diff --git a/server/channels/api4/post_test.go b/server/channels/api4/post_test.go
index 5931993c74..8ef72a08b0 100644
--- a/server/channels/api4/post_test.go
+++ b/server/channels/api4/post_test.go
@@ -16,6 +16,7 @@ import (
 	"reflect"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -27,6 +28,7 @@ import (
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/mattermost/mattermost/server/v8/channels/app"
 	"github.com/mattermost/mattermost/server/v8/channels/store/storetest/mocks"
+	"github.com/mattermost/mattermost/server/v8/channels/testlib"
 	"github.com/mattermost/mattermost/server/v8/channels/utils"
 	"github.com/mattermost/mattermost/server/v8/channels/utils/testutils"
 )
@@ -2937,6 +2939,155 @@ func TestPermanentDeletePost(t *testing.T) {
 	})
 }
 
+func TestWebHubMembership(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	u1 := th.CreateUser()
+	th.LinkUserToTeam(u1, th.BasicTeam)
+	th.AddUserToChannel(u1, th.BasicChannel)
+
+	ch2 := th.CreatePrivateChannel()
+	u2 := th.CreateUser()
+	th.LinkUserToTeam(u2, th.BasicTeam)
+	th.AddUserToChannel(u2, ch2)
+
+	quitChan := make(chan struct{})
+	var wg sync.WaitGroup
+
+	wg.Add(3)
+	for _, obj := range []struct {
+		testName string
+		user     *model.User
+	}{
+		{
+			testName: "basicUser",
+			user:     th.BasicUser,
+		},
+		{
+			testName: "u1",
+			user:     u1,
+		},
+		{
+			testName: "u2",
+			user:     u2,
+		},
+	} {
+		cli := th.CreateClient()
+		_, _, err := cli.Login(context.Background(), obj.user.Username, obj.user.Password)
+		require.NoError(t, err)
+
+		wsClient, err := th.CreateWebSocketClientWithClient(cli)
+		require.NoError(t, err)
+		defer wsClient.Close()
+
+		wsClient.Listen()
+
+		go func(testName string) {
+			defer wg.Done()
+			var cnt int
+			for {
+				select {
+				case event := <-wsClient.EventChannel:
+					if event.EventType() == model.WebsocketEventPosted {
+						var post model.Post
+						err := json.Unmarshal([]byte(event.GetData()["post"].(string)), &post)
+						require.NoError(t, err)
+
+						cnt++
+						// Cases:
+						// Post to basicChannel should go to u1 and basicUser.
+						// Add u1 to ch2.
+						// Post to ch2 should go to u1, u2 and basicUser.
+						// Remove u1 from ch2.
+						// Post to ch2 should go to u2 and basicUser.
+						switch testName {
+						case "basicUser":
+							if cnt == 1 {
+								assert.Equal(t, th.BasicChannel.Id, post.ChannelId)
+							} else if cnt == 2 {
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else if cnt == 3 {
+								// After removing, there will be a "removed from channel post"
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else if cnt == 4 {
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else {
+								assert.Fail(t, "more than 4 messages arrived for basicUser")
+							}
+						case "u1":
+							// First msg should be from basicChannel
+							if cnt == 1 {
+								assert.Equal(t, th.BasicChannel.Id, post.ChannelId)
+							} else if cnt == 2 {
+								// second should be from ch2
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else {
+								assert.Fail(t, "more than 2 messages arrived for u1")
+							}
+						case "u2":
+							if cnt == 1 {
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else if cnt == 2 {
+								// After removing, there will be a "removed from channel post"
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else if cnt == 3 {
+								assert.Equal(t, ch2.Id, post.ChannelId)
+							} else {
+								assert.Fail(t, "more than 3 messages arrived for u2")
+							}
+						}
+					}
+				case <-quitChan:
+					return
+				}
+			}
+		}(obj.testName)
+	}
+
+	// Will send to basic channel
+	th.CreatePost()
+	// Add u1 to ch2
+	th.AddUserToChannel(u1, ch2)
+	// Send post to ch2
+	th.CreatePostWithClient(th.Client, ch2)
+	// Remove u1 from ch2
+	th.RemoveUserFromChannel(u1, ch2)
+	// Send post to ch2
+	th.CreatePostWithClient(th.Client, ch2)
+
+	// It is possible to create a signalling mechanism from the goroutines
+	// after all events are received, but we also want to verify that no additional
+	// events are being sent.
+	time.Sleep(2 * time.Second)
+	close(quitChan)
+	wg.Wait()
+}
+
+func TestWebHubCloseConnOnDBFail(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer func() {
+		th.TearDown()
+		// Asserting that the error message is present in the log
+		testlib.AssertLog(t, th.LogBuffer, mlog.LvlError.Name, "Error while registering to hub")
+		_, err := th.Server.Store().GetInternalMasterDB().Exec(`ALTER TABLE dummy RENAME to ChannelMembers`)
+		require.NoError(t, err)
+	}()
+
+	cli := th.CreateClient()
+	_, _, err := cli.Login(context.Background(), th.BasicUser.Username, th.BasicUser.Password)
+	require.NoError(t, err)
+
+	_, err = th.Server.Store().GetInternalMasterDB().Exec(`ALTER TABLE ChannelMembers RENAME to dummy`)
+	require.NoError(t, err)
+
+	wsClient, err := th.CreateWebSocketClientWithClient(cli)
+	require.NoError(t, err)
+	defer wsClient.Close()
+
+	require.NoError(t, th.TestLogger.Flush())
+}
+
 func TestDeletePostEvent(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()
diff --git a/server/channels/api4/websocket.go b/server/channels/api4/websocket.go
index 2496ad9e1a..1b02d1bea3 100644
--- a/server/channels/api4/websocket.go
+++ b/server/channels/api4/websocket.go
@@ -70,7 +70,7 @@ func connectWebSocket(c *Context, w http.ResponseWriter, r *http.Request) {
 	} else {
 		cfg, err = c.App.Srv().Platform().PopulateWebConnConfig(c.AppContext.Session(), cfg, r.URL.Query().Get(sequenceNumberParam))
 		if err != nil {
-			c.Logger.Warn("Error while populating webconn config", mlog.String("id", r.URL.Query().Get(connectionIDParam)), mlog.Err(err))
+			c.Logger.Error("Error while populating webconn config", mlog.String("id", r.URL.Query().Get(connectionIDParam)), mlog.Err(err))
 			ws.Close()
 			return
 		}
@@ -78,7 +78,12 @@ func connectWebSocket(c *Context, w http.ResponseWriter, r *http.Request) {
 
 	wc := c.App.Srv().Platform().NewWebConn(cfg, c.App, c.App.Srv().Channels())
 	if c.AppContext.Session().UserId != "" {
-		c.App.Srv().Platform().HubRegister(wc)
+		err = c.App.Srv().Platform().HubRegister(wc)
+		if err != nil {
+			c.Logger.Error("Error while registering to hub", mlog.String("id", r.URL.Query().Get(connectionIDParam)), mlog.Err(err))
+			ws.Close()
+			return
+		}
 	}
 
