CS API: Support for /messages, fixes for /sync (#847)

* Merge forward

* Tidy up a bit

* TODO: What to do with NextBatch here?

* Replace SyncPosition with PaginationToken throughout syncapi

* Fix PaginationTokens

* Fix lint errors

* Add a couple of missing functions into the syncapi external storage interface

* Some updates based on review comments from @babolivier

* Some updates based on review comments from @babolivier

* argh whitespacing

* Fix opentracing span

* Remove dead code

* Don't overshadow err (fix lint issue)

* Handle extremities after inserting event into topology

* Try insert event topology as ON CONFLICT DO NOTHING

* Prevent OOB error in addRoomDeltaToResponse

* Thwarted by gocyclo again

* Fix NewPaginationTokenFromString, define unit test for it

* Update pagination token test

* Update sytest-whitelist

* Hopefully fix some of the sync batch tokens

* Remove extraneous sync position func

* Revert to topology tokens in addRoomDeltaToResponse etc

* Fix typo

* Remove prevPDUPos as dead now that backwardTopologyPos is used instead

* Fix selectEventsWithEventIDsSQL

* Update sytest-blacklist

* Update sytest-whitelist

21 files changed, 1502 insertions, 281 deletions

diff --git a/syncapi/consumers/clientapi.go b/syncapi/consumers/clientapi.go
index ed39cd2d..17f2c522 100644
--- a/syncapi/consumers/clientapi.go
+++ b/syncapi/consumers/clientapi.go
@@ -90,7 +90,7 @@ func (s *OutputClientDataConsumer) onMessage(msg *sarama.ConsumerMessage) error
 		}).Panicf("could not save account data")
 	}
 
-	s.notifier.OnNewEvent(nil, "", []string{string(msg.Key)}, types.SyncPosition{PDUPosition: pduPos})
+	s.notifier.OnNewEvent(nil, "", []string{string(msg.Key)}, types.PaginationToken{PDUPosition: pduPos})
 
 	return nil
 }
diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index cde2f508..ba1b7dc5 100644
--- a/syncapi/consumers/roomserver.go
+++ b/syncapi/consumers/roomserver.go
@@ -133,6 +133,7 @@ func (s *OutputRoomEventConsumer) onNewRoomEvent(
 		msg.AddsStateEventIDs,
 		msg.RemovesStateEventIDs,
 		msg.TransactionID,
+		false,
 	)
 	if err != nil {
 		// panic rather than continue with an inconsistent database
@@ -144,7 +145,7 @@ func (s *OutputRoomEventConsumer) onNewRoomEvent(
 		}).Panicf("roomserver output log: write event failure")
 		return nil
 	}
-	s.notifier.OnNewEvent(&ev, "", nil, types.SyncPosition{PDUPosition: pduPos})
+	s.notifier.OnNewEvent(&ev, "", nil, types.PaginationToken{PDUPosition: pduPos})
 
 	return nil
 }
@@ -161,7 +162,7 @@ func (s *OutputRoomEventConsumer) onNewInviteEvent(
 		}).Panicf("roomserver output log: write invite failure")
 		return nil
 	}
-	s.notifier.OnNewEvent(&msg.Event, "", nil, types.SyncPosition{PDUPosition: pduPos})
+	s.notifier.OnNewEvent(&msg.Event, "", nil, types.PaginationToken{PDUPosition: pduPos})
 	return nil
 }
 
diff --git a/syncapi/consumers/typingserver.go b/syncapi/consumers/typingserver.go
index 392f7987..36925441 100644
--- a/syncapi/consumers/typingserver.go
+++ b/syncapi/consumers/typingserver.go
@@ -63,7 +63,12 @@ func NewOutputTypingEventConsumer(
 // Start consuming from typing api
 func (s *OutputTypingEventConsumer) Start() error {
 	s.db.SetTypingTimeoutCallback(func(userID, roomID string, latestSyncPosition int64) {
-		s.notifier.OnNewEvent(nil, roomID, nil, types.SyncPosition{TypingPosition: latestSyncPosition})
+		s.notifier.OnNewEvent(
+			nil, roomID, nil,
+			types.PaginationToken{
+				EDUTypingPosition: types.StreamPosition(latestSyncPosition),
+			},
+		)
 	})
 
 	return s.typingConsumer.Start()
@@ -83,7 +88,7 @@ func (s *OutputTypingEventConsumer) onMessage(msg *sarama.ConsumerMessage) error
 		"typing":  output.Event.Typing,
 	}).Debug("received data from typing server")
 
-	var typingPos int64
+	var typingPos types.StreamPosition
 	typingEvent := output.Event
 	if typingEvent.Typing {
 		typingPos = s.db.AddTypingUser(typingEvent.UserID, typingEvent.RoomID, output.ExpireTime)
@@ -91,6 +96,6 @@ func (s *OutputTypingEventConsumer) onMessage(msg *sarama.ConsumerMessage) error
 		typingPos = s.db.RemoveTypingUser(typingEvent.UserID, typingEvent.RoomID)
 	}
 
-	s.notifier.OnNewEvent(nil, output.Event.RoomID, nil, types.SyncPosition{TypingPosition: typingPos})
+	s.notifier.OnNewEvent(nil, output.Event.RoomID, nil, types.PaginationToken{EDUTypingPosition: typingPos})
 	return nil
 }
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
new file mode 100644
index 00000000..26f48ca4
--- /dev/null
+++ b/syncapi/routing/messages.go
@@ -0,0 +1,482 @@
+// Copyright 2018 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package routing
+
+import (
+	"context"
+	"net/http"
+	"sort"
+	"strconv"
+
+	"github.com/matrix-org/dendrite/clientapi/httputil"
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/syncapi/storage"
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	log "github.com/sirupsen/logrus"
+)
+
+type messagesReq struct {
+	ctx              context.Context
+	db               storage.Database
+	queryAPI         api.RoomserverQueryAPI
+	federation       *gomatrixserverlib.FederationClient
+	cfg              *config.Dendrite
+	roomID           string
+	from             *types.PaginationToken
+	to               *types.PaginationToken
+	wasToProvided    bool
+	limit            int
+	backwardOrdering bool
+}
+
+type messagesResp struct {
+	Start string                          `json:"start"`
+	End   string                          `json:"end"`
+	Chunk []gomatrixserverlib.ClientEvent `json:"chunk"`
+}
+
+const defaultMessagesLimit = 10
+
+// OnIncomingMessagesRequest implements the /messages endpoint from the
+// client-server API.
+// See: https://matrix.org/docs/spec/client_server/latest.html#get-matrix-client-r0-rooms-roomid-messages
+func OnIncomingMessagesRequest(
+	req *http.Request, db storage.Database, roomID string,
+	federation *gomatrixserverlib.FederationClient,
+	queryAPI api.RoomserverQueryAPI,
+	cfg *config.Dendrite,
+) util.JSONResponse {
+	var err error
+
+	// Extract parameters from the request's URL.
+	// Pagination tokens.
+	from, err := types.NewPaginationTokenFromString(req.URL.Query().Get("from"))
+	if err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.InvalidArgumentValue("Invalid from parameter: " + err.Error()),
+		}
+	}
+
+	// Direction to return events from.
+	dir := req.URL.Query().Get("dir")
+	if dir != "b" && dir != "f" {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.MissingArgument("Bad or missing dir query parameter (should be either 'b' or 'f')"),
+		}
+	}
+	// A boolean is easier to handle in this case, especially since dir is sure
+	// to have one of the two accepted values (so dir == "f" <=> !backwardOrdering).
+	backwardOrdering := (dir == "b")
+
+	// Pagination tokens. To is optional, and its default value depends on the
+	// direction ("b" or "f").
+	var to *types.PaginationToken
+	wasToProvided := true
+	if s := req.URL.Query().Get("to"); len(s) > 0 {
+		to, err = types.NewPaginationTokenFromString(s)
+		if err != nil {
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidArgumentValue("Invalid to parameter: " + err.Error()),
+			}
+		}
+	} else {
+		// If "to" isn't provided, it defaults to either the earliest stream
+		// position (if we're going backward) or to the latest one (if we're
+		// going forward).
+		to, err = setToDefault(req.Context(), db, backwardOrdering, roomID)
+		if err != nil {
+			return httputil.LogThenError(req, err)
+		}
+		wasToProvided = false
+	}
+
+	// Maximum number of events to return; defaults to 10.
+	limit := defaultMessagesLimit
+	if len(req.URL.Query().Get("limit")) > 0 {
+		limit, err = strconv.Atoi(req.URL.Query().Get("limit"))
+
+		if err != nil {
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.InvalidArgumentValue("limit could not be parsed into an integer: " + err.Error()),
+			}
+		}
+	}
+	// TODO: Implement filtering (#587)
+
+	// Check the room ID's format.
+	if _, _, err = gomatrixserverlib.SplitID('!', roomID); err != nil {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.MissingArgument("Bad room ID: " + err.Error()),
+		}
+	}
+
+	mReq := messagesReq{
+		ctx:              req.Context(),
+		db:               db,
+		queryAPI:         queryAPI,
+		federation:       federation,
+		cfg:              cfg,
+		roomID:           roomID,
+		from:             from,
+		to:               to,
+		wasToProvided:    wasToProvided,
+		limit:            limit,
+		backwardOrdering: backwardOrdering,
+	}
+
+	clientEvents, start, end, err := mReq.retrieveEvents()
+	if err != nil {
+		return httputil.LogThenError(req, err)
+	}
+
+	// Respond with the events.
+	return util.JSONResponse{
+		Code: http.StatusOK,
+		JSON: messagesResp{
+			Chunk: clientEvents,
+			Start: start.String(),
+			End:   end.String(),
+		},
+	}
+}
+
+// retrieveEvents retrieve events from the local database for a request on
+// /messages. If there's not enough events to retrieve, it asks another
+// homeserver in the room for older events.
+// Returns an error if there was an issue talking to the database or with the
+// remote homeserver.
+func (r *messagesReq) retrieveEvents() (
+	clientEvents []gomatrixserverlib.ClientEvent, start,
+	end *types.PaginationToken, err error,
+) {
+	// Retrieve the events from the local database.
+	streamEvents, err := r.db.GetEventsInRange(
+		r.ctx, r.from, r.to, r.roomID, r.limit, r.backwardOrdering,
+	)
+	if err != nil {
+		return
+	}
+
+	var events []gomatrixserverlib.Event
+
+	// There can be two reasons for streamEvents to be empty: either we've
+	// reached the oldest event in the room (or the most recent one, depending
+	// on the ordering), or we've reached a backward extremity.
+	if len(streamEvents) == 0 {
+		if events, err = r.handleEmptyEventsSlice(); err != nil {
+			return
+		}
+	} else {
+		if events, err = r.handleNonEmptyEventsSlice(streamEvents); err != nil {
+			return
+		}
+	}
+
+	// If we didn't get any event, we don't need to proceed any further.
+	if len(events) == 0 {
+		return []gomatrixserverlib.ClientEvent{}, r.from, r.to, nil
+	}
+
+	// Sort the events to ensure we send them in the right order. We currently
+	// do that based on the event's timestamp.
+	if r.backwardOrdering {
+		sort.SliceStable(events, func(i int, j int) bool {
+			// Backward ordering is antichronological (latest event to oldest
+			// one).
+			return sortEvents(&(events[j]), &(events[i]))
+		})
+	} else {
+		sort.SliceStable(events, func(i int, j int) bool {
+			// Forward ordering is chronological (oldest event to latest one).
+			return sortEvents(&(events[i]), &(events[j]))
+		})
+	}
+
+	// Convert all of the events into client events.
+	clientEvents = gomatrixserverlib.ToClientEvents(events, gomatrixserverlib.FormatAll)
+	// Get the position of the first and the last event in the room's topology.
+	// This position is currently determined by the event's depth, so we could
+	// also use it instead of retrieving from the database. However, if we ever
+	// change the way topological positions are defined (as depth isn't the most
+	// reliable way to define it), it would be easier and less troublesome to
+	// only have to change it in one place, i.e. the database.
+	startPos, err := r.db.EventPositionInTopology(
+		r.ctx, streamEvents[0].EventID(),
+	)
+	if err != nil {
+		return
+	}
+	endPos, err := r.db.EventPositionInTopology(
+		r.ctx, streamEvents[len(streamEvents)-1].EventID(),
+	)
+	if err != nil {
+		return
+	}
+	// Generate pagination tokens to send to the client using the positions
+	// retrieved previously.
+	start = types.NewPaginationTokenFromTypeAndPosition(
+		types.PaginationTokenTypeTopology, startPos, 0,
+	)
+	end = types.NewPaginationTokenFromTypeAndPosition(
+		types.PaginationTokenTypeTopology, endPos, 0,
+	)
+
+	if r.backwardOrdering {
+		// A stream/topological position is a cursor located between two events.
+		// While they are identified in the code by the event on their right (if
+		// we consider a left to right chronological order), tokens need to refer
+		// to them by the event on their left, therefore we need to decrement the
+		// end position we send in the response if we're going backward.
+		end.PDUPosition--
+	}
+
+	// The lowest token value is 1, therefore we need to manually set it to that
+	// value if we're below it.
+	if end.PDUPosition < types.StreamPosition(1) {
+		end.PDUPosition = types.StreamPosition(1)
+	}
+
+	return clientEvents, start, end, err
+}
+
+// handleEmptyEventsSlice handles the case where the initial request to the
+// database returned an empty slice of events. It does so by checking whether
+// the set is empty because we've reached a backward extremity, and if that is
+// the case, by retrieving as much events as requested by backfilling from
+// another homeserver.
+// Returns an error if there was an issue talking with the database or
+// backfilling.
+func (r *messagesReq) handleEmptyEventsSlice() (
+	events []gomatrixserverlib.Event, err error,
+) {
+	backwardExtremities, err := r.db.BackwardExtremitiesForRoom(r.ctx, r.roomID)
+
+	// Check if we have backward extremities for this room.
+	if len(backwardExtremities) > 0 {
+		// If so, retrieve as much events as needed through backfilling.
+		events, err = r.backfill(backwardExtremities, r.limit)
+		if err != nil {
+			return
+		}
+	} else {
+		// If not, it means the slice was empty because we reached the room's
+		// creation, so return an empty slice.
+		events = []gomatrixserverlib.Event{}
+	}
+
+	return
+}
+
+// handleNonEmptyEventsSlice handles the case where the initial request to the
+// database returned a non-empty slice of events. It does so by checking whether
+// events are missing from the expected result, and retrieve missing events
+// through backfilling if needed.
+// Returns an error if there was an issue while backfilling.
+func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent) (
+	events []gomatrixserverlib.Event, err error,
+) {
+	// Check if we have enough events.
+	isSetLargeEnough := true
+	if len(streamEvents) < r.limit {
+		if r.backwardOrdering {
+			if r.wasToProvided {
+				// The condition in the SQL query is a strict "greater than" so
+				// we need to check against to-1.
+				streamPos := types.StreamPosition(streamEvents[len(streamEvents)-1].StreamPosition)
+				isSetLargeEnough = (r.to.PDUPosition-1 == streamPos)
+			}
+		} else {
+			streamPos := types.StreamPosition(streamEvents[0].StreamPosition)
+			isSetLargeEnough = (r.from.PDUPosition-1 == streamPos)
+		}
+	}
+
+	// Check if the slice contains a backward extremity.
+	backwardExtremities, err := r.db.BackwardExtremitiesForRoom(r.ctx, r.roomID)
+	if err != nil {
+		return
+	}
+
+	// Backfill is needed if we've reached a backward extremity and need more
+	// events. It's only needed if the direction is backward.
+	if len(backwardExtremities) > 0 && !isSetLargeEnough && r.backwardOrdering {
+		var pdus []gomatrixserverlib.Event
+		// Only ask the remote server for enough events to reach the limit.
+		pdus, err = r.backfill(backwardExtremities, r.limit-len(streamEvents))
+		if err != nil {
+			return
+		}
+
+		// Append the PDUs to the list to send back to the client.
+		events = append(events, pdus...)
+	}
+
+	// Append the events ve previously retrieved locally.
+	events = append(events, r.db.StreamEventsToEvents(nil, streamEvents)...)
+
+	return
+}
+
+// containsBackwardExtremity checks if a slice of StreamEvent contains a
+// backward extremity. It does so by selecting the earliest event in the slice
+// and by checking the presence in the database of all of its parent events, and
+// considers the event itself a backward extremity if at least one of the parent
+// events doesn't exist in the database.
+// Returns an error if there was an issue with talking to the database.
+func (r *messagesReq) containsBackwardExtremity(events []types.StreamEvent) (bool, error) {
+	// Select the earliest retrieved event.
+	var ev *types.StreamEvent
+	if r.backwardOrdering {
+		ev = &(events[len(events)-1])
+	} else {
+		ev = &(events[0])
+	}
+	// Get the earliest retrieved event's parents.
+	prevIDs := ev.PrevEventIDs()
+	prevs, err := r.db.Events(r.ctx, prevIDs)
+	if err != nil {
+		return false, nil
+	}
+	// Check if we have all of the events we requested. If not, it means we've
+	// reached a backward extremity.
+	var eventInDB bool
+	var id string
+	// Iterate over the IDs we used in the request.
+	for _, id = range prevIDs {
+		eventInDB = false
+		// Iterate over the events we got in response.
+		for _, ev := range prevs {
+			if ev.EventID() == id {
+				eventInDB = true
+			}
+		}
+		// One occurrence of one the event's parents not being present in the
+		// database is enough to say that the event is a backward extremity.
+		if !eventInDB {
+			return true, nil
+		}
+	}
+
+	return false, nil
+}
+
+// backfill performs a backfill request over the federation on another
+// homeserver in the room.
+// See: https://matrix.org/docs/spec/server_server/latest#get-matrix-federation-v1-backfill-roomid
+// It also stores the PDUs retrieved from the remote homeserver's response to
+// the database.
+// Returns with an empty string if the remote homeserver didn't return with any
+// event, or if there is no remote homeserver to contact.
+// Returns an error if there was an issue with retrieving the list of servers in
+// the room or sending the request.
+func (r *messagesReq) backfill(fromEventIDs []string, limit int) ([]gomatrixserverlib.Event, error) {
+	// Query the list of servers in the room when one of the backward extremities
+	// was sent.
+	var serversResponse api.QueryServersInRoomAtEventResponse
+	serversRequest := api.QueryServersInRoomAtEventRequest{
+		RoomID:  r.roomID,
+		EventID: fromEventIDs[0],
+	}
+	if err := r.queryAPI.QueryServersInRoomAtEvent(r.ctx, &serversRequest, &serversResponse); err != nil {
+		return nil, err
+	}
+
+	// Use the first server from the response, except if that server is us.
+	// In that case, use the second one if the roomserver responded with
+	// enough servers. If not, use an empty string to prevent the backfill
+	// from happening as there's no server to direct the request towards.
+	// TODO: Be smarter at selecting the server to direct the request
+	// towards.
+	srvToBackfillFrom := serversResponse.Servers[0]
+	if srvToBackfillFrom == r.cfg.Matrix.ServerName {
+		if len(serversResponse.Servers) > 1 {
+			srvToBackfillFrom = serversResponse.Servers[1]
+		} else {
+			srvToBackfillFrom = gomatrixserverlib.ServerName("")
+			log.Warn("Not enough servers to backfill from")
+		}
+	}
+
+	pdus := make([]gomatrixserverlib.Event, 0)
+
+	// If the roomserver responded with at least one server that isn't us,
+	// send it a request for backfill.
+	if len(srvToBackfillFrom) > 0 {
+		txn, err := r.federation.Backfill(
+			r.ctx, srvToBackfillFrom, r.roomID, limit, fromEventIDs,
+		)
+		if err != nil {
+			return nil, err
+		}
+
+		pdus = txn.PDUs
+
+		// Store the events in the database, while marking them as unfit to show
+		// up in responses to sync requests.
+		for _, pdu := range pdus {
+			if _, err = r.db.WriteEvent(
+				r.ctx, &pdu, []gomatrixserverlib.Event{}, []string{}, []string{},
+				nil, true,
+			); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return pdus, nil
+}
+
+// setToDefault returns the default value for the "to" query parameter of a
+// request to /messages if not provided. It defaults to either the earliest
+// topological position (if we're going backward) or to the latest one (if we're
+// going forward).
+// Returns an error if there was an issue with retrieving the latest position
+// from the database
+func setToDefault(
+	ctx context.Context, db storage.Database, backwardOrdering bool,
+	roomID string,
+) (to *types.PaginationToken, err error) {
+	if backwardOrdering {
+		to = types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, 1, 0)
+	} else {
+		var pos types.StreamPosition
+		pos, err = db.MaxTopologicalPosition(ctx, roomID)
+		if err != nil {
+			return
+		}
+
+		to = types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, pos, 0)
+	}
+
+	return
+}
+
+// sortEvents is a function to give to sort.SliceStable, and compares the
+// timestamp of two Matrix events.
+// Returns true if the first event happened before the second one, false
+// otherwise.
+func sortEvents(e1 *gomatrixserverlib.Event, e2 *gomatrixserverlib.Event) bool {
+	t := e1.OriginServerTS().Time()
+	return e2.OriginServerTS().Time().After(t)
+}
diff --git a/syncapi/routing/routing.go b/syncapi/routing/routing.go
index bd9389bd..8916565d 100644
--- a/syncapi/routing/routing.go
+++ b/syncapi/routing/routing.go
@@ -22,8 +22,11 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/devices"
 	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/sync"
+	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
 )
 
