From 490c40f5f38ddc7c63bff70ea9ecc677fb650c7b Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Mon, 20 Jan 2020 15:56:40 +0000
Subject: [PATCH] Merge forward

---
 cmd/dendrite-monolith-server/main.go          |   2 +-
 cmd/dendrite-sync-api-server/main.go          |   3 +-
 roomserver/api/query.go                       |  40 +-
 roomserver/query/query.go                     |  49 ++
 syncapi/consumers/roomserver.go               |   1 +
 syncapi/routing/messages.go                   | 480 ++++++++++++++++++
 syncapi/routing/routing.go                    |  15 +-
 .../postgres/backward_extremities_table.go    | 118 +++++
 .../postgres/current_room_state_table.go      |   3 +-
 .../postgres/output_room_events_table.go      | 149 ++++--
 .../output_room_events_topology_table.go      | 187 +++++++
 syncapi/storage/postgres/syncserver.go        | 316 ++++++++++--
 syncapi/storage/storage.go                    |   7 +-
 syncapi/sync/request.go                       |  24 +
 syncapi/syncapi.go                            |   7 +-
 syncapi/types/types.go                        |  93 ++++
 16 files changed, 1396 insertions(+), 98 deletions(-)
 create mode 100644 syncapi/routing/messages.go
 create mode 100644 syncapi/storage/postgres/backward_extremities_table.go
 create mode 100644 syncapi/storage/postgres/output_room_events_topology_table.go

diff --git a/cmd/dendrite-monolith-server/main.go b/cmd/dendrite-monolith-server/main.go
index 5ea6b154a..7515ec5c0 100644
--- a/cmd/dendrite-monolith-server/main.go
+++ b/cmd/dendrite-monolith-server/main.go
@@ -70,7 +70,7 @@ func main() {
 	federationapi.SetupFederationAPIComponent(base, accountDB, deviceDB, federation, &keyRing, alias, input, query, asQuery, fedSenderAPI)
 	mediaapi.SetupMediaAPIComponent(base, deviceDB)
 	publicroomsapi.SetupPublicRoomsAPIComponent(base, deviceDB)
-	syncapi.SetupSyncAPIComponent(base, deviceDB, accountDB, query)
+	syncapi.SetupSyncAPIComponent(base, deviceDB, accountDB, query, federation, cfg)
 
 	httpHandler := common.WrapHandlerInCORS(base.APIMux)
 
diff --git a/cmd/dendrite-sync-api-server/main.go b/cmd/dendrite-sync-api-server/main.go
index 1c47ec525..55e9faeef 100644
--- a/cmd/dendrite-sync-api-server/main.go
+++ b/cmd/dendrite-sync-api-server/main.go
@@ -26,10 +26,11 @@ func main() {
 
 	deviceDB := base.CreateDeviceDB()
 	accountDB := base.CreateAccountsDB()
+	federation := base.CreateFederationClient()
 
 	_, _, query := base.CreateHTTPRoomserverAPIs()
 
-	syncapi.SetupSyncAPIComponent(base, deviceDB, accountDB, query)
+	syncapi.SetupSyncAPIComponent(base, deviceDB, accountDB, query, federation, cfg)
 
 	base.SetupAndServeHTTP(string(base.Cfg.Bind.SyncAPI), string(base.Cfg.Listen.SyncAPI))
 
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index e52c74ac3..a990cd0b4 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -230,6 +230,20 @@ type QueryBackfillResponse struct {
 	Events []gomatrixserverlib.Event `json:"events"`
 }
 
+// QueryServersInRoomAtEventRequest is a request to QueryServersInRoomAtEvent
+type QueryServersInRoomAtEventRequest struct {
+	// ID of the room to retrieve member servers for.
+	RoomID string `json:"room_id"`
+	// ID of the event for which to retrieve member servers.
+	EventID string `json:"event_id"`
+}
+
+// QueryServersInRoomAtEventResponse is a response to QueryServersInRoomAtEvent
+type QueryServersInRoomAtEventResponse struct {
+	// Servers present in the room for these events.
+	Servers []gomatrixserverlib.ServerName `json:"servers"`
+}
+
 // RoomserverQueryAPI is used to query information from the room server.
 type RoomserverQueryAPI interface {
 	// Query the latest events and state for a room from the room server.
@@ -303,6 +317,12 @@ type RoomserverQueryAPI interface {
 		request *QueryBackfillRequest,
 		response *QueryBackfillResponse,
 	) error
+
+	QueryServersInRoomAtEvent(
+		ctx context.Context,
+		request *QueryServersInRoomAtEventRequest,
+		response *QueryServersInRoomAtEventResponse,
+	) error
 }
 