 	wc.Pump()
diff --git a/server/channels/app/app_iface.go b/server/channels/app/app_iface.go
index 18abd21242..f573d82bfb 100644
--- a/server/channels/app/app_iface.go
+++ b/server/channels/app/app_iface.go
@@ -249,10 +249,6 @@ type AppIface interface {
 	GetUserStatusesByIds(userIDs []string) ([]*model.Status, *model.AppError)
 	// HasRemote returns whether a given channelID is present in the channel remotes or not.
 	HasRemote(channelID string, remoteID string) (bool, error)
-	// HubRegister registers a connection to a hub.
-	HubRegister(webConn *platform.WebConn)
-	// HubUnregister unregisters a connection from a hub.
-	HubUnregister(webConn *platform.WebConn)
 	// InstallPlugin unpacks and installs a plugin but does not enable or activate it unless the the
 	// plugin was already enabled.
 	InstallPlugin(pluginFile io.ReadSeeker, replace bool) (*model.Manifest, *model.AppError)
diff --git a/server/channels/app/notification_test.go b/server/channels/app/notification_test.go
index 534148e384..5bb9af521e 100644
--- a/server/channels/app/notification_test.go
+++ b/server/channels/app/notification_test.go
@@ -523,7 +523,7 @@ func connectFakeWebSocket(t *testing.T, th *TestHelper, userID string, connectio
 		Locale:       "en",
 		ConnectionID: connectionID,
 	}, th.App, th.App.Channels())
-	th.App.Srv().Platform().HubRegister(webConn)
+	require.NoError(t, th.App.Srv().Platform().HubRegister(webConn))
 
 	// Start reading from it
 	go webConn.Pump()
diff --git a/server/channels/app/opentracing/opentracing_layer.go b/server/channels/app/opentracing/opentracing_layer.go
index b66a26edb0..80e1088cec 100644
--- a/server/channels/app/opentracing/opentracing_layer.go
+++ b/server/channels/app/opentracing/opentracing_layer.go
@@ -11982,36 +11982,6 @@ func (a *OpenTracingAppLayer) HasSharedChannel(channelID string) (bool, error) {
 	return resultVar0, resultVar1
 }
 
-func (a *OpenTracingAppLayer) HubRegister(webConn *platform.WebConn) {
-	origCtx := a.ctx
-	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.HubRegister")
-
-	a.ctx = newCtx
-	a.app.Srv().Store().SetContext(newCtx)
-	defer func() {
-		a.app.Srv().Store().SetContext(origCtx)
-		a.ctx = origCtx
-	}()
-
-	defer span.Finish()
-	a.app.HubRegister(webConn)
-}
-
-func (a *OpenTracingAppLayer) HubUnregister(webConn *platform.WebConn) {
-	origCtx := a.ctx
-	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.HubUnregister")
-
-	a.ctx = newCtx
-	a.app.Srv().Store().SetContext(newCtx)
-	defer func() {
-		a.app.Srv().Store().SetContext(origCtx)
-		a.ctx = origCtx
-	}()
-
-	defer span.Finish()
-	a.app.HubUnregister(webConn)
-}
-
 func (a *OpenTracingAppLayer) IPFiltering() einterfaces.IPFilteringInterface {
 	origCtx := a.ctx
 	span, newCtx := tracing.StartSpanWithParentByContext(a.ctx, "app.IPFiltering")
diff --git a/server/channels/app/platform/web_conn.go b/server/channels/app/platform/web_conn.go
index 9c2bd3e149..d83417a874 100644
--- a/server/channels/app/platform/web_conn.go
+++ b/server/channels/app/platform/web_conn.go
@@ -26,7 +26,6 @@ import (
 	"github.com/mattermost/mattermost/server/public/shared/i18n"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/mattermost/mattermost/server/public/shared/request"
-	"github.com/mattermost/mattermost/server/v8/channels/store/sqlstore"
 )
 
 const (
@@ -95,10 +94,8 @@ type WebConn struct {
 	UserId           string
 	PostedAck        bool
 
-	allChannelMembers         map[string]string
-	lastAllChannelMembersTime int64
-	lastUserActivityAt        int64
-	send                      chan model.WebSocketMessage
+	lastUserActivityAt int64
+	send               chan model.WebSocketMessage
 	// deadQueue behaves like a queue of a finite size
 	// which is used to store all messages that are sent via the websocket.
 	// It basically acts as the user-space socket buffer, and is used
@@ -218,7 +215,7 @@ func (ps *PlatformService) NewWebConn(cfg *WebConnConfig, suite SuiteIFace, runn
 	if tcpConn != nil {
 		err := tcpConn.SetNoDelay(false)
 		if err != nil {
-			mlog.Warn("Error in setting NoDelay socket opts", mlog.Err(err))
+			ps.logger.Warn("Error in setting NoDelay socket opts", mlog.Err(err))
 		}
 	}
 
@@ -321,6 +318,9 @@ func (wc *WebConn) SetConnectionID(id string) {
 
 // GetConnectionID returns the connection id of the connection.
 func (wc *WebConn) GetConnectionID() string {
+	if wc.connectionID.Load() == nil {
+		return ""
+	}
 	return wc.connectionID.Load().(string)
 }
 
@@ -566,7 +566,7 @@ func (wc *WebConn) writePump() {
 				err = enc.Encode(msg)
 			}
 			if err != nil {
-				mlog.Warn("Error in encoding websocket message", mlog.Err(err))
+				wc.Platform.logger.Warn("Error in encoding websocket message", mlog.Err(err))
 				continue
 			}
 
@@ -581,7 +581,7 @@ func (wc *WebConn) writePump() {
 					logData = append(logData, mlog.String("channel_id", evt.GetBroadcast().ChannelId))
 				}
 
-				mlog.Warn("websocket.full", logData...)
+				wc.Platform.logger.Warn("websocket.full", logData...)
 				wc.lastLogTimeFull = time.Now()
 			}
 
@@ -608,7 +608,7 @@ func (wc *WebConn) writePump() {
 
 		case <-authTicker.C:
 			if wc.GetSessionToken() == "" {
-				mlog.Debug("websocket.authTicker: did not authenticate", mlog.Stringer("ip_address", wc.WebSocket.RemoteAddr()))
+				wc.Platform.logger.Debug("websocket.authTicker: did not authenticate", mlog.Stringer("ip_address", wc.WebSocket.RemoteAddr()))
 				return
 			}
 			authTicker.Stop()
@@ -629,7 +629,7 @@ func (wc *WebConn) writeMessage(msg *model.WebSocketEvent) error {
 	var buf bytes.Buffer
 	err := msg.Encode(json.NewEncoder(&buf), &buf)
 	if err != nil {
-		mlog.Warn("Error in encoding websocket message", mlog.Err(err))
+		wc.Platform.logger.Warn("Error in encoding websocket message", mlog.Err(err))
 		return nil
 	}
 	wc.Sequence++
@@ -734,8 +734,6 @@ func (wc *WebConn) drainDeadQueue(index int) error {
 