@@ -34,7 +37,12 @@ const pathPrefixR0 = "/_matrix/client/r0"
 // Due to Setup being used to call many other functions, a gocyclo nolint is
 // applied:
 // nolint: gocyclo
-func Setup(apiMux *mux.Router, srp *sync.RequestPool, syncDB storage.Database, deviceDB *devices.Database) {
+func Setup(
+	apiMux *mux.Router, srp *sync.RequestPool, syncDB storage.Database,
+	deviceDB *devices.Database, federation *gomatrixserverlib.FederationClient,
+	queryAPI api.RoomserverQueryAPI,
+	cfg *config.Dendrite,
+) {
 	r0mux := apiMux.PathPrefix(pathPrefixR0).Subrouter()
 
 	authData := auth.Data{
@@ -71,4 +79,12 @@ func Setup(apiMux *mux.Router, srp *sync.RequestPool, syncDB storage.Database, d
 		}
 		return OnIncomingStateTypeRequest(req, syncDB, vars["roomID"], vars["type"], vars["stateKey"])
 	})).Methods(http.MethodGet, http.MethodOptions)
+
+	r0mux.Handle("/rooms/{roomID}/messages", common.MakeAuthAPI("room_messages", authData, func(req *http.Request, device *authtypes.Device) util.JSONResponse {
+		vars, err := common.URLDecodeMapValues(mux.Vars(req))
+		if err != nil {
+			return util.ErrorResponse(err)
+		}
+		return OnIncomingMessagesRequest(req, syncDB, vars["roomID"], federation, queryAPI, cfg)
+	})).Methods(http.MethodGet, http.MethodOptions)
 }
diff --git a/syncapi/storage/postgres/account_data_table.go b/syncapi/storage/postgres/account_data_table.go
index 36ba88cd..94e6ac41 100644
--- a/syncapi/storage/postgres/account_data_table.go
+++ b/syncapi/storage/postgres/account_data_table.go
@@ -21,6 +21,7 @@ import (
 
 	"github.com/lib/pq"
 	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrix"
 )
 
@@ -89,7 +90,7 @@ func (s *accountDataStatements) prepare(db *sql.DB) (err error) {
 func (s *accountDataStatements) insertAccountData(
 	ctx context.Context,
 	userID, roomID, dataType string,
-) (pos int64, err error) {
+) (pos types.StreamPosition, err error) {
 	err = s.insertAccountDataStmt.QueryRowContext(ctx, userID, roomID, dataType).Scan(&pos)
 	return
 }
@@ -97,7 +98,7 @@ func (s *accountDataStatements) insertAccountData(
 func (s *accountDataStatements) selectAccountDataInRange(
 	ctx context.Context,
 	userID string,
-	oldPos, newPos int64,
+	oldPos, newPos types.StreamPosition,
 	accountDataFilterPart *gomatrix.FilterPart,
 ) (data map[string][]string, err error) {
 	data = make(map[string][]string)
diff --git a/syncapi/storage/postgres/backward_extremities_table.go b/syncapi/storage/postgres/backward_extremities_table.go
new file mode 100644
index 00000000..476d26fa
--- /dev/null
+++ b/syncapi/storage/postgres/backward_extremities_table.go
@@ -0,0 +1,118 @@
+// Copyright 2018 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package postgres
+
+import (
+	"context"
+	"database/sql"
+)
+
+const backwardExtremitiesSchema = `
+-- Stores output room events received from the roomserver.
+CREATE TABLE IF NOT EXISTS syncapi_backward_extremities (
+	-- The 'room_id' key for the event.
+	room_id TEXT NOT NULL,
+	-- The event ID for the event.
+	event_id TEXT NOT NULL,
+
+	PRIMARY KEY(room_id, event_id)
+);
+`
+
+const insertBackwardExtremitySQL = "" +
+	"INSERT INTO syncapi_backward_extremities (room_id, event_id)" +
+	" VALUES ($1, $2)"
+
+const selectBackwardExtremitiesForRoomSQL = "" +
+	"SELECT event_id FROM syncapi_backward_extremities WHERE room_id = $1"
+
+const isBackwardExtremitySQL = "" +
+	"SELECT EXISTS (" +
+	" SELECT TRUE FROM syncapi_backward_extremities" +
+	" WHERE room_id = $1 AND event_id = $2" +
+	")"
+
+const deleteBackwardExtremitySQL = "" +
+	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND event_id = $2"
+
+type backwardExtremitiesStatements struct {
+	insertBackwardExtremityStmt          *sql.Stmt
+	selectBackwardExtremitiesForRoomStmt *sql.Stmt
+	isBackwardExtremityStmt              *sql.Stmt
+	deleteBackwardExtremityStmt          *sql.Stmt
+}
+
+func (s *backwardExtremitiesStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(backwardExtremitiesSchema)
+	if err != nil {
+		return
+	}
+	if s.insertBackwardExtremityStmt, err = db.Prepare(insertBackwardExtremitySQL); err != nil {
+		return
+	}
+	if s.selectBackwardExtremitiesForRoomStmt, err = db.Prepare(selectBackwardExtremitiesForRoomSQL); err != nil {
+		return
+	}
+	if s.isBackwardExtremityStmt, err = db.Prepare(isBackwardExtremitySQL); err != nil {
+		return
+	}
+	if s.deleteBackwardExtremityStmt, err = db.Prepare(deleteBackwardExtremitySQL); err != nil {
+		return
+	}
+	return
+}
+
+func (s *backwardExtremitiesStatements) insertsBackwardExtremity(
+	ctx context.Context, roomID, eventID string,
+) (err error) {
+	_, err = s.insertBackwardExtremityStmt.ExecContext(ctx, roomID, eventID)
+	return
+}
+
+func (s *backwardExtremitiesStatements) selectBackwardExtremitiesForRoom(
+	ctx context.Context, roomID string,
+) (eventIDs []string, err error) {
+	eventIDs = make([]string, 0)
+
+	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
+	if err != nil {
+		return
+	}
+
+	for rows.Next() {
+		var eID string
+		if err = rows.Scan(&eID); err != nil {
+			return
+		}
+
+		eventIDs = append(eventIDs, eID)
+	}
+
+	return
+}
+
+func (s *backwardExtremitiesStatements) isBackwardExtremity(
+	ctx context.Context, roomID, eventID string,
+) (isBE bool, err error) {
+	err = s.isBackwardExtremityStmt.QueryRowContext(ctx, roomID, eventID).Scan(&isBE)
+	return
+}
+
+func (s *backwardExtremitiesStatements) deleteBackwardExtremity(
+	ctx context.Context, roomID, eventID string,
+) (err error) {
+	_, err = s.insertBackwardExtremityStmt.ExecContext(ctx, roomID, eventID)
+	return
+}
diff --git a/syncapi/storage/postgres/current_room_state_table.go b/syncapi/storage/postgres/current_room_state_table.go
index 8b208043..816cbb44 100644
--- a/syncapi/storage/postgres/current_room_state_table.go
+++ b/syncapi/storage/postgres/current_room_state_table.go
@@ -22,6 +22,7 @@ import (
 