 // RoomserverQueryLatestEventsAndStatePath is the HTTP path for the QueryLatestEventsAndState API.
@@ -332,8 +352,11 @@ const RoomserverQueryMissingEventsPath = "/api/roomserver/queryMissingEvents"
 // RoomserverQueryStateAndAuthChainPath is the HTTP path for the QueryStateAndAuthChain API
 const RoomserverQueryStateAndAuthChainPath = "/api/roomserver/queryStateAndAuthChain"
 
-// RoomserverQueryBackfillPath is the HTTP path for the QueryBackfill API
-const RoomserverQueryBackfillPath = "/api/roomserver/QueryBackfill"
+// RoomserverQueryBackfillPath is the HTTP path for the QueryMissingEvents API
+const RoomserverQueryBackfillPath = "/api/roomserver/queryBackfill"
+
+// RoomserverQueryServersInRoomAtEvent is the HTTP path for the QueryServersInRoomAtEvent API
+const RoomserverQueryServersInRoomAtEvent = "/api/roomserver/queryServersInRoomAtEvents"
 
 // NewRoomserverQueryAPIHTTP creates a RoomserverQueryAPI implemented by talking to a HTTP POST API.
 // If httpClient is nil then it uses the http.DefaultClient
@@ -478,3 +501,16 @@ func (h *httpRoomserverQueryAPI) QueryBackfill(
 	apiURL := h.roomserverURL + RoomserverQueryBackfillPath
 	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
 }
+
+// QueryServersInRoomAtEvent implements RoomServerQueryAPI
+func (h *httpRoomserverQueryAPI) QueryServersInRoomAtEvent(
+	ctx context.Context,
+	request *QueryServersInRoomAtEventRequest,
+	response *QueryServersInRoomAtEventResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "QueryBackfill")
+	defer span.Finish()
+
+	apiURL := h.roomserverURL + RoomserverQueryServersInRoomAtEvent
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}
diff --git a/roomserver/query/query.go b/roomserver/query/query.go
index caa7a95bb..9dbf3a44b 100644
--- a/roomserver/query/query.go
+++ b/roomserver/query/query.go
@@ -660,6 +660,41 @@ func getAuthChain(
 	return authEvents, nil
 }
 
