Playing With Fire

Exploring the web one Elixir at a time

Phoenix - Building a Chat Server in 15 minutes

This post will try to show how quickly you can build a web application using Elixir and Phoenix.

For those that don’t know what Phoenix is, its a web application framework based on the MVC paradigm. In some ways its similar to Ruby on Rails (RoR). It has a similar structure, there are some generators for common code. Unlike RoR, Phoenix is fast - really fast and extremely stable The Road to 2 Million Websocket Connections in Phoenix.

I originally wrote this as a tutorial for my place of work to demonstrate how quickly applications can be built using Phoenix. Its set up in a number of steps.

This post was originally written for Phoenix v1, later versions seem to have a requirement for the DB to be configured.


Step 0

Make sure the following prerequisites are in place: Erlang, Elixir (see below), Node.JS, NPM, PostgreSQL. If you’re running linux you will need inotify-tools as well.

These are prerequisites of the framework, but we won’t be using any PostgreSQL connectivity in this example application.

Install Elixir

Quick installation instructions are given below

$ brew update
$ brew install elixir

$ wget
$ sudo dpkg -i erlang-solutions_1.0_all.deb
$ sudo apt-get update
$ sudo apt-get install elixir

Download installer from link provided
These should all install Erlang as a dependency. If this is not the case, go here:

Full instructions can be found here:

Install Phoenix

The following are quick installation instructions (I’m using a Mac, so you’re mileage might vary). Full instructions can be found at

On your command line/terminal

$ mix local.hex
$ mix archive.install


Step 1

With Elixir and Phoenix Framework installed we can now start building out the application.

Create new Phoenix Framework application

On your command line/terminal

$ cd ~/Projects
$ mix mychat

At the prompt type: y and then when complete change to the newly minted mychat dir

Fetch and install dependencies? [Yn] y
$ cd mychat

Update: Phoenix 1.2 requires DB set up

Update the config/dev.exs file with your DB credentials

config :myChat, MyChat.Repo,
  adapter: Ecto.Adapters.Postgres,
  username: "postgres",                 <---- PUT YOUR USER HERE
  password: "postgres",                 <---- PUT YOUR PASSWORD HERE
  database: "mychat_dev",
  hostname: "localhost",
  pool_size: 10

Once this is done run:

$ mix ecto.create

Fire phoenix up to make sure that all is in order:

$ mix phoenix.server

The first run will always take a little longer because it needs to compile everything.

If you receive this error:

error: Compiling of web/static/js/socket.js failed. Couldn't find preset "es2015" relative to directory "web/static/js" ; Compiling of web/static/js/app.js failed. Couldn't find preset "es2015" relative to directory "web/static/js"

then you need to add in the babel preset for ES2015 as follows:

npm install --save babel-preset-es2015

and then restart the server

$ mix phoenix.server

Open up a browser

Use your favourite web browser and navigate to http://localhost:4000

If successfully installed and running you should now see this in your browser

Phoenix home page


Step 2

Whilst Phoenix Framework is running in development mode, it automatically performs code reloads… (most of the time)

Prepare vendor js and css

Because I’m lazy, I’m going to short-cut some of the browser manipulations using jQuery. In order to use jQuery with Phoenix, it needs to be placed in the web/static/vendor folder. This is so that when brunch picks it up to compile, it doesnt try and push it through an ES6 transpiler.

Copy this file to web/static/vendor: jquery.min.js

Whilst you’re at it, replace the app.css file in web/static/css with the following file: app.css

As these are new assets, you will need to force a page reload in order to pull them down.

Add a new Controller

Add the following file: web/controllers/chat_controller.ex

defmodule Mychat.ChatController do
    use Mychat.Web, :controller

    def index(conn, _params) do
        render conn, "lobby.html"

Update Routing

Change the default scope in the routing file (web/router.ex)to the following

scope "/", Mychat do
    pipe_through :browser # Use the default browser stack
    get "/", ChatController, :index

Add a new View

Add the following file: web/views/chat_view.ex

defmodule Mychat.ChatView do
    use Mychat.Web, :view

Add a new Template

And this one: web/templates/chat/lobby.html.eex

<div id="messages" class="container">
<div id="footer">
    <div class="container">
        <div class="row">
            <div class="col-sm-2">
                <div class="input-group">
                    <span class="input-group-addon">@</span>
                    <input id="username" type="text" class="form-control" placeholder="username">
            <div class="col-sm-10">
                <input id="message-input" class="form-control" />

Update the Layout

If you were to check your browser now, you would see something that looks a little bit screwy.

This is because we’re still using the old layout.

Change web/templates/layout/app.html.eex to this:

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <meta name="description" content="">
    <meta name="author" content="">
    <title>My Chat App</title>
    <link rel="stylesheet" href="<%= static_path(@conn, "/css/app.css") %>">
    <div class="navbar navbar-default navbar-fixed-top" role="navigation">
        <div class="container">
            <div class="navbar-header">
                <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
                    <span class="sr-only">Toggle navigation</span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                    <span class="icon-bar"></span>
                <a class="navbar-brand" href="#">My Chat App</a>

    <%= render @view_module, @view_template, assigns %>

    <script src="<%= static_path(@conn, "/js/app.js") %>"></script>


All being well, you should now see in your browser something not too distant from this

MyChat Application window


Step 3

Lets prepare Phoenix Framework to operate through channels. A Channel is essentially a wrapper around websockets.