 	"github.com/lib/pq"
 	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrix"
 	"github.com/matrix-org/gomatrixserverlib"
 )
@@ -87,10 +88,10 @@ const selectStateEventSQL = "" +
 
 const selectEventsWithEventIDsSQL = "" +
 	// TODO: The session_id and transaction_id blanks are here because otherwise
-	// the rowsToStreamEvents expects there to be exactly four columns. We need to
+	// the rowsToStreamEvents expects there to be exactly five columns. We need to
 	// figure out if these really need to be in the DB, and if so, we need a
 	// better permanent fix for this. - neilalexander, 2 Jan 2020
-	"SELECT added_at, event_json, 0 AS session_id, '' AS transaction_id" +
+	"SELECT added_at, event_json, 0 AS session_id, false AS exclude_from_sync, '' AS transaction_id" +
 	" FROM syncapi_current_room_state WHERE event_id = ANY($1)"
 
 type currentRoomStateStatements struct {
@@ -213,7 +214,7 @@ func (s *currentRoomStateStatements) deleteRoomStateByEventID(
 
 func (s *currentRoomStateStatements) upsertRoomState(
 	ctx context.Context, txn *sql.Tx,
-	event gomatrixserverlib.Event, membership *string, addedAt int64,
+	event gomatrixserverlib.Event, membership *string, addedAt types.StreamPosition,
 ) error {
 	// Parse content as JSON and search for an "url" key
 	containsURL := false
@@ -242,7 +243,7 @@ func (s *currentRoomStateStatements) upsertRoomState(
 
 func (s *currentRoomStateStatements) selectEventsWithEventIDs(
 	ctx context.Context, txn *sql.Tx, eventIDs []string,
-) ([]streamEvent, error) {
+) ([]types.StreamEvent, error) {
 	stmt := common.TxStmt(txn, s.selectEventsWithEventIDsStmt)
 	rows, err := stmt.QueryContext(ctx, pq.StringArray(eventIDs))
 	if err != nil {
diff --git a/syncapi/storage/postgres/invites_table.go b/syncapi/storage/postgres/invites_table.go
index ced4bfc4..ca4bbeb5 100644
--- a/syncapi/storage/postgres/invites_table.go
+++ b/syncapi/storage/postgres/invites_table.go
@@ -20,6 +20,7 @@ import (
 	"database/sql"
 
 	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
@@ -86,7 +87,7 @@ func (s *inviteEventsStatements) prepare(db *sql.DB) (err error) {
 
 func (s *inviteEventsStatements) insertInviteEvent(
 	ctx context.Context, inviteEvent gomatrixserverlib.Event,
-) (streamPos int64, err error) {
+) (streamPos types.StreamPosition, err error) {
 	err = s.insertInviteEventStmt.QueryRowContext(
 		ctx,
 		inviteEvent.RoomID(),
@@ -107,7 +108,7 @@ func (s *inviteEventsStatements) deleteInviteEvent(
 // selectInviteEventsInRange returns a map of room ID to invite event for the
 // active invites for the target user ID in the supplied range.
 func (s *inviteEventsStatements) selectInviteEventsInRange(
-	ctx context.Context, txn *sql.Tx, targetUserID string, startPos, endPos int64,
+	ctx context.Context, txn *sql.Tx, targetUserID string, startPos, endPos types.StreamPosition,
 ) (map[string]gomatrixserverlib.Event, error) {
 	stmt := common.TxStmt(txn, s.selectInviteEventsInRangeStmt)
 	rows, err := stmt.QueryContext(ctx, targetUserID, startPos, endPos)
diff --git a/syncapi/storage/postgres/output_room_events_table.go b/syncapi/storage/postgres/output_room_events_table.go
index ca271593..be302d73 100644
--- a/syncapi/storage/postgres/output_room_events_table.go
+++ b/syncapi/storage/postgres/output_room_events_table.go
@@ -22,6 +22,7 @@ import (
 	"sort"
 
 	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrix"
 
 	"github.com/lib/pq"
@@ -36,28 +37,35 @@ CREATE SEQUENCE IF NOT EXISTS syncapi_stream_id;
 
 -- Stores output room events received from the roomserver.
 CREATE TABLE IF NOT EXISTS syncapi_output_room_events (
-    -- An incrementing ID which denotes the position in the log that this event resides at.
-    -- NB: 'serial' makes no guarantees to increment by 1 every time, only that it increments.
-    --     This isn't a problem for us since we just want to order by this field.
-    id BIGINT PRIMARY KEY DEFAULT nextval('syncapi_stream_id'),
-    -- The event ID for the event
-    event_id TEXT NOT NULL,
-    -- The 'room_id' key for the event.
-    room_id TEXT NOT NULL,
-    -- The JSON for the event. Stored as TEXT because this should be valid UTF-8.
-    event_json TEXT NOT NULL,
-    -- The event type e.g 'm.room.member'.
-    type TEXT NOT NULL,
-    -- The 'sender' property of the event.
-    sender TEXT NOT NULL,
-	-- true if the event content contains a url key.
-    contains_url BOOL NOT NULL,
-    -- A list of event IDs which represent a delta of added/removed room state. This can be NULL
-    -- if there is no delta.
-    add_state_ids TEXT[],
-    remove_state_ids TEXT[],
-    session_id BIGINT,  -- The client session that sent the event, if any
-    transaction_id TEXT  -- The transaction id used to send the event, if any
+  -- An incrementing ID which denotes the position in the log that this event resides at.
+  -- NB: 'serial' makes no guarantees to increment by 1 every time, only that it increments.
+  --     This isn't a problem for us since we just want to order by this field.
+  id BIGINT PRIMARY KEY DEFAULT nextval('syncapi_stream_id'),
+  -- The event ID for the event
+  event_id TEXT NOT NULL,
+  -- The 'room_id' key for the event.
+  room_id TEXT NOT NULL,
+  -- The JSON for the event. Stored as TEXT because this should be valid UTF-8.
+  event_json TEXT NOT NULL,
+  -- The event type e.g 'm.room.member'.
+  type TEXT NOT NULL,
+  -- The 'sender' property of the event.
+  sender TEXT NOT NULL,
+  -- true if the event content contains a url key.
+  contains_url BOOL NOT NULL,
+  -- A list of event IDs which represent a delta of added/removed room state. This can be NULL
+  -- if there is no delta.
+  add_state_ids TEXT[],
+  remove_state_ids TEXT[],
+  -- The client session that sent the event, if any
+  session_id BIGINT,
+  -- The transaction id used to send the event, if any
+  transaction_id TEXT,
+  -- Should the event be excluded from responses to /sync requests. Useful for
+  -- events retrieved through backfilling that have a position in the stream
+  -- that relates to the moment these were retrieved rather than the moment these
+  -- were emitted.
+  exclude_from_sync BOOL DEFAULT FALSE
 );
 -- for event selection
 CREATE UNIQUE INDEX IF NOT EXISTS syncapi_event_id_idx ON syncapi_output_room_events(event_id);
@@ -65,23 +73,33 @@ CREATE UNIQUE INDEX IF NOT EXISTS syncapi_event_id_idx ON syncapi_output_room_ev
 
 const insertEventSQL = "" +
 	"INSERT INTO syncapi_output_room_events (" +
-	"room_id, event_id, event_json, type, sender, contains_url, add_state_ids, remove_state_ids, session_id, transaction_id" +
-	") VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10) RETURNING id"
+	"room_id, event_id, event_json, type, sender, contains_url, add_state_ids, remove_state_ids, session_id, transaction_id, exclude_from_sync" +
+	") VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11) RETURNING id"
 
 const selectEventsSQL = "" +
-	"SELECT id, event_json, session_id, transaction_id FROM syncapi_output_room_events WHERE event_id = ANY($1)"
+	"SELECT id, event_json, session_id, exclude_from_sync, transaction_id FROM syncapi_output_room_events WHERE event_id = ANY($1)"
 
 const selectRecentEventsSQL = "" +
-	"SELECT id, event_json, session_id, transaction_id FROM syncapi_output_room_events" +
+	"SELECT id, event_json, session_id, exclude_from_sync, transaction_id FROM syncapi_output_room_events" +
 	" WHERE room_id = $1 AND id > $2 AND id <= $3" +
 	" ORDER BY id DESC LIMIT $4"
 
+const selectRecentEventsForSyncSQL = "" +
+	"SELECT id, event_json, session_id, exclude_from_sync, transaction_id FROM syncapi_output_room_events" +
+	" WHERE room_id = $1 AND id > $2 AND id <= $3 AND exclude_from_sync = FALSE" +
+	" ORDER BY id DESC LIMIT $4"
+
+const selectEarlyEventsSQL = "" +
+	"SELECT id, event_json, session_id, exclude_from_sync, transaction_id FROM syncapi_output_room_events" +
+	" WHERE room_id = $1 AND id > $2 AND id <= $3" +
+	" ORDER BY id ASC LIMIT $4"
+
 const selectMaxEventIDSQL = "" +
 	"SELECT MAX(id) FROM syncapi_output_room_events"
 
 // In order for us to apply the state updates correctly, rows need to be ordered in the order they were received (id).
 const selectStateInRangeSQL = "" +
-	"SELECT id, event_json, add_state_ids, remove_state_ids" +
+	"SELECT id, event_json, exclude_from_sync, add_state_ids, remove_state_ids" +
 	" FROM syncapi_output_room_events" +
 	" WHERE (id > $1 AND id <= $2) AND (add_state_ids IS NOT NULL OR remove_state_ids IS NOT NULL)" +
 	" AND ( $3::text[] IS NULL OR     sender  = ANY($3)  )" +
@@ -93,11 +111,13 @@ const selectStateInRangeSQL = "" +
 	" LIMIT $8"
 
 type outputRoomEventsStatements struct {
-	insertEventStmt        *sql.Stmt
-	selectEventsStmt       *sql.Stmt
-	selectMaxEventIDStmt   *sql.Stmt
-	selectRecentEventsStmt *sql.Stmt
-	selectStateInRangeStmt *sql.Stmt
+	insertEventStmt               *sql.Stmt
+	selectEventsStmt              *sql.Stmt
+	selectMaxEventIDStmt          *sql.Stmt
+	selectRecentEventsStmt        *sql.Stmt
+	selectRecentEventsForSyncStmt *sql.Stmt
+	selectEarlyEventsStmt         *sql.Stmt
+	selectStateInRangeStmt        *sql.Stmt
 }
 
 func (s *outputRoomEventsStatements) prepare(db *sql.DB) (err error) {
@@ -117,6 +137,12 @@ func (s *outputRoomEventsStatements) prepare(db *sql.DB) (err error) {
 	if s.selectRecentEventsStmt, err = db.Prepare(selectRecentEventsSQL); err != nil {
 		return
 	}
+	if s.selectRecentEventsForSyncStmt, err = db.Prepare(selectRecentEventsForSyncSQL); err != nil {
+		return
+	}
+	if s.selectEarlyEventsStmt, err = db.Prepare(selectEarlyEventsSQL); err != nil {
+		return
+	}
 	if s.selectStateInRangeStmt, err = db.Prepare(selectStateInRangeSQL); err != nil {
 		return
 	}
@@ -127,9 +153,9 @@ func (s *outputRoomEventsStatements) prepare(db *sql.DB) (err error) {
 // Results are bucketed based on the room ID. If the same state is overwritten multiple times between the
 // two positions, only the most recent state is returned.
 func (s *outputRoomEventsStatements) selectStateInRange(
-	ctx context.Context, txn *sql.Tx, oldPos, newPos int64,
+	ctx context.Context, txn *sql.Tx, oldPos, newPos types.StreamPosition,
 	stateFilterPart *gomatrix.FilterPart,
-) (map[string]map[string]bool, map[string]streamEvent, error) {
+) (map[string]map[string]bool, map[string]types.StreamEvent, error) {
 	stmt := common.TxStmt(txn, s.selectStateInRangeStmt)
 
 	rows, err := stmt.QueryContext(
@@ -149,19 +175,20 @@ func (s *outputRoomEventsStatements) selectStateInRange(
 	//  - For each room ID, build up an array of event IDs which represents cumulative adds/removes
 	// For each room, map cumulative event IDs to events and return. This may need to a batch SELECT based on event ID
 	// if they aren't in the event ID cache. We don't handle state deletion yet.
-	eventIDToEvent := make(map[string]streamEvent)
+	eventIDToEvent := make(map[string]types.StreamEvent)
 
 	// RoomID => A set (map[string]bool) of state event IDs which are between the two positions
 	stateNeeded := make(map[string]map[string]bool)
 
 	for rows.Next() {
 		var (
-			streamPos  int64
-			eventBytes []byte
-			addIDs     pq.StringArray
-			delIDs     pq.StringArray
+			streamPos       types.StreamPosition
+			eventBytes      []byte
+			excludeFromSync bool
+			addIDs          pq.StringArray
+			delIDs          pq.StringArray
 		)
-		if err := rows.Scan(&streamPos, &eventBytes, &addIDs, &delIDs); err != nil {
+		if err := rows.Scan(&streamPos, &eventBytes, &excludeFromSync, &addIDs, &delIDs); err != nil {
 			return nil, nil, err
 		}
 		// Sanity check for deleted state and whine if we see it. We don't need to do anything
@@ -192,9 +219,10 @@ func (s *outputRoomEventsStatements) selectStateInRange(
 		}
 		stateNeeded[ev.RoomID()] = needSet
 
-		eventIDToEvent[ev.EventID()] = streamEvent{
-			Event:          ev,
-			streamPosition: streamPos,
+		eventIDToEvent[ev.EventID()] = types.StreamEvent{
+			Event:           ev,
+			StreamPosition:  streamPos,
+			ExcludeFromSync: excludeFromSync,
 		}
 	}
 
@@ -221,8 +249,8 @@ func (s *outputRoomEventsStatements) selectMaxEventID(
 func (s *outputRoomEventsStatements) insertEvent(
 	ctx context.Context, txn *sql.Tx,
 	event *gomatrixserverlib.Event, addState, removeState []string,
-	transactionID *api.TransactionID,
-) (streamPos int64, err error) {
+	transactionID *api.TransactionID, excludeFromSync bool,
+) (streamPos types.StreamPosition, err error) {
 	var txnID *string
 	var sessionID *int64
 	if transactionID != nil {
@@ -251,16 +279,53 @@ func (s *outputRoomEventsStatements) insertEvent(
 		pq.StringArray(removeState),
 		sessionID,
 		txnID,
+		excludeFromSync,
 	).Scan(&streamPos)
 	return
 }
 
