code: function connect(mongoDBConnectionUrl, mongoDBConnectionOption, redisUrl, cb) {
    if (arguments.length < 4) throw new Error('connect requires 4 arguments');
  
    async.series([
      function connectToMongoDB(callback) {
        MongoClient.connect(mongoDBConnectionUrl, mongoDBConnectionOption || {}, callback);
      },
      function connectToRedis(callback) {
        var redisOpts = {
          url: redisUrl
        };
        var client = redis.createClient(redisOpts);
        // We don't want the app to crash if Redis is not available.
        client.on('error', handleRedisError);
        return callback(null, client);
      }
    ], function(err, results) {
      if (err) {
        // If there was any problem with connecting to mongo or redis on startup,
        // the app should crash as its in an unknown state.
        throw err;
      }
      var mongoDbClient = results[0];
      var redisClient = results[1];
  
      server.setClients(mongoDbClient, redisClient);
  
      syncConnected = true;
      eventEmitter.emit('sync:ready');
      return cb(null, mongoDbClient, redisClient);
    });
  }
docs: Connect to mongodb & redis with the given connection urls.
  This should be called before `start()` is called
  
  @param {string} mongoDBConnectionUrl A MongoDB connection uril in a format supported by the `mongodb` module
  @param {Object} mongoDBConnectionOption mongodb connection options.
  @param {string} redisUrl A redis connection url in a format supported by the `redis` module
  @param {function} cb