 // InvalidateCache resets all internal data of the WebConn.
 func (wc *WebConn) InvalidateCache() {
-	wc.allChannelMembers = nil
-	wc.lastAllChannelMembersTime = 0
 	wc.SetSession(nil)
 	wc.SetSessionExpiresAt(0)
 }
@@ -751,9 +749,9 @@ func (wc *WebConn) IsAuthenticated() bool {
 		session, err := wc.Suite.GetSession(wc.GetSessionToken())
 		if err != nil {
 			if err.StatusCode >= http.StatusBadRequest && err.StatusCode < http.StatusInternalServerError {
-				mlog.Debug("Invalid session.", mlog.Err(err))
+				wc.Platform.logger.Debug("Invalid session.", mlog.Err(err))
 			} else {
-				mlog.Error("Could not get session", mlog.String("session_token", wc.GetSessionToken()), mlog.Err(err))
+				wc.Platform.logger.Error("Could not get session", mlog.String("session_token", wc.GetSessionToken()), mlog.Err(err))
 			}
 
 			wc.SetSessionToken("")
@@ -789,7 +787,7 @@ func (wc *WebConn) ShouldSendEventToGuest(msg *model.WebSocketEvent) bool {
 	case model.WebsocketEventUserUpdated:
 		user, ok := msg.GetData()["user"].(*model.User)
 		if !ok {
-			mlog.Debug("webhub.shouldSendEvent: user not found in message", mlog.Any("user", msg.GetData()["user"]))
+			wc.Platform.logger.Debug("webhub.shouldSendEvent: user not found in message", mlog.Any("user", msg.GetData()["user"]))
 			return false
 		}
 		userID = user.Id
@@ -828,7 +826,7 @@ func (wc *WebConn) ShouldSendEvent(msg *model.WebSocketEvent) bool {
 			model.WebsocketEventStatusChange,
 			model.WebsocketEventMultipleChannelsViewed:
 			if wc.Active.Load() && time.Since(wc.lastLogTimeSlow) > websocketSuppressWarnThreshold {
-				mlog.Warn(
+				wc.Platform.logger.Warn(
 					"websocket.slow: dropping message",
 					mlog.String("user_id", wc.UserId),
 					mlog.String("conn_id", wc.GetConnectionID()),
@@ -893,8 +891,8 @@ func (wc *WebConn) ShouldSendEvent(msg *model.WebSocketEvent) bool {
 
 	// Only report events to users who are in the channel for the event
 	if chID := msg.GetBroadcast().ChannelId; chID != "" {
-		// For typing events, we don't send them to users who don't have
-		// that channel or thread opened.
+		// For typing/reaction_added/reaction_removed events, we don't send them to users
+		// who don't have that channel or thread opened.
 		if wc.Platform.Config().FeatureFlags.WebSocketEventScope &&
 			slices.Contains([]model.WebsocketEventType{
 				model.WebsocketEventTyping,
@@ -904,30 +902,9 @@ func (wc *WebConn) ShouldSendEvent(msg *model.WebSocketEvent) bool {
 			return false
 		}
 
-		if model.GetMillis()-wc.lastAllChannelMembersTime > webConnMemberCacheTime {
-			wc.allChannelMembers = nil
-			wc.lastAllChannelMembersTime = 0
-		}
-
-		if wc.allChannelMembers == nil {
-			result, err := wc.Platform.Store.Channel().GetAllChannelMembersForUser(
-				sqlstore.RequestContextWithMaster(request.EmptyContext(wc.Platform.logger)),
-				wc.UserId,
-				false,
-				false,
-			)
-			if err != nil {
-				mlog.Error("webhub.shouldSendEvent.", mlog.Err(err))
-				return false
-			}
-			wc.allChannelMembers = result
-			wc.lastAllChannelMembersTime = model.GetMillis()
-		}
-
-		if _, ok := wc.allChannelMembers[chID]; ok {
-			return true
-		}
-		return false
+		// We don't need to do any further checks because this is already scoped
+		// to channel members from web_hub.
+		return true
 	}
 
 	// Only report events to users who are in the team for the event
@@ -960,9 +937,9 @@ func (wc *WebConn) isMemberOfTeam(teamID string) bool {
 		session, err := wc.Suite.GetSession(wc.GetSessionToken())
 		if err != nil {
 			if err.StatusCode >= http.StatusBadRequest && err.StatusCode < http.StatusInternalServerError {
-				mlog.Debug("Invalid session.", mlog.Err(err))
+				wc.Platform.logger.Debug("Invalid session.", mlog.Err(err))
 			} else {
-				mlog.Error("Could not get session", mlog.String("session_token", wc.GetSessionToken()), mlog.Err(err))
+				wc.Platform.logger.Error("Could not get session", mlog.String("session_token", wc.GetSessionToken()), mlog.Err(err))
 			}
 			return false
 		}
@@ -976,12 +953,12 @@ func (wc *WebConn) isMemberOfTeam(teamID string) bool {
 func (wc *WebConn) logSocketErr(source string, err error) {
 	// browsers will appear as CloseNoStatusReceived
 	if websocket.IsCloseError(err, websocket.CloseNormalClosure, websocket.CloseNoStatusReceived) {
-		mlog.Debug(source+": client side closed socket",
+		wc.Platform.logger.Debug(source+": client side closed socket",
 			mlog.String("user_id", wc.UserId),
 			mlog.String("conn_id", wc.GetConnectionID()),
 			mlog.String("origin_client", wc.originClient))
 	} else {
-		mlog.Debug(source+": closing websocket",
+		wc.Platform.logger.Debug(source+": closing websocket",
 			mlog.String("user_id", wc.UserId),
 			mlog.String("conn_id", wc.GetConnectionID()),
 			mlog.String("origin_client", wc.originClient),
diff --git a/server/channels/app/platform/web_hub.go b/server/channels/app/platform/web_hub.go
index 7ed5622379..a73f611067 100644
--- a/server/channels/app/platform/web_hub.go
+++ b/server/channels/app/platform/web_hub.go
@@ -4,6 +4,7 @@
 package platform
 
 import (
+	"fmt"
 	"hash/maphash"
 	"runtime"
 	"runtime/debug"
@@ -14,6 +15,7 @@ import (
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/mattermost/mattermost/server/public/shared/request"
+	"github.com/mattermost/mattermost/server/v8/channels/store"
 )
 
 const (
@@ -45,6 +47,11 @@ type webConnSessionMessage struct {
 	isRegistered chan bool
 }
 
+type webConnRegisterMessage struct {
+	conn *WebConn
+	err  chan error
+}
+
 type webConnCheckMessage struct {
 	userID       string
 	connectionID string
@@ -65,7 +72,7 @@ type Hub struct {
 	connectionCount int64
 	platform        *PlatformService
 	connectionIndex int
-	register        chan *WebConn
+	register        chan *webConnRegisterMessage
 	unregister      chan *WebConn
 	broadcast       chan *model.WebSocketEvent
 	stop            chan struct{}
@@ -84,7 +91,7 @@ type Hub struct {
 func newWebHub(ps *PlatformService) *Hub {
 	return &Hub{
 		platform:        ps,
-		register:        make(chan *WebConn),
+		register:        make(chan *webConnRegisterMessage),
 		unregister:      make(chan *WebConn),
 		broadcast:       make(chan *model.WebSocketEvent, broadcastQueueSize),
 		stop:            make(chan struct{}),
@@ -150,14 +157,15 @@ func (ps *PlatformService) GetHubForUserId(userID string) *Hub {
 }
 
 // HubRegister registers a connection to a hub.
-func (ps *PlatformService) HubRegister(webConn *WebConn) {
+func (ps *PlatformService) HubRegister(webConn *WebConn) error {
 	hub := ps.GetHubForUserId(webConn.UserId)
 	if hub != nil {
 		if metrics := ps.metricsIFace; metrics != nil {
 			metrics.IncrementWebSocketBroadcastUsersRegistered(strconv.Itoa(hub.connectionIndex), 1)
 		}
-		hub.Register(webConn)
+		return hub.Register(webConn)
 	}
+	return nil
 }
 