-// RecentEventsInRoom returns the most recent events in the given room, up to a maximum of 'limit'.
+// selectRecentEvents returns the most recent events in the given room, up to a maximum of 'limit'.
+// If onlySyncEvents has a value of true, only returns the events that aren't marked as to exclude
+// from sync.
 func (s *outputRoomEventsStatements) selectRecentEvents(
 	ctx context.Context, txn *sql.Tx,
-	roomID string, fromPos, toPos int64, limit int,
-) ([]streamEvent, error) {
-	stmt := common.TxStmt(txn, s.selectRecentEventsStmt)
+	roomID string, fromPos, toPos types.StreamPosition, limit int,
+	chronologicalOrder bool, onlySyncEvents bool,
+) ([]types.StreamEvent, error) {
+	var stmt *sql.Stmt
+	if onlySyncEvents {
+		stmt = common.TxStmt(txn, s.selectRecentEventsForSyncStmt)
+	} else {
+		stmt = common.TxStmt(txn, s.selectRecentEventsStmt)
+	}
+
+	rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close() // nolint: errcheck
+	events, err := rowsToStreamEvents(rows)
+	if err != nil {
+		return nil, err
+	}
+	if chronologicalOrder {
+		// The events need to be returned from oldest to latest, which isn't
+		// necessary the way the SQL query returns them, so a sort is necessary to
+		// ensure the events are in the right order in the slice.
+		sort.SliceStable(events, func(i int, j int) bool {
+			return events[i].StreamPosition < events[j].StreamPosition
+		})
+	}
+	return events, nil
+}
+
+// selectEarlyEvents returns the earliest events in the given room, starting
+// from a given position, up to a maximum of 'limit'.
+func (s *outputRoomEventsStatements) selectEarlyEvents(
+	ctx context.Context, txn *sql.Tx,
+	roomID string, fromPos, toPos types.StreamPosition, limit int,
+) ([]types.StreamEvent, error) {
+	stmt := common.TxStmt(txn, s.selectEarlyEventsStmt)
 	rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
 	if err != nil {
 		return nil, err
@@ -274,16 +339,16 @@ func (s *outputRoomEventsStatements) selectRecentEvents(
 	// necessarily the way the SQL query returns them, so a sort is necessary to
 	// ensure the events are in the right order in the slice.
 	sort.SliceStable(events, func(i int, j int) bool {
-		return events[i].streamPosition < events[j].streamPosition
+		return events[i].StreamPosition < events[j].StreamPosition
 	})
 	return events, nil
 }
 
-// Events returns the events for the given event IDs. Returns an error if any one of the event IDs given are missing
-// from the database.
+// selectEvents returns the events for the given event IDs. If an event is
+// missing from the database, it will be omitted.
 func (s *outputRoomEventsStatements) selectEvents(
 	ctx context.Context, txn *sql.Tx, eventIDs []string,
-) ([]streamEvent, error) {
+) ([]types.StreamEvent, error) {
 	stmt := common.TxStmt(txn, s.selectEventsStmt)
 	rows, err := stmt.QueryContext(ctx, pq.StringArray(eventIDs))
 	if err != nil {
@@ -293,17 +358,18 @@ func (s *outputRoomEventsStatements) selectEvents(
 	return rowsToStreamEvents(rows)
 }
 
-func rowsToStreamEvents(rows *sql.Rows) ([]streamEvent, error) {
-	var result []streamEvent
+func rowsToStreamEvents(rows *sql.Rows) ([]types.StreamEvent, error) {
+	var result []types.StreamEvent
 	for rows.Next() {
 		var (
-			streamPos     int64
-			eventBytes    []byte
-			sessionID     *int64
-			txnID         *string
-			transactionID *api.TransactionID
+			streamPos       types.StreamPosition
+			eventBytes      []byte
+			excludeFromSync bool
+			sessionID       *int64
+			txnID           *string
+			transactionID   *api.TransactionID
 		)
-		if err := rows.Scan(&streamPos, &eventBytes, &sessionID, &txnID); err != nil {
+		if err := rows.Scan(&streamPos, &eventBytes, &sessionID, &excludeFromSync, &txnID); err != nil {
 			return nil, err
 		}
 		// TODO: Handle redacted events
@@ -319,10 +385,11 @@ func rowsToStreamEvents(rows *sql.Rows) ([]streamEvent, error) {
 			}
 		}
 
-		result = append(result, streamEvent{
-			Event:          ev,
-			streamPosition: streamPos,
-			transactionID:  transactionID,
+		result = append(result, types.StreamEvent{
+			Event:           ev,
+			StreamPosition:  streamPos,
+			TransactionID:   transactionID,
+			ExcludeFromSync: excludeFromSync,
 		})
 	}
 	return result, nil
diff --git a/syncapi/storage/postgres/output_room_events_topology_table.go b/syncapi/storage/postgres/output_room_events_topology_table.go
new file mode 100644
index 00000000..4a50b9a0
--- /dev/null
+++ b/syncapi/storage/postgres/output_room_events_topology_table.go
@@ -0,0 +1,188 @@
+// Copyright 2018 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package postgres
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+const outputRoomEventsTopologySchema = `
+-- Stores output room events received from the roomserver.
+CREATE TABLE IF NOT EXISTS syncapi_output_room_events_topology (
+	-- The event ID for the event.
+    event_id TEXT PRIMARY KEY,
+	-- The place of the event in the room's topology. This can usually be determined
+	-- from the event's depth.
+	topological_position BIGINT NOT NULL,
+    -- The 'room_id' key for the event.
+    room_id TEXT NOT NULL
+);
+-- The topological order will be used in events selection and ordering
+CREATE UNIQUE INDEX IF NOT EXISTS syncapi_event_topological_position_idx ON syncapi_output_room_events_topology(topological_position, room_id);
+`
+
+const insertEventInTopologySQL = "" +
+	"INSERT INTO syncapi_output_room_events_topology (event_id, topological_position, room_id)" +
+	" VALUES ($1, $2, $3)" +
+	" ON CONFLICT DO NOTHING"
+
+const selectEventIDsInRangeASCSQL = "" +
+	"SELECT event_id FROM syncapi_output_room_events_topology" +
+	" WHERE room_id = $1 AND topological_position > $2 AND topological_position <= $3" +
+	" ORDER BY topological_position ASC LIMIT $4"
+
+const selectEventIDsInRangeDESCSQL = "" +
+	"SELECT event_id FROM syncapi_output_room_events_topology" +
+	" WHERE room_id = $1 AND topological_position > $2 AND topological_position <= $3" +
+	" ORDER BY topological_position DESC LIMIT $4"
+
+const selectPositionInTopologySQL = "" +
+	"SELECT topological_position FROM syncapi_output_room_events_topology" +
+	" WHERE event_id = $1"
+
+const selectMaxPositionInTopologySQL = "" +
+	"SELECT MAX(topological_position) FROM syncapi_output_room_events_topology" +
+	" WHERE room_id = $1"
+
+const selectEventIDsFromPositionSQL = "" +
+	"SELECT event_id FROM syncapi_output_room_events_topology" +
+	" WHERE room_id = $1 AND topological_position = $2"
+
+type outputRoomEventsTopologyStatements struct {
+	insertEventInTopologyStmt       *sql.Stmt
+	selectEventIDsInRangeASCStmt    *sql.Stmt
+	selectEventIDsInRangeDESCStmt   *sql.Stmt
+	selectPositionInTopologyStmt    *sql.Stmt
+	selectMaxPositionInTopologyStmt *sql.Stmt
+	selectEventIDsFromPositionStmt  *sql.Stmt
+}
+
+func (s *outputRoomEventsTopologyStatements) prepare(db *sql.DB) (err error) {
+	_, err = db.Exec(outputRoomEventsTopologySchema)
+	if err != nil {
+		return
+	}
+	if s.insertEventInTopologyStmt, err = db.Prepare(insertEventInTopologySQL); err != nil {
+		return
+	}
+	if s.selectEventIDsInRangeASCStmt, err = db.Prepare(selectEventIDsInRangeASCSQL); err != nil {
+		return
+	}
+	if s.selectEventIDsInRangeDESCStmt, err = db.Prepare(selectEventIDsInRangeDESCSQL); err != nil {
+		return
+	}
+	if s.selectPositionInTopologyStmt, err = db.Prepare(selectPositionInTopologySQL); err != nil {
+		return
+	}
+	if s.selectMaxPositionInTopologyStmt, err = db.Prepare(selectMaxPositionInTopologySQL); err != nil {
+		return
+	}
+	if s.selectEventIDsFromPositionStmt, err = db.Prepare(selectEventIDsFromPositionSQL); err != nil {
+		return
+	}
+	return
+}
+
+// insertEventInTopology inserts the given event in the room's topology, based
+// on the event's depth.
+func (s *outputRoomEventsTopologyStatements) insertEventInTopology(
+	ctx context.Context, event *gomatrixserverlib.Event,
+) (err error) {
+	_, err = s.insertEventInTopologyStmt.ExecContext(
+		ctx, event.EventID(), event.Depth(), event.RoomID(),
+	)
+	return
+}
+
+// selectEventIDsInRange selects the IDs of events which positions are within a
+// given range in a given room's topological order.
+// Returns an empty slice if no events match the given range.
+func (s *outputRoomEventsTopologyStatements) selectEventIDsInRange(
+	ctx context.Context, roomID string, fromPos, toPos types.StreamPosition,
+	limit int, chronologicalOrder bool,
+) (eventIDs []string, err error) {
+	// Decide on the selection's order according to whether chronological order
+	// is requested or not.
+	var stmt *sql.Stmt
+	if chronologicalOrder {
+		stmt = s.selectEventIDsInRangeASCStmt
+	} else {
+		stmt = s.selectEventIDsInRangeDESCStmt
+	}
+
+	// Query the event IDs.
+	rows, err := stmt.QueryContext(ctx, roomID, fromPos, toPos, limit)
+	if err == sql.ErrNoRows {
+		// If no event matched the request, return an empty slice.
+		return []string{}, nil
+	} else if err != nil {
+		return
+	}
+
+	// Return the IDs.
+	var eventID string
+	for rows.Next() {
+		if err = rows.Scan(&eventID); err != nil {
+			return
+		}
+		eventIDs = append(eventIDs, eventID)
+	}
+
+	return
+}
+
+// selectPositionInTopology returns the position of a given event in the
+// topology of the room it belongs to.
+func (s *outputRoomEventsTopologyStatements) selectPositionInTopology(
+	ctx context.Context, eventID string,
+) (pos types.StreamPosition, err error) {
+	err = s.selectPositionInTopologyStmt.QueryRowContext(ctx, eventID).Scan(&pos)
+	return
+}
+
+func (s *outputRoomEventsTopologyStatements) selectMaxPositionInTopology(
+	ctx context.Context, roomID string,
+) (pos types.StreamPosition, err error) {
+	err = s.selectMaxPositionInTopologyStmt.QueryRowContext(ctx, roomID).Scan(&pos)
+	return
+}
+
+// selectEventIDsFromPosition returns the IDs of all events that have a given
+// position in the topology of a given room.
+func (s *outputRoomEventsTopologyStatements) selectEventIDsFromPosition(
+	ctx context.Context, roomID string, pos types.StreamPosition,
+) (eventIDs []string, err error) {
+	// Query the event IDs.
+	rows, err := s.selectEventIDsFromPositionStmt.QueryContext(ctx, roomID, pos)
+	if err == sql.ErrNoRows {
+		// If no event matched the request, return an empty slice.
+		return []string{}, nil
+	} else if err != nil {
+		return
+	}
+	// Return the IDs.
+	var eventID string
+	for rows.Next() {
+		if err = rows.Scan(&eventID); err != nil {
+			return
+		}
+		eventIDs = append(eventIDs, eventID)
+	}
+	return
+}
diff --git a/syncapi/storage/postgres/syncserver.go b/syncapi/storage/postgres/syncserver.go
index 3a62d136..621aec95 100644
--- a/syncapi/storage/postgres/syncserver.go
+++ b/syncapi/storage/postgres/syncserver.go
@@ -20,7 +20,6 @@ import (
 	"database/sql"
 	"encoding/json"
 	"fmt"
-	"strconv"
 	"time"
 
 	"github.com/sirupsen/logrus"
@@ -43,29 +42,24 @@ type stateDelta struct {
 	membership  string
 	// The PDU stream position of the latest membership event for this user, if applicable.
 	// Can be 0 if there is no membership event in this delta.
-	membershipPos int64
+	membershipPos types.StreamPosition
 }
 
-// Same as gomatrixserverlib.Event but also has the PDU stream position for this event.
-type streamEvent struct {
-	gomatrixserverlib.Event
-	streamPosition int64
-	transactionID  *api.TransactionID
-}
-
-// SyncServerDatabase represents a sync server datasource which manages
+// SyncServerDatasource represents a sync server datasource which manages
 // both the database for PDUs and caches for EDUs.
 type SyncServerDatasource struct {
 	db *sql.DB
 	common.PartitionOffsetStatements
-	accountData accountDataStatements
-	events      outputRoomEventsStatements
-	roomstate   currentRoomStateStatements
-	invites     inviteEventsStatements
-	typingCache *cache.TypingCache
+	accountData         accountDataStatements
+	events              outputRoomEventsStatements
+	roomstate           currentRoomStateStatements
+	invites             inviteEventsStatements
+	typingCache         *cache.TypingCache
+	topology            outputRoomEventsTopologyStatements
+	backwardExtremities backwardExtremitiesStatements
 }
 
-// NewSyncServerDatabase creates a new sync server database
+// NewSyncServerDatasource creates a new sync server database
 func NewSyncServerDatasource(dbDataSourceName string) (*SyncServerDatasource, error) {
 	var d SyncServerDatasource
 	var err error
@@ -87,6 +81,12 @@ func NewSyncServerDatasource(dbDataSourceName string) (*SyncServerDatasource, er
 	if err := d.invites.prepare(d.db); err != nil {
 		return nil, err
 	}
+	if err := d.topology.prepare(d.db); err != nil {
+		return nil, err
+	}
+	if err := d.backwardExtremities.prepare(d.db); err != nil {
+		return nil, err
+	}
 	d.typingCache = cache.NewTypingCache()
 	return &d, nil
 }
@@ -109,7 +109,46 @@ func (d *SyncServerDatasource) Events(ctx context.Context, eventIDs []string) ([
 
