A tale of friendly competition and Server-Sent Events

Me

Espen Hovlandsdal

Developer at VG

I believe in Javascript

espen.codes

Background

is a broadcaster

Traditionally, journalists ➜ users

"Push" makes more sense than "poll"

More and more "realtime" services

is a broadcaster

Use case is usually very similar:

Broadcast some arbitrary data

The tale begins...

Websockets is starting to sound like a good idea

Spend 10% of time
on a prototype

"Websockets are awesome, but does it scale?"

var app = require('express')();
var http = require('http').Server(app);
var io = require('socket.io')(http);

http.listen(3000, function() {
    console.log('listening on *:3000');
});

setInterval(function() {
    io.emit('ping', { timestamp: Date.now() });
}, 5000);

Server

// On VG.no:

var chance = 50;
if ((Math.random() * 100) < chance) {
    var script = d.createElement('script');
    script.type = 'text/javascript';
    script.async = true;
    script.src = '<url-to-client-script>.js';
}

Client

Result? Not encouraging

Latency spikes
High CPU usage
Hard to debug
Server "crashed"
Monster server,
~60k connection cap?

Some time later...

Let's investigate

WSS: Not HTTP

handshake resembles HTTP

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13
Origin: http://example.com

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Sec-WebSocket-Protocol: chat

Full-duplex

On top of TCP
Port 80/443

6 different protocols

RFC since
Dec 2011

Proxies and firewalls still cause issues

Server-Sent Events?

One-way -
Server ➜ Client

Extremely simple,
HTTP-based

GET /chat HTTP/1.1
Host: server.example.com
Accept: text/event-stream


HTTP/1.1 200 OK
Connection: Keep-Alive
Content-Type: text/event-stream
Cache-Control: no-cache

id: 123
data: some arbitrary data

id: 124
data: {"we could also":"push json"}

Additional features