Implement backfill in the roomserver (#983)

* Initial cut for backfilling

The syncserver now asks the roomserver via QueryBackfill (which already
existed to *handle* backfill requests) which then makes federation requests
via gomatrixserverlib.RequestBackfill.

Currently, tests fail on subsequent /messages requests because we don't know
which servers are in the room, because we are unable to get state snapshots
from a backfilled event because that code doesn't exist yet.

* WIP backfill, doesn't work

* Make initial backfill pass checks

* Persist backfilled events with state snapshots

* Remove debug lines

* Linting

* Review comments

13 files changed, 367 insertions, 193 deletions

diff --git a/cmd/dendrite-demo-libp2p/main.go b/cmd/dendrite-demo-libp2p/main.go
index 0365a6f2..f1d18d0e 100644
--- a/cmd/dendrite-demo-libp2p/main.go
+++ b/cmd/dendrite-demo-libp2p/main.go
@@ -148,7 +148,7 @@ func main() {
 	federation := createFederationClient(base)
 	keyRing := keydb.CreateKeyRing(federation.Client, keyDB, cfg.Matrix.KeyPerspectives)
 
-	alias, input, query := roomserver.SetupRoomServerComponent(&base.Base)
+	alias, input, query := roomserver.SetupRoomServerComponent(&base.Base, keyRing)
 	eduInputAPI := eduserver.SetupEDUServerComponent(&base.Base, cache.New())
 	asQuery := appservice.SetupAppServiceAPIComponent(
 		&base.Base, accountDB, deviceDB, federation, alias, query, transactions.New(),
diff --git a/cmd/dendrite-monolith-server/main.go b/cmd/dendrite-monolith-server/main.go
index 70a59ed6..8cfb7566 100644
--- a/cmd/dendrite-monolith-server/main.go
+++ b/cmd/dendrite-monolith-server/main.go
@@ -57,7 +57,7 @@ func main() {
 	federation := base.CreateFederationClient()
 	keyRing := keydb.CreateKeyRing(federation.Client, keyDB, cfg.Matrix.KeyPerspectives)
 
-	alias, input, query := roomserver.SetupRoomServerComponent(base)
+	alias, input, query := roomserver.SetupRoomServerComponent(base, keyRing)
 	eduInputAPI := eduserver.SetupEDUServerComponent(base, cache.New())
 	asQuery := appservice.SetupAppServiceAPIComponent(
 		base, accountDB, deviceDB, federation, alias, query, transactions.New(),
diff --git a/cmd/dendrite-room-server/main.go b/cmd/dendrite-room-server/main.go
index 41b70575..f33a2b88 100644
--- a/cmd/dendrite-room-server/main.go
+++ b/cmd/dendrite-room-server/main.go
@@ -18,6 +18,7 @@ import (
 	_ "net/http/pprof"
 
 	"github.com/matrix-org/dendrite/common/basecomponent"
+	"github.com/matrix-org/dendrite/common/keydb"
 	"github.com/matrix-org/dendrite/roomserver"
 )
 
@@ -25,8 +26,11 @@ func main() {
 	cfg := basecomponent.ParseFlags()
 	base := basecomponent.NewBaseDendrite(cfg, "RoomServerAPI")
 	defer base.Close() // nolint: errcheck
+	keyDB := base.CreateKeyDB()
+	federation := base.CreateFederationClient()
+	keyRing := keydb.CreateKeyRing(federation.Client, keyDB, cfg.Matrix.KeyPerspectives)
 
-	roomserver.SetupRoomServerComponent(base)
+	roomserver.SetupRoomServerComponent(base, keyRing)
 
 	base.SetupAndServeHTTP(string(base.Cfg.Bind.RoomServer), string(base.Cfg.Listen.RoomServer))
 
diff --git a/federationapi/routing/backfill.go b/federationapi/routing/backfill.go
index 6f85ba4b..6f49b9a8 100644
--- a/federationapi/routing/backfill.go
+++ b/federationapi/routing/backfill.go
@@ -69,6 +69,7 @@ func Backfill(
 