 	// We don't include a device here as we only include transaction IDs in
 	// incremental syncs.
-	return streamEventsToEvents(nil, streamEvents), nil
+	return d.StreamEventsToEvents(nil, streamEvents), nil
+}
+
+func (d *SyncServerDatasource) handleBackwardExtremities(ctx context.Context, ev *gomatrixserverlib.Event) error {
+	// If the event is already known as a backward extremity, don't consider
+	// it as such anymore now that we have it.
+	isBackwardExtremity, err := d.backwardExtremities.isBackwardExtremity(ctx, ev.RoomID(), ev.EventID())
+	if err != nil {
+		return err
+	}
+	if isBackwardExtremity {
+		if err = d.backwardExtremities.deleteBackwardExtremity(ctx, ev.RoomID(), ev.EventID()); err != nil {
+			return err
+		}
+	}
+
+	// Check if we have all of the event's previous events. If an event is
+	// missing, add it to the room's backward extremities.
+	prevEvents, err := d.events.selectEvents(ctx, nil, ev.PrevEventIDs())
+	if err != nil {
+		return err
+	}
+	var found bool
+	for _, eID := range ev.PrevEventIDs() {
+		found = false
+		for _, prevEv := range prevEvents {
+			if eID == prevEv.EventID() {
+				found = true
+			}
+		}
+
+		// If the event is missing, consider it a backward extremity.
+		if !found {
+			if err = d.backwardExtremities.insertsBackwardExtremity(ctx, ev.RoomID(), ev.EventID()); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
 }
 
 // WriteEvent into the database. It is not safe to call this function from multiple goroutines, as it would create races
@@ -120,16 +159,26 @@ func (d *SyncServerDatasource) WriteEvent(
 	ev *gomatrixserverlib.Event,
 	addStateEvents []gomatrixserverlib.Event,
 	addStateEventIDs, removeStateEventIDs []string,
-	transactionID *api.TransactionID,
-) (pduPosition int64, returnErr error) {
+	transactionID *api.TransactionID, excludeFromSync bool,
+) (pduPosition types.StreamPosition, returnErr error) {
 	returnErr = common.WithTransaction(d.db, func(txn *sql.Tx) error {
 		var err error
-		pos, err := d.events.insertEvent(ctx, txn, ev, addStateEventIDs, removeStateEventIDs, transactionID)
+		pos, err := d.events.insertEvent(
+			ctx, txn, ev, addStateEventIDs, removeStateEventIDs, transactionID, excludeFromSync,
+		)
 		if err != nil {
 			return err
 		}
 		pduPosition = pos
 
+		if err = d.topology.insertEventInTopology(ctx, ev); err != nil {
+			return err
+		}
+
+		if err = d.handleBackwardExtremities(ctx, ev); err != nil {
+			return err
+		}
+
 		if len(addStateEvents) == 0 && len(removeStateEventIDs) == 0 {
 			// Nothing to do, the event may have just been a message event.
 			return nil
@@ -137,14 +186,15 @@ func (d *SyncServerDatasource) WriteEvent(
 
 		return d.updateRoomState(ctx, txn, removeStateEventIDs, addStateEvents, pduPosition)
 	})
-	return
+
+	return pduPosition, returnErr
 }
 
 func (d *SyncServerDatasource) updateRoomState(
 	ctx context.Context, txn *sql.Tx,
 	removedEventIDs []string,
 	addedEvents []gomatrixserverlib.Event,
-	pduPosition int64,
+	pduPosition types.StreamPosition,
 ) error {
 	// remove first, then add, as we do not ever delete state, but do replace state which is a remove followed by an add.
 	for _, eventID := range removedEventIDs {
@@ -196,14 +246,141 @@ func (d *SyncServerDatasource) GetStateEventsForRoom(
 	return
 }
 
+// GetEventsInRange retrieves all of the events on a given ordering using the
+// given extremities and limit.
+func (d *SyncServerDatasource) GetEventsInRange(
+	ctx context.Context,
+	from, to *types.PaginationToken,
+	roomID string, limit int,
+	backwardOrdering bool,
+) (events []types.StreamEvent, err error) {
+	// If the pagination token's type is types.PaginationTokenTypeTopology, the
+	// events must be retrieved from the rooms' topology table rather than the
+	// table contaning the syncapi server's whole stream of events.
+	if from.Type == types.PaginationTokenTypeTopology {
+		// Determine the backward and forward limit, i.e. the upper and lower
+		// limits to the selection in the room's topology, from the direction.
+		var backwardLimit, forwardLimit types.StreamPosition
+		if backwardOrdering {
+			// Backward ordering is antichronological (latest event to oldest
+			// one).
+			backwardLimit = to.PDUPosition
+			forwardLimit = from.PDUPosition
+		} else {
+			// Forward ordering is chronological (oldest event to latest one).
+			backwardLimit = from.PDUPosition
+			forwardLimit = to.PDUPosition
+		}
+
+		// Select the event IDs from the defined range.
+		var eIDs []string
+		eIDs, err = d.topology.selectEventIDsInRange(
+			ctx, roomID, backwardLimit, forwardLimit, limit, !backwardOrdering,
+		)
+		if err != nil {
+			return
+		}
+
+		// Retrieve the events' contents using their IDs.
+		events, err = d.events.selectEvents(ctx, nil, eIDs)
+		return
+	}
+
+	// If the pagination token's type is types.PaginationTokenTypeStream, the
+	// events must be retrieved from the table contaning the syncapi server's
+	// whole stream of events.
+
+	if backwardOrdering {
+		// When using backward ordering, we want the most recent events first.
+		if events, err = d.events.selectRecentEvents(
+			ctx, nil, roomID, to.PDUPosition, from.PDUPosition, limit, false, false,
+		); err != nil {
+			return
+		}
+	} else {
+		// When using forward ordering, we want the least recent events first.
+		if events, err = d.events.selectEarlyEvents(
+			ctx, nil, roomID, from.PDUPosition, to.PDUPosition, limit,
+		); err != nil {
+			return
+		}
+	}
+
+	return
+}
+
 // SyncPosition returns the latest positions for syncing.
-func (d *SyncServerDatasource) SyncPosition(ctx context.Context) (types.SyncPosition, error) {
+func (d *SyncServerDatasource) SyncPosition(ctx context.Context) (types.PaginationToken, error) {
 	return d.syncPositionTx(ctx, nil)
 }
 
+// BackwardExtremitiesForRoom returns the event IDs of all of the backward
+// extremities we know of for a given room.
+func (d *SyncServerDatasource) BackwardExtremitiesForRoom(
+	ctx context.Context, roomID string,
+) (backwardExtremities []string, err error) {
+	return d.backwardExtremities.selectBackwardExtremitiesForRoom(ctx, roomID)
+}
+
+// MaxTopologicalPosition returns the highest topological position for a given
+// room.
+func (d *SyncServerDatasource) MaxTopologicalPosition(
+	ctx context.Context, roomID string,
+) (types.StreamPosition, error) {
+	return d.topology.selectMaxPositionInTopology(ctx, roomID)
+}
+
+// EventsAtTopologicalPosition returns all of the events matching a given
+// position in the topology of a given room.
+func (d *SyncServerDatasource) EventsAtTopologicalPosition(
+	ctx context.Context, roomID string, pos types.StreamPosition,
+) ([]types.StreamEvent, error) {
+	eIDs, err := d.topology.selectEventIDsFromPosition(ctx, roomID, pos)
+	if err != nil {
+		return nil, err
+	}
+
+	return d.events.selectEvents(ctx, nil, eIDs)
+}
+
+func (d *SyncServerDatasource) EventPositionInTopology(
+	ctx context.Context, eventID string,
+) (types.StreamPosition, error) {
+	return d.topology.selectPositionInTopology(ctx, eventID)
+}
+
+// SyncStreamPosition returns the latest position in the sync stream. Returns 0 if there are no events yet.
+func (d *SyncServerDatasource) SyncStreamPosition(ctx context.Context) (types.StreamPosition, error) {
+	return d.syncStreamPositionTx(ctx, nil)
+}
+
+func (d *SyncServerDatasource) syncStreamPositionTx(
+	ctx context.Context, txn *sql.Tx,
+) (types.StreamPosition, error) {
+	maxID, err := d.events.selectMaxEventID(ctx, txn)
+	if err != nil {
+		return 0, err
+	}
+	maxAccountDataID, err := d.accountData.selectMaxAccountDataID(ctx, txn)
+	if err != nil {
+		return 0, err
+	}
+	if maxAccountDataID > maxID {
+		maxID = maxAccountDataID
+	}
+	maxInviteID, err := d.invites.selectMaxInviteID(ctx, txn)
+	if err != nil {
+		return 0, err
+	}
+	if maxInviteID > maxID {
+		maxID = maxInviteID
+	}
+	return types.StreamPosition(maxID), nil
+}
+
 func (d *SyncServerDatasource) syncPositionTx(
 	ctx context.Context, txn *sql.Tx,
-) (sp types.SyncPosition, err error) {
+) (sp types.PaginationToken, err error) {
 
 	maxEventID, err := d.events.selectMaxEventID(ctx, txn)
 	if err != nil {
@@ -223,10 +400,8 @@ func (d *SyncServerDatasource) syncPositionTx(
 	if maxInviteID > maxEventID {
 		maxEventID = maxInviteID
 	}
-	sp.PDUPosition = maxEventID
-
-	sp.TypingPosition = d.typingCache.GetLatestSyncPosition()
-
+	sp.PDUPosition = types.StreamPosition(maxEventID)
+	sp.EDUTypingPosition = types.StreamPosition(d.typingCache.GetLatestSyncPosition())
 	return
 }
 
@@ -235,7 +410,7 @@ func (d *SyncServerDatasource) syncPositionTx(
 func (d *SyncServerDatasource) addPDUDeltaToResponse(
 	ctx context.Context,
 	device authtypes.Device,
-	fromPos, toPos int64,
+	fromPos, toPos types.StreamPosition,
 	numRecentEventsPerRoom int,
 	wantFullState bool,
 	res *types.Response,
@@ -287,7 +462,7 @@ func (d *SyncServerDatasource) addPDUDeltaToResponse(
 // addTypingDeltaToResponse adds all typing notifications to a sync response
 // since the specified position.
 func (d *SyncServerDatasource) addTypingDeltaToResponse(
-	since int64,
+	since types.PaginationToken,
 	joinedRoomIDs []string,
 	res *types.Response,
 ) error {
@@ -296,7 +471,7 @@ func (d *SyncServerDatasource) addTypingDeltaToResponse(
 	var err error
 	for _, roomID := range joinedRoomIDs {
 		if typingUsers, updated := d.typingCache.GetTypingUsersIfUpdatedAfter(
-			roomID, since,
+			roomID, int64(since.EDUTypingPosition),
 		); updated {
 			ev := gomatrixserverlib.ClientEvent{
 				Type: gomatrixserverlib.MTyping,
@@ -321,14 +496,14 @@ func (d *SyncServerDatasource) addTypingDeltaToResponse(
 // addEDUDeltaToResponse adds updates for EDUs of each type since fromPos if
 // the positions of that type are not equal in fromPos and toPos.
 func (d *SyncServerDatasource) addEDUDeltaToResponse(
-	fromPos, toPos types.SyncPosition,
+	fromPos, toPos types.PaginationToken,
 	joinedRoomIDs []string,
 	res *types.Response,
 ) (err error) {
 
-	if fromPos.TypingPosition != toPos.TypingPosition {
+	if fromPos.EDUTypingPosition != toPos.EDUTypingPosition {
 		err = d.addTypingDeltaToResponse(
-			fromPos.TypingPosition, joinedRoomIDs, res,
+			fromPos, joinedRoomIDs, res,
 		)
 	}
 
@@ -343,7 +518,7 @@ func (d *SyncServerDatasource) addEDUDeltaToResponse(
 func (d *SyncServerDatasource) IncrementalSync(
 	ctx context.Context,
 	device authtypes.Device,
-	fromPos, toPos types.SyncPosition,
+	fromPos, toPos types.PaginationToken,
 	numRecentEventsPerRoom int,
 	wantFullState bool,
 ) (*types.Response, error) {
@@ -383,7 +558,7 @@ func (d *SyncServerDatasource) getResponseWithPDUsForCompleteSync(
 	numRecentEventsPerRoom int,
 ) (
 	res *types.Response,
-	toPos types.SyncPosition,
+	toPos types.PaginationToken,
 	joinedRoomIDs []string,
 	err error,
 ) {
@@ -423,27 +598,37 @@ func (d *SyncServerDatasource) getResponseWithPDUsForCompleteSync(
 		}
 		// 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 []streamEvent
+		var recentStreamEvents []types.StreamEvent
 		recentStreamEvents, err = d.events.selectRecentEvents(
-			ctx, txn, roomID, 0, toPos.PDUPosition, numRecentEventsPerRoom,
+			ctx, txn, roomID, types.StreamPosition(0), toPos.PDUPosition,
+			numRecentEventsPerRoom, true, true,
+			//ctx, txn, roomID, 0, toPos.PDUPosition, numRecentEventsPerRoom,
 		)
 		if err != nil {
 			return
 		}
 
+		// Retrieve the backward topology position, i.e. the position of the
+		// oldest event in the room's topology.
+		var backwardTopologyPos types.StreamPosition
+		backwardTopologyPos, err = d.topology.selectPositionInTopology(ctx, recentStreamEvents[0].EventID())
+		if err != nil {
+			return nil, types.PaginationToken{}, []string{}, err
+		}
+		if backwardTopologyPos-1 <= 0 {
+			backwardTopologyPos = types.StreamPosition(1)
+		} else {
+			backwardTopologyPos = backwardTopologyPos - 1
+		}
+
 		// We don't include a device here as we don't need to send down
 		// transaction IDs for complete syncs
-		recentEvents := streamEventsToEvents(nil, recentStreamEvents)
-
+		recentEvents := d.StreamEventsToEvents(nil, recentStreamEvents)
 		stateEvents = removeDuplicates(stateEvents, recentEvents)
 		jr := types.NewJoinResponse()
-		if prevPDUPos := recentStreamEvents[0].streamPosition - 1; prevPDUPos > 0 {
-			// Use the short form of batch token for prev_batch
-			jr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
-		} else {
-			// Use the short form of batch token for prev_batch
-			jr.Timeline.PrevBatch = "1"
-		}
+		jr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
+			types.PaginationTokenTypeTopology, backwardTopologyPos, 0,
+		).String()
 		jr.Timeline.Events = gomatrixserverlib.ToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 		jr.Timeline.Limited = true
 		jr.State.Events = gomatrixserverlib.ToClientEvents(stateEvents, gomatrixserverlib.FormatSync)
@@ -471,7 +656,7 @@ func (d *SyncServerDatasource) CompleteSync(
 
