Most languages which I start to learn, I’ve found I learn the basics of the language (enough to feel at relative ease with the language) but then want to see it in real world scenarios. One of those is usually a web API or the likes. So today I’m looking at using Elixir along with the Phoenix framework.

Note: I’m new to Elixir and Phoenix, this post is based upon my learnings, trying to get a basic web API/service working and there may be better ways to achieve this that I’m not aware of yet.

Phoenix is a way to (as they say on their site) to “build rich, interactive web applications”. Actually I find it builds too much as by default it will create a website, code for working with a DB (PostgresQL by default) etc. In this post I want to create something more akin to a web API or microservice.

If you’re after the default application, then run

mix phx.server

In this post I want to create a simple API service so instead we’ll use phx.new to create a service named my_api and well remove the website/HTML and ecto (the DB) side of things

mix phx.new my_api --no-html --no-ecto --no-mailer

If you run the command above you’ll get a new application generated. Just cd my_api to allow us to run the service etc.

If you’d like to see what the default generated application is then run the following

mix phx.server

By default this will start a server against localhost:4000. If you open the browser you’ll see a default dashboard/page which likely says there’s no route for GET / and then lists some available routes.

The /dev/dashboard route takes you to a nice LiveDashboard showing information about the Elixir and Phoenix.

To shutdown the Phoenix server CTRL+C twice within the terminal that you ran it up from.

For my very simple web service, I do not even what the live dashboard. So if you created that new app. delete your new app folder and then run this minimal code version (unless you’d prefer to keep live dashboard etc.)

mix phx.new my_api --no-html --no-ecto --no-mailer --no-dashboard --no-assets --no-gettext

This will then generate a fairly minimal server which is a good starting point for our service. You’ll notice first off that there are now, no routes when you run this via mix phx.server
.

Let’s add a controller, this will acts as the controller for our web service, so within the /lib/my_api_web/controllers folder add a new file named math-controller.ex and past the following code into it (obviously change the module name to suite your application name)

defmodule MyApiWeb.MathController do
  #use MyApiWeb, :controller
  use Phoenix.Controller, formats: [:html, :json]

  def index(conn, _params) do
   json(conn, "{name: Scooby}")
  end
end

We now need to hook up our controller to a route, so go to the router.ex file within the /lib/my_api_web/ folder and alter the scope section to look like this

scope "/", MyApiWeb do
  pipe_through :api

  resources "/api", MathController, except: [:new, :edit, :create, :delete, :update, :show]
end

If you run mix phx.server you should see a route to /api, typing http://localhost:4000/api will return “{name: Scooby}” as defined in the math-controller index. This is not very math-like so let’s create a couple of functions, one for adding numbers and one for subtracting.

Remove the resources section (or comment out using #) in the scope then add the following routes

get "/add", MathController, :add
get "/sub", MathController, :subtract

Go to the math-controler.ex and add the following functions

def add(conn, %{"a" => a, "b" => b}) do
  text(conn, String.to_integer(a) + String.to_integer(b))
end

def subtract(conn, %{"a" => a, "b" => b}) do
  text(conn, String.to_integer(a) - String.to_integer(b))
end

Notice we destructuring params to values a and b – we’ll convert those values to integers and use the text function to return raw text (previously we expected JSON hence uses the json function). Now when you browse the add method, for example http://localhost:4000/add?a=10&b=5 or subtract method, for example http://localhost:4000/sub?a=10&b=5 you should see raw text returned with answers to the math functions.

What routes are we exposing

Another useful way of checking the available routes (without running the server) is, as follows

mix phx.routes

Config

If you’ve looked around the generated code you’ll notice the config folder.

One thing you might like to do now is change localhost to 0.0.0.0 so edit dev.exs and replace

http: [ip: {127, 0, 0, 1}, port: 4000],

with

http: [ip: {0, 0, 0, 0}, port: 4000],

If you do NOT do this and you decide to deploy the dev release to Docker, you’ll find you cannot access your service from outside of Docker (which ofcourse is quite standard).

Releases

Generating a release will precompile any files that can be compiled and allows us to run the server without the source code (as you’d expect) you will need to tell the compiler what configuration to use, we do that by setting the MIX_ENV like this

export MIX_ENV=prod

(No MIX_ENV environment variable will default dev)

Then running

mix release

This will create and assemble your compiled files to _build/prod/rel/my_api/bin/my_api (obviously replacing the last part with your app name). The results of a release build show using

Note: Replace /prod/ with /dev/ above etc. as per the environment you’ve compiled for

_build/prod/rel/my_api/bin/my_api start

to start your application, this will need start a server. By default the above does not start a server so instead we need to set the following environment variable

export PHX_SERVER=true

You’ll also able to run following it will automatically generate the bin/server and sets the PHX_SERVER environment variable

mix phx.gen.release

One last thing, you may find when you use the start command (against PROD) that this fails saying you are missing the SECRET_KEY_BASE. We can generate this using

mix phx.gen.secret

Then simply

export SECRET_KEY_BASE=your-generated-key

This is for signing cookies etc. and you can see where the exception comes from within the runtime.exs file. This is set as an environment variable, best not to check into source control.

Dockerizing our service

Okay, it’s not Elixir specific, but I feel that the natural conclusion to our API/service development is to have it all running in a container. Let’s start by creating a container image based upon the build and using the phx.server call…

Create yourself a Dockerfile which looks like this

FROM elixir:latest

RUN mkdir /app
COPY . /app
WORKDIR /app

RUN mix local.hex --force
RUN mix do compile

EXPOSE 4000

CMD ["mix", "phx.server"]

I’m assuming we’re going to stick with port 4000 in the above and in the commands below, so I’ll document this via the EXPOSE command.

Now to build and run our container let’s use the following

docker build -t pp/my-api:0.1.0 .
docker run --rm --name my-api -p 4000:4000 -d pp/my-api:0.1.0

Now you should be able to uses http://localhost:4000 to access your shiny new Elixir/Phoenix API/service.

Note: Remember that if you cannot access the service outside of the docker image, ensure you’ve set the http ip in dev.exs to 0.0.0.0

If we want to instead containerize our release build then we could use the following

FROM elixir:latest

ENV PHX_SERVER=true

RUN mkdir /app
COPY /_build/dev/ /app
WORKDIR /app/rel/my_api/bin

EXPOSE 4000

CMD ["./my_api", "start"]

Again using the previous build and run commands, will start the server (if all went to plan).

Code

Code is available in my GitHub blog project repo.