 // HubUnregister unregisters a connection from a hub.
@@ -262,11 +270,17 @@ func (ps *PlatformService) WebConnCountForUser(userID string) int {
 }
 
 // Register registers a connection to the hub.
-func (h *Hub) Register(webConn *WebConn) {
+func (h *Hub) Register(webConn *WebConn) error {
+	wr := &webConnRegisterMessage{
+		conn: webConn,
+		err:  make(chan error),
+	}
 	select {
-	case h.register <- webConn:
+	case h.register <- wr:
+		return <-wr.err
 	case <-h.stop:
 	}
+	return nil
 }
 
 // Unregister unregisters a connection from the hub.
@@ -389,7 +403,10 @@ func (h *Hub) Start() {
 		ticker := time.NewTicker(inactiveConnReaperInterval)
 		defer ticker.Stop()
 
-		connIndex := newHubConnectionIndex(inactiveConnReaperInterval)
+		connIndex := newHubConnectionIndex(inactiveConnReaperInterval,
+			h.platform.Store,
+			h.platform.logger,
+		)
 
 		for {
 			select {
@@ -423,22 +440,27 @@ func (h *Hub) Start() {
 				req.result <- connIndex.ForUserActiveCount(req.userID)
 			case <-ticker.C:
 				connIndex.RemoveInactiveConnections()
-			case webConn := <-h.register:
+			case webConnReg := <-h.register:
 				// Mark the current one as active.
 				// There is no need to check if it was inactive or not,
 				// we will anyways need to make it active.
-				webConn.Active.Store(true)
+				webConnReg.conn.Active.Store(true)
 
-				connIndex.Add(webConn)
+				err := connIndex.Add(webConnReg.conn)
+				if err != nil {
+					webConnReg.err <- err
+					continue
+				}
 				atomic.StoreInt64(&h.connectionCount, int64(connIndex.AllActive()))
 
-				if webConn.IsAuthenticated() && webConn.reuseCount == 0 {
+				if webConnReg.conn.IsAuthenticated() && webConnReg.conn.reuseCount == 0 {
 					// The hello message should only be sent when the reuseCount is 0.
 					// i.e in server restart, or long timeout, or fresh connection case.
 					// In case of seq number not found in dead queue, it is handled by
 					// the webconn write pump.
-					webConn.send <- webConn.createHelloMessage()
+					webConnReg.conn.send <- webConnReg.conn.createHelloMessage()
 				}
+				webConnReg.err <- nil
 			case webConn := <-h.unregister:
 				// If already removed (via queue full), then removing again becomes a noop.
 				// But if not removed, mark inactive.
@@ -497,6 +519,13 @@ func (h *Hub) Start() {
 				for _, webConn := range connIndex.ForUser(userID) {
 					webConn.InvalidateCache()
 				}
+				err := connIndex.InvalidateCMCacheForUser(userID)
+				if err != nil {
+					h.platform.Log().Error("Error while invalidating channel member cache", mlog.String("user_id", userID), mlog.Err(err))
+					for _, webConn := range connIndex.ForUser(userID) {
+						closeAndRemoveConn(connIndex, webConn)
+					}
+				}
 			case activity := <-h.activity:
 				for _, webConn := range connIndex.ForUser(activity.userID) {
 					if !webConn.Active.Load() {
@@ -519,8 +548,7 @@ func (h *Hub) Start() {
 							mlog.String("user_id", directMsg.conn.UserId),
 							mlog.String("conn_id", directMsg.conn.GetConnectionID()))
 					}
-					close(directMsg.conn.send)
-					connIndex.Remove(directMsg.conn)
+					closeAndRemoveConn(connIndex, directMsg.conn)
 				}
 			case msg := <-h.broadcast:
 				if metrics := h.platform.metricsIFace; metrics != nil {
@@ -546,27 +574,29 @@ func (h *Hub) Start() {
 									mlog.String("user_id", webConn.UserId),
 									mlog.String("conn_id", webConn.GetConnectionID()))
 							}
-							close(webConn.send)
-							connIndex.Remove(webConn)
+							closeAndRemoveConn(connIndex, webConn)
 						}
 					}
 				}
 
+				var targetConns []*WebConn
 				if connID := msg.GetBroadcast().ConnectionId; connID != "" {
 					if webConn := connIndex.ForConnection(connID); webConn != nil {
-						broadcast(webConn)
-						continue
+						targetConns = append(targetConns, webConn)
 					}
-				} else if msg.GetBroadcast().UserId != "" {
-					candidates := connIndex.ForUser(msg.GetBroadcast().UserId)
-					for _, webConn := range candidates {
+				} else if userID := msg.GetBroadcast().UserId; userID != "" {
+					targetConns = connIndex.ForUser(userID)
+				} else if channelID := msg.GetBroadcast().ChannelId; channelID != "" {
+					targetConns = connIndex.ForChannel(channelID)
+				}
+				if targetConns != nil {
+					for _, webConn := range targetConns {
 						broadcast(webConn)
 					}
 					continue
 				}
 
-				candidates := connIndex.All()
-				for webConn := range candidates {
+				for webConn := range connIndex.All() {
 					broadcast(webConn)
 				}
 			case <-h.stop:
@@ -616,14 +646,24 @@ func areAllInactive(conns []*WebConn) bool {
 	return true
 }
 
+// closeAndRemoveConn closes the send channel which will close the
+// websocket connection, and then it removes the webConn from the conn index.
+func closeAndRemoveConn(connIndex *hubConnectionIndex, conn *WebConn) {
+	close(conn.send)
+	connIndex.Remove(conn)
+}
+
 // hubConnectionIndex provides fast addition, removal, and iteration of web connections.
-// It requires 3 functionalities which need to be very fast:
+// It requires 4 functionalities which need to be very fast:
 // - check if a connection exists or not.
 // - get all connections for a given userID.
+// - get all connections for a given channelID.
 // - get all connections.
 type hubConnectionIndex struct {
 	// byUserId stores the list of connections for a given userID
 	byUserId map[string][]*WebConn
+	// byChannelID stores the list of connections for a given channelID.
+	byChannelID map[string][]*WebConn
 	// byConnection serves the dual purpose of storing the index of the webconn
 	// in the value of byUserId map, and also to get all connections.
 	byConnection   map[*WebConn]int
@@ -631,21 +671,39 @@ type hubConnectionIndex struct {
 	// staleThreshold is the limit beyond which inactive connections
 	// will be deleted.
 	staleThreshold time.Duration
+
+	store  store.Store
+	logger mlog.LoggerIFace
 }
 
-func newHubConnectionIndex(interval time.Duration) *hubConnectionIndex {
+func newHubConnectionIndex(interval time.Duration,
+	store store.Store,
+	logger mlog.LoggerIFace,
+) *hubConnectionIndex {
 	return &hubConnectionIndex{
 		byUserId:       make(map[string][]*WebConn),
+		byChannelID:    make(map[string][]*WebConn),
 		byConnection:   make(map[*WebConn]int),
 		byConnectionId: make(map[string]*WebConn),
 		staleThreshold: interval,
+		store:          store,
+		logger:         logger,
 	}
 }
 