 	// Use a zero value SyncPosition for fromPos so all EDU states are added.
 	err = d.addEDUDeltaToResponse(
-		types.SyncPosition{}, toPos, joinedRoomIDs, res,
+		types.PaginationToken{}, toPos, joinedRoomIDs, res,
 	)
 	if err != nil {
 		return nil, err
@@ -496,7 +681,7 @@ var txReadOnlySnapshot = sql.TxOptions{
 // If no data is retrieved, returns an empty map
 // If there was an issue with the retrieval, returns an error
 func (d *SyncServerDatasource) GetAccountDataInRange(
-	ctx context.Context, userID string, oldPos, newPos int64,
+	ctx context.Context, userID string, oldPos, newPos types.StreamPosition,
 	accountDataFilterPart *gomatrix.FilterPart,
 ) (map[string][]string, error) {
 	return d.accountData.selectAccountDataInRange(ctx, userID, oldPos, newPos, accountDataFilterPart)
@@ -510,7 +695,7 @@ func (d *SyncServerDatasource) GetAccountDataInRange(
 // Returns an error if there was an issue with the upsert
 func (d *SyncServerDatasource) UpsertAccountData(
 	ctx context.Context, userID, roomID, dataType string,
-) (int64, error) {
+) (types.StreamPosition, error) {
 	return d.accountData.insertAccountData(ctx, userID, roomID, dataType)
 }
 
@@ -519,7 +704,7 @@ func (d *SyncServerDatasource) UpsertAccountData(
 // Returns an error if there was a problem communicating with the database.
 func (d *SyncServerDatasource) AddInviteEvent(
 	ctx context.Context, inviteEvent gomatrixserverlib.Event,
-) (int64, error) {
+) (types.StreamPosition, error) {
 	return d.invites.insertInviteEvent(ctx, inviteEvent)
 }
 
@@ -542,26 +727,26 @@ func (d *SyncServerDatasource) SetTypingTimeoutCallback(fn cache.TimeoutCallback
 // Returns the newly calculated sync position for typing notifications.
 func (d *SyncServerDatasource) AddTypingUser(
 	userID, roomID string, expireTime *time.Time,
-) int64 {
-	return d.typingCache.AddTypingUser(userID, roomID, expireTime)
+) types.StreamPosition {
+	return types.StreamPosition(d.typingCache.AddTypingUser(userID, roomID, expireTime))
 }
 
 // RemoveTypingUser removes a typing user from the typing cache.
 // Returns the newly calculated sync position for typing notifications.
 func (d *SyncServerDatasource) RemoveTypingUser(
 	userID, roomID string,
-) int64 {
-	return d.typingCache.RemoveUser(userID, roomID)
+) types.StreamPosition {
+	return types.StreamPosition(d.typingCache.RemoveUser(userID, roomID))
 }
 
 func (d *SyncServerDatasource) addInvitesToResponse(
 	ctx context.Context, txn *sql.Tx,
 	userID string,
-	fromPos, toPos int64,
+	fromPos, toPos types.StreamPosition,
 	res *types.Response,
 ) error {
 	invites, err := d.invites.selectInviteEventsInRange(
-		ctx, txn, userID, int64(fromPos), int64(toPos),
+		ctx, txn, userID, fromPos, toPos,
 	)
 	if err != nil {
 		return err
@@ -577,12 +762,32 @@ func (d *SyncServerDatasource) addInvitesToResponse(
 	return nil
 }
 
+// Retrieve the backward topology position, i.e. the position of the
+// oldest event in the room's topology.
+func (d *SyncServerDatasource) getBackwardTopologyPos(
+	ctx context.Context,
+	events []types.StreamEvent,
+) (pos types.StreamPosition, err error) {
+	if len(events) > 0 {
+		pos, err = d.topology.selectPositionInTopology(ctx, events[0].EventID())
+		if err != nil {
+			return
+		}
+	}
+	if pos-1 <= 0 {
+		pos = types.StreamPosition(1)
+	} else {
+		pos = pos - 1
+	}
+	return
+}
+
 // addRoomDeltaToResponse adds a room state delta to a sync response
 func (d *SyncServerDatasource) addRoomDeltaToResponse(
 	ctx context.Context,
 	device *authtypes.Device,
 	txn *sql.Tx,
-	fromPos, toPos int64,
+	fromPos, toPos types.StreamPosition,
 	delta stateDelta,
 	numRecentEventsPerRoom int,
 	res *types.Response,
@@ -598,38 +803,28 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 		endPos = delta.membershipPos
 	}
 	recentStreamEvents, err := d.events.selectRecentEvents(
-		ctx, txn, delta.roomID, fromPos, endPos, numRecentEventsPerRoom,
+		ctx, txn, delta.roomID, types.StreamPosition(fromPos), types.StreamPosition(endPos),
+		numRecentEventsPerRoom, true, true,
 	)
 	if err != nil {
 		return err
 	}
-	recentEvents := streamEventsToEvents(device, recentStreamEvents)
+	recentEvents := d.StreamEventsToEvents(device, recentStreamEvents)
 	delta.stateEvents = removeDuplicates(delta.stateEvents, recentEvents) // roll back
 
-	var prevPDUPos int64
-
-	if len(recentEvents) == 0 {
-		if len(delta.stateEvents) == 0 {
-			// Don't bother appending empty room entries
-			return nil
-		}
-
-		// If full_state=true and since is already up to date, then we'll have
-		// state events but no recent events.
-		prevPDUPos = toPos - 1
-	} else {
-		prevPDUPos = recentStreamEvents[0].streamPosition - 1
-	}
-
-	if prevPDUPos <= 0 {
-		prevPDUPos = 1
+	var backwardTopologyPos types.StreamPosition
+	backwardTopologyPos, err = d.getBackwardTopologyPos(ctx, recentStreamEvents)
+	if err != nil {
+		return err
 	}
 
 	switch delta.membership {
 	case gomatrixserverlib.Join:
 		jr := types.NewJoinResponse()
-		// Use the short form of batch token for prev_batch
-		jr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
+
+		jr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
+			types.PaginationTokenTypeTopology, backwardTopologyPos, 0,
+		).String()
 		jr.Timeline.Events = gomatrixserverlib.ToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 		jr.Timeline.Limited = false // TODO: if len(events) >= numRecents + 1 and then set limited:true
 		jr.State.Events = gomatrixserverlib.ToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
@@ -640,8 +835,9 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 		// TODO: recentEvents may contain events that this user is not allowed to see because they are
 		//       no longer in the room.
 		lr := types.NewLeaveResponse()
-		// Use the short form of batch token for prev_batch
-		lr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
+		lr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
+			types.PaginationTokenTypeTopology, backwardTopologyPos, 0,
+		).String()
 		lr.Timeline.Events = gomatrixserverlib.ToClientEvents(recentEvents, gomatrixserverlib.FormatSync)
 		lr.Timeline.Limited = false // TODO: if len(events) >= numRecents + 1 and then set limited:true
 		lr.State.Events = gomatrixserverlib.ToClientEvents(delta.stateEvents, gomatrixserverlib.FormatSync)
@@ -656,9 +852,9 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 func (d *SyncServerDatasource) fetchStateEvents(
 	ctx context.Context, txn *sql.Tx,
 	roomIDToEventIDSet map[string]map[string]bool,
-	eventIDToEvent map[string]streamEvent,
-) (map[string][]streamEvent, error) {
-	stateBetween := make(map[string][]streamEvent)
+	eventIDToEvent map[string]types.StreamEvent,
+) (map[string][]types.StreamEvent, error) {
+	stateBetween := make(map[string][]types.StreamEvent)
 	missingEvents := make(map[string][]string)
 	for roomID, ids := range roomIDToEventIDSet {
 		events := stateBetween[roomID]
@@ -700,7 +896,7 @@ func (d *SyncServerDatasource) fetchStateEvents(
 
 func (d *SyncServerDatasource) fetchMissingStateEvents(
 	ctx context.Context, txn *sql.Tx, eventIDs []string,
-) ([]streamEvent, error) {
+) ([]types.StreamEvent, error) {
 	// Fetch from the events table first so we pick up the stream ID for the
 	// event.
 	events, err := d.events.selectEvents(ctx, txn, eventIDs)
@@ -743,7 +939,7 @@ func (d *SyncServerDatasource) fetchMissingStateEvents(
 // A list of joined room IDs is also returned in case the caller needs it.
 func (d *SyncServerDatasource) getStateDeltas(
 	ctx context.Context, device *authtypes.Device, txn *sql.Tx,
-	fromPos, toPos int64, userID string,
+	fromPos, toPos types.StreamPosition, userID string,
 	stateFilterPart *gomatrix.FilterPart,
 ) ([]stateDelta, []string, error) {
 	// Implement membership change algorithm: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L821
@@ -776,7 +972,7 @@ func (d *SyncServerDatasource) getStateDeltas(
 			if membership := getMembershipFromEvent(&ev.Event, userID); membership != "" {
 				if membership == gomatrixserverlib.Join {
 					// send full room state down instead of a delta
-					var s []streamEvent
+					var s []types.StreamEvent
 					s, err = d.currentStateStreamEventsForRoom(ctx, txn, roomID, stateFilterPart)
 					if err != nil {
 						return nil, nil, err
@@ -787,8 +983,8 @@ func (d *SyncServerDatasource) getStateDeltas(
 
 				deltas = append(deltas, stateDelta{
 					membership:    membership,
-					membershipPos: ev.streamPosition,
-					stateEvents:   streamEventsToEvents(device, stateStreamEvents),
+					membershipPos: ev.StreamPosition,
+					stateEvents:   d.StreamEventsToEvents(device, stateStreamEvents),
 					roomID:        roomID,
 				})
 				break
@@ -804,7 +1000,7 @@ func (d *SyncServerDatasource) getStateDeltas(
 	for _, joinedRoomID := range joinedRoomIDs {
 		deltas = append(deltas, stateDelta{
 			membership:  gomatrixserverlib.Join,
-			stateEvents: streamEventsToEvents(device, state[joinedRoomID]),
+			stateEvents: d.StreamEventsToEvents(device, state[joinedRoomID]),
 			roomID:      joinedRoomID,
 		})
 	}
@@ -818,7 +1014,7 @@ func (d *SyncServerDatasource) getStateDeltas(
 // updates for other rooms.
 func (d *SyncServerDatasource) getStateDeltasForFullStateSync(
 	ctx context.Context, device *authtypes.Device, txn *sql.Tx,
-	fromPos, toPos int64, userID string,
+	fromPos, toPos types.StreamPosition, userID string,
 	stateFilterPart *gomatrix.FilterPart,
 ) ([]stateDelta, []string, error) {
 	joinedRoomIDs, err := d.roomstate.selectRoomIDsWithMembership(ctx, txn, userID, gomatrixserverlib.Join)
@@ -837,7 +1033,7 @@ func (d *SyncServerDatasource) getStateDeltasForFullStateSync(
 		}
 		deltas = append(deltas, stateDelta{
 			membership:  gomatrixserverlib.Join,
-			stateEvents: streamEventsToEvents(device, s),
+			stateEvents: d.StreamEventsToEvents(device, s),
 			roomID:      joinedRoomID,
 		})
 	}
@@ -858,8 +1054,8 @@ func (d *SyncServerDatasource) getStateDeltasForFullStateSync(
 				if membership != gomatrixserverlib.Join { // We've already added full state for all joined rooms above.
 					deltas = append(deltas, stateDelta{
 						membership:    membership,
-						membershipPos: ev.streamPosition,
-						stateEvents:   streamEventsToEvents(device, stateStreamEvents),
+						membershipPos: ev.StreamPosition,
+						stateEvents:   d.StreamEventsToEvents(device, stateStreamEvents),
 						roomID:        roomID,
 					})
 				}
@@ -875,29 +1071,29 @@ func (d *SyncServerDatasource) getStateDeltasForFullStateSync(
 func (d *SyncServerDatasource) currentStateStreamEventsForRoom(
 	ctx context.Context, txn *sql.Tx, roomID string,
 	stateFilterPart *gomatrix.FilterPart,
-) ([]streamEvent, error) {
+) ([]types.StreamEvent, error) {
 	allState, err := d.roomstate.selectCurrentState(ctx, txn, roomID, stateFilterPart)
 	if err != nil {
 		return nil, err
 	}
-	s := make([]streamEvent, len(allState))
+	s := make([]types.StreamEvent, len(allState))
 	for i := 0; i < len(s); i++ {
-		s[i] = streamEvent{Event: allState[i], streamPosition: 0}
+		s[i] = types.StreamEvent{Event: allState[i], StreamPosition: 0}
 	}
 	return s, nil
 }
 