 	// Populate the request.
 	req := api.QueryBackfillRequest{
+		RoomID:            roomID,
 		EarliestEventsIDs: eIDs,
 		ServerName:        request.Origin(),
 	}
diff --git a/go.mod b/go.mod
index f566e6d9..73171d89 100644
--- a/go.mod
+++ b/go.mod
@@ -26,6 +26,7 @@ require (
 	github.com/opentracing/opentracing-go v1.1.0
 	github.com/pkg/errors v0.9.1
 	github.com/prometheus/client_golang v1.4.1
+	github.com/prometheus/common v0.9.1
 	github.com/sirupsen/logrus v1.4.2
 	github.com/tidwall/gjson v1.6.0
 	github.com/uber/jaeger-client-go v2.15.0+incompatible
diff --git a/go.sum b/go.sum
index 535a999d..9c721312 100644
--- a/go.sum
+++ b/go.sum
@@ -367,6 +367,16 @@ github.com/matrix-org/gomatrix v0.0.0-20190528120928-7df988a63f26 h1:Hr3zjRsq2bh
 github.com/matrix-org/gomatrix v0.0.0-20190528120928-7df988a63f26/go.mod h1:3fxX6gUjWyI/2Bt7J1OLhpCzOfO/bB3AiX0cJtEKud0=
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200124100636-0c2ec91d1df5 h1:kmRjpmFOenVpOaV/DRlo9p6z/IbOKlUC+hhKsAAh8Qg=
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200124100636-0c2ec91d1df5/go.mod h1:FsKa2pWE/bpQql9H7U4boOPXFoJX/QcqaZZ6ijLkaZI=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200422082552-d7b4202c47f3 h1:xis1ojN99vjygwqudzB9VQq3cM2SJ7aCAMlXj/YN+88=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200422082552-d7b4202c47f3/go.mod h1:JsAzE1Ll3+gDWS9JSUHPJiiyAksvOOnGWF2nXdg4ZzU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200423090438-562549dbe799 h1:OsoUMTirIpeuZJdYkKKiYe6jm0E5viZR7aOS9K465QI=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200423090438-562549dbe799/go.mod h1:JsAzE1Ll3+gDWS9JSUHPJiiyAksvOOnGWF2nXdg4ZzU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200424154222-2827b39252bd h1:243fMfK0XqTQsdUY3IIqtxPX5g9MfPTaAP92CseqOek=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200424154222-2827b39252bd/go.mod h1:JsAzE1Ll3+gDWS9JSUHPJiiyAksvOOnGWF2nXdg4ZzU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200427134702-21db6d1430e3 h1:aJMAKjfXG5I8TqPxJQbQIkGSWM770oxkpgsPHE8C06E=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200427134702-21db6d1430e3/go.mod h1:JsAzE1Ll3+gDWS9JSUHPJiiyAksvOOnGWF2nXdg4ZzU=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200427152419-6a0535cc473a h1:tlXCVU3eab9kksGYBRA3oyrmIRwD/aPujo5KJCdlCVQ=
+github.com/matrix-org/gomatrixserverlib v0.0.0-20200427152419-6a0535cc473a/go.mod h1:JsAzE1Ll3+gDWS9JSUHPJiiyAksvOOnGWF2nXdg4ZzU=
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200428095012-a95e289995b1 h1:TB4V69eOtvmHdFp0+BgLNrDCcCwq6QDUOTjmi8fjC/M=
 github.com/matrix-org/gomatrixserverlib v0.0.0-20200428095012-a95e289995b1/go.mod h1:JsAzE1Ll3+gDWS9JSUHPJiiyAksvOOnGWF2nXdg4ZzU=
 github.com/matrix-org/naffka v0.0.0-20200127221512-0716baaabaf1 h1:osLoFdOy+ChQqVUn2PeTDETFftVkl4w9t/OW18g3lnk=
@@ -677,6 +687,7 @@ google.golang.org/grpc v1.19.0/go.mod h1:mqu4LbDTu4XGKhr4mRzUsmM4RtVoemTSY81AxZi
 google.golang.org/grpc v1.20.1/go.mod h1:10oTOabMzJvdu6/UiuZezV6QK5dSlG84ov/aaiqXj38=
 gopkg.in/Shopify/sarama.v1 v1.20.1 h1:Gi09A3fJXm0Jgt8kuKZ8YK+r60GfYn7MQuEmI3oq6hE=
 gopkg.in/Shopify/sarama.v1 v1.20.1/go.mod h1:AxnvoaevB2nBjNK17cG61A3LleFcWFwVBHBt+cot4Oc=
+gopkg.in/alecthomas/kingpin.v2 v2.2.6 h1:jMFz6MfLP0/4fUyZle81rXUoxOBFi19VUFKVDOQfozc=
 gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLkstjWtayDeSgw=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
diff --git a/roomserver/api/query.go b/roomserver/api/query.go
index b272b1eb..11fa5c9c 100644
--- a/roomserver/api/query.go
+++ b/roomserver/api/query.go
@@ -229,6 +229,8 @@ type QueryStateAndAuthChainResponse struct {
 
 // QueryBackfillRequest is a request to QueryBackfill.
 type QueryBackfillRequest struct {
+	// The room to backfill
+	RoomID string `json:"room_id"`
 	// Events to start paginating from.
 	EarliestEventsIDs []string `json:"earliest_event_ids"`
 	// The maximum number of events to retrieve.
@@ -243,21 +245,7 @@ type QueryBackfillResponse struct {
 	Events []gomatrixserverlib.HeaderedEvent `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"`
-}
-
-// QueryRoomVersionCapabilities asks for the default room version
+// QueryRoomVersionCapabilitiesRequest asks for the default room version
 type QueryRoomVersionCapabilitiesRequest struct{}
 
 // QueryRoomVersionCapabilitiesResponse is a response to QueryRoomVersionCapabilitiesRequest
@@ -266,12 +254,12 @@ type QueryRoomVersionCapabilitiesResponse struct {
 	AvailableRoomVersions map[gomatrixserverlib.RoomVersion]string `json:"available"`
 }
 