-func (i *hubConnectionIndex) Add(wc *WebConn) {
+func (i *hubConnectionIndex) Add(wc *WebConn) error {
+	cm, err := i.store.Channel().GetAllChannelMembersForUser(request.EmptyContext(i.logger), wc.UserId, false, false)
+	if err != nil {
+		return fmt.Errorf("error getChannelMembersForUser: %v", err)
+	}
+	for chID := range cm {
+		i.byChannelID[chID] = append(i.byChannelID[chID], wc)
+	}
+
 	i.byUserId[wc.UserId] = append(i.byUserId[wc.UserId], wc)
 	i.byConnection[wc] = len(i.byUserId[wc.UserId]) - 1
 	i.byConnectionId[wc.GetConnectionID()] = wc
+	return nil
 }
 
 func (i *hubConnectionIndex) Remove(wc *WebConn) {
@@ -654,21 +712,67 @@ func (i *hubConnectionIndex) Remove(wc *WebConn) {
 		return
 	}
 
+	// Remove the wc from i.byUserId
 	// get the conn slice.
 	userConnections := i.byUserId[wc.UserId]
 	// get the last connection.
 	last := userConnections[len(userConnections)-1]
-	// set the slot that we are trying to remove to be the last connection.
+	// https://go.dev/wiki/SliceTricks#delete-without-preserving-order
 	userConnections[userConnIndex] = last
-	// remove the last connection pointer from slice.
 	userConnections[len(userConnections)-1] = nil
-	// remove the last connection from the slice.
 	i.byUserId[wc.UserId] = userConnections[:len(userConnections)-1]
 	// set the index of the connection that was moved to the new index.
 	i.byConnection[last] = userConnIndex
 
+	connectionID := wc.GetConnectionID()
+	// Remove webconns from i.byChannelID
+	// This has O(n) complexity. We are trading off speed while removing
+	// a connection, to improve broadcasting a message.
+	for chID, webConns := range i.byChannelID {
+		// https://go.dev/wiki/SliceTricks#filtering-without-allocating
+		filtered := webConns[:0]
+		for _, conn := range webConns {
+			if conn.GetConnectionID() != connectionID {
+				filtered = append(filtered, conn)
+			}
+		}
+		for i := len(filtered); i < len(webConns); i++ {
+			webConns[i] = nil
+		}
+		i.byChannelID[chID] = filtered
+	}
+
 	delete(i.byConnection, wc)
-	delete(i.byConnectionId, wc.GetConnectionID())
+	delete(i.byConnectionId, connectionID)
+}
+
+func (i *hubConnectionIndex) InvalidateCMCacheForUser(userID string) error {
+	// We make this query first to fail fast in case of an error.
+	cm, err := i.store.Channel().GetAllChannelMembersForUser(request.EmptyContext(i.logger), userID, false, false)
+	if err != nil {
+		return err
+	}
+
+	// Clear out all user entries which belong to channels.
+	for chID, webConns := range i.byChannelID {
+		// https://go.dev/wiki/SliceTricks#filtering-without-allocating
+		filtered := webConns[:0]
+		for _, conn := range webConns {
+			if conn.UserId != userID {
+				filtered = append(filtered, conn)
+			}
+		}
+		for i := len(filtered); i < len(webConns); i++ {
+			webConns[i] = nil
+		}
+		i.byChannelID[chID] = filtered
+	}
+
+	// re-populate the cache
+	for chID := range cm {
+		i.byChannelID[chID] = append(i.byChannelID[chID], i.ForUser(userID)...)
+	}
+	return nil
 }
 
 func (i *hubConnectionIndex) Has(wc *WebConn) bool {
@@ -691,6 +795,16 @@ func (i *hubConnectionIndex) ForUser(id string) []*WebConn {
 	return conns
 }
 
+// ForChannel returns all connections for a channelID.
+func (i *hubConnectionIndex) ForChannel(channelID string) []*WebConn {
+	// Note: this is expensive because usually there will be
+	// more than 1 member for a channel, and broadcasting
+	// is a hot path, but worth it.
+	conns := make([]*WebConn, len(i.byChannelID[channelID]))
+	copy(conns, i.byChannelID[channelID])
+	return conns
+}
+
 // ForUserActiveCount returns the number of active connections for a userID
 func (i *hubConnectionIndex) ForUserActiveCount(id string) int {
 	cnt := 0
diff --git a/server/channels/app/platform/web_hub_test.go b/server/channels/app/platform/web_hub_test.go
index 99a903c7b2..83eda9a465 100644
--- a/server/channels/app/platform/web_hub_test.go
+++ b/server/channels/app/platform/web_hub_test.go
@@ -15,13 +15,11 @@ import (
 
 	"github.com/gorilla/websocket"
 	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/mock"
 	"github.com/stretchr/testify/require"
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/i18n"
 	platform_mocks "github.com/mattermost/mattermost/server/v8/channels/app/platform/mocks"
-	"github.com/mattermost/mattermost/server/v8/channels/store/storetest/mocks"
 	"github.com/mattermost/mattermost/server/v8/channels/testlib"
 )
 
@@ -53,7 +51,7 @@ func registerDummyWebConn(t *testing.T, th *TestHelper, addr net.Addr, session *
 		Locale:    "en",
 	}
 	wc := th.Service.NewWebConn(cfg, th.Suite, &hookRunner{})
-	th.Service.HubRegister(wc)
+	require.NoError(t, th.Service.HubRegister(wc))
 	go wc.Pump()
 	return wc
 }
@@ -105,8 +103,8 @@ func TestHubStopRaceCondition(t *testing.T) {
 	go func() {
 		wc4 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
 		wc5 := registerDummyWebConn(t, th, s.Listener.Addr(), session)
-		hub.Register(wc4)
-		hub.Register(wc5)
+		require.NoError(t, hub.Register(wc4))
+		require.NoError(t, hub.Register(wc5))
 
 		hub.UpdateActivity("userId", "sessionToken", 0)
 
@@ -128,50 +126,9 @@ func TestHubStopRaceCondition(t *testing.T) {
 }
 
 func TestHubSessionRevokeRace(t *testing.T) {
-	th := SetupWithStoreMock(t)
+	th := Setup(t)
 	defer th.TearDown()
 