-// streamEventsToEvents converts streamEvent to Event. If device is non-nil and
+// StreamEventsToEvents converts streamEvent to Event. If device is non-nil and
 // matches the streamevent.transactionID device then the transaction ID gets
 // added to the unsigned section of the output event.
-func streamEventsToEvents(device *authtypes.Device, in []streamEvent) []gomatrixserverlib.Event {
+func (d *SyncServerDatasource) StreamEventsToEvents(device *authtypes.Device, in []types.StreamEvent) []gomatrixserverlib.Event {
 	out := make([]gomatrixserverlib.Event, len(in))
 	for i := 0; i < len(in); i++ {
 		out[i] = in[i].Event
-		if device != nil && in[i].transactionID != nil {
-			if device.UserID == in[i].Sender() && device.SessionID == in[i].transactionID.SessionID {
+		if device != nil && in[i].TransactionID != nil {
+			if device.UserID == in[i].Sender() && device.SessionID == in[i].TransactionID.SessionID {
 				err := out[i].SetUnsignedField(
-					"transaction_id", in[i].transactionID.TransactionID,
+					"transaction_id", in[i].TransactionID.TransactionID,
 				)
 				if err != nil {
 					logrus.WithFields(logrus.Fields{
diff --git a/syncapi/storage/storage.go b/syncapi/storage/storage.go
index 5db4b3a1..4e8a2c83 100644
--- a/syncapi/storage/storage.go
+++ b/syncapi/storage/storage.go
@@ -33,19 +33,26 @@ type Database interface {
 	common.PartitionStorer
 	AllJoinedUsersInRooms(ctx context.Context) (map[string][]string, error)
 	Events(ctx context.Context, eventIDs []string) ([]gomatrixserverlib.Event, error)
-	WriteEvent(ctx context.Context, ev *gomatrixserverlib.Event, addStateEvents []gomatrixserverlib.Event, addStateEventIDs, removeStateEventIDs []string, transactionID *api.TransactionID) (pduPosition int64, returnErr error)
+	WriteEvent(context.Context, *gomatrixserverlib.Event, []gomatrixserverlib.Event, []string, []string, *api.TransactionID, bool) (types.StreamPosition, error)
 	GetStateEvent(ctx context.Context, roomID, evType, stateKey string) (*gomatrixserverlib.Event, error)
 	GetStateEventsForRoom(ctx context.Context, roomID string, stateFilterPart *gomatrix.FilterPart) (stateEvents []gomatrixserverlib.Event, err error)
-	SyncPosition(ctx context.Context) (types.SyncPosition, error)
-	IncrementalSync(ctx context.Context, device authtypes.Device, fromPos, toPos types.SyncPosition, numRecentEventsPerRoom int, wantFullState bool) (*types.Response, error)
+	SyncPosition(ctx context.Context) (types.PaginationToken, error)
+	IncrementalSync(ctx context.Context, device authtypes.Device, fromPos, toPos types.PaginationToken, numRecentEventsPerRoom int, wantFullState bool) (*types.Response, error)
 	CompleteSync(ctx context.Context, userID string, numRecentEventsPerRoom int) (*types.Response, error)
-	GetAccountDataInRange(ctx context.Context, userID string, oldPos, newPos int64, accountDataFilterPart *gomatrix.FilterPart) (map[string][]string, error)
-	UpsertAccountData(ctx context.Context, userID, roomID, dataType string) (int64, error)
-	AddInviteEvent(ctx context.Context, inviteEvent gomatrixserverlib.Event) (int64, error)
+	GetAccountDataInRange(ctx context.Context, userID string, oldPos, newPos types.StreamPosition, accountDataFilterPart *gomatrix.FilterPart) (map[string][]string, error)
+	UpsertAccountData(ctx context.Context, userID, roomID, dataType string) (types.StreamPosition, error)
+	AddInviteEvent(ctx context.Context, inviteEvent gomatrixserverlib.Event) (types.StreamPosition, error)
 	RetireInviteEvent(ctx context.Context, inviteEventID string) error
 	SetTypingTimeoutCallback(fn cache.TimeoutCallbackFn)
-	AddTypingUser(userID, roomID string, expireTime *time.Time) int64
-	RemoveTypingUser(userID, roomID string) int64
+	AddTypingUser(userID, roomID string, expireTime *time.Time) types.StreamPosition
+	RemoveTypingUser(userID, roomID string) types.StreamPosition
+	GetEventsInRange(ctx context.Context, from, to *types.PaginationToken, roomID string, limit int, backwardOrdering bool) (events []types.StreamEvent, err error)
+	EventPositionInTopology(ctx context.Context, eventID string) (types.StreamPosition, error)
+	EventsAtTopologicalPosition(ctx context.Context, roomID string, pos types.StreamPosition) ([]types.StreamEvent, error)
+	BackwardExtremitiesForRoom(ctx context.Context, roomID string) (backwardExtremities []string, err error)
+	MaxTopologicalPosition(ctx context.Context, roomID string) (types.StreamPosition, error)
+	StreamEventsToEvents(device *authtypes.Device, in []types.StreamEvent) []gomatrixserverlib.Event
+	SyncStreamPosition(ctx context.Context) (types.StreamPosition, error)
 }
 
 // NewPublicRoomsServerDatabase opens a database connection.
diff --git a/syncapi/sync/notifier.go b/syncapi/sync/notifier.go
index 548a17ac..aaee49d3 100644
--- a/syncapi/sync/notifier.go
+++ b/syncapi/sync/notifier.go
@@ -36,7 +36,7 @@ type Notifier struct {
 	// Protects currPos and userStreams.
 	streamLock *sync.Mutex
 	// The latest sync position
-	currPos types.SyncPosition
+	currPos types.PaginationToken
 	// A map of user_id => UserStream which can be used to wake a given user's /sync request.
 	userStreams map[string]*UserStream
 	// The last time we cleaned out stale entries from the userStreams map
@@ -46,7 +46,7 @@ type Notifier struct {
 // NewNotifier creates a new notifier set to the given sync position.
 // In order for this to be of any use, the Notifier needs to be told all rooms and
 // the joined users within each of them by calling Notifier.Load(*storage.SyncServerDatabase).
-func NewNotifier(pos types.SyncPosition) *Notifier {
+func NewNotifier(pos types.PaginationToken) *Notifier {
 	return &Notifier{
 		currPos:             pos,
 		roomIDToJoinedUsers: make(map[string]userIDSet),
@@ -68,7 +68,7 @@ func NewNotifier(pos types.SyncPosition) *Notifier {
 // event type it handles, leaving other fields as 0.
 func (n *Notifier) OnNewEvent(
 	ev *gomatrixserverlib.Event, roomID string, userIDs []string,
-	posUpdate types.SyncPosition,
+	posUpdate types.PaginationToken,
 ) {
 	// update the current position then notify relevant /sync streams.
 	// This needs to be done PRIOR to waking up users as they will read this value.
@@ -151,7 +151,7 @@ func (n *Notifier) Load(ctx context.Context, db storage.Database) error {
 }
 
 // CurrentPosition returns the current sync position
-func (n *Notifier) CurrentPosition() types.SyncPosition {
+func (n *Notifier) CurrentPosition() types.PaginationToken {
 	n.streamLock.Lock()
 	defer n.streamLock.Unlock()
 
@@ -173,7 +173,7 @@ func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
 	}
 }
 
-func (n *Notifier) wakeupUsers(userIDs []string, newPos types.SyncPosition) {
+func (n *Notifier) wakeupUsers(userIDs []string, newPos types.PaginationToken) {
 	for _, userID := range userIDs {
 		stream := n.fetchUserStream(userID, false)
 		if stream != nil {
diff --git a/syncapi/sync/notifier_test.go b/syncapi/sync/notifier_test.go
index 808e07cc..02da0f7e 100644
--- a/syncapi/sync/notifier_test.go
+++ b/syncapi/sync/notifier_test.go
@@ -32,11 +32,11 @@ var (
 	randomMessageEvent  gomatrixserverlib.Event
 	aliceInviteBobEvent gomatrixserverlib.Event
 	bobLeaveEvent       gomatrixserverlib.Event
-	syncPositionVeryOld types.SyncPosition
-	syncPositionBefore  types.SyncPosition
-	syncPositionAfter   types.SyncPosition
-	syncPositionNewEDU  types.SyncPosition
-	syncPositionAfter2  types.SyncPosition
+	syncPositionVeryOld types.PaginationToken
+	syncPositionBefore  types.PaginationToken
+	syncPositionAfter   types.PaginationToken
+	syncPositionNewEDU  types.PaginationToken
+	syncPositionAfter2  types.PaginationToken
 )
 
 var (
@@ -46,9 +46,9 @@ var (
 )
 
 func init() {
-	baseSyncPos := types.SyncPosition{
-		PDUPosition:    0,
-		TypingPosition: 0,
+	baseSyncPos := types.PaginationToken{
+		PDUPosition:       0,
+		EDUTypingPosition: 0,
 	}
 
 	syncPositionVeryOld = baseSyncPos
@@ -61,7 +61,7 @@ func init() {
 	syncPositionAfter.PDUPosition = 12
 
 	syncPositionNewEDU = syncPositionAfter
-	syncPositionNewEDU.TypingPosition = 1
+	syncPositionNewEDU.EDUTypingPosition = 1
 
 	syncPositionAfter2 = baseSyncPos
 	syncPositionAfter2.PDUPosition = 13
@@ -119,7 +119,7 @@ func TestImmediateNotification(t *testing.T) {
 		t.Fatalf("TestImmediateNotification error: %s", err)
 	}
 	if pos != syncPositionBefore {
-		t.Fatalf("TestImmediateNotification want %d, got %d", syncPositionBefore, pos)
+		t.Fatalf("TestImmediateNotification want %v, got %v", syncPositionBefore, pos)
 	}
 }
 
@@ -138,7 +138,7 @@ func TestNewEventAndJoinedToRoom(t *testing.T) {
 			t.Errorf("TestNewEventAndJoinedToRoom error: %s", err)
 		}
 		if pos != syncPositionAfter {
-			t.Errorf("TestNewEventAndJoinedToRoom want %d, got %d", syncPositionAfter, pos)
+			t.Errorf("TestNewEventAndJoinedToRoom want %v, got %v", syncPositionAfter, pos)
 		}
 		wg.Done()
 	}()
@@ -166,7 +166,7 @@ func TestNewInviteEventForUser(t *testing.T) {
 			t.Errorf("TestNewInviteEventForUser error: %s", err)
 		}
 		if pos != syncPositionAfter {
-			t.Errorf("TestNewInviteEventForUser want %d, got %d", syncPositionAfter, pos)
+			t.Errorf("TestNewInviteEventForUser want %v, got %v", syncPositionAfter, pos)
 		}
 		wg.Done()
 	}()
@@ -194,7 +194,7 @@ func TestEDUWakeup(t *testing.T) {
 			t.Errorf("TestNewInviteEventForUser error: %s", err)
 		}
 		if pos != syncPositionNewEDU {
-			t.Errorf("TestNewInviteEventForUser want %d, got %d", syncPositionNewEDU, pos)
+			t.Errorf("TestNewInviteEventForUser want %v, got %v", syncPositionNewEDU, pos)
 		}
 		wg.Done()
 	}()
@@ -222,7 +222,7 @@ func TestMultipleRequestWakeup(t *testing.T) {
 			t.Errorf("TestMultipleRequestWakeup error: %s", err)
 		}
 		if pos != syncPositionAfter {
-			t.Errorf("TestMultipleRequestWakeup want %d, got %d", syncPositionAfter, pos)
+			t.Errorf("TestMultipleRequestWakeup want %v, got %v", syncPositionAfter, pos)
 		}
 		wg.Done()
 	}
@@ -262,7 +262,7 @@ func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom error: %s", err)
 		}
 		if pos != syncPositionAfter {
-			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom want %d, got %d", syncPositionAfter, pos)
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom want %v, got %v", syncPositionAfter, pos)
 		}
 		leaveWG.Done()
 	}()
@@ -281,7 +281,7 @@ func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom error: %s", err)
 		}
 		if pos != syncPositionAfter2 {
-			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom want %d, got %d", syncPositionAfter2, pos)
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom want %v, got %v", syncPositionAfter2, pos)
 		}
 		aliceWG.Done()
 	}()
@@ -305,14 +305,14 @@ func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 	time.Sleep(1 * time.Millisecond)
 }
 
-func waitForEvents(n *Notifier, req syncRequest) (types.SyncPosition, error) {
+func waitForEvents(n *Notifier, req syncRequest) (types.PaginationToken, error) {
 	listener := n.GetListener(req)
 	defer listener.Close()
 
 	select {
 	case <-time.After(5 * time.Second):
-		return types.SyncPosition{}, fmt.Errorf(
-			"waitForEvents timed out waiting for %s (pos=%d)", req.device.UserID, req.since,
+		return types.PaginationToken{}, fmt.Errorf(
+			"waitForEvents timed out waiting for %s (pos=%v)", req.device.UserID, req.since,
 		)
 	case <-listener.GetNotifyChannel(*req.since):
 		p := listener.GetSyncPosition()
@@ -337,7 +337,7 @@ func lockedFetchUserStream(n *Notifier, userID string) *UserStream {
 	return n.fetchUserStream(userID, true)
 }
 
-func newTestSyncRequest(userID string, since types.SyncPosition) syncRequest {
+func newTestSyncRequest(userID string, since types.PaginationToken) syncRequest {
 	return syncRequest{
 		device:        authtypes.Device{UserID: userID},
 		timeout:       1 * time.Minute,
diff --git a/syncapi/sync/request.go b/syncapi/sync/request.go
index a5d2f60f..f2e199d2 100644
--- a/syncapi/sync/request.go
+++ b/syncapi/sync/request.go
@@ -16,10 +16,8 @@ package sync
 
 import (
 	"context"
-	"errors"
 	"net/http"
 	"strconv"
-	"strings"
 	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
@@ -38,7 +36,7 @@ type syncRequest struct {
 	device        authtypes.Device
 	limit         int
 	timeout       time.Duration
-	since         *types.SyncPosition // nil means that no since token was supplied
+	since         *types.PaginationToken // nil means that no since token was supplied
 	wantFullState bool
 	log           *log.Entry
 }
@@ -47,7 +45,7 @@ func newSyncRequest(req *http.Request, device authtypes.Device) (*syncRequest, e
 	timeout := getTimeout(req.URL.Query().Get("timeout"))
 	fullState := req.URL.Query().Get("full_state")
 	wantFullState := fullState != "" && fullState != "false"
-	since, err := getSyncStreamPosition(req.URL.Query().Get("since"))
+	since, err := getPaginationToken(req.URL.Query().Get("since"))
 	if err != nil {
 		return nil, err
 	}
@@ -75,41 +73,14 @@ func getTimeout(timeoutMS string) time.Duration {
 }
 
 // getSyncStreamPosition tries to parse a 'since' token taken from the API to a
-// types.SyncPosition. If the string is empty then (nil, nil) is returned.
+// types.PaginationToken. If the string is empty then (nil, nil) is returned.
 // There are two forms of tokens: The full length form containing all PDU and EDU
 // positions separated by "_", and the short form containing only the PDU
 // position. Short form can be used for, e.g., `prev_batch` tokens.
-func getSyncStreamPosition(since string) (*types.SyncPosition, error) {
+func getPaginationToken(since string) (*types.PaginationToken, error) {
 	if since == "" {
 		return nil, nil
 	}
 
-	posStrings := strings.Split(since, "_")
-	if len(posStrings) != 2 && len(posStrings) != 1 {
-		// A token can either be full length or short (PDU-only).
-		return nil, errors.New("malformed batch token")
-	}
-
-	positions := make([]int64, len(posStrings))
-	for i, posString := range posStrings {
-		pos, err := strconv.ParseInt(posString, 10, 64)
-		if err != nil {
-			return nil, err
-		}
-		positions[i] = pos
-	}
-
-	if len(positions) == 2 {
-		// Full length token; construct SyncPosition with every entry in
-		// `positions`. These entries must have the same order with the fields
-		// in struct SyncPosition, so we disable the govet check below.
-		return &types.SyncPosition{ //nolint:govet
-			positions[0], positions[1],
-		}, nil
-	} else {
-		// Token with PDU position only
-		return &types.SyncPosition{
-			PDUPosition: positions[0],
-		}, nil
-	}
+	return types.NewPaginationTokenFromString(since)
 }
diff --git a/syncapi/sync/requestpool.go b/syncapi/sync/requestpool.go
index d75f07e6..5a3ae880 100644
--- a/syncapi/sync/requestpool.go
+++ b/syncapi/sync/requestpool.go
@@ -130,7 +130,7 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 	}
 }
 