-// QueryRoomVersionForRoom asks for the room version for a given room.
+// QueryRoomVersionForRoomRequest asks for the room version for a given room.
 type QueryRoomVersionForRoomRequest struct {
 	RoomID string `json:"room_id"`
 }
 
-// QueryRoomVersionCapabilitiesResponse is a response to QueryServersInRoomAtEventResponse
+// QueryRoomVersionForRoomResponse is a response to QueryRoomVersionForRoomRequest
 type QueryRoomVersionForRoomResponse struct {
 	RoomVersion gomatrixserverlib.RoomVersion `json:"room_version"`
 }
@@ -350,12 +338,6 @@ type RoomserverQueryAPI interface {
 		response *QueryBackfillResponse,
 	) error
 
-	QueryServersInRoomAtEvent(
-		ctx context.Context,
-		request *QueryServersInRoomAtEventRequest,
-		response *QueryServersInRoomAtEventResponse,
-	) error
-
 	// Asks for the default room version as preferred by the server.
 	QueryRoomVersionCapabilities(
 		ctx context.Context,
@@ -401,13 +383,10 @@ const RoomserverQueryStateAndAuthChainPath = "/api/roomserver/queryStateAndAuthC
 // RoomserverQueryBackfillPath is the HTTP path for the QueryBackfillPath API
 const RoomserverQueryBackfillPath = "/api/roomserver/queryBackfill"
 
-// RoomserverQueryServersInRoomAtEventPath is the HTTP path for the QueryServersInRoomAtEvent API
-const RoomserverQueryServersInRoomAtEventPath = "/api/roomserver/queryServersInRoomAtEvents"
-
 // RoomserverQueryRoomVersionCapabilitiesPath is the HTTP path for the QueryRoomVersionCapabilities API
 const RoomserverQueryRoomVersionCapabilitiesPath = "/api/roomserver/queryRoomVersionCapabilities"
 
-// RoomserverQueryRoomVersionCapabilitiesPath is the HTTP path for the QueryRoomVersionCapabilities API
+// RoomserverQueryRoomVersionForRoomPath is the HTTP path for the QueryRoomVersionForRoom API
 const RoomserverQueryRoomVersionForRoomPath = "/api/roomserver/queryRoomVersionForRoom"
 
 // NewRoomserverQueryAPIHTTP creates a RoomserverQueryAPI implemented by talking to a HTTP POST API.
@@ -555,19 +534,6 @@ func (h *httpRoomserverQueryAPI) QueryBackfill(
 	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, "QueryServersInRoomAtEvent")
-	defer span.Finish()
-
-	apiURL := h.roomserverURL + RoomserverQueryServersInRoomAtEventPath
-	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
-}
-
 // QueryRoomVersionCapabilities implements RoomServerQueryAPI
 func (h *httpRoomserverQueryAPI) QueryRoomVersionCapabilities(
 	ctx context.Context,
diff --git a/roomserver/input/events.go b/roomserver/input/events.go
index 205035d9..69828d9f 100644
--- a/roomserver/input/events.go
+++ b/roomserver/input/events.go
@@ -26,6 +26,7 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/storage"
 	"github.com/matrix-org/dendrite/roomserver/types"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/sirupsen/logrus"
 	log "github.com/sirupsen/logrus"
 )
 
@@ -53,6 +54,7 @@ func processRoomEvent(
 	// Check that the event passes authentication checks and work out the numeric IDs for the auth events.
 	authEventNIDs, err := checkAuthEvents(ctx, db, headered, input.AuthEventIDs)
 	if err != nil {
+		logrus.WithError(err).WithField("event_id", event.EventID()).Error("processRoomEvent.checkAuthEvents failed for event")
 		return
 	}
 
@@ -77,6 +79,7 @@ func processRoomEvent(
 		// For outliers we can stop after we've stored the event itself as it
 		// doesn't have any associated state to store and we don't need to
 		// notify anyone about it.
+		logrus.WithField("event_id", event.EventID()).WithField("type", event.Type()).WithField("room", event.RoomID()).Info("Stored outlier")
 		return event.EventID(), nil
 	}
 
