Elixir: Your package should survive in production!

  • Dmitry Rubinstein
  • 30 November 2016
  • Comments

You are very good person! Great person! You have just created you first (or second, or third, etc.) package in Elixir language and published it to the Hex. You are nice enough to follow some conventions. At least this one.

The work is done, isn’t it? You’ve done everything as in official tutorial™. What problems can it lead to? “Nothing” is wrong answer…

Do you want to know why? Well… I’ll try to answer this question.

Your mix.exs is wrong!

Of course it is. Let’s find some problems!

Build consolidation and application termination

Take a look at the function project/0 in our Hex tutorial:

def project do
  [app: :postgrex,
   version: "0.1.0",
   elixir: "0.13.0",
   description: description(),
   package: package(),
   deps: deps()]

Probably, you decided to follow this tutorial literally. And deleted some incredibly important lines from project/0 function.

[build_embedded: Mix.env == :prod,
 start_permanent: Mix.env == :prod]

:buid_embedded tells our compiler to consolidate our project with all it’s dependencies in _build directory. Moreover it consolidates all protocols. It’s not needed in you development environment, while all dependencies are installed on your working machine, but imagine that you miss some of them on production server without Elixir or Erlang installation?

:start_permanent tells your virtual machine to terminate after your application is terminated. This is not necessary in development, because you need your iex to survive even when your application dies. But in production you should supervise your application at the point of the whole virtual machine too. So, it’s good when it crashes with your application.

What is application’s behaviour in production without these lines? Nobody knows. May be OK. May be not. Just don’t forget to put them in your configuration, of course if they needed.

What’s in your application/0?

The second critical step. application/0 function. In tutorial it’s empty:

def application do

Is it empty in your code? Seems to me, that no. You have there :logger, maybe :httpoison or :ecto, and without these deps your application even doesn’t start.

def application do
  [applications: [:logger, :httpoison, :gproc]]

The questions are:

  • Should you add mod of your own application?
  • Or, maybe you should remove :logger from :applications list?

This is the problem for your brains. You can solve it - but you should know, that this is a problem.

Don’t add :mod of your application in application/0 function, if the main module of your application doesn’t have start_link or start functions! That is redundantly, and can cause bad effects in the future.

And of course, fix your README. Don’t force package users to add your application in applications list. Mix will try to start you package before user’s application, but how will it perform that without start function? Nobody knows.

The answer on the second question is simple - remove everything, that is not needed to be started before your application. Some packages are not startable - they doesn’t have start or start_link functions in application module. How will Mix start them? You already know the answer.

Update (thanks to George Guimarães):

Elixir v1.4 changed the way you should list deps. Check it out here. You still have to think about applications that shouldn’t be started at runtime, and list application from OTP dist that you use.

Look at you deps/0!

I can bet that your package uses some dependencies. They are defined in deps/0 function, and of course they are up to date. The problem is that then next day after you published the package your deps are NOT up to date! Welcome to the reality!

You decided to use your friend Bob’s package, and this is your deps/0 function:

defp deps do
  [{:bobs_lib, "= 0.2.1",
    :ex_doc, github: "elixir-lang/ex_doc"}]

Pretty good! But Bob unexpectedly released new bugfix with version v0.2.2. And your