-	sess1 := &model.Session{
-		Id:             "id1",
-		UserId:         "user1",
-		DeviceId:       "",
-		Token:          "sesstoken",
-		ExpiresAt:      model.GetMillis() + 300000,
-		LastActivityAt: 10000,
-	}
-
-	mockStore := th.Service.Store.(*mocks.Store)
-
-	mockUserStore := mocks.UserStore{}
-	mockUserStore.On("Count", mock.Anything).Return(int64(10), nil)
-	mockUserStore.On("GetUnreadCount", mock.AnythingOfType("string"), mock.AnythingOfType("bool")).Return(int64(1), nil)
-	mockPostStore := mocks.PostStore{}
-	mockPostStore.On("GetMaxPostSize").Return(65535, nil)
-	mockSystemStore := mocks.SystemStore{}
-	mockSystemStore.On("GetByName", "UpgradedFromTE").Return(&model.System{Name: "UpgradedFromTE", Value: "false"}, nil)
-	mockSystemStore.On("GetByName", "InstallationDate").Return(&model.System{Name: "InstallationDate", Value: "10"}, nil)
-	mockSystemStore.On("GetByName", "FirstServerRunTimestamp").Return(&model.System{Name: "FirstServerRunTimestamp", Value: "10"}, nil)
-
-	mockSessionStore := mocks.SessionStore{}
-	mockSessionStore.On("UpdateLastActivityAt", "id1", mock.Anything).Return(nil)
-	mockSessionStore.On("Save", mock.AnythingOfType("*request.Context"), mock.AnythingOfType("*model.Session")).Return(sess1, nil)
-	mockSessionStore.On("Get", mock.AnythingOfType("*request.Context"), mock.Anything, "id1").Return(sess1, nil)
-	mockSessionStore.On("Remove", "id1").Return(nil)
-
-	mockStatusStore := mocks.StatusStore{}
-	mockStatusStore.On("Get", "user1").Return(&model.Status{UserId: "user1", Status: model.StatusOnline}, nil)
-	mockStatusStore.On("UpdateLastActivityAt", "user1", mock.Anything).Return(nil)
-	mockStatusStore.On("SaveOrUpdate", mock.AnythingOfType("*model.Status")).Return(nil)
-
-	mockOAuthStore := mocks.OAuthStore{}
-	mockStore.On("Session").Return(&mockSessionStore)
-	mockStore.On("OAuth").Return(&mockOAuthStore)
-	mockStore.On("Status").Return(&mockStatusStore)
-	mockStore.On("User").Return(&mockUserStore)
-	mockStore.On("Post").Return(&mockPostStore)
-	mockStore.On("System").Return(&mockSystemStore)
-	mockStore.On("GetDBSchemaVersion").Return(1, nil)
-
 	// This needs to be false for the condition to trigger
 	th.Service.UpdateConfig(func(cfg *model.Config) {
 		*cfg.ServiceSettings.ExtendSessionLengthWithActivity = false
@@ -181,7 +138,7 @@ func TestHubSessionRevokeRace(t *testing.T) {
 	defer s.Close()
 
 	session, err := th.Service.CreateSession(th.Context, &model.Session{
-		UserId: "testid",
+		UserId: model.NewId(),
 	})
 	require.NoError(t, err)
 
@@ -193,7 +150,7 @@ func TestHubSessionRevokeRace(t *testing.T) {
 	time.Sleep(2 * time.Second)
 	// We override the LastActivityAt which happens in NewWebConn.
 	// This is needed to call RevokeSessionById which triggers the race.
-	th.Service.AddSessionToCache(sess1)
+	th.Service.AddSessionToCache(session)
 
 	go func() {
 		for i := 0; i <= broadcastQueueSize; i++ {
@@ -203,7 +160,7 @@ func TestHubSessionRevokeRace(t *testing.T) {
 	}()
 
 	// This call should happen _after_ !wc.IsAuthenticated() and _before_wc.isMemberOfTeam().
-	// There's no guarantee this will happen. But that's out best bet to trigger this race.
+	// There's no guarantee this will happen. But that's our best bet to trigger this race.
 	wc1.InvalidateCache()
 
 	for i := 0; i < 10; i++ {
@@ -220,92 +177,236 @@ func TestHubSessionRevokeRace(t *testing.T) {
 }
 
 func TestHubConnIndex(t *testing.T) {
-	th := Setup(t)
+	th := Setup(t).InitBasic()
 	defer th.TearDown()
 
-	connIndex := newHubConnectionIndex(1 * time.Second)
-
-	// User1
-	wc1 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   model.NewId(),
-	}
-	wc1.SetConnectionID(model.NewId())
-	wc1.SetSession(&model.Session{})
-
-	// User2
-	wc2 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   model.NewId(),
-	}
-	wc2.SetConnectionID(model.NewId())
-	wc2.SetSession(&model.Session{})
-
-	wc3 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   wc2.UserId,
-	}
-	wc3.SetConnectionID(model.NewId())
-	wc3.SetSession(&model.Session{})
-
-	wc4 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   wc2.UserId,
-	}
-	wc4.SetConnectionID(model.NewId())
-	wc4.SetSession(&model.Session{})
-
-	connIndex.Add(wc1)
-	connIndex.Add(wc2)
-	connIndex.Add(wc3)
-	connIndex.Add(wc4)
+	_, err := th.Service.Store.Channel().SaveMember(th.Context, &model.ChannelMember{
+		ChannelId:   th.BasicChannel.Id,
+		UserId:      th.BasicUser.Id,
+		NotifyProps: model.GetDefaultChannelNotifyProps(),
+		SchemeGuest: th.BasicUser.IsGuest(),
+		SchemeUser:  !th.BasicUser.IsGuest(),
+	})
+	require.NoError(t, err)
+	_, err = th.Service.Store.Channel().SaveMember(th.Context, &model.ChannelMember{
+		ChannelId:   th.BasicChannel.Id,
+		UserId:      th.BasicUser2.Id,
+		NotifyProps: model.GetDefaultChannelNotifyProps(),
+		SchemeGuest: th.BasicUser2.IsGuest(),
+		SchemeUser:  !th.BasicUser2.IsGuest(),
+	})
+	require.NoError(t, err)
 
 	t.Run("Basic", func(t *testing.T) {
-		assert.True(t, connIndex.Has(wc1))
-		assert.True(t, connIndex.Has(wc2))
+		connIndex := newHubConnectionIndex(1*time.Second, th.Service.Store, th.Service.logger)
 
-		assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2, wc3, wc4})
-		assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{wc1})
-		assert.True(t, connIndex.Has(wc2))
-		assert.True(t, connIndex.Has(wc1))
-		assert.Len(t, connIndex.All(), 4)
+		// User1
+		wc1 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   model.NewId(),
+		}
+		wc1.SetConnectionID(model.NewId())
+		wc1.SetSession(&model.Session{})
+
+		// User2
+		wc2 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   model.NewId(),
+		}
+		wc2.SetConnectionID(model.NewId())
+		wc2.SetSession(&model.Session{})
+
+		wc3 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   wc2.UserId,
+		}
+		wc3.SetConnectionID(model.NewId())
+		wc3.SetSession(&model.Session{})
+
+		wc4 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   wc2.UserId,
+		}
+		wc4.SetConnectionID(model.NewId())
+		wc4.SetSession(&model.Session{})
+
+		connIndex.Add(wc1)
+		connIndex.Add(wc2)
+		connIndex.Add(wc3)
+		connIndex.Add(wc4)
+
+		t.Run("Basic", func(t *testing.T) {
+			assert.True(t, connIndex.Has(wc1))
+			assert.True(t, connIndex.Has(wc2))
+
+			assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2, wc3, wc4})
+			assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{wc1})
+			assert.True(t, connIndex.Has(wc2))
+			assert.True(t, connIndex.Has(wc1))
+			assert.Len(t, connIndex.All(), 4)
+		})
+
+		t.Run("RemoveMiddleUser2", func(t *testing.T) {
+			connIndex.Remove(wc3) // Remove from middle from user2
+
+			assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2, wc4})
+			assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{wc1})
+			assert.True(t, connIndex.Has(wc2))
+			assert.False(t, connIndex.Has(wc3))
+			assert.True(t, connIndex.Has(wc4))
+			assert.Len(t, connIndex.All(), 3)
+		})
+
+		t.Run("RemoveUser1", func(t *testing.T) {
+			connIndex.Remove(wc1) // Remove sole connection from user1
+
+			assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2, wc4})
+			assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{})
+			assert.Len(t, connIndex.ForUser(wc1.UserId), 0)
+			assert.Len(t, connIndex.All(), 2)
+			assert.False(t, connIndex.Has(wc1))
+			assert.True(t, connIndex.Has(wc2))
+		})
+
+		t.Run("RemoveEndUser2", func(t *testing.T) {
+			connIndex.Remove(wc4) // Remove from end from user2
+
+			assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2})
+			assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{})
+			assert.True(t, connIndex.Has(wc2))
+			assert.False(t, connIndex.Has(wc3))
+			assert.False(t, connIndex.Has(wc4))
+			assert.Len(t, connIndex.All(), 1)
+		})
 	})
 