@@ -89,11 +92,6 @@ func processRoomEvent(
 		}
 	}
 
-	if input.Kind == api.KindBackfill {
-		// Backfill is not implemented.
-		panic("Not implemented")
-	}
-
 	// Update the extremities of the event graph for the room
 	return event.EventID(), updateLatestEvents(
 		ctx, db, ow, roomNID, stateAtEvent, event, input.SendAsServer, input.TransactionID,
diff --git a/roomserver/query/backfill.go b/roomserver/query/backfill.go
new file mode 100644
index 00000000..09a515e9
--- /dev/null
+++ b/roomserver/query/backfill.go
@@ -0,0 +1,217 @@
+package query
+
+import (
+	"context"
+
+	"github.com/matrix-org/dendrite/roomserver/storage"
+	"github.com/matrix-org/dendrite/roomserver/types"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/sirupsen/logrus"
+)
+
+// backfillRequester implements gomatrixserverlib.BackfillRequester
+type backfillRequester struct {
+	db         storage.Database
+	fedClient  *gomatrixserverlib.FederationClient
+	thisServer gomatrixserverlib.ServerName
+
+	// per-request state
+	servers                 []gomatrixserverlib.ServerName
+	eventIDToBeforeStateIDs map[string][]string
+	eventIDMap              map[string]gomatrixserverlib.Event
+}
+
+func newBackfillRequester(db storage.Database, fedClient *gomatrixserverlib.FederationClient, thisServer gomatrixserverlib.ServerName) *backfillRequester {
+	return &backfillRequester{
+		db:                      db,
+		fedClient:               fedClient,
+		thisServer:              thisServer,
+		eventIDToBeforeStateIDs: make(map[string][]string),
+		eventIDMap:              make(map[string]gomatrixserverlib.Event),
+	}
+}
+
+func (b *backfillRequester) StateIDsBeforeEvent(ctx context.Context, targetEvent gomatrixserverlib.HeaderedEvent) ([]string, error) {
+	b.eventIDMap[targetEvent.EventID()] = targetEvent.Unwrap()
+	if ids, ok := b.eventIDToBeforeStateIDs[targetEvent.EventID()]; ok {
+		return ids, nil
+	}
+	// if we have exactly 1 prev event and we know the state of the room at that prev event, then just roll forward the prev event.
+	// Else, we have to hit /state_ids because either we don't know the state at all at this event (new backwards extremity) or
+	// we don't know the result of state res to merge forks (2 or more prev_events)
+	if len(targetEvent.PrevEventIDs()) == 1 {
+		prevEventID := targetEvent.PrevEventIDs()[0]
+		prevEvent, ok := b.eventIDMap[prevEventID]
+		if !ok {
+			goto FederationHit
+		}
+		prevEventStateIDs, ok := b.eventIDToBeforeStateIDs[prevEventID]
+		if !ok {
+			goto FederationHit
+		}
+		newStateIDs := b.calculateNewStateIDs(targetEvent.Unwrap(), prevEvent, prevEventStateIDs)
+		if newStateIDs != nil {
+			b.eventIDToBeforeStateIDs[targetEvent.EventID()] = newStateIDs
+			return newStateIDs, nil
+		}
+		// else we failed to calculate the new state, so fallthrough
+	}
+
+FederationHit:
+	var lastErr error
+	logrus.WithField("event_id", targetEvent.EventID()).Info("Requesting /state_ids at event")
+	for _, srv := range b.servers { // hit any valid server
+		c := gomatrixserverlib.FederatedStateProvider{
+			FedClient:      b.fedClient,
+			AuthEventsOnly: false,
+			Server:         srv,
+		}
+		res, err := c.StateIDsBeforeEvent(ctx, targetEvent)
+		if err != nil {
+			lastErr = err
+			continue
+		}
+		b.eventIDToBeforeStateIDs[targetEvent.EventID()] = res
+		return res, nil
+	}
+	return nil, lastErr
+}
+
+func (b *backfillRequester) calculateNewStateIDs(targetEvent, prevEvent gomatrixserverlib.Event, prevEventStateIDs []string) []string {
+	newStateIDs := prevEventStateIDs[:]
+	if prevEvent.StateKey() == nil {
+		// state is the same as the previous event
+		b.eventIDToBeforeStateIDs[targetEvent.EventID()] = newStateIDs
+		return newStateIDs
+	}
+
+	missingState := false // true if we are missing the info for a state event ID
+	foundEvent := false   // true if we found a (type, state_key) match
+	// find which state ID to replace, if any
+	for i, id := range newStateIDs {
+		ev, ok := b.eventIDMap[id]
+		if !ok {
+			missingState = true
+			continue
+		}
+		// The state IDs BEFORE the target event are the state IDs BEFORE the prev_event PLUS the prev_event itself
+		if ev.Type() == prevEvent.Type() && ev.StateKey() != nil && *ev.StateKey() == *prevEvent.StateKey() {
+			newStateIDs[i] = prevEvent.EventID()
+			foundEvent = true
+			break
+		}
+	}
+	if !foundEvent && !missingState {
