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server.go
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server.go
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// Golang HTML5 Server Side Events Example
//
// Run this code like:
// > go run server.go
//
// Then open up your browser to http://localhost:8000
// Your browser must support HTML5 SSE, of course.
package main
import (
"fmt"
"html/template"
"log"
"net/http"
"time"
)
// A single Broker will be created in this program. It is responsible
// for keeping a list of which clients (browsers) are currently attached
// and broadcasting events (messages) to those clients.
type Broker struct {
// Create a map of clients, the keys of the map are the channels
// over which we can push messages to attached clients. (The values
// are just booleans and are meaningless.)
//
clients map[chan string]bool
// Channel into which new clients can be pushed
//
newClients chan chan string
// Channel into which disconnected clients should be pushed
//
defunctClients chan chan string
// Channel into which messages are pushed to be broadcast out
// to attahed clients.
//
messages chan string
}
// This Broker method starts a new goroutine. It handles
// the addition & removal of clients, as well as the broadcasting
// of messages out to clients that are currently attached.
func (b *Broker) Start() {
// Start a goroutine
//
go func() {
// Loop endlessly
//
for {
// Block until we receive from one of the
// three following channels.
select {
case s := <-b.newClients:
// There is a new client attached and we
// want to start sending them messages.
b.clients[s] = true
log.Println("Added new client")
case s := <-b.defunctClients:
// A client has dettached and we want to
// stop sending them messages.
delete(b.clients, s)
close(s)
log.Println("Removed client")
case msg := <-b.messages:
// There is a new message to send. For each
// attached client, push the new message
// into the client's message channel.
for s := range b.clients {
s <- msg
}
log.Printf("Broadcast message to %d clients", len(b.clients))
}
}
}()
}
// This Broker method handles and HTTP request at the "/events/" URL.
func (b *Broker) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// Make sure that the writer supports flushing.
//
f, ok := w.(http.Flusher)
if !ok {
http.Error(w, "Streaming unsupported!", http.StatusInternalServerError)
return
}
// Create a new channel, over which the broker can
// send this client messages.
messageChan := make(chan string)
// Add this client to the map of those that should
// receive updates
b.newClients <- messageChan
// Listen to the closing of the http connection via the Request.Context
notify := r.Context().Done()
go func() {
<-notify
// Remove this client from the map of attached clients
// when `EventHandler` exits.
b.defunctClients <- messageChan
log.Println("HTTP connection just closed.")
}()
// 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("Transfer-Encoding", "chunked")
// Don't close the connection, instead loop endlessly.
for {
// Read from our messageChan.
msg, open := <-messageChan
if !open {
// If our messageChan was closed, this means that the client has
// disconnected.
break
}
// Write to the ResponseWriter, `w`.
fmt.Fprintf(w, "data: Message: %s\n\n", msg)
// Flush the response. This is only possible if
// the repsonse supports streaming.
f.Flush()
}
// Done.
log.Println("Finished HTTP request at ", r.URL.Path)
}
// Handler for the main page, which we wire up to the
// route at "/" below in `main`.
func handler(w http.ResponseWriter, r *http.Request) {
// Did you know Golang's ServeMux matches only the
// prefix of the request URL? It's true. Here we
// insist the path is just "/".
if r.URL.Path != "/" {
w.WriteHeader(http.StatusNotFound)
return
}
// Read in the template with our SSE JavaScript code.
t, err := template.ParseFiles("templates/index.html")
if err != nil {
log.Fatal("Error parsing your template.")
}
// Render the template, writing to `w`.
t.Execute(w, "friend")
// Done.
log.Println("Finished HTTP request at", r.URL.Path)
}
// Main routine
func main() {
// Make a new Broker instance
b := &Broker{
make(map[chan string]bool),
make(chan (chan string)),
make(chan (chan string)),
make(chan string),
}
// Start processing events
b.Start()
// Make b the HTTP handler for "/events/". It can do
// this because it has a ServeHTTP method. That method
// is called in a separate goroutine for each
// request to "/events/".
http.Handle("/events/", b)
// Generate a constant stream of events that get pushed
// into the Broker's messages channel and are then broadcast
// out to any clients that are attached.
go func() {
for i := 0; ; i++ {
// Create a little message to send to clients,
// including the current time.
b.messages <- fmt.Sprintf("%d - the time is %v", i, time.Now())
// Print a nice log message and sleep for 5s.
log.Printf("Sent message %d ", i)
time.Sleep(5e9)
}
}()
// When we get a request at "/", call `handler`
// in a new goroutine.
http.Handle("/", http.HandlerFunc(handler))
// Start the server and listen forever on port 8000.
http.ListenAndServe(":8000", nil)
}