMaxPeekIDStmt      *sql.Stmt
+}
+
+func NewSqlitePeeksTable(db *sql.DB, streamID *streamIDStatements) (tables.Peeks, error) {
+	_, err := db.Exec(peeksSchema)
+	if err != nil {
+		return nil, err
+	}
+	s := &peekStatements{
+		db:                 db,
+		streamIDStatements: streamID,
+	}
+	if s.insertPeekStmt, err = db.Prepare(insertPeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeekStmt, err = db.Prepare(deletePeekSQL); err != nil {
+		return nil, err
+	}
+	if s.deletePeeksStmt, err = db.Prepare(deletePeeksSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeeksInRangeStmt, err = db.Prepare(selectPeeksInRangeSQL); err != nil {
+		return nil, err
+	}
+	if s.selectPeekingDevicesStmt, err = db.Prepare(selectPeekingDevicesSQL); err != nil {
+		return nil, err
+	}
+	if s.selectMaxPeekIDStmt, err = db.Prepare(selectMaxPeekIDSQL); err != nil {
+		return nil, err
+	}
+	return s, nil
+}
+
+func (s *peekStatements) InsertPeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	streamPos, err = s.streamIDStatements.nextStreamID(ctx, txn)
+	if err != nil {
+		return
+	}
+	nowMilli := time.Now().UnixNano() / int64(time.Millisecond)
+	_, err = sqlutil.TxStmt(txn, s.insertPeekStmt).ExecContext(ctx, streamPos, roomID, userID, deviceID, nowMilli)
+	return
+}
+
+func (s *peekStatements) DeletePeek(
+	ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string,
+) (streamPos types.StreamPosition, err error) {
+	streamPos, err = s.streamIDStatements.nextStreamID(ctx, txn)
+	if err != nil {
+		return
+	}
+	_, err = sqlutil.TxStmt(txn, s.deletePeekStmt).ExecContext(ctx, streamPos, roomID, userID, deviceID)
+	return
+}
+
+func (s *peekStatements) DeletePeeks(
+	ctx context.Context, txn *sql.Tx, roomID, userID string,
+) (types.StreamPosition, error) {
+	streamPos, err := s.streamIDStatements.nextStreamID(ctx, txn)
+	if err != nil {
+		return 0, err
+	}
+	result, err := sqlutil.TxStmt(txn, s.deletePeeksStmt).ExecContext(ctx, streamPos, roomID, userID)
+	if err != nil {
+		return 0, err
+	}
+	numAffected, err := result.RowsAffected()
+	if err != nil {
+		return 0, err
+	}
+	if numAffected == 0 {
+		return 0, sql.ErrNoRows
+	}
+	return streamPos, nil
+}
+
+func (s *peekStatements) SelectPeeksInRange(
+	ctx context.Context, txn *sql.Tx, userID, deviceID string, r types.Range,
+) (peeks []types.Peek, err error) {
+	rows, err := sqlutil.TxStmt(txn, s.selectPeeksInRangeStmt).QueryContext(ctx, userID, deviceID, r.Low(), r.High())
+	if err != nil {
+		return
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeeksInRange: rows.close() failed")
+
+	for rows.Next() {
+		peek := types.Peek{}
+		var id types.StreamPosition
+		if err = rows.Scan(&id, &peek.RoomID, &peek.Deleted); err != nil {
+			return
+		}
+		peek.New = (id > r.Low() && id <= r.High()) && !peek.Deleted
+		peeks = append(peeks, peek)
+	}
+
+	return peeks, rows.Err()
+}
+
+func (s *peekStatements) SelectPeekingDevices(
+	ctx context.Context,
+) (peekingDevices map[string][]types.PeekingDevice, err error) {
+	rows, err := s.selectPeekingDevicesStmt.QueryContext(ctx)
+	if err != nil {
+		return nil, err
+	}
+	defer internal.CloseAndLogIfError(ctx, rows, "SelectPeekingDevices: rows.close() failed")
+
+	result := make(map[string][]types.PeekingDevice)
+	for rows.Next() {
+		var roomID, userID, deviceID string
+		if err := rows.Scan(&roomID, &userID, &deviceID); err != nil {
+			return nil, err
+		}
+		devices := result[roomID]
+		devices = append(devices, types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+		result[roomID] = devices
+	}
+	return result, nil
+}
+
+func (s *peekStatements) SelectMaxPeekID(
+	ctx context.Context, txn *sql.Tx,
+) (id int64, err error) {
+	var nullableID sql.NullInt64
+	stmt := sqlutil.TxStmt(txn, s.selectMaxPeekIDStmt)
+	err = stmt.QueryRowContext(ctx).Scan(&nullableID)
+	if nullableID.Valid {
+		id = nullableID.Int64
+	}
+	return
+}
diff --git a/syncapi/storage/sqlite3/syncserver.go b/syncapi/storage/sqlite3/syncserver.go
index f68bf737..86d83ec9 100644
--- a/syncapi/storage/sqlite3/syncserver.go
+++ b/syncapi/storage/sqlite3/syncserver.go
@@ -75,6 +75,10 @@ func (d *SyncServerDatasource) prepare() (err error) {
 	if err != nil {
 		return err
 	}
+	peeks, err := NewSqlitePeeksTable(d.db, &d.streamID)
+	if err != nil {
+		return err
+	}
 	topology, err := NewSqliteTopologyTable(d.db)
 	if err != nil {
 		return err
@@ -95,6 +99,7 @@ func (d *SyncServerDatasource) prepare() (err error) {
 		DB:                  d.db,
 		Writer:              d.writer,
 		Invites:             invites,
+		Peeks:               peeks,
 		AccountData:         accountData,
 		OutputEvents:        events,
 		BackwardExtremities: bwExtrem,
diff --git a/syncapi/storage/tables/interface.go b/syncapi/storage/tables/interface.go
index 38f6d848..631746c6 100644
--- a/syncapi/storage/tables/interface.go
+++ b/syncapi/storage/tables/interface.go
@@ -39,6 +39,15 @@ type Invites interface {
 	SelectMaxInviteID(ctx context.Context, txn *sql.Tx) (id int64, err error)
 }
 