+		// we can be certain that this is new state
+		newStateIDs = append(newStateIDs, prevEvent.EventID())
+		foundEvent = true
+	}
+
+	if foundEvent {
+		b.eventIDToBeforeStateIDs[targetEvent.EventID()] = newStateIDs
+		return newStateIDs
+	}
+	return nil
+}
+
+func (b *backfillRequester) StateBeforeEvent(ctx context.Context, roomVer gomatrixserverlib.RoomVersion, event gomatrixserverlib.HeaderedEvent, eventIDs []string) (map[string]*gomatrixserverlib.Event, error) {
+	// try to fetch the events from the database first
+	events, err := b.ProvideEvents(roomVer, eventIDs)
+	if err != nil {
+		// non-fatal, fallthrough
+		logrus.WithError(err).Info("Failed to fetch events")
+	} else {
+		logrus.Infof("Fetched %d/%d events from the database", len(events), len(eventIDs))
+		if len(events) == len(eventIDs) {
+			result := make(map[string]*gomatrixserverlib.Event)
+			for i := range events {
+				result[events[i].EventID()] = &events[i]
+				b.eventIDMap[events[i].EventID()] = events[i]
+			}
+			return result, nil
+		}
+	}
+
+	c := gomatrixserverlib.FederatedStateProvider{
+		FedClient:      b.fedClient,
+		AuthEventsOnly: false,
+		Server:         b.servers[0],
+	}
+	result, err := c.StateBeforeEvent(ctx, roomVer, event, eventIDs)
+	if err != nil {
+		return nil, err
+	}
+	for eventID, ev := range result {
+		b.eventIDMap[eventID] = *ev
+	}
+	return result, nil
+}
+
+// ServersAtEvent is called when trying to determine which server to request from.
+// It returns a list of servers which can be queried for backfill requests. These servers
+// will be servers that are in the room already. The entries at the beginning are preferred servers
+// and will be tried first. An empty list will fail the request.
+func (b *backfillRequester) ServersAtEvent(ctx context.Context, roomID, eventID string) (servers []gomatrixserverlib.ServerName) {
+	// getMembershipsBeforeEventNID requires a NID, so retrieving the NID for
+	// the event is necessary.
+	NIDs, err := b.db.EventNIDs(ctx, []string{eventID})
+	if err != nil {
+		logrus.WithField("event_id", eventID).WithError(err).Error("ServersAtEvent: failed to get event NID for event")
+		return
+	}
+
+	// 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 := getMembershipsBeforeEventNID(ctx, b.db, NIDs[eventID], true)
+	if err != nil {
+		logrus.WithField("event_id", eventID).WithError(err).Error("ServersAtEvent: failed to get memberships before event")
+		return
+	}
+
+	// Store the server names in a temporary map to avoid duplicates.
+	serverSet := make(map[gomatrixserverlib.ServerName]bool)
+	for _, event := range events {
+		serverSet[event.Origin()] = true
+	}
+	for server := range serverSet {
+		if server == b.thisServer {
+			continue
+		}
+		servers = append(servers, server)
+	}
+	b.servers = servers
+	return
+}
+
+// Backfill performs a backfill request to the given server.
+// https://matrix.org/docs/spec/server_server/latest#get-matrix-federation-v1-backfill-roomid
+func (b *backfillRequester) Backfill(ctx context.Context, server gomatrixserverlib.ServerName, roomID string, fromEventIDs []string, limit int) (*gomatrixserverlib.Transaction, error) {
+	tx, err := b.fedClient.Backfill(ctx, server, roomID, limit, fromEventIDs)
+	return &tx, err
+}
+
+func (b *backfillRequester) ProvideEvents(roomVer gomatrixserverlib.RoomVersion, eventIDs []string) ([]gomatrixserverlib.Event, error) {
+	ctx := context.Background()
+	nidMap, err := b.db.EventNIDs(ctx, eventIDs)
+	if err != nil {
+		logrus.WithError(err).WithField("event_ids", eventIDs).Error("Failed to find events")
+		return nil, err
+	}
+	eventNIDs := make([]types.EventNID, len(nidMap))
+	i := 0
+	for _, nid := range nidMap {
+		eventNIDs[i] = nid
+		i++
+	}
+	eventsWithNids, err := b.db.Events(ctx, eventNIDs)
+	if err != nil {
+		logrus.WithError(err).WithField("event_nids", eventNIDs).Error("Failed to load events")
+		return nil, err
+	}
+	events := make([]gomatrixserverlib.Event, len(eventsWithNids))
+	for i := range eventsWithNids {
+		events[i] = eventsWithNids[i].Event
+	}
+	return events, nil
+}
diff --git a/roomserver/query/query.go b/roomserver/query/query.go
index 7508d790..a54fa58d 100644
--- a/roomserver/query/query.go
+++ b/roomserver/query/query.go
@@ -19,6 +19,7 @@ package query
 import (
 	"context"
 	"encoding/json"
+	"fmt"
 	"net/http"
 
