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Example SSE server in Golang
// v2 of the great example of SSE in go by @ismasan.
// includes fixes:
// * infinite loop ending in panic
// * closing a client twice
// * potentially blocked listen() from closing a connection during multiplex step.
package main
import (
"fmt"
"log"
"net/http"
"time"
)
// the amount of time to wait when pushing a message to
// a slow client or a client that closed after `range clients` started.
const patience time.Duration = time.Second*1
// Example SSE server in Golang.
// $ go run sse.go
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
}
func (broker *Broker) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
// Make sure that the writer supports flushing.
//
flusher, ok := rw.(http.Flusher)
if !ok {
http.Error(rw, "Streaming unsupported!", http.StatusInternalServerError)
return
}
rw.Header().Set("Content-Type", "text/event-stream")
rw.Header().Set("Cache-Control", "no-cache")
rw.Header().Set("Connection", "keep-alive")
rw.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 := rw.(http.CloseNotifier).CloseNotify()
for {
select {
case <-notify:
return
default:
// Write to the ResponseWriter
// Server Sent Events compatible
fmt.Fprintf(rw, "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 {
select {
case clientMessageChan <- event:
case <-time.After(patience):
log.Print("Skipping client.")
}
}
}
}
}
func main() {
broker := NewServer()
go func() {
for {
time.Sleep(time.Second * 2)
eventString := fmt.Sprintf("the time is %v", time.Now())
log.Println("Receiving event")
broker.Notifier <- []byte(eventString)
}
}()
log.Fatal("HTTP server error: ", http.ListenAndServe("localhost:3000", broker))
}
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