+// QueryServersInRoomAtEvent implements api.RoomserverQueryAPI
+func (r *RoomserverQueryAPI) QueryServersInRoomAtEvent(
+	ctx context.Context,
+	request *api.QueryServersInRoomAtEventRequest,
+	response *api.QueryServersInRoomAtEventResponse,
+) error {
+	// getMembershipsBeforeEventNID requires a NID, so retrieving the NID for
+	// the event is necessary.
+	NIDs, err := r.DB.EventNIDs(ctx, []string{request.EventID})
+	if err != nil {
+		return err
+	}
+
+	// Retrieve all "m.room.member" state events of "join" membership, which
+	// contains the list of users in the room before the event, therefore all
+	// the servers in it at that moment.
+	events, err := r.getMembershipsBeforeEventNID(ctx, NIDs[request.EventID], true)
+	if err != nil {
+		return err
+	}
+
+	// Store the server names in a temporary map to avoid duplicates.
+	servers := make(map[gomatrixserverlib.ServerName]bool)
+	for _, event := range events {
+		servers[event.Origin()] = true
+	}
+
+	// Populate the response.
+	for server := range servers {
+		response.Servers = append(response.Servers, server)
+	}
+
+	return nil
+}
+
 // SetupHTTP adds the RoomserverQueryAPI handlers to the http.ServeMux.
 // nolint: gocyclo
 func (r *RoomserverQueryAPI) SetupHTTP(servMux *http.ServeMux) {
@@ -803,4 +838,18 @@ func (r *RoomserverQueryAPI) SetupHTTP(servMux *http.ServeMux) {
 			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
 		}),
 	)
+	servMux.Handle(
+		api.RoomserverQueryServersInRoomAtEvent,
+		common.MakeInternalAPI("QueryServersInRoomAtEvent", func(req *http.Request) util.JSONResponse {
+			var request api.QueryServersInRoomAtEventRequest
+			var response api.QueryServersInRoomAtEventResponse
+			if err := json.NewDecoder(req.Body).Decode(&request); err != nil {
+				return util.ErrorResponse(err)
+			}
+			if err := r.QueryServersInRoomAtEvent(req.Context(), &request, &response); err != nil {
+				return util.ErrorResponse(err)
+			}
+			return util.JSONResponse{Code: http.StatusOK, JSON: &response}
+		}),
+	)
 }
diff --git a/syncapi/consumers/roomserver.go b/syncapi/consumers/roomserver.go
index cde2f5080..c9f572d14 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
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
new file mode 100644
index 000000000..c61b5322b
--- /dev/null
+++ b/syncapi/routing/messages.go
@@ -0,0 +1,480 @@
+// 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/r0.4.0.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,
+	)
+	end = types.NewPaginationTokenFromTypeAndPosition(
+		types.PaginationTokenTypeTopology, endPos,
+	)
+
+	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.Position--
+	}
+
+	// The lowest token value is 1, therefore we need to manually set it to that
+	// value if we're below it.
+	if end.Position < types.StreamPosition(1) {
+		end.Position = types.StreamPosition(1)
+	}
+
+	return
+}
+
+// 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.
+				isSetLargeEnough = (r.to.Position-1 == types.StreamPosition(streamEvents[len(streamEvents)-1].StreamPosition))
+			}
+		} else {
+			isSetLargeEnough = (r.from.Position-1 == types.StreamPosition(streamEvents[0].StreamPosition))
+		}
+	}
+
+	// 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/unstable.html#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)
+	} else {
+		var pos types.StreamPosition
+		pos, err = db.MaxTopologicalPosition(ctx, roomID)
+		if err != nil {
+			return
+		}
+
+		to = types.NewPaginationTokenFromTypeAndPosition(types.PaginationTokenTypeTopology, pos)
+	}
+
+	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 bd9389bdd..922a7df6e 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 +79,9 @@ 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 := mux.Vars(req)
+		return OnIncomingMessagesRequest(req, syncDB, vars["roomID"], federation, queryAPI, cfg)
+	})).Methods(http.MethodGet, http.MethodOptions)
 }
diff --git a/syncapi/storage/postgres/backward_extremities_table.go b/syncapi/storage/postgres/backward_extremities_table.go
new file mode 100644
index 000000000..476d26faa
--- /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 8b2080438..f857c0141 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"
 )
@@ -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/output_room_events_table.go b/syncapi/storage/postgres/output_room_events_table.go
index ca2715934..e1b1df5ec 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"
@@ -57,7 +58,12 @@ CREATE TABLE IF NOT EXISTS syncapi_output_room_events (
     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
+    transaction_id TEXT,  -- The transaction id used to send the event, if any
+		-- 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 +71,33 @@ 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 +109,13 @@ 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 +135,12 @@ 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
 	}
@@ -129,7 +153,7 @@ func (s *outputRoomEventsStatements) prepare(db *sql.DB) (err error) {
 func (s *outputRoomEventsStatements) selectStateInRange(
 	ctx context.Context, txn *sql.Tx, oldPos, newPos int64,
 	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 +173,20 @@ 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       int64
+			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 +217,10 @@ 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,
 		}
 	}
 
@@ -221,7 +247,7 @@ func (s *outputRoomEventsStatements) selectMaxEventID(
 func (s *outputRoomEventsStatements) insertEvent(
 	ctx context.Context, txn *sql.Tx,
 	event *gomatrixserverlib.Event, addState, removeState []string,
-	transactionID *api.TransactionID,
+	transactionID *api.TransactionID, excludeFromSync bool,
 ) (streamPos int64, err error) {
 	var txnID *string
 	var sessionID *int64
@@ -251,16 +277,53 @@ 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 +337,16 @@ 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 +356,18 @@ 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       int64
+			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 +383,11 @@ 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 000000000..4362cd3f0
--- /dev/null
+++ b/syncapi/storage/postgres/output_room_events_topology_table.go
@@ -0,0 +1,187 @@
+// 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)"
+
+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 3a62d1364..207d28735 100644
--- a/syncapi/storage/postgres/syncserver.go
+++ b/syncapi/storage/postgres/syncserver.go
@@ -46,26 +46,21 @@ type stateDelta struct {
 	membershipPos int64
 }
 
-// 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 +82,12 @@ 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 +110,7 @@ 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
 }
 
