This is the 4th in a series of posts leading up to Node.js Knockout on how to use Socket.IO. This post was written by socket.io creator Guillermo Rauch.

Knocking out Socket.IO

Ready to rock the Node Knockout 3? Considering making a real-time app or game? Then you're likely considering Socket.IO

Socket.IO makes realtime easy and cross-browser

If the web 2.0 was about AJAX requests and responses, the real-time web is about events.

With Socket.IO, you can emit events from the server to the client and vice-versa, at any time.

Consider the following example, in which we push data from the server to the client every 3 seconds. We're going to get the latest tweets and render them on the client.

On the server side, we create a directory called example/ with the following package.json inside:

{
    "name": "example"
  , "version": "0.0.2"
  , "dependencies": {
        "socket.io": "~0.9.10"
      , "request": "~2.11.4"
    }
}

Then run npm install. That will get Socket.IO and a library to make http requests (to Twitter) very simple to write.

Create an app.js with the following:

var sio = require('socket.io')
  , http = require('http')
  , index = require('fs').readFileSync(__dirname + '/index.html')
  , request = require('request')

var app = http.createServer(function (req, res) {
      res.writeHead(200, { 'Content-Type': 'text/html' });
      res.end(index);
    })
  , io = sio.listen(app);

app.listen(3000);

So far, we initialized a HTTP server that serves the index file, and we attach Socket.IO to it. We then make it listen on port 3000.

We're now gonna search twitter every 2 seconds to send tweets to the client. The clients don't explicitly request it like in traditional Ajax, but we push it to them:

setInterval(function () {
  request({
    url: 'http://search.twitter.com/search.json?q=today'
  }, function (err, res, data) {
    var obj = JSON.parse(data);
    io.sockets.emit('tweets', obj.results.map(function (v) { return v.text; }));
  });
}, 2000);

Notice that I accessed the io.sockets property. That's what we call the Manager of sockets, which also emits a connection event for individual people that connect:

io.sockets.on('connection', function (socket) {
  socket.broadcast.emit('someone connected');

  socket.on('some event', function () {
    console.log('I got an event');
  });
});

In this case, I'm making this socket broadcast to others that it connected. Broadcasting from the socket object means in this case we're sending a message to everyone else, except for that particular socket.

Now that we're done with the server part, we capture these events on the client and do some jQuery juggling. Create an index.html with this code:

<!doctype html>
<html>
  <head>
    <script src="/socket.io/socket.io.js"></script>
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>

    <script>
      var socket = io.connect();

      $(function () {
        socket.on('tweets', function (tweets) {
          $.each(tweets, function (i, tweet) {
            $('#tweets').prepend($('<li>').text(tweet));
          });
        });

        socket.on('someone connected', function () {
          $('body').prepend('<p>Someone just connected!</p>');
        });

        socket.emit('some event');
      });
    </script>
  </head>
  <body>
    <h1>Tweets</h1>
    <ul id="tweets"></ul>
  </body>
</html>

Putting sockets in rooms

Many times you need to emit information to certain people and not others. Use socket.join:

io.sockets.on('connection', function (socket) {
  socket.join('fighters');
});

Then you can broadcast and emit to those people:

// to all sockets in a group
io.sockets.in('fighters').emit('something');

// from a socket to the rest of the sockets
socket.broadcast.to('fighters').emit('something else');

Tools for game makers.

Remember the broadcast property that I accessed above? That's what we call a flag:

A useful one for game developers is called the volatile flag. If you ever have data that it's not absolutely crucial that the client gets if the client has not finished receiving previous messages, you can make that volatile:

socket.volatile.emit('position', x, y);

If the player changes its x and y coordinates really quickly, and we're not done sending a particular position, some packets will be dropped. This will make things faster in many scenarios.

Going beyond, with callbacks

On both ends (server and client side), you can pass a function to request a explicit data response. Consider an example where the chat client sets a nickname, but you want to validate whether it's available or not.

On the client:

socket.emit('set nickname', $('#my-input').val(), function (available) {
  if (available) {
    alert('Nick is available. Congrats!');
  } else {
    alert('Nick not available');
  }
});

On the server:

io.sockets.on('connection', function (socket) {
  socket.on('set nickname', function (fn) {
    isNickNameAvailable(function (bool) {
      fn(bool);
    });
  });
});

Wrapping up

You can check out the example on GitHub. Clone the repo, then run npm install and node app and point your browser to http://localhost:3000!

These are all the tools in your toolbox. Go make great realtime apps!

nko3/knocking-out-socket.io

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