Update the endpoints file

Depending on the version of Phoenix Framework that you have installed, you might need to update the endpoints file (lib/mychat/endpoint.ex) with the following line

defmodule Mychat.Endpoint do
    use Phoenix.Endpoint, otp_app: :mychat

    socket "/socket", Mychat.UserSocket      <--- MAKE SURE THIS LINE IS IN PLACE


If you’re using the latest version of Phoenix then this should not be necessary, but its still good to check.

Update the user_socket file

This file (web/channels/user_socket.ex) is too complicated at the moment for what we need it for. Change it to this

defmodule Mychat.UserSocket do
    use Phoenix.Socket

    ## Channels
    channel "rooms:*", Mychat.RoomChannel

    ## Transports
    transport :websocket, Phoenix.Transports.WebSocket
    transport :longpoll, Phoenix.Transports.LongPoll

    def connect(_params, socket), do: {:ok, socket}

    def id(_socket), do: nil


Add in the room_channel file

You will notice that the file above makes reference to the Mychat.RoomChannel module - this currently doesn’t exist. If you check the browser at this point, it might have gotten all explody.

Add the web/channels/room_channel.ex file to change that

defmodule Mychat.RoomChannel do
    use Mychat.Web, :channel

    def join("rooms:lobby", message, socket) do
        Process.flag(:trap_exit, true)
        send(self, {:after_join, message})
        {:ok, socket}

    def join("rooms:" <> _something_else, _msg, _socket) do
        {:error, %{reason: "can't do this"}}

    def handle_info({:after_join, msg}, socket) do
        broadcast! socket, "user:entered", %{user: msg["user"]}
        push socket, "join", %{status: "connected"}
        {:noreply, socket}

    def terminate(_reason, _socket) do

    def handle_in("new:msg", msg, socket) do
        broadcast! socket, "new:msg", %{user: msg["user"], body: msg["body"]}
        {:reply, {:ok, %{msg: msg["body"]}}, assign(socket, :user, msg["user"])}


If you check the browser at this point, nothing much will have changed. We didn’t add any new pages.

The next section will add the client side of the conversation.


Step 4

Adding in the final JS. Time to get down with some ES2015.

Update socket.js

If you are at all nosey and have already taken a look at the directory structure of the project, you might have noticed the socket.js file. We’re going to update that file now in order to start wiring up this part of our two way conversation.

Its important to remember at this point that Phoenix Framework uses ES2015 out of the box, so to some this javascript might look a little odd…

Update web/static/js/socket.js to reflect the following code

import {Socket} from "phoenix"

let socket = new Socket("/socket")


export default socket

Create the worker file

Because I have a twisted sense of irony, I have called the file that holds the code that does all the work gossip.js.

Add this in next to the socket.js file that you amended above, in web/static/js/.

class Gossip {

    static init(socket){
        var $status    = $("#status")
        var $messages  = $("#messages")
        var $input     = $("#message-input")
        var $username  = $("#username")

        socket.onOpen( ev => console.log("OPEN", ev) )
        socket.onError( ev => console.log("ERROR", ev) )
        socket.onClose( e => console.log("CLOSE", e) )

        var chan ="rooms:lobby", {})
            .receive("ignore", () => console.log("auth error"))
            .receive("ok", () => console.log("join ok"))
            .receive("timeout", () => console.log("Connection interruption"))
        chan.onError(e => console.log("something went wrong", e))
        chan.onClose(e => console.log("channel closed", e))

        $"keypress").on("keypress", e => {
            if (e.keyCode == 13) {
                chan.push("new:msg", {user: $username.val(), body: $input.val()}, 10000)

        chan.on("new:msg", msg => {
            scrollTo(0, document.body.scrollHeight)

        chan.on("user:entered", msg => {
            var username = this.sanitize(msg.user || "anonymous")
            $messages.append(`<br/><i>[${username} entered]</i>`)

    static sanitize(html){ return $("<div/>").text(html).html() }

    static messageTemplate(msg){
        let username = this.sanitize(msg.user || "anonymous")
        let body     = this.sanitize(msg.body)

        return(`<p><a href='#'>[${username}]</a>&nbsp; ${body}</p>`)


export default Gossip

Complete the imports and start the process

The last code change that needs to be made is as follows - in the app.js file, update the content to

import "phoenix_html"

// Import local files
// Local files can be imported directly using relative
// paths "./socket" or full ones "web/static/js/socket".

import socket from "./socket"
import Gossip from "./gossip"


If you return to your browser session (you’ll probably need to refresh the page), you will now be able to enter a name and a message. When the message box loses focus, the main section of the page will update.

Whats more, if you open more than one browser session to the application you’ll be able to mimic multiple users talking to each other.


The End

Thats it. Really. A working chat application that allows people to talk to one another. Granted its not Skype, but its a start.