-	t.Run("RemoveMiddleUser2", func(t *testing.T) {
-		connIndex.Remove(wc3) // Remove from middle from user2
+	t.Run("ByConnectionId", func(t *testing.T) {
+		connIndex := newHubConnectionIndex(1*time.Second, th.Service.Store, th.Service.logger)
 
-		assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2, wc4})
-		assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{wc1})
-		assert.True(t, connIndex.Has(wc2))
-		assert.False(t, connIndex.Has(wc3))
-		assert.True(t, connIndex.Has(wc4))
-		assert.Len(t, connIndex.All(), 3)
+		// User1
+		wc1ID := model.NewId()
+		wc1 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   th.BasicUser.Id,
+		}
+		wc1.SetConnectionID(wc1ID)
+		wc1.SetSession(&model.Session{})
+
+		// User2
+		wc2ID := model.NewId()
+		wc2 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   th.BasicUser2.Id,
+		}
+		wc2.SetConnectionID(wc2ID)
+		wc2.SetSession(&model.Session{})
+
+		wc3ID := model.NewId()
+		wc3 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   wc2.UserId,
+		}
+		wc3.SetConnectionID(wc3ID)
+		wc3.SetSession(&model.Session{})
+
+		t.Run("no connections", func(t *testing.T) {
+			assert.False(t, connIndex.Has(wc1))
+			assert.False(t, connIndex.Has(wc2))
+			assert.False(t, connIndex.Has(wc3))
+			assert.Empty(t, connIndex.byConnectionId)
+		})
+
+		t.Run("adding", func(t *testing.T) {
+			connIndex.Add(wc1)
+			connIndex.Add(wc3)
+
+			assert.Len(t, connIndex.byConnectionId, 2)
+			assert.Equal(t, wc1, connIndex.ForConnection(wc1ID))
+			assert.Equal(t, wc3, connIndex.ForConnection(wc3ID))
+			assert.Equal(t, (*WebConn)(nil), connIndex.ForConnection(wc2ID))
+		})
+
+		t.Run("removing", func(t *testing.T) {
+			connIndex.Remove(wc3)
+
+			assert.Len(t, connIndex.byConnectionId, 1)
+			assert.Equal(t, wc1, connIndex.ForConnection(wc1ID))
+			assert.Equal(t, (*WebConn)(nil), connIndex.ForConnection(wc3ID))
+			assert.Equal(t, (*WebConn)(nil), connIndex.ForConnection(wc2ID))
+		})
 	})
 
-	t.Run("RemoveUser1", func(t *testing.T) {
-		connIndex.Remove(wc1) // Remove sole connection from user1
+	t.Run("ByChannelId", func(t *testing.T) {
+		connIndex := newHubConnectionIndex(1*time.Second, th.Service.Store, th.Service.logger)
 
-		assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2, wc4})
-		assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{})
-		assert.Len(t, connIndex.ForUser(wc1.UserId), 0)
-		assert.Len(t, connIndex.All(), 2)
-		assert.False(t, connIndex.Has(wc1))
-		assert.True(t, connIndex.Has(wc2))
-	})
+		// User1
+		wc1ID := model.NewId()
+		wc1 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   th.BasicUser.Id,
+		}
+		wc1.SetConnectionID(wc1ID)
+		wc1.SetSession(&model.Session{})
 
-	t.Run("RemoveEndUser2", func(t *testing.T) {
-		connIndex.Remove(wc4) // Remove from end from user2
+		// User2
+		wc2ID := model.NewId()
+		wc2 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   th.BasicUser2.Id,
+		}
+		wc2.SetConnectionID(wc2ID)
+		wc2.SetSession(&model.Session{})
 
-		assert.ElementsMatch(t, connIndex.ForUser(wc2.UserId), []*WebConn{wc2})
-		assert.ElementsMatch(t, connIndex.ForUser(wc1.UserId), []*WebConn{})
-		assert.True(t, connIndex.Has(wc2))
-		assert.False(t, connIndex.Has(wc3))
-		assert.False(t, connIndex.Has(wc4))
-		assert.Len(t, connIndex.All(), 1)
+		wc3ID := model.NewId()
+		wc3 := &WebConn{
+			Platform: th.Service,
+			Suite:    th.Suite,
+			UserId:   wc2.UserId,
+		}
+		wc3.SetConnectionID(wc3ID)
+		wc3.SetSession(&model.Session{})
+
+		connIndex.Add(wc1)
+		connIndex.Add(wc2)
+		connIndex.Add(wc3)
+
+		t.Run("ForChannel", func(t *testing.T) {
+			require.Len(t, connIndex.byChannelID, 1)
+			require.Equal(t, []*WebConn{wc1, wc2, wc3}, connIndex.ForChannel(th.BasicChannel.Id))
+			require.Len(t, connIndex.ForChannel("notexist"), 0)
+		})
+
+		ch := th.CreateChannel(th.BasicTeam)
+		_, err = th.Service.Store.Channel().SaveMember(th.Context, &model.ChannelMember{
+			ChannelId:   ch.Id,
+			UserId:      th.BasicUser2.Id,
+			NotifyProps: model.GetDefaultChannelNotifyProps(),
+			SchemeGuest: th.BasicUser2.IsGuest(),
+			SchemeUser:  !th.BasicUser2.IsGuest(),
+		})
+		require.NoError(t, err)
+
+		t.Run("InvalidateCMCacheForUser", func(t *testing.T) {
+			require.NoError(t, connIndex.InvalidateCMCacheForUser(th.BasicUser2.Id))
+			require.Len(t, connIndex.byChannelID, 2)
+			require.Len(t, connIndex.ForChannel(th.BasicChannel.Id), 3)
+			require.Len(t, connIndex.ForChannel(ch.Id), 2)
+		})
+
+		t.Run("Remove", func(t *testing.T) {
+			connIndex.Remove(wc3)
+			require.Len(t, connIndex.byChannelID, 2)
+			require.Len(t, connIndex.ForChannel(th.BasicChannel.Id), 2)
+		})
 	})
 }
 
@@ -313,7 +414,7 @@ func TestHubConnIndexIncorrectRemoval(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()
 
-	connIndex := newHubConnectionIndex(1 * time.Second)
+	connIndex := newHubConnectionIndex(1*time.Second, th.Service.Store, th.Service.logger)
 
 	// User2
 	wc2 := &WebConn{
@@ -356,73 +457,11 @@ func TestHubConnIndexIncorrectRemoval(t *testing.T) {
 	}
 }
 