+type Peeks interface {
+	InsertPeek(ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string) (streamPos types.StreamPosition, err error)
+	DeletePeek(ctx context.Context, txn *sql.Tx, roomID, userID, deviceID string) (streamPos types.StreamPosition, err error)
+	DeletePeeks(ctx context.Context, txn *sql.Tx, roomID, userID string) (streamPos types.StreamPosition, err error)
+	SelectPeeksInRange(ctxt context.Context, txn *sql.Tx, userID, deviceID string, r types.Range) (peeks []types.Peek, err error)
+	SelectPeekingDevices(ctxt context.Context) (peekingDevices map[string][]types.PeekingDevice, err error)
+	SelectMaxPeekID(ctx context.Context, txn *sql.Tx) (id int64, err error)
+}
+
 type Events interface {
 	SelectStateInRange(ctx context.Context, txn *sql.Tx, r types.Range, stateFilter *gomatrixserverlib.StateFilter) (map[string]map[string]bool, map[string]types.StreamEvent, error)
 	SelectMaxEventID(ctx context.Context, txn *sql.Tx) (id int64, err error)
diff --git a/syncapi/sync/notifier.go b/syncapi/sync/notifier.go
index df23a2f4..fcac3f16 100644
--- a/syncapi/sync/notifier.go
+++ b/syncapi/sync/notifier.go
@@ -33,6 +33,8 @@ import (
 type Notifier struct {
 	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
 	roomIDToJoinedUsers map[string]userIDSet
+	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
+	roomIDToPeekingDevices map[string]peekingDeviceSet
 	// Protects currPos and userStreams.
 	streamLock *sync.Mutex
 	// The latest sync position
@@ -48,11 +50,12 @@ type Notifier struct {
 // the joined users within each of them by calling Notifier.Load(*storage.SyncServerDatabase).
 func NewNotifier(pos types.StreamingToken) *Notifier {
 	return &Notifier{
-		currPos:             pos,
-		roomIDToJoinedUsers: make(map[string]userIDSet),
-		userDeviceStreams:   make(map[string]map[string]*UserDeviceStream),
-		streamLock:          &sync.Mutex{},
-		lastCleanUpTime:     time.Now(),
+		currPos:                pos,
+		roomIDToJoinedUsers:    make(map[string]userIDSet),
+		roomIDToPeekingDevices: make(map[string]peekingDeviceSet),
+		userDeviceStreams:      make(map[string]map[string]*UserDeviceStream),
+		streamLock:             &sync.Mutex{},
+		lastCleanUpTime:        time.Now(),
 	}
 }
 
