Limit the number of SSE Subscribers to 16 by default

Signed-off-by: Julien <roidelapluie@o11y.eu>

cmd/prometheus/main.go

@@ -153,6 +153,7 @@ type flagConfig struct {
 	queryMaxSamples             int
 	RemoteFlushDeadline         model.Duration
 	nameEscapingScheme          string
+	maxNotificationsSubscribers int
 
 	enableAutoReload   bool
 	autoReloadInterval model.Duration
@@ -274,8 +275,6 @@ func main() {
 		)
 	}
 
-	notifs := api.NewNotifications(prometheus.DefaultRegisterer)
-
 	cfg := flagConfig{
 		notifier: notifier.Options{
 			Registerer: prometheus.DefaultRegisterer,
@@ -283,8 +282,6 @@ func main() {
 		web: web.Options{
 			Registerer: prometheus.DefaultRegisterer,
 			Gatherer:   prometheus.DefaultGatherer,
-			NotificationsSub:    notifs.Sub,
-			NotificationsGetter: notifs.Get,
 		},
 		promlogConfig: promlog.Config{},
 	}
@@ -319,6 +316,9 @@ func main() {
 	a.Flag("web.max-connections", "Maximum number of simultaneous connections across all listeners.").
 		Default("512").IntVar(&cfg.web.MaxConnections)
 
+	a.Flag("web.max-notifications-subscribers", "Limits the maximum number of subscribers that can concurrently receive live notifications. If the limit is reached, new subscription requests will be denied until existing connections close.").
+		Default("16").IntVar(&cfg.maxNotificationsSubscribers)
+
 	a.Flag("web.external-url",
 		"The URL under which Prometheus is externally reachable (for example, if Prometheus is served via a reverse proxy). Used for generating relative and absolute links back to Prometheus itself. If the URL has a path portion, it will be used to prefix all HTTP endpoints served by Prometheus. If omitted, relevant URL components will be derived automatically.").
 		PlaceHolder("<URL>").StringVar(&cfg.prometheusURL)
@@ -500,6 +500,10 @@ func main() {
 
 	logger := promlog.New(&cfg.promlogConfig)
 
+	notifs := api.NewNotifications(cfg.maxNotificationsSubscribers, prometheus.DefaultRegisterer)
+	cfg.web.NotificationsSub = notifs.Sub
+	cfg.web.NotificationsGetter = notifs.Get
+
 	if err := cfg.setFeatureListOptions(logger); err != nil {
 		fmt.Fprintln(os.Stderr, fmt.Errorf("Error parsing feature list: %w", err))
 		os.Exit(1)

docs/command-line/prometheus.md

@@ -21,6 +21,7 @@ The Prometheus monitoring server
 | <code class="text-nowrap">--web.config.file</code> | [EXPERIMENTAL] Path to configuration file that can enable TLS or authentication. |  |
 | <code class="text-nowrap">--web.read-timeout</code> | Maximum duration before timing out read of the request, and closing idle connections. | `5m` |
 | <code class="text-nowrap">--web.max-connections</code> | Maximum number of simultaneous connections across all listeners. | `512` |
+| <code class="text-nowrap">--web.max-notifications-subscribers</code> | Limits the maximum number of subscribers that can concurrently receive live notifications. If the limit is reached, new subscription requests will be denied until existing connections close. | `16` |
 | <code class="text-nowrap">--web.external-url</code> | The URL under which Prometheus is externally reachable (for example, if Prometheus is served via a reverse proxy). Used for generating relative and absolute links back to Prometheus itself. If the URL has a path portion, it will be used to prefix all HTTP endpoints served by Prometheus. If omitted, relevant URL components will be derived automatically. |  |
 | <code class="text-nowrap">--web.route-prefix</code> | Prefix for the internal routes of web endpoints. Defaults to path of --web.external-url. |  |
 | <code class="text-nowrap">--web.user-assets</code> | Path to static asset directory, available at /user. |  |

web/api/notifications.go

@@ -37,6 +37,7 @@ type Notifications struct {
 	mu             sync.Mutex
 	notifications  []Notification
 	subscribers    map[chan Notification]struct{} // Active subscribers.
+	maxSubscribers int
 
 	subscriberGauge      prometheus.Gauge
 	notificationsSent    prometheus.Counter
@@ -44,9 +45,10 @@ type Notifications struct {
 }
 