-func (rp *RequestPool) currentSyncForUser(req syncRequest, latestPos types.SyncPosition) (res *types.Response, err error) {
+func (rp *RequestPool) currentSyncForUser(req syncRequest, latestPos types.PaginationToken) (res *types.Response, err error) {
 	// TODO: handle ignored users
 	if req.since == nil {
 		res, err = rp.db.CompleteSync(req.ctx, req.device.UserID, req.limit)
@@ -143,7 +143,7 @@ func (rp *RequestPool) currentSyncForUser(req syncRequest, latestPos types.SyncP
 	}
 
 	accountDataFilter := gomatrix.DefaultFilterPart() // TODO: use filter provided in req instead
-	res, err = rp.appendAccountData(res, req.device.UserID, req, latestPos.PDUPosition, &accountDataFilter)
+	res, err = rp.appendAccountData(res, req.device.UserID, req, int64(latestPos.PDUPosition), &accountDataFilter)
 	return
 }
 
@@ -183,7 +183,11 @@ func (rp *RequestPool) appendAccountData(
 	}
 
 	// Sync is not initial, get all account data since the latest sync
-	dataTypes, err := rp.db.GetAccountDataInRange(req.ctx, userID, req.since.PDUPosition, currentPos, accountDataFilter)
+	dataTypes, err := rp.db.GetAccountDataInRange(
+		req.ctx, userID,
+		types.StreamPosition(req.since.PDUPosition), types.StreamPosition(currentPos),
+		accountDataFilter,
+	)
 	if err != nil {
 		return nil, err
 	}
diff --git a/syncapi/sync/userstream.go b/syncapi/sync/userstream.go
index beb10e48..6eef8644 100644
--- a/syncapi/sync/userstream.go
+++ b/syncapi/sync/userstream.go
@@ -35,7 +35,7 @@ type UserStream struct {
 	// Closed when there is an update.
 	signalChannel chan struct{}
 	// The last sync position that there may have been an update for the user
-	pos types.SyncPosition
+	pos types.PaginationToken
 	// The last time when we had some listeners waiting
 	timeOfLastChannel time.Time
 	// The number of listeners waiting
@@ -51,7 +51,7 @@ type UserStreamListener struct {
 }
 
 // NewUserStream creates a new user stream
-func NewUserStream(userID string, currPos types.SyncPosition) *UserStream {
+func NewUserStream(userID string, currPos types.PaginationToken) *UserStream {
 	return &UserStream{
 		UserID:            userID,
 		timeOfLastChannel: time.Now(),
@@ -85,7 +85,7 @@ func (s *UserStream) GetListener(ctx context.Context) UserStreamListener {
 }
 
 // Broadcast a new sync position for this user.
-func (s *UserStream) Broadcast(pos types.SyncPosition) {
+func (s *UserStream) Broadcast(pos types.PaginationToken) {
 	s.lock.Lock()
 	defer s.lock.Unlock()
 
@@ -120,7 +120,7 @@ func (s *UserStream) TimeOfLastNonEmpty() time.Time {
 
 // GetStreamPosition returns last sync position which the UserStream was
 // notified about
-func (s *UserStreamListener) GetSyncPosition() types.SyncPosition {
+func (s *UserStreamListener) GetSyncPosition() types.PaginationToken {
 	s.userStream.lock.Lock()
 	defer s.userStream.lock.Unlock()
 
@@ -132,7 +132,7 @@ func (s *UserStreamListener) GetSyncPosition() types.SyncPosition {
 // sincePos specifies from which point we want to be notified about. If there
 // has already been an update after sincePos we'll return a closed channel
 // immediately.
-func (s *UserStreamListener) GetNotifyChannel(sincePos types.SyncPosition) <-chan struct{} {
+func (s *UserStreamListener) GetNotifyChannel(sincePos types.PaginationToken) <-chan struct{} {
 	s.userStream.lock.Lock()
 	defer s.userStream.lock.Unlock()
 
diff --git a/syncapi/syncapi.go b/syncapi/syncapi.go
index 4738feea..ecf532ca 100644
--- a/syncapi/syncapi.go
+++ b/syncapi/syncapi.go
@@ -21,7 +21,9 @@ import (
 
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
 	"github.com/matrix-org/dendrite/common/basecomponent"
+	"github.com/matrix-org/dendrite/common/config"
 	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/devices"
 	"github.com/matrix-org/dendrite/syncapi/consumers"
@@ -37,6 +39,8 @@ func SetupSyncAPIComponent(
 	deviceDB *devices.Database,
 	accountsDB *accounts.Database,
 	queryAPI api.RoomserverQueryAPI,
+	federation *gomatrixserverlib.FederationClient,
+	cfg *config.Dendrite,
 ) {
 	syncDB, err := storage.NewSyncServerDatasource(string(base.Cfg.Database.SyncAPI))
 	if err != nil {
@@ -77,5 +81,5 @@ func SetupSyncAPIComponent(
 		logrus.WithError(err).Panicf("failed to start typing server consumer")
 	}
 
-	routing.Setup(base.APIMux, requestPool, syncDB, deviceDB)
+	routing.Setup(base.APIMux, requestPool, syncDB, deviceDB, federation, queryAPI, cfg)
 }
diff --git a/syncapi/types/types.go b/syncapi/types/types.go
index af7ec865..c25a38cd 100644
--- a/syncapi/types/types.go
+++ b/syncapi/types/types.go
@@ -16,45 +16,144 @@ package types
 
 import (
 	"encoding/json"
+	"errors"
+	"fmt"
 	"strconv"
+	"strings"
 
+	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
-// SyncPosition contains the PDU and EDU stream sync positions for a client.
-type SyncPosition struct {
-	// PDUPosition is the stream position for PDUs the client is at.
-	PDUPosition int64
-	// TypingPosition is the client's position for typing notifications.
-	TypingPosition int64
+var (
+	// ErrInvalidPaginationTokenType is returned when an attempt at creating a
+	// new instance of PaginationToken with an invalid type (i.e. neither "s"
+	// nor "t").
+	ErrInvalidPaginationTokenType = fmt.Errorf("Pagination token has an unknown prefix (should be either s or t)")
+	// ErrInvalidPaginationTokenLen is returned when the pagination token is an
+	// invalid length
+	ErrInvalidPaginationTokenLen = fmt.Errorf("Pagination token has an invalid length")
+)
+
+// StreamPosition represents the offset in the sync stream a client is at.
+type StreamPosition int64
+
+// Same as gomatrixserverlib.Event but also has the PDU stream position for this event.
+type StreamEvent struct {
+	gomatrixserverlib.Event
+	StreamPosition  StreamPosition
+	TransactionID   *api.TransactionID
+	ExcludeFromSync bool
 }
 
-// String implements the Stringer interface.
-func (sp SyncPosition) String() string {
-	return strconv.FormatInt(sp.PDUPosition, 10) + "_" +
-		strconv.FormatInt(sp.TypingPosition, 10)
+// PaginationTokenType represents the type of a pagination token.
+// It can be either "s" (representing a position in the whole stream of events)
+// or "t" (representing a position in a room's topology/depth).
+type PaginationTokenType string
+
+const (
+	// PaginationTokenTypeStream represents a position in the server's whole
+	// stream of events
+	PaginationTokenTypeStream PaginationTokenType = "s"
+	// PaginationTokenTypeTopology represents a position in a room's topology.
+	PaginationTokenTypeTopology PaginationTokenType = "t"
+)
+
+// PaginationToken represents a pagination token, used for interactions with
+// /sync or /messages, for example.
+type PaginationToken struct {
+	//Position StreamPosition
+	Type              PaginationTokenType
+	PDUPosition       StreamPosition
+	EDUTypingPosition StreamPosition
 }
 
-// IsAfter returns whether one SyncPosition refers to states newer than another SyncPosition.
-func (sp SyncPosition) IsAfter(other SyncPosition) bool {
-	return sp.PDUPosition > other.PDUPosition ||
-		sp.TypingPosition > other.TypingPosition
+// NewPaginationTokenFromString takes a string of the form "xyyyy..." where "x"
+// represents the type of a pagination token and "yyyy..." the token itself, and
+// parses it in order to create a new instance of PaginationToken. Returns an
+// error if the token couldn't be parsed into an int64, or if the token type
+// isn't a known type (returns ErrInvalidPaginationTokenType in the latter
+// case).
+func NewPaginationTokenFromString(s string) (token *PaginationToken, err error) {
+	if len(s) == 0 {
+		return nil, ErrInvalidPaginationTokenLen
+	}
+
+	token = new(PaginationToken)
+	var positions []string
+
+	switch t := PaginationTokenType(s[:1]); t {
+	case PaginationTokenTypeStream, PaginationTokenTypeTopology:
+		token.Type = t
+		positions = strings.Split(s[1:], "_")
+	default:
+		token.Type = PaginationTokenTypeStream
+		positions = strings.Split(s, "_")
+	}
+
+	// Try to get the PDU position.
+	if len(positions) >= 1 {
+		if pduPos, err := strconv.ParseInt(positions[0], 10, 64); err != nil {
+			return nil, err
+		} else if pduPos < 0 {
+			return nil, errors.New("negative PDU position not allowed")
+		} else {
+			token.PDUPosition = StreamPosition(pduPos)
+		}
+	}
+
+	// Try to get the typing position.
+	if len(positions) >= 2 {
+		if typPos, err := strconv.ParseInt(positions[1], 10, 64); err != nil {
+			return nil, err
+		} else if typPos < 0 {
+			return nil, errors.New("negative EDU typing position not allowed")
+		} else {
+			token.EDUTypingPosition = StreamPosition(typPos)
+		}
+	}
+
+	return
+}
+
+// NewPaginationTokenFromTypeAndPosition takes a PaginationTokenType and a
+// StreamPosition and returns an instance of PaginationToken.
+func NewPaginationTokenFromTypeAndPosition(
+	t PaginationTokenType, pdupos StreamPosition, typpos StreamPosition,
+) (p *PaginationToken) {
+	return &PaginationToken{
+		Type:              t,
+		PDUPosition:       pdupos,
+		EDUTypingPosition: typpos,
+	}
 }
 
-// WithUpdates returns a copy of the SyncPosition with updates applied from another SyncPosition.
-// If the latter SyncPosition contains a field that is not 0, it is considered an update,
-// and its value will replace the corresponding value in the SyncPosition on which WithUpdates is called.
-func (sp SyncPosition) WithUpdates(other SyncPosition) SyncPosition {
-	ret := sp
+// String translates a PaginationToken to a string of the "xyyyy..." (see
+// NewPaginationToken to know what it represents).
+func (p *PaginationToken) String() string {
+	return fmt.Sprintf("%s%d_%d", p.Type, p.PDUPosition, p.EDUTypingPosition)
+}
+
+// WithUpdates returns a copy of the PaginationToken with updates applied from another PaginationToken.
+// If the latter PaginationToken contains a field that is not 0, it is considered an update,
+// and its value will replace the corresponding value in the PaginationToken on which WithUpdates is called.
+func (pt *PaginationToken) WithUpdates(other PaginationToken) PaginationToken {
+	ret := *pt
 	if other.PDUPosition != 0 {
 		ret.PDUPosition = other.PDUPosition
 	}
-	if other.TypingPosition != 0 {
-		ret.TypingPosition = other.TypingPosition
+	if other.EDUTypingPosition != 0 {
+		ret.EDUTypingPosition = other.EDUTypingPosition
 	}
 	return ret
 }
 
+// IsAfter returns whether one PaginationToken refers to states newer than another PaginationToken.
+func (sp *PaginationToken) IsAfter(other PaginationToken) bool {
+	return sp.PDUPosition > other.PDUPosition ||
+		sp.EDUTypingPosition > other.EDUTypingPosition
+}
+
 // PrevEventRef represents a reference to a previous event in a state event upgrade
 type PrevEventRef struct {
 	PrevContent   json.RawMessage `json:"prev_content"`
@@ -79,9 +178,9 @@ type Response struct {
 }
 
 // NewResponse creates an empty response with initialised maps.
-func NewResponse(pos SyncPosition) *Response {
+func NewResponse(token PaginationToken) *Response {
 	res := Response{
-		NextBatch: pos.String(),
+		NextBatch: token.String(),
 	}
 	// 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.
@@ -96,6 +195,14 @@ func NewResponse(pos SyncPosition) *Response {
 	res.AccountData.Events = make([]gomatrixserverlib.ClientEvent, 0)
 	res.Presence.Events = make([]gomatrixserverlib.ClientEvent, 0)
 
+	// Fill next_batch with a pagination token. Since this is a response to a sync request, we can assume
+	// we'll always return a stream token.
+	res.NextBatch = NewPaginationTokenFromTypeAndPosition(
+		PaginationTokenTypeStream,
+		StreamPosition(token.PDUPosition),
+		StreamPosition(token.EDUTypingPosition),
+	).String()
+
 	return &res
 }
 
diff --git a/syncapi/types/types_test.go b/syncapi/types/types_test.go
new file mode 100644
index 00000000..f4c84e0d
--- /dev/null
+++ b/syncapi/types/types_test.go
@@ -0,0 +1,52 @@
+package types
+
+import "testing"
+
+func TestNewPaginationTokenFromString(t *testing.T) {
+	shouldPass := map[string]PaginationToken{
+		"2": PaginationToken{
+			Type:        PaginationTokenTypeStream,
+			PDUPosition: 2,
+		},
+		"s4": PaginationToken{
+			Type:        PaginationTokenTypeStream,
+			PDUPosition: 4,
+		},
+		"s3_1": PaginationToken{
+			Type:              PaginationTokenTypeStream,
+			PDUPosition:       3,
+			EDUTypingPosition: 1,
+		},
+		"t3_1_4": PaginationToken{
+			Type:              PaginationTokenTypeTopology,
+			PDUPosition:       3,
+			EDUTypingPosition: 1,
+		},
+	}
+
+	shouldFail := []string{
+		"",
+		"s_1",
+		"s_",
+		"a3_4",
+		"b",
+		"b-1",
+		"-4",
+	}
+
+	for test, expected := range shouldPass {
+		result, err := NewPaginationTokenFromString(test)
+		if err != nil {
+			t.Error(err)
+		}
+		if *result != expected {
+			t.Errorf("expected %v but got %v", expected.String(), result.String())
+		}
+	}
+
+	for _, test := range shouldFail {
+		if _, err := NewPaginationTokenFromString(test); err == nil {
+			t.Errorf("input '%v' should have errored but didn't", test)
+		}
+	}
+}