@@ -82,6 +85,8 @@ func (n *Notifier) OnNewEvent(
 	if ev != nil {
 		// Map this event's room_id to a list of joined users, and wake them up.
 		usersToNotify := n.joinedUsers(ev.RoomID())
+		// Map this event's room_id to a list of peeking devices, and wake them up.
+		peekingDevicesToNotify := n.PeekingDevices(ev.RoomID())
 		// If this is an invite, also add in the invitee to this list.
 		if ev.Type() == "m.room.member" && ev.StateKey() != nil {
 			targetUserID := *ev.StateKey()
@@ -108,11 +113,11 @@ func (n *Notifier) OnNewEvent(
 			}
 		}
 
-		n.wakeupUsers(usersToNotify, latestPos)
+		n.wakeupUsers(usersToNotify, peekingDevicesToNotify, latestPos)
 	} else if roomID != "" {
-		n.wakeupUsers(n.joinedUsers(roomID), latestPos)
+		n.wakeupUsers(n.joinedUsers(roomID), n.PeekingDevices(roomID), latestPos)
 	} else if len(userIDs) > 0 {
-		n.wakeupUsers(userIDs, latestPos)
+		n.wakeupUsers(userIDs, nil, latestPos)
 	} else {
 		log.WithFields(log.Fields{
 			"posUpdate": posUpdate.String,
@@ -120,6 +125,18 @@ func (n *Notifier) OnNewEvent(
 	}
 }
 
+func (n *Notifier) OnNewPeek(
+	roomID, userID, deviceID string,
+) {
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
+
+	n.addPeekingDevice(roomID, userID, deviceID)
+
+	// we don't wake up devices here given the roomserver consumer will do this shortly afterwards
+	// by calling OnNewEvent.
+}
+
 func (n *Notifier) OnNewSendToDevice(
 	userID string, deviceIDs []string,
 	posUpdate types.StreamingToken,
@@ -139,7 +156,7 @@ func (n *Notifier) OnNewKeyChange(
 	defer n.streamLock.Unlock()
 	latestPos := n.currPos.WithUpdates(posUpdate)
 	n.currPos = latestPos
-	n.wakeupUsers([]string{wakeUserID}, latestPos)
+	n.wakeupUsers([]string{wakeUserID}, nil, latestPos)
 }
 
 // GetListener returns a UserStreamListener that can be used to wait for
@@ -169,6 +186,13 @@ func (n *Notifier) Load(ctx context.Context, db storage.Database) error {
 		return err
 	}
 	n.setUsersJoinedToRooms(roomToUsers)
+
+	roomToPeekingDevices, err := db.AllPeekingDevicesInRooms(ctx)
+	if err != nil {
+		return err
+	}
+	n.setPeekingDevices(roomToPeekingDevices)
+
 	return nil
 }
 
@@ -195,9 +219,24 @@ func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
 	}
 }
 
+// setPeekingDevices marks the given devices as peeking in the given rooms, such that new events from
+// these rooms will wake the given devices' /sync requests. This should be called prior to ANY calls to
+// OnNewEvent (eg on startup) to prevent racing.
+func (n *Notifier) setPeekingDevices(roomIDToPeekingDevices map[string][]types.PeekingDevice) {
+	// This is just the bulk form of addPeekingDevice
+	for roomID, peekingDevices := range roomIDToPeekingDevices {
+		if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+			n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+		}
+		for _, peekingDevice := range peekingDevices {
+			n.roomIDToPeekingDevices[roomID].add(peekingDevice)
+		}
+	}
+}
+
 // wakeupUsers will wake up the sync strems for all of the devices for all of the
-// specified user IDs.
-func (n *Notifier) wakeupUsers(userIDs []string, newPos types.StreamingToken) {
+// specified user IDs, and also the specified peekingDevices
+func (n *Notifier) wakeupUsers(userIDs []string, peekingDevices []types.PeekingDevice, newPos types.StreamingToken) {
 	for _, userID := range userIDs {
 		for _, stream := range n.fetchUserStreams(userID) {
 			if stream == nil {
@@ -206,6 +245,13 @@ func (n *Notifier) wakeupUsers(userIDs []string, newPos types.StreamingToken) {
 			stream.Broadcast(newPos) // wake up all goroutines Wait()ing on this stream
 		}
 	}
+
+	for _, peekingDevice := range peekingDevices {
+		// TODO: don't bother waking up for devices whose users we already woke up
+		if stream := n.fetchUserDeviceStream(peekingDevice.UserID, peekingDevice.DeviceID, false); stream != nil {
+			stream.Broadcast(newPos) // wake up all goroutines Wait()ing on this stream
+		}
+	}
 }
 