 // NewNotifications creates a new Notifications instance.
-func NewNotifications(reg prometheus.Registerer) *Notifications {
+func NewNotifications(maxSubscribers int, reg prometheus.Registerer) *Notifications {
 	n := &Notifications{
 		subscribers:    make(map[chan Notification]struct{}),
+		maxSubscribers: maxSubscribers,
 		subscriberGauge: prometheus.NewGauge(prometheus.GaugeOpts{
 			Namespace: "prometheus",
 			Subsystem: "api",
@@ -147,10 +149,16 @@ func (n *Notifications) Get() []Notification {
 // Sub allows a client to subscribe to live notifications.
 // It returns a channel where the subscriber will receive notifications and a function to unsubscribe.
 // Each subscriber has its own goroutine to handle notifications and prevent blocking.
-func (n *Notifications) Sub() (<-chan Notification, func()) {
+func (n *Notifications) Sub() (<-chan Notification, func(), bool) {
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	if len(n.subscribers) >= n.maxSubscribers {
+		return nil, nil, false
+	}
+
 	ch := make(chan Notification, 10) // Buffered channel to prevent blocking.
 
-	n.mu.Lock()
 	// Add the new subscriber to the list.
 	n.subscribers[ch] = struct{}{}
 	n.subscriberGauge.Set(float64(len(n.subscribers)))
@@ -159,7 +167,6 @@ func (n *Notifications) Sub() (<-chan Notification, func()) {
 	for _, notification := range n.notifications {
 		ch <- notification
 	}
-	n.mu.Unlock()
 
 	// Unsubscribe function to remove the channel from subscribers.
 	unsubscribe := func() {
@@ -172,5 +179,5 @@ func (n *Notifications) Sub() (<-chan Notification, func()) {
 		n.subscriberGauge.Set(float64(len(n.subscribers)))
 	}
 
-	return ch, unsubscribe
+	return ch, unsubscribe, true
 }

web/api/notifications_test.go

@@ -23,7 +23,7 @@ import (
 
 // TestNotificationLifecycle tests adding, modifying, and deleting notifications.
 func TestNotificationLifecycle(t *testing.T) {
-	notifs := NewNotifications(nil)
+	notifs := NewNotifications(10, nil)
 
 	// Add a notification.
 	notifs.AddNotification("Test Notification 1")
@@ -47,10 +47,11 @@ func TestNotificationLifecycle(t *testing.T) {
 
 // TestSubscriberReceivesNotifications tests that a subscriber receives notifications, including modifications and deletions.
 func TestSubscriberReceivesNotifications(t *testing.T) {
-	notifs := NewNotifications(nil)
+	notifs := NewNotifications(10, nil)
 
 	// Subscribe to notifications.
-	sub, unsubscribe := notifs.Sub()
+	sub, unsubscribe, ok := notifs.Sub()
+	require.True(t, ok)
 
 	var wg sync.WaitGroup
 	wg.Add(1)
@@ -103,12 +104,14 @@ func TestSubscriberReceivesNotifications(t *testing.T) {
 
 // TestMultipleSubscribers tests that multiple subscribers receive notifications independently.
 func TestMultipleSubscribers(t *testing.T) {
-	notifs := NewNotifications(nil)
+	notifs := NewNotifications(10, nil)
 
 	// Subscribe two subscribers to notifications.
-	sub1, unsubscribe1 := notifs.Sub()
+	sub1, unsubscribe1, ok1 := notifs.Sub()
+	require.True(t, ok1)
 
-	sub2, unsubscribe2 := notifs.Sub()
+	sub2, unsubscribe2, ok2 := notifs.Sub()
+	require.True(t, ok2)
 
 	var wg sync.WaitGroup
 	wg.Add(2)
@@ -157,10 +160,11 @@ func TestMultipleSubscribers(t *testing.T) {
 
