🟢 Node -

WebSockets

WebSockets allow duplexed connection between client and server. Module socket.io gives abstract WebSockets with fallbacks.

Basic connection:

// SERVER
var express = require('express');
var app = express.createServer();
var socket = require('socket.io');
var io = socket.listen(app);

io.sockets.on('connection', function(client) {

  console.log('Client connected');

  client.on('answer', function(question, answer) {
    client.broadcast.emit('answer', question, answer);
  });

  // When client sends event named question.
  client.on('question', function(question) {

    // See if this client has already asked a question.
    client.get('question_asked', function(err, asked) {

      // If has not asked, send question to all other clients
      if ( ! asked ) {
        client.broadcast.emit('question', question);
        client.set('question_asked', true);
      }

    });

  });

});

<!-- CLIENT -->
<script src="/socket.io/socket.io.js"></script>
<script>

    var server = io.connect('http://localhost:8080');

    server.on('question', function(question) {
        insertQuestion(question);
    });

    server.on('answer', function(question, answer) {
        answerQuestion(question, answer);
    });

</script>

Using redis to as persisting data storage with socket.io

    npm install redis

var redis = require('redis');
var client = redis.createClient();

client.set('message1', 'hello!');
client.set('message2', 'yo!');

// Non-blocking!
client.get('message1', function(error, reply) {
  console.log(reply);
});

More complex:

var express = require('express');
var app = express.createServer();
var socket = require('socket.io');
var io = socket.listen(app);

var redis = require('redis');
var redisClient = redis.createClient();

io.sockets.on('connection', function(client) {

  // Send saved questions to new clients.
  redisClient.lrange('questions', 0, -1, function(err, questions) {
    questions.forEach(function(question) {
      client.emit('question', question);
    });
  })

  // On 'answer', sent same event to all other clients.
  client.on('answer', function(question, answer) {
    client.broadcast.emit('answer', question, answer);
  });

  //
  client.on('question', function(question) {

    client.get('question_asked', function(asked) {

      if ( ! asked ) {
        client.set('question_asked', true);
        client.broadcast.emit('question', question);

        redisClient.lpush(
          'questions',
          question,
          function(error, reply) {
            redisClient.ltrim('questions', 0, 19)
          }
        );
      }

    });
  });

});