Use server-sent-events implementation from thoughtbot

New endpoint action "action=events" opens an SSE channel.

Implementation based on https://thoughtbot.com/blog/writing-a-server-sent-events-server-in-go

cmd/eksterd/main.go

@@ -116,11 +116,13 @@ func NewApp(options AppOptions) *App {
 		Backend: app.backend,
 	})
 
-	handler := server.NewMicrosubHandler(app.backend)
+	handler, broker := server.NewMicrosubHandler(app.backend)
 	if options.AuthEnabled {
 		handler = WithAuth(handler, app.backend)
 	}
 
+	app.backend.broker = broker
+
 	http.Handle("/microsub", handler)
 
 	http.Handle("/incoming/", &incomingHandler{

cmd/eksterd/memory.go

@@ -35,6 +35,7 @@ import (
 	"p83.nl/go/ekster/pkg/auth"
 	"p83.nl/go/ekster/pkg/fetch"
 	"p83.nl/go/ekster/pkg/microsub"
+	"p83.nl/go/ekster/pkg/server"
 	"p83.nl/go/ekster/pkg/util"
 
 	"github.com/gomodule/redigo/redis"
@@ -59,7 +60,7 @@ type memoryBackend struct {
 	ticker *time.Ticker
 	quit   chan struct{}
 
-	listeners []microsub.EventListener
+	broker *server.Broker
 }
 
 type channelSetting struct {
@@ -646,7 +647,12 @@ func matchItemText(item microsub.Item, re *regexp.Regexp) bool {
 
 func (b *memoryBackend) channelAddItem(channel string, item microsub.Item) error {
 	timelineBackend := b.getTimeline(channel)
-	return timelineBackend.AddItem(item)
+	err := timelineBackend.AddItem(item)
+
+	// Sent message to Server-Sent-Events
+	b.broker.Notifier <- server.Message{Event: "new item", Object: item}
+
+	return err
 }
 
 func (b *memoryBackend) updateChannelUnreadCount(channel string) error {
@@ -717,17 +723,6 @@ func Fetch2(fetchURL string) (*http.Response, error) {
 	return cachedResp, err
 }
 
-func (b *memoryBackend) sendMessage(msg microsub.Message) {
-	for _, l := range b.listeners {
-		l.WriteMessage(microsub.Event{Msg: msg})
-	}
-}
-
-func (b *memoryBackend) AddEventListener(el microsub.EventListener) error {
-	b.listeners = append(b.listeners, el)
-	return nil
-}
-
 func (b *memoryBackend) createChannel(name string) microsub.Channel {
 	uid := fmt.Sprintf("%012d", b.NextUid)
 	channel := microsub.Channel{

pkg/microsub/protocol.go

@@ -145,8 +145,6 @@ type Microsub interface {
 
 	Search(query string) ([]Feed, error)
 	PreviewURL(url string) (Timeline, error)
-
-	AddEventListener(el EventListener) error
 }
 
 func (unread Unread) MarshalJSON() ([]byte, error) {

pkg/server/events.go

@@ -1,51 +1,74 @@
 package server
 
 import (
-	"encoding/json"
-	"fmt"
-	"net"
-	"time"
-
-	"p83.nl/go/ekster/pkg/microsub"
+	"log"
 )
 
-type Consumer struct {
-	conn   net.Conn
-	output chan microsub.Message
+// A MessageChan is a channel of channels
+// Each connection sends a channel of bytes to a global MessageChan
+// The main broker listen() loop listens on new connections on MessageChan
+// New event messages are broadcast to all registered connection channels
+type MessageChan chan Message
+
+type Message struct {
+	Event  string
+	Object interface{}
 }
 
-func newConsumer(conn net.Conn) *Consumer {
-	cons := &Consumer{conn, make(chan microsub.Message)}
+// A Broker holds open client connections,
+// listens for incoming events on its Notifier channel
+// and broadcast event data to all registered connections
+type Broker struct {
+	// Events are pushed to this channel by the main UDP daemon
+	Notifier chan Message
 
-	fmt.Fprint(conn, "HTTP/1.0 200 OK\r\n")
-	fmt.Fprint(conn, "Content-Type: text/event-stream\r\n")
-	fmt.Fprint(conn, "Access-Control-Allow-Origin: *\r\n")
-	fmt.Fprint(conn, "\r\n")
+	// New client connections
+	newClients chan MessageChan
 