 // TestUnsubscribe tests that unsubscribing prevents further notifications from being received.
 func TestUnsubscribe(t *testing.T) {
-	notifs := NewNotifications(nil)
+	notifs := NewNotifications(10, nil)
 
 	// Subscribe to notifications.
-	sub, unsubscribe := notifs.Sub()
+	sub, unsubscribe, ok := notifs.Sub()
+	require.True(t, ok)
 
 	var wg sync.WaitGroup
 	wg.Add(1)
@@ -190,3 +194,30 @@ func TestUnsubscribe(t *testing.T) {
 	require.Len(t, receivedNotifications, 1, "Expected 1 notification before unsubscribe.")
 	require.Equal(t, "Test Notification 1", receivedNotifications[0].Text, "Unexpected notification text.")
 }
+
+// TestMaxSubscribers tests that exceeding the max subscribers limit prevents additional subscriptions.
+func TestMaxSubscribers(t *testing.T) {
+	maxSubscribers := 2
+	notifs := NewNotifications(maxSubscribers, nil)
+
+	// Subscribe the maximum number of subscribers.
+	_, unsubscribe1, ok1 := notifs.Sub()
+	require.True(t, ok1, "Expected first subscription to succeed.")
+
+	_, unsubscribe2, ok2 := notifs.Sub()
+	require.True(t, ok2, "Expected second subscription to succeed.")
+
+	// Try to subscribe more than the max allowed.
+	_, _, ok3 := notifs.Sub()
+	require.False(t, ok3, "Expected third subscription to fail due to max subscriber limit.")
+
+	// Unsubscribe one subscriber and try again.
+	unsubscribe1()
+
+	_, unsubscribe4, ok4 := notifs.Sub()
+	require.True(t, ok4, "Expected subscription to succeed after unsubscribing a subscriber.")
+
+	// Clean up the subscriptions.
+	unsubscribe2()
+	unsubscribe4()
+}

web/api/v1/api.go

@@ -215,7 +215,7 @@ type API struct {
 	isAgent             bool
 	statsRenderer       StatsRenderer
 	notificationsGetter func() []api.Notification
-	notificationsSub    func() (<-chan api.Notification, func())
+	notificationsSub    func() (<-chan api.Notification, func(), bool)
 
 	remoteWriteHandler http.Handler
 	remoteReadHandler  http.Handler
@@ -250,7 +250,7 @@ func NewAPI(
 	runtimeInfo func() (RuntimeInfo, error),
 	buildInfo *PrometheusVersion,
 	notificationsGetter func() []api.Notification,
-	notificationsSub func() (<-chan api.Notification, func()),
+	notificationsSub func() (<-chan api.Notification, func(), bool),
 	gatherer prometheus.Gatherer,
 	registerer prometheus.Registerer,
 	statsRenderer StatsRenderer,
@@ -1690,7 +1690,11 @@ func (api *API) notificationsSSE(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Connection", "keep-alive")
 
 	// Subscribe to notifications.
-	notifications, unsubscribe := api.notificationsSub()
+	notifications, unsubscribe, ok := api.notificationsSub()
+	if !ok {
+		w.WriteHeader(http.StatusNoContent)
+		return
+	}
 	defer unsubscribe()
 
 	// Set up a flusher to push the response to the client.

web/ui/mantine-ui/src/components/NotificationsProvider.tsx

@@ -42,7 +42,8 @@ export const NotificationsProvider: React.FC<{ children: React.ReactNode }> = ({
 
     eventSource.onerror = () => {
       eventSource.close();
-      setIsConnectionError(true);
+      // We do not call setIsConnectionError(true), we only set it to true if
+      // the fallback API does not work either.
       setShouldFetchFromAPI(true);
     };
 

web/web.go

@@ -268,7 +268,7 @@ type Options struct {
 	Notifier              *notifier.Manager
 	Version               *PrometheusVersion
 	NotificationsGetter   func() []api.Notification
-	NotificationsSub      func() (<-chan api.Notification, func())
+	NotificationsSub      func() (<-chan api.Notification, func(), bool)
 	Flags                 map[string]string
 
 	ListenAddresses            []string