server.js

const io = require('socket.io').listen(8001)
const redis = require('redis')

io.of('/notifications')
.on('connection', function (socket) {
  console.log('New connection: ' + socket.id)
  // Send the message of connection for receiving the user ID
  socket.emit('connected')

  // Receive the ID
  socket.on('join', function (userId) {
    const channel = 'push:notifications:' + userId
    console.log('Connecting to redis: ' + channel)

    // store in the socket our connection
    socket.redisClient = redis.createClient()
    socket.redisClient.subscribe(channel)

    // subscribe to our channel (We don't need to check because we have a
    // connection per channel/user)
    socket.redisClient.on('message', (channel, message) => {
      console.log(channel + ': ' + message)
      socket.emit('notification', channel, message)
    })
  })
})