 // WriteEvent into the database. It is not safe to call this function from multiple goroutines, as it would create races
@@ -120,16 +121,57 @@ func (d *SyncServerDatasource) WriteEvent(
 	ev *gomatrixserverlib.Event,
 	addStateEvents []gomatrixserverlib.Event,
 	addStateEventIDs, removeStateEventIDs []string,
-	transactionID *api.TransactionID,
+	transactionID *api.TransactionID, excludeFromSync bool,
 ) (pduPosition int64, 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 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
+				}
+			}
+		}
+
 		if len(addStateEvents) == 0 && len(removeStateEventIDs) == 0 {
 			// Nothing to do, the event may have just been a message event.
 			return nil
@@ -196,11 +238,138 @@ 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.Position
+			forwardLimit = from.Position
+		} else {
+			// Forward ordering is chronological (oldest event to latest one).
+			backwardLimit = from.Position
+			forwardLimit = to.Position
+		}
+
+		// 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.Position, from.Position, 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.Position, to.Position, limit,
+		); err != nil {
+			return
+		}
+	}
+
+	return
+}
+
 // SyncPosition returns the latest positions for syncing.
 func (d *SyncServerDatasource) SyncPosition(ctx context.Context) (types.SyncPosition, 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) {
@@ -399,9 +568,16 @@ func (d *SyncServerDatasource) getResponseWithPDUsForCompleteSync(
 	defer common.EndTransaction(txn, &succeeded)
 
 	// Get the current sync position which we will base the sync response on.
-	toPos, err = d.syncPositionTx(ctx, txn)
+	/*
+		toPos, err = d.syncPositionTx(ctx, txn)
+		if err != nil {
+			return
+		}
+	*/
+	// Get the current stream position which we will base the sync response on.
+	pos, err := d.syncStreamPositionTx(ctx, txn)
 	if err != nil {
-		return
+		return nil, types.SyncPosition{}, []string{}, err
 	}
 
 	res = types.NewResponse(toPos)
@@ -423,21 +599,38 @@ 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), pos,
+			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.SyncPosition{}, []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 {
+		jr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
+			types.PaginationTokenTypeTopology, backwardTopologyPos,
+		).String()
+		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 {
@@ -598,12 +791,13 @@ 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
@@ -618,18 +812,35 @@ func (d *SyncServerDatasource) addRoomDeltaToResponse(
 		// state events but no recent events.
 		prevPDUPos = toPos - 1
 	} else {
-		prevPDUPos = recentStreamEvents[0].streamPosition - 1
+		prevPDUPos = recentStreamEvents[0].StreamPosition - 1
 	}
 
 	if prevPDUPos <= 0 {
 		prevPDUPos = 1
 	}
 
+	// 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 err
+	}
+	if backwardTopologyPos-1 <= 0 {
+		backwardTopologyPos = types.StreamPosition(1)
+	} else {
+		backwardTopologyPos = backwardTopologyPos - 1
+	}
+
 	switch delta.membership {
 	case gomatrixserverlib.Join:
 		jr := types.NewJoinResponse()
+
+		jr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
+			types.PaginationTokenTypeTopology, backwardTopologyPos,
+		).String()
 		// Use the short form of batch token for prev_batch
-		jr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
+		//jr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
 		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 +851,11 @@ 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()
+		lr.Timeline.PrevBatch = types.NewPaginationTokenFromTypeAndPosition(
+			types.PaginationTokenTypeStream, backwardTopologyPos,
+		).String()
 		// Use the short form of batch token for prev_batch
-		lr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
+		//lr.Timeline.PrevBatch = strconv.FormatInt(prevPDUPos, 10)
 		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 +870,9 @@ 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 +914,7 @@ 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)
@@ -776,19 +990,25 @@ 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
 					}
+					/*
+						s = make([]StreamEvent, len(allState))
+						for i := 0; i < len(s); i++ {
+							s[i] = StreamEvent{Event: allState[i], StreamPosition: types.StreamPosition(0)}
+						}
+					*/
 					state[roomID] = s
 					continue // we'll add this room in when we do joined rooms
 				}
 
 				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 +1024,7 @@ 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,
 		})
 	}
@@ -837,7 +1057,7 @@ func (d *SyncServerDatasource) getStateDeltasForFullStateSync(
 		}
 		deltas = append(deltas, stateDelta{
 			membership:  gomatrixserverlib.Join,
-			stateEvents: streamEventsToEvents(device, s),
+			stateEvents: d.StreamEventsToEvents(device, s),
 			roomID:      joinedRoomID,
 		})
 	}
@@ -858,8 +1078,8 @@ 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 +1095,29 @@ 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 5db4b3a1b..9bd35e28a 100644
--- a/syncapi/storage/storage.go
+++ b/syncapi/storage/storage.go
@@ -33,7 +33,7 @@ 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) (int64, 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)
@@ -46,6 +46,11 @@ type Database interface {
 	SetTypingTimeoutCallback(fn cache.TimeoutCallbackFn)
 	AddTypingUser(userID, roomID string, expireTime *time.Time) int64
 	RemoveTypingUser(userID, roomID string) int64
+	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)
+	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
 }
 