 // wakeupUserDevice will wake up the sync stream for a specific user device. Other
@@ -284,6 +330,32 @@ func (n *Notifier) joinedUsers(roomID string) (userIDs []string) {
 	return n.roomIDToJoinedUsers[roomID].values()
 }
 
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) addPeekingDevice(roomID, userID, deviceID string) {
+	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+		n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+	}
+	n.roomIDToPeekingDevices[roomID].add(types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+// nolint:unused
+func (n *Notifier) removePeekingDevice(roomID, userID, deviceID string) {
+	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+		n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+	}
+	// XXX: is this going to work as a key?
+	n.roomIDToPeekingDevices[roomID].remove(types.PeekingDevice{UserID: userID, DeviceID: deviceID})
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) PeekingDevices(roomID string) (peekingDevices []types.PeekingDevice) {
+	if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
+		return
+	}
+	return n.roomIDToPeekingDevices[roomID].values()
+}
+
 // removeEmptyUserStreams iterates through the user stream map and removes any
 // that have been empty for a certain amount of time. This is a crude way of
 // ensuring that the userStreams map doesn't grow forver.
@@ -329,3 +401,23 @@ func (s userIDSet) values() (vals []string) {
 	}
 	return
 }
+
+// A set of PeekingDevices, similar to userIDSet
+
+type peekingDeviceSet map[types.PeekingDevice]bool
+
+func (s peekingDeviceSet) add(d types.PeekingDevice) {
+	s[d] = true
+}
+
+// nolint:unused
+func (s peekingDeviceSet) remove(d types.PeekingDevice) {
+	delete(s, d)
+}
+
+func (s peekingDeviceSet) values() (vals []types.PeekingDevice) {
+	for d := range s {
+		vals = append(vals, d)
+	}
+	return
+}
diff --git a/syncapi/types/types.go b/syncapi/types/types.go
index f3324800..2499976e 100644
--- a/syncapi/types/types.go
+++ b/syncapi/types/types.go
@@ -388,6 +388,7 @@ type Response struct {
 	} `json:"presence,omitempty"`
 	Rooms struct {
 		Join   map[string]JoinResponse   `json:"join"`
+		Peek   map[string]JoinResponse   `json:"peek"`
 		Invite map[string]InviteResponse `json:"invite"`
 		Leave  map[string]LeaveResponse  `json:"leave"`
 	} `json:"rooms"`
@@ -407,6 +408,7 @@ func NewResponse() *Response {
 	// Pre-initialise the maps. Synapse will return {} even if there are no rooms under a specific section,
 	// so let's do the same thing. Bonus: this means we can't get dreaded 'assignment to entry in nil map' errors.
 	res.Rooms.Join = make(map[string]JoinResponse)
+	res.Rooms.Peek = make(map[string]JoinResponse)
 	res.Rooms.Invite = make(map[string]InviteResponse)
 	res.Rooms.Leave = make(map[string]LeaveResponse)
 
@@ -433,7 +435,7 @@ func (r *Response) IsEmpty() bool {
 		len(r.ToDevice.Events) == 0
 }
 
-// JoinResponse represents a /sync response for a room which is under the 'join' key.
+// JoinResponse represents a /sync response for a room which is under the 'join' or 'peek' key.
 type JoinResponse struct {
 	State struct {
 		Events []gomatrixserverlib.ClientEvent `json:"events"`
@@ -507,3 +509,14 @@ type SendToDeviceEvent struct {
 	DeviceID    string
 	SentByToken *StreamingToken
 }
+
+type PeekingDevice struct {
+	UserID   string
+	DeviceID string
+}
+
+type Peek struct {
+	RoomID  string
+	New     bool
+	Deleted bool
+}