[Node.js]33. Level 7: Persisting Questions

Let's go back to our live-moderation app and add some persistence, first to the questions people ask.

Use the lpush command to add new questions to the list named questions. Do this inside thequestion listener.

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) {
  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);

        // add the question to the list here
        redisClient.lpush('questions', question);
      }
    });
  });
});

Now that we have questions stored in redis, let's emit them whenever a new client connects to the server through socket.io.

Use the lrange command to retrieve an array of questions that represent the questions list in redis. Inside of the lrange callback, use forEach to loop through each question and emit it on the client. Remember, don't use broadcast.emit because we only want to send the questions to the client that is connecting to the server.

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

  redisClient.lrange('questions', 0, -1, function(err, messages){
    messages.forEach(function(message){
        client.emit('question', message);
    });
  });

  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);
      }
    });
  });
});

Great work! One last thing though, since every time a new question comes in we store it in thequestions list, we might run into a problem where there are just too many questions stored in that list.

Add a callback to the lpush command, and inside that callback use the ltrim command to make sure the questions list always has at most 20 items.