 // NewPublicRoomsServerDatabase opens a database connection.
diff --git a/syncapi/sync/request.go b/syncapi/sync/request.go
index a5d2f60f4..07b3d3cfa 100644
--- a/syncapi/sync/request.go
+++ b/syncapi/sync/request.go
@@ -17,6 +17,7 @@ package sync
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net/http"
 	"strconv"
 	"strings"
@@ -32,6 +33,12 @@ import (
 const defaultSyncTimeout = time.Duration(0)
 const defaultTimelineLimit = 20
 
+var (
+	// ErrNotStreamToken is returned if a pagination token isn't of type
+	// types.PaginationTokenTypeStream
+	ErrNotStreamToken = fmt.Errorf("The provided pagination token has the wrong prefix (should be s)")
+)
+
 // syncRequest represents a /sync request, with sensible defaults/sanity checks applied.
 type syncRequest struct {
 	ctx           context.Context
@@ -74,6 +81,23 @@ func getTimeout(timeoutMS string) time.Duration {
 	return time.Duration(i) * time.Millisecond
 }
 
+// getPaginationToken tries to parse a 'since' token taken from the API to a
+// pagination token. If the string is empty then (nil, nil) is returned.
+// Returns an error if the parsed token's type isn't types.PaginationTokenTypeStream.
+func getPaginationToken(since string) (*types.StreamPosition, error) {
+	if since == "" {
+		return nil, nil
+	}
+	p, err := types.NewPaginationTokenFromString(since)
+	if err != nil {
+		return nil, err
+	}
+	if p.Type != types.PaginationTokenTypeStream {
+		return nil, ErrNotStreamToken
+	}
+	return &(p.Position), nil
+}
+
 // 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.
 // There are two forms of tokens: The full length form containing all PDU and EDU
diff --git a/syncapi/syncapi.go b/syncapi/syncapi.go
index 4738feea2..81987f5f1 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 +81,6 @@ 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)
+	//routing.Setup(base.APIMux, requestPool, syncDB, deviceDB)
 }
diff --git a/syncapi/types/types.go b/syncapi/types/types.go
index af7ec865f..08a0239b6 100644
--- a/syncapi/types/types.go
+++ b/syncapi/types/types.go
@@ -16,11 +16,23 @@ package types
 
 import (
 	"encoding/json"
+	"fmt"
 	"strconv"
 
+	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
+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)")
+)
+
+// StreamPosition represents the offset in the sync stream a client is at.
+type StreamPosition int64
+
 // 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.
@@ -29,6 +41,14 @@ type SyncPosition struct {
 	TypingPosition int64
 }
 
+// Same as gomatrixserverlib.Event but also has the PDU stream position for this event.
+type StreamEvent struct {
+	gomatrixserverlib.Event
+	StreamPosition  int64
+	TransactionID   *api.TransactionID
+	ExcludeFromSync bool
+}
+
 // String implements the Stringer interface.
 func (sp SyncPosition) String() string {
 	return strconv.FormatInt(sp.PDUPosition, 10) + "_" +
@@ -55,6 +75,68 @@ func (sp SyncPosition) WithUpdates(other SyncPosition) SyncPosition {
 	return ret
 }
 
+// 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
+}
+
+// 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) (p *PaginationToken, err error) {
+	p = new(PaginationToken)
+
+	// Parse the token (aka position).
+	position, err := strconv.ParseInt(s[1:], 10, 64)
+	if err != nil {
+		return
+	}
+	p.Position = StreamPosition(position)
+
+	// Check if the type is among the known ones.
+	p.Type = PaginationTokenType(s[:1])
+	if p.Type != PaginationTokenTypeStream && p.Type != PaginationTokenTypeTopology {
+		err = ErrInvalidPaginationTokenType
+	}
+
+	return
+}
+
+// NewPaginationTokenFromTypeAndPosition takes a PaginationTokenType and a
+// StreamPosition and returns an instance of PaginationToken.
+func NewPaginationTokenFromTypeAndPosition(
+	t PaginationTokenType, pos StreamPosition,
+) (p *PaginationToken) {
+	return &PaginationToken{
+		Type:     t,
+		Position: pos,
+	}
+}
+
+// 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", p.Type, p.Position)
+}
+
 // PrevEventRef represents a reference to a previous event in a state event upgrade
 type PrevEventRef struct {
 	PrevContent   json.RawMessage `json:"prev_content"`
@@ -78,6 +160,17 @@ type Response struct {
 	} `json:"rooms"`
 }
 
+/*
+
+func NewResponse(pos StreamPosition) *Response {
+	res := Response{}
+	// 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, pos).String()
+}
+
+*/
+
 // NewResponse creates an empty response with initialised maps.
 func NewResponse(pos SyncPosition) *Response {
 	res := Response{