 	"github.com/matrix-org/dendrite/common"
@@ -31,12 +32,16 @@ import (
 	"github.com/matrix-org/dendrite/roomserver/version"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
+	"github.com/sirupsen/logrus"
 )
 
 // RoomserverQueryAPI is an implementation of api.RoomserverQueryAPI
 type RoomserverQueryAPI struct {
 	DB             storage.Database
 	ImmutableCache caching.ImmutableCache
+	ServerName     gomatrixserverlib.ServerName
+	KeyRing        gomatrixserverlib.JSONVerifier
+	FedClient      *gomatrixserverlib.FederationClient
 }
 
 // QueryLatestEventsAndState implements api.RoomserverQueryAPI
@@ -281,7 +286,7 @@ func (r *RoomserverQueryAPI) QueryMembershipsForRoom(
 
 		events, err = r.DB.Events(ctx, eventNIDs)
 	} else {
-		events, err = r.getMembershipsBeforeEventNID(ctx, membershipEventNID, request.JoinedOnly)
+		events, err = getMembershipsBeforeEventNID(ctx, r.DB, membershipEventNID, request.JoinedOnly)
 	}
 
 	if err != nil {
@@ -300,19 +305,19 @@ func (r *RoomserverQueryAPI) QueryMembershipsForRoom(
 // of the event's room as it was when this event was fired, then filters the state events to
 // only keep the "m.room.member" events with a "join" membership. These events are returned.
 // Returns an error if there was an issue fetching the events.
-func (r *RoomserverQueryAPI) getMembershipsBeforeEventNID(
-	ctx context.Context, eventNID types.EventNID, joinedOnly bool,
+func getMembershipsBeforeEventNID(
+	ctx context.Context, db storage.Database, eventNID types.EventNID, joinedOnly bool,
 ) ([]types.Event, error) {
-	roomState := state.NewStateResolution(r.DB)
+	roomState := state.NewStateResolution(db)
 	events := []types.Event{}
 	// Lookup the event NID
-	eIDs, err := r.DB.EventIDs(ctx, []types.EventNID{eventNID})
+	eIDs, err := db.EventIDs(ctx, []types.EventNID{eventNID})
 	if err != nil {
 		return nil, err
 	}
 	eventIDs := []string{eIDs[eventNID]}
 
-	prevState, err := r.DB.StateAtEventIDs(ctx, eventIDs)
+	prevState, err := db.StateAtEventIDs(ctx, eventIDs)
 	if err != nil {
 		return nil, err
 	}
@@ -332,7 +337,7 @@ func (r *RoomserverQueryAPI) getMembershipsBeforeEventNID(
 	}
 
 	// Get all of the events in this state
-	stateEvents, err := r.DB.Events(ctx, eventNIDs)
+	stateEvents, err := db.Events(ctx, eventNIDs)
 	if err != nil {
 		return nil, err
 	}
@@ -484,6 +489,13 @@ func (r *RoomserverQueryAPI) QueryBackfill(
 	request *api.QueryBackfillRequest,
 	response *api.QueryBackfillResponse,
 ) error {
+	// if we are requesting the backfill then we need to do a federation hit
+	// TODO: we could be more sensible and fetch as many events we already have then request the rest
+	//       which is what the syncapi does already.
+	if request.ServerName == r.ServerName {
+		return r.backfillViaFederation(ctx, request, response)
+	}
+	// someone else is requesting the backfill, try to service their request.
 	var err error
 	var front []string
 
@@ -525,6 +537,55 @@ func (r *RoomserverQueryAPI) QueryBackfill(
 	return err
 }
 
+func (r *RoomserverQueryAPI) backfillViaFederation(ctx context.Context, req *api.QueryBackfillRequest, res *api.QueryBackfillResponse) error {
+	roomVer, err := r.DB.GetRoomVersionForRoom(ctx, req.RoomID)
+	if err != nil {
+		return fmt.Errorf("backfillViaFederation: unknown room version for room %s : %w", req.RoomID, err)
+	}
+	requester := newBackfillRequester(r.DB, r.FedClient, r.ServerName)
+	events, err := gomatrixserverlib.RequestBackfill(
+		ctx, requester,
+		r.KeyRing, req.RoomID, roomVer, req.EarliestEventsIDs, req.Limit)
+	if err != nil {
+		return err
+	}
+	logrus.WithField("room_id", req.RoomID).Infof("backfilled %d events", len(events))
+
+	// persist these new events - auth checks have already been done
+	roomNID, backfilledEventMap := persistEvents(ctx, r.DB, events)
+	if err != nil {
+		return err
+	}
+
+	for _, ev := range backfilledEventMap {
+		// now add state for these events
+		stateIDs, ok := requester.eventIDToBeforeStateIDs[ev.EventID()]
+		if !ok {
+			// this should be impossible as all events returned must have pass Step 5 of the PDU checks
+			// which requires a list of state IDs.
+			logrus.WithError(err).WithField("event_id", ev.EventID()).Error("Failed to find state IDs for event which passed auth checks")
+			continue
+		}
+		var entries []types.StateEntry
+		if entries, err = r.DB.StateEntriesForEventIDs(ctx, stateIDs); err != nil {
+			return err
+		}
+
+		var beforeStateSnapshotNID types.StateSnapshotNID
+		if beforeStateSnapshotNID, err = r.DB.AddState(ctx, roomNID, nil, entries); err != nil {
+			return err
+		}
+		if err = r.DB.SetState(ctx, ev.EventNID, beforeStateSnapshotNID); err != nil {
+			logrus.WithError(err).WithField("event_id", ev.EventID()).Error("Failed to set state before event")
+		}
+	}
+
+	// TODO: update backwards extremities, as that should be moved from syncapi to roomserver at some point.
+
+	res.Events = events
+	return nil
+}
+
 func (r *RoomserverQueryAPI) isServerCurrentlyInRoom(ctx context.Context, serverName gomatrixserverlib.ServerName, roomID string) (bool, error) {
 	roomNID, err := r.DB.RoomNID(ctx, roomID)
 	if err != nil {
@@ -778,39 +839,33 @@ 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)
+func persistEvents(ctx context.Context, db storage.Database, events []gomatrixserverlib.HeaderedEvent) (types.RoomNID, map[string]types.Event) {
+	var roomNID types.RoomNID
+	backfilledEventMap := make(map[string]types.Event)
+	for _, ev := range events {
+		nidMap, err := db.EventNIDs(ctx, ev.AuthEventIDs())
+		if err != nil { // this shouldn't happen as RequestBackfill already found them
+			logrus.WithError(err).WithField("auth_events", ev.AuthEventIDs()).Error("Failed to find one or more auth events")
+			continue
+		}
+		authNids := make([]types.EventNID, len(nidMap))
+		i := 0
+		for _, nid := range nidMap {
+			authNids[i] = nid
+			i++
+		}
+		var stateAtEvent types.StateAtEvent
+		roomNID, stateAtEvent, err = db.StoreEvent(ctx, ev.Unwrap(), nil, authNids)
+		if err != nil {
+			logrus.WithError(err).WithField("event_id", ev.EventID()).Error("Failed to store backfilled event")
+			continue
+		}
+		backfilledEventMap[ev.EventID()] = types.Event{
+			EventNID: stateAtEvent.StateEntry.EventNID,
+			Event:    ev.Unwrap(),
+		}
 	}
-
-	return nil
+	return roomNID, backfilledEventMap
 }
 