-	go func() {
-		ticker := time.NewTicker(10 * time.Second).C
+	// Closed client connections
+	closingClients chan MessageChan
+
+	// Client connections registry
+	clients map[MessageChan]bool
+}
+
+// Listen on different channels and act accordingly
+func (broker *Broker) listen() {
 	for {
 		select {
-			case <-ticker:
-				fmt.Fprint(conn, `event: ping`)
-				fmt.Fprint(conn, "\r\n")
-				fmt.Fprint(conn, "\r\n")
-
-			case msg := <-cons.output:
-				fmt.Fprint(conn, `event: message`)
-				fmt.Fprint(conn, "\r\n")
-				fmt.Fprint(conn, `data:`)
-				json.NewEncoder(conn).Encode(msg)
-				fmt.Fprint(conn, "\r\n")
-				fmt.Fprint(conn, "\r\n")
+		case s := <-broker.newClients:
+			// A new client has connected.
+			// Register their message channel
+			broker.clients[s] = true
+			log.Printf("Client added. %d registered clients", len(broker.clients))
+		case s := <-broker.closingClients:
+			// A client has detached and we want to
+			// stop sending them messages.
+			delete(broker.clients, s)
+			log.Printf("Removed client. %d registered clients", len(broker.clients))
+		case event := <-broker.Notifier:
+			// We got a new event from the outside!
+			// Send event to all connected clients
+			for clientMessageChan := range broker.clients {
+				clientMessageChan <- event
 			}
 		}
-		conn.Close()
-	}()
-
-	return cons
 	}
 
-func (cons *Consumer) WriteMessage(evt microsub.Event) {
-	cons.output <- evt.Msg
+}
+
+// Broker factory
+func NewServer() (broker *Broker) {
+	// Instantiate a broker
+	broker = &Broker{
+		Notifier:       make(chan Message, 1),
+		newClients:     make(chan MessageChan),
+		closingClients: make(chan MessageChan),
+		clients:        make(map[MessageChan]bool),
+	}
+
+	// Set it running - listening and broadcasting events
+	go broker.listen()
+
+	return
 }

pkg/server/microsub.go

@@ -37,6 +37,7 @@ const (
 
 type microsubHandler struct {
 	backend microsub.Microsub
+	Broker  *Broker
 }
 
 func respondJSON(w http.ResponseWriter, value interface{}) {
@@ -50,8 +51,9 @@ func respondJSON(w http.ResponseWriter, value interface{}) {
 	}
 }
 
-func NewMicrosubHandler(backend microsub.Microsub) http.Handler {
-	return &microsubHandler{backend}
+func NewMicrosubHandler(backend microsub.Microsub) (http.Handler, *Broker) {
+	broker := NewServer()
+	return &microsubHandler{backend, broker}, broker
 }
 
 func (h *microsubHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -106,9 +108,56 @@ func (h *microsubHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				"items": following,
 			})
 		} else if action == "events" {
-			conn, _, _ := w.(http.Hijacker).Hijack()
-			cons := newConsumer(conn)
-			h.backend.AddEventListener(cons)
+			// Make sure that the writer supports flushing.
+			//
+			flusher, ok := w.(http.Flusher)
+
+			if !ok {
+				http.Error(w, "Streaming unsupported!", http.StatusInternalServerError)
+				return
+			}
+
+			// Set the headers related to event streaming.
+			w.Header().Set("Content-Type", "text/event-stream")
+			w.Header().Set("Cache-Control", "no-cache")
+			w.Header().Set("Connection", "keep-alive")
+			w.Header().Set("Access-Control-Allow-Origin", "*")
+
+			// Each connection registers its own message channel with the Broker's connections registry
+			messageChan := make(MessageChan)
+
+			// Signal the broker that we have a new connection
+			h.Broker.newClients <- messageChan
+
+			// Remove this client from the map of connected clients
+			// when this handler exits.
+			defer func() {
+				h.Broker.closingClients <- messageChan
+			}()
+
+			// Listen to connection close and un-register messageChan
+			notify := w.(http.CloseNotifier).CloseNotify()
+
+			go func() {
+				<-notify
+				h.Broker.closingClients <- messageChan
+			}()
+
+			// block waiting or messages broadcast on this connection's messageChan
+			for {
+				// Write to the ResponseWriter, Server Sent Events compatible
+				message := <-messageChan
+				output, err := json.Marshal(message.Object)
+				if err != nil {
+					log.Println(err)
+					continue
+				}
+				fmt.Fprintf(w, "event: %s\n", message.Event)
+				fmt.Fprintf(w, "data: %s\n\n", output)
+
+				// Flush the data immediately instead of buffering it for later.
+				flusher.Flush()
+			}
 		} else {
 			http.Error(w, fmt.Sprintf("unknown action %s\n", action), 400)
 			return