Introduction to Mix
In this guide, we will learn how to build a complete Elixir application, with its own supervision tree, configuration, tests and more.
The application works as a distributed key-value store. We are going to organize key-value pairs into buckets and distribute those buckets across multiple nodes. We will also build a simple client that allows us to connect to any of those nodes and send requests such as:
CREATE shopping OK PUT shopping milk 1 OK PUT shopping eggs 3 OK GET shopping milk 1 OK DELETE shopping eggs OK
In order to build our key-value application, we are going to use three main tools:
OTP (Open Telecom Platform) is a set of libraries that ships with Erlang. Erlang developers use OTP to build robust, fault-tolerant applications. In this chapter we will explore how many aspects from OTP integrate with Elixir, including supervision trees, event managers and more;
Mix is a build tool that ships with Elixir that provides tasks for creating, compiling, testing your application, managing its dependencies and much more;
ExUnit is a test-unit based framework that ships with Elixir;
In this chapter, we will create our first project using Mix and explore different features in OTP, Mix and ExUnit as we go.
Note: this guide requires Elixir v1.2.0 or later. You can check your Elixir version with
elixir --versionand install a more recent version if required by following the steps described in the first chapter of the Getting Started guide.
If you have any questions or improvements to the guide, please reach discussion channels such as the Elixir Forum or the issues tracker. Your input is really important to help us guarantee the guides are accessible and up to date!
The final code for this guide is in this repository and can be used as a reference.
Our first project
When you install Elixir, besides getting the
iex executables, you also get an executable Elixir script named
Let’s create our first project by invoking
mix new from the command line. We’ll pass the project name as argument (
kv, in this case), and tell Mix that our main module should be the all-uppercase
KV, instead of the default, which would have been
$ mix new kv --module KV
Mix will create a directory named
kv with a few files in it:
* creating README.md * creating .gitignore * creating mix.exs * creating config * creating config/config.exs * creating lib * creating lib/kv.ex * creating test * creating test/test_helper.exs * creating test/kv_test.exs
Let’s take a brief look at those generated files.
Note: Mix is an Elixir executable. This means that in order to run
mix, you need to have Elixir’s executable in your PATH. If not, you can run it by passing the script as argument to
$ bin/elixir bin/mix new kv --module KV
Note that you can also execute any script in your PATH from Elixir via the -S option:
$ bin/elixir -S mix new kv --module KV
When using -S,
elixirfinds the script wherever it is in your PATH and executes it.
A file named
mix.exs was generated inside our new project folder (
kv) and its main responsibility is to configure our project. Let’s take a look at it (comments removed):
defmodule KV.Mixfile do use Mix.Project def project do [app: :kv, version: "0.1.0", elixir: "~> 1.3", build_embedded: Mix.env == :prod, start_permanent: Mix.env == :prod, deps: deps()] end def application do [extra_applications: [:logger]] end defp deps do  end end
mix.exs defines two public functions:
project, which returns project configuration like the project name and version, and
application, which is used to generate an application file.
There is also a private function named
deps, which is invoked from the
project function, that defines our project dependencies. Defining
deps as a separate function is not required, but it helps keep the project configuration tidy.
Mix also generates a file at
lib/kv.ex with a simple module definition:
defmodule KV do end
This structure is enough to compile our project:
$ cd kv $ mix compile
Compiling 1 file (.ex) Generated kv app
lib/kv.ex file was compiled, an application manifest named
kv.app was generated and all protocols were consolidated as described in the Getting Started guide. All compilation artifacts are placed inside the
_build directory using the options defined in the
Once the project is compiled, you can start an
iex session inside the project by running:
$ iex -S mix
Mix also generated the appropriate structure for running our project tests. Mix projects usually follow the convention of having a
<filename>_test.exs file in the
test directory for each file in the
lib directory. For this reason, we can already find a
test/kv_test.exs corresponding to our
lib/kv.ex file. It doesn’t do much at this point:
defmodule KVTest do use ExUnit.Case doctest KV test "the truth" do assert 1 + 1 == 2 end end
It is important to note a couple things:
the test file is an Elixir script file (
.exs). This is convenient because we don’t need to compile test files before running them;
we define a test module named
ExUnit.Caseto inject the testing API and define a simple test using the
Mix also generated a file named
test/test_helper.exs which is responsible for setting up the test framework:
This file will be automatically required by Mix every time before we run our tests. We can run tests with
Compiled lib/kv.ex Generated kv app [...] . Finished in 0.04 seconds (0.04s on load, 0.00s on tests) 1 test, 0 failures Randomized with seed 540224
Notice that by running
mix test, Mix has compiled the source files and generated the application file once again. This happens because Mix supports multiple environments, which we will explore in the next section.
Furthermore, you can see that ExUnit prints a dot for each successful test and automatically randomizes tests too. Let’s make the test fail on purpose and see what happens.
Change the assertion in
test/kv_test.exs to the following:
assert 1 + 1 == 3
mix test again (notice this time there will be no compilation):
1) test the truth (KVTest) test/kv_test.exs:5 Assertion with == failed code: 1 + 1 == 3 lhs: 2 rhs: 3 stacktrace: test/kv_test.exs:6 Finished in 0.05 seconds (0.05s on load, 0.00s on tests) 1 test, 1 failure
For each failure, ExUnit prints a detailed report, containing the test name with the test case, the code that failed and the values for the left-hand side (lhs) and right-hand side (rhs) of the
In the second line of the failure, right below the test name, there is the location where the test was defined. If you copy the test location in this full second line (including the file and line number) and append it to
mix test, Mix will load and run just that particular test:
$ mix test test/kv_test.exs:5
This shortcut will be extremely useful as we build our project, allowing us to quickly iterate by running just a specific test.
Finally, the stacktrace relates to the failure itself, giving information about the test and often the place the failure was generated from within the source files.
Mix supports the concept of “environments”. They allow a developer to customize compilation and other options for specific scenarios. By default, Mix understands three environments:
:dev- the one in which Mix tasks (like
compile) run by default
:test- used by
:prod- the one you will use to run your project in production
The environment applies only to the current project. As we will see later on, any dependency you add to your project will by default run in the
Customization per environment can be done by accessing the
Mix.env function in your
mix.exs file, which returns the current environment as an atom. That’s what we have used in both
def project do [..., build_embedded: Mix.env == :prod, start_permanent: Mix.env == :prod, ...] end
When you compile your source code, Elixir compiles artifacts to the
_build directory. However, in many occasions to avoid unnecessary copying, Elixir will create filesystem links from
_build to actual source files. When true,
:build_embedded disables this behaviour as it aims to provide everything you need to run your application inside
Similarly, when true, the
:start_permanent option starts your application in permanent mode, which means the Erlang VM will crash if your application’s supervision tree shuts down. Notice we don’t want this behaviour in dev and test because it is useful to keep the VM instance running in those environments for troubleshooting purposes.
Mix will default to the
:dev environment, except for the
test task that will default to the
:test environment. The environment can be changed via the
MIX_ENV environment variable:
$ MIX_ENV=prod mix compile
Or on Windows:
> set "MIX_ENV=prod" && mix compile
There is much more to Mix, and we will continue to explore it as we build our project. A general overview is available on the Mix documentation.
Keep in mind that you can always invoke the help task to list all available tasks:
$ mix help
You can get further information about a particular task by invoking
mix help TASK.
Let’s write some code!