 // QueryRoomVersionCapabilities implements api.RoomserverQueryAPI
@@ -995,20 +1050,6 @@ func (r *RoomserverQueryAPI) SetupHTTP(servMux *http.ServeMux) {
 		}),
 	)
 	servMux.Handle(
-		api.RoomserverQueryServersInRoomAtEventPath,
-		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}
-		}),
-	)
-	servMux.Handle(
 		api.RoomserverQueryRoomVersionCapabilitiesPath,
 		common.MakeInternalAPI("QueryRoomVersionCapabilities", func(req *http.Request) util.JSONResponse {
 			var request api.QueryRoomVersionCapabilitiesRequest
diff --git a/roomserver/roomserver.go b/roomserver/roomserver.go
index fa4f2062..ea1c5c4c 100644
--- a/roomserver/roomserver.go
+++ b/roomserver/roomserver.go
@@ -18,6 +18,7 @@ import (
 	"net/http"
 
 	"github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrixserverlib"
 
 	asQuery "github.com/matrix-org/dendrite/appservice/query"
 	"github.com/matrix-org/dendrite/common/basecomponent"
@@ -33,7 +34,7 @@ import (
 // allowing other components running in the same process to hit the query the
 // APIs directly instead of having to use HTTP.
 func SetupRoomServerComponent(
-	base *basecomponent.BaseDendrite,
+	base *basecomponent.BaseDendrite, keyRing gomatrixserverlib.JSONVerifier,
 ) (api.RoomserverAliasAPI, api.RoomserverInputAPI, api.RoomserverQueryAPI) {
 	roomserverDB, err := storage.Open(string(base.Cfg.Database.RoomServer))
 	if err != nil {
@@ -51,6 +52,11 @@ func SetupRoomServerComponent(
 	queryAPI := query.RoomserverQueryAPI{
 		DB:             roomserverDB,
 		ImmutableCache: base.ImmutableCache,
+		ServerName:     base.Cfg.Matrix.ServerName,
+		FedClient:      base.CreateFederationClient(),
+		// TODO: We should have a key server so we don't keep adding components
+		// which talk to the same DB.
+		KeyRing: keyRing,
 	}
 
