SSE message stream in Go

sse.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"net/http"

	"github.com/gorilla/mux"
)

// Example SSE server in Golang.
//     $ go run sse.go
// Inspired from https://gist.github.com/ismasan/3fb75381cd2deb6bfa9c

type Broker struct {

	// Events are pushed to this channel by the main events-gathering routine
	Notifier chan []byte

	// New client connections
	newClients chan chan []byte

	// Closed client connections
	closingClients chan chan []byte

	// Client connections registry
	clients map[chan []byte]bool
}

func NewServer() (broker *Broker) {
	// Instantiate a broker
	broker = &Broker{
		Notifier:       make(chan []byte, 1),
		newClients:     make(chan chan []byte),
		closingClients: make(chan chan []byte),
		clients:        make(map[chan []byte]bool),
	}

	// Set it running - listening and broadcasting events
	go broker.listen()

	return
}

type Message struct {
	Name    string `json:"name"`
	Message string `json:"msg"`
}

func (broker *Broker) Stream(w http.ResponseWriter, r *http.Request) {
	flusher, ok := w.(http.Flusher)

	if !ok {
		http.Error(w, "Streaming unsupported!", http.StatusInternalServerError)
		return
	}

	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(chan []byte)

	// Signal the broker that we have a new connection
	broker.newClients <- messageChan

	// Remove this client from the map of connected clients
	// when this handler exits.
	defer func() {
		broker.closingClients <- messageChan
	}()

	// Listen to connection close and un-register messageChan
	notify := w.(http.CloseNotifier).CloseNotify()

	go func() {
		<-notify
		broker.closingClients <- messageChan
	}()

	for {

		// Write to the ResponseWriter
		// Server Sent Events compatible
		fmt.Fprintf(w, "data: %s\n\n", <-messageChan)

		// Flush the data immediatly instead of buffering it for later.
		flusher.Flush()
	}

}

func (broker *Broker) listen() {
	for {
		select {
		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 dettached 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
			}
		}
	}

}

func (broker *Broker) BroadcastMessage(w http.ResponseWriter, r *http.Request) {
	// params := mux.Vars(r)
	var msg Message
	_ = json.NewDecoder(r.Body).Decode(&msg)

	// broker.Notifier <- []byte(fmt.Sprintf("the time is %v", msg))
	j, _ := json.Marshal(msg)
	broker.Notifier <- []byte(j)
	json.NewEncoder(w).Encode(msg)
}

func init() {

}
func main() {
	broker := NewServer()
	router := mux.NewRouter()

	router.HandleFunc("/messages", broker.BroadcastMessage).Methods("POST")

	router.HandleFunc("/stream", broker.Stream).Methods("GET")

	log.Fatal(http.ListenAndServe(":8000", router))

}

Hey, this is absolutely fantastic. Saved me hours (probably days!) of hassle & pain juggling channels and connect closing problems.
Thanks for sharing. Nice clear code too!

Hi @benc-uk thanks, this is actually an old example (also most of it is copied :D) I wanted to update it but forgot time to time (your comment made me aware of this!). We might want to use a sync.Map for the clients registry. And we can have channel example as in, subscribe to a /channel/<name> and publish to a /channel/<name> which is more useful example :D