-func TestHubConnIndexByConnectionId(t *testing.T) {
-	th := Setup(t)
-	defer th.TearDown()
-
-	connIndex := newHubConnectionIndex(1 * time.Second)
-
-	// User1
-	wc1ID := model.NewId()
-	wc1 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   model.NewId(),
-	}
-	wc1.SetConnectionID(wc1ID)
-	wc1.SetSession(&model.Session{})
-
-	// User2
-	wc2ID := model.NewId()
-	wc2 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   model.NewId(),
-	}
-	wc2.SetConnectionID(wc2ID)
-	wc2.SetSession(&model.Session{})
-
-	wc3ID := model.NewId()
-	wc3 := &WebConn{
-		Platform: th.Service,
-		Suite:    th.Suite,
-		UserId:   wc2.UserId,
-	}
-	wc3.SetConnectionID(wc3ID)
-	wc3.SetSession(&model.Session{})
-
-	t.Run("no connections", func(t *testing.T) {
-		assert.False(t, connIndex.Has(wc1))
-		assert.False(t, connIndex.Has(wc2))
-		assert.False(t, connIndex.Has(wc3))
-		assert.Empty(t, connIndex.byConnectionId)
-	})
-
-	t.Run("adding", func(t *testing.T) {
-		connIndex.Add(wc1)
-		connIndex.Add(wc3)
-
-		assert.Len(t, connIndex.byConnectionId, 2)
-		assert.Equal(t, wc1, connIndex.ForConnection(wc1ID))
-		assert.Equal(t, wc3, connIndex.ForConnection(wc3ID))
-		assert.Equal(t, (*WebConn)(nil), connIndex.ForConnection(wc2ID))
-	})
-
-	t.Run("removing", func(t *testing.T) {
-		connIndex.Remove(wc3)
-
-		assert.Len(t, connIndex.byConnectionId, 1)
-		assert.Equal(t, wc1, connIndex.ForConnection(wc1ID))
-		assert.Equal(t, (*WebConn)(nil), connIndex.ForConnection(wc3ID))
-		assert.Equal(t, (*WebConn)(nil), connIndex.ForConnection(wc2ID))
-	})
-}
-
 func TestHubConnIndexInactive(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()
 
-	connIndex := newHubConnectionIndex(2 * time.Second)
+	connIndex := newHubConnectionIndex(2*time.Second, th.Service.Store, th.Service.logger)
 
 	// User1
 	wc1 := &WebConn{
@@ -582,7 +621,7 @@ func TestHubWebConnCount(t *testing.T) {
 func BenchmarkHubConnIndex(b *testing.B) {
 	th := Setup(b).InitBasic()
 	defer th.TearDown()
-	connIndex := newHubConnectionIndex(1 * time.Second)
+	connIndex := newHubConnectionIndex(1*time.Second, th.Service.Store, th.Service.logger)
 
 	// User1
 	wc1 := &WebConn{
@@ -627,7 +666,7 @@ func TestHubConnIndexRemoveMemLeak(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()
 
-	connIndex := newHubConnectionIndex(1 * time.Second)
+	connIndex := newHubConnectionIndex(1*time.Second, th.Service.Store, th.Service.logger)
 
 	wc := &WebConn{
 		Platform: th.Service,
diff --git a/server/channels/app/platform/websocket_router.go b/server/channels/app/platform/websocket_router.go
index 9e4fc626f9..c264d5c1bb 100644
--- a/server/channels/app/platform/websocket_router.go
+++ b/server/channels/app/platform/websocket_router.go
@@ -49,6 +49,7 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 
 		session, err := conn.Suite.GetSession(token)
 		if err != nil {
+			conn.Platform.Log().Warn("Error while getting session token", mlog.Err(err))
 			conn.WebSocket.Close()
 			return
 		}
@@ -56,7 +57,12 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 		conn.SetSessionToken(session.Token)
 		conn.UserId = session.UserId
 
-		conn.Platform.HubRegister(conn)
+		nErr := conn.Platform.HubRegister(conn)
+		if nErr != nil {
+			conn.Platform.Log().Error("Error while registering to hub", mlog.String("user_id", conn.UserId), mlog.Err(nErr))
+			conn.WebSocket.Close()
+			return
+		}
 
 		conn.Platform.Go(func() {
 			conn.Platform.SetStatusOnline(session.UserId, false)
diff --git a/server/channels/app/web_conn_test.go b/server/channels/app/web_conn_test.go
index e0e9988bd8..1b0c02de55 100644
--- a/server/channels/app/web_conn_test.go
+++ b/server/channels/app/web_conn_test.go
@@ -15,6 +15,9 @@ import (
 	"github.com/mattermost/mattermost/server/v8/channels/app/platform"
 )
 
+// TestWebConnShouldSendEvent is not exhaustive because some of the checks
+// happen inside web_hub.go before the event is actually broadcasted, and checked
+// via ShouldSendEvent.
 func TestWebConnShouldSendEvent(t *testing.T) {
 	os.Setenv("MM_FEATUREFLAGS_WEBSOCKETEVENTSCOPE", "true")
 	defer os.Unsetenv("MM_FEATUREFLAGS_WEBSOCKETEVENTSCOPE")
@@ -157,14 +160,6 @@ func TestWebConnShouldSendEvent(t *testing.T) {
 		})
 	}
 
-	t.Run("should send to basic user in basic channel", func(t *testing.T) {
-		event = event.SetBroadcast(&model.WebsocketBroadcast{ChannelId: th.BasicChannel.Id})
-
-		assert.True(t, basicUserWc.ShouldSendEvent(event), "expected user 1")
-		assert.False(t, basicUser2Wc.ShouldSendEvent(event), "did not expect user 2")
-		assert.False(t, adminUserWc.ShouldSendEvent(event), "did not expect admin")
-	})
-
 	t.Run("should not send typing event unless in scope", func(t *testing.T) {
 		event2 := model.NewWebSocketEvent(model.WebsocketEventTyping, "", th.BasicChannel.Id, "", nil, "")
 		// Basic, unset case
@@ -222,14 +217,6 @@ func TestWebConnShouldSendEvent(t *testing.T) {
 		assert.False(t, basicUserWc.ShouldSendEvent(event2))
 	})
 
-	t.Run("should send to basic user and admin in channel2", func(t *testing.T) {
-		event = event.SetBroadcast(&model.WebsocketBroadcast{ChannelId: channel2.Id})
-
-		assert.True(t, basicUserWc.ShouldSendEvent(event), "expected user 1")
-		assert.False(t, basicUser2Wc.ShouldSendEvent(event), "did not expect user 2")
-		assert.True(t, adminUserWc.ShouldSendEvent(event), "expected admin")
-	})
-
 	t.Run("channel member cache invalidated after user added to channel", func(t *testing.T) {
 		th.AddUserToChannel(th.BasicUser2, channel2)
 		basicUser2Wc.InvalidateCache()
diff --git a/server/channels/app/web_hub.go b/server/channels/app/web_hub.go
index 1f16f81b63..c46b9cb074 100644
--- a/server/channels/app/web_hub.go
+++ b/server/channels/app/web_hub.go
@@ -16,16 +16,6 @@ func (a *App) GetHubForUserId(userID string) *platform.Hub {
 	return a.Srv().Platform().GetHubForUserId(userID)
 }
 
-// HubRegister registers a connection to a hub.
-func (a *App) HubRegister(webConn *platform.WebConn) {
-	a.Srv().Platform().HubRegister(webConn)
-}
-
-// HubUnregister unregisters a connection from a hub.
-func (a *App) HubUnregister(webConn *platform.WebConn) {
-	a.Srv().Platform().HubUnregister(webConn)
-}
-
 func (a *App) Publish(message *model.WebSocketEvent) {
 	a.Srv().Platform().Publish(message)
 }