 	queryAPI.SetupHTTP(http.DefaultServeMux)
diff --git a/roomserver/storage/interface.go b/roomserver/storage/interface.go
index a13c44d6..fb39eca6 100644
--- a/roomserver/storage/interface.go
+++ b/roomserver/storage/interface.go
@@ -31,7 +31,8 @@ type Database interface {
 		state []types.StateEntry,
 	) (types.StateSnapshotNID, error)
 	// Look up the state of a room at each event for a list of string event IDs.
-	// Returns an error if there is an error talking to the database
+	// Returns an error if there is an error talking to the database.
+	// The length of []types.StateAtEvent is guaranteed to equal the length of eventIDs if no error is returned.
 	// Returns a types.MissingEventError if the room state for the event IDs aren't in the database
 	StateAtEventIDs(ctx context.Context, eventIDs []string) ([]types.StateAtEvent, error)
 	// Look up the numeric IDs for a list of string event types.
diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
index 53a9a963..c48414ab 100644
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@@ -285,7 +285,7 @@ func (r *messagesReq) handleEmptyEventsSlice() (
 	// 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)
+		events, err = r.backfill(r.roomID, backwardExtremities, r.limit)
 		if err != nil {
 			return
 		}
@@ -334,7 +334,7 @@ func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent
 	if len(backwardExtremities) > 0 && !isSetLargeEnough && r.backwardOrdering {
 		var pdus []gomatrixserverlib.HeaderedEvent
 		// Only ask the remote server for enough events to reach the limit.
-		pdus, err = r.backfill(backwardExtremities, r.limit-len(streamEvents))
+		pdus, err = r.backfill(r.roomID, backwardExtremities, r.limit-len(streamEvents))
 		if err != nil {
 			return
 		}
@@ -358,45 +358,29 @@ func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent
 // 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.HeaderedEvent, error) {
-	verReq := api.QueryRoomVersionForRoomRequest{RoomID: r.roomID}
-	verRes := api.QueryRoomVersionForRoomResponse{}
-	if err := r.queryAPI.QueryRoomVersionForRoom(r.ctx, &verReq, &verRes); err != nil {
-		return nil, err
-	}
-
-	srvToBackfillFrom, err := r.serverToBackfillFrom(fromEventIDs)
-	if err != nil {
-		return nil, fmt.Errorf("Cannot find server to backfill from: %w", err)
-	}
-
-	headered := make([]gomatrixserverlib.HeaderedEvent, 0)
-
-	// If the roomserver responded with at least one server that isn't us,
-	// send it a request for backfill.
-	util.GetLogger(r.ctx).WithField("server", srvToBackfillFrom).WithField("limit", limit).Info("Backfilling from server")
-	txn, err := r.federation.Backfill(
-		r.ctx, srvToBackfillFrom, r.roomID, limit, fromEventIDs,
-	)
+func (r *messagesReq) backfill(roomID string, fromEventIDs []string, limit int) ([]gomatrixserverlib.HeaderedEvent, error) {
+	var res api.QueryBackfillResponse
+	err := r.queryAPI.QueryBackfill(context.Background(), &api.QueryBackfillRequest{
+		RoomID:            roomID,
+		EarliestEventsIDs: fromEventIDs,
+		Limit:             limit,
+		ServerName:        r.cfg.Matrix.ServerName,
+	}, &res)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("QueryBackfill failed: %w", err)
 	}
+	util.GetLogger(r.ctx).WithField("new_events", len(res.Events)).Info("Storing new events from backfill")
 
-	for _, p := range txn.PDUs {
-		event, e := gomatrixserverlib.NewEventFromUntrustedJSON(p, verRes.RoomVersion)
-		if e != nil {
-			continue
-		}
-		headered = append(headered, event.Headered(verRes.RoomVersion))
-	}
-	util.GetLogger(r.ctx).WithField("server", srvToBackfillFrom).WithField("new_events", len(headered)).Info("Storing new events from backfill")
+	// TODO: we should only be inserting events into the database from the roomserver's kafka output stream.
+	// Currently, this can race with live events for the room and cause problems. It's also just a bit unclear
+	// when you have multiple entry points to write events.
 
 	// Store the events in the database, while marking them as unfit to show
 	// up in responses to sync requests.
-	for i := range headered {
+	for i := range res.Events {
 		if _, err = r.db.WriteEvent(
 			r.ctx,
-			&headered[i],
+			&res.Events[i],
 			[]gomatrixserverlib.HeaderedEvent{},
 			[]string{},
 			[]string{},
@@ -406,63 +390,7 @@ func (r *messagesReq) backfill(fromEventIDs []string, limit int) ([]gomatrixserv
 		}
 	}
 
-	return headered, nil
-}
-
-func (r *messagesReq) serverToBackfillFrom(fromEventIDs []string) (gomatrixserverlib.ServerName, 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 {
-		util.GetLogger(r.ctx).WithError(err).Warn("Failed to query servers in room at event, falling back to event sender")
-		// FIXME: We shouldn't be doing this but in situations where we have already backfilled once
-		// the query API doesn't work as backfilled events do not make it to the room server.
-		// This means QueryServersInRoomAtEvent returns an error as it doesn't have the event ID in question.
-		// We need to inject backfilled events into the room server and store them appropriately.
-		events, err := r.db.Events(r.ctx, fromEventIDs)
-		if err != nil {
-			return "", err
-		}
-		if len(events) == 0 {
-			// should be impossible as these event IDs are backwards extremities
-			return "", fmt.Errorf("backfill: missing backwards extremities, event IDs: %s", fromEventIDs)
-		}
-		// The rationale here is that the last event was unlikely to be sent by us, so poke the server who sent it.
-		// We shouldn't be doing this really, but as a heuristic it should work pretty well for now.
-		for _, e := range events {
-			_, srv, srverr := gomatrixserverlib.SplitID('@', e.Sender())
-			if srverr != nil {
-				util.GetLogger(r.ctx).WithError(srverr).Warn("Failed to extract domain from event sender")
-				continue
-			}
-			if srv != r.cfg.Matrix.ServerName {
-				return srv, nil
-			}
-		}
-		// no valid events which have a remote server, fail.
-		return "", 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 {
-			util.GetLogger(r.ctx).Info("Not enough servers to backfill from")
-			return "", nil
-		}
-	}
-	return srvToBackfillFrom, nil
+	return res.Events, nil
 }
 
 // setToDefault returns the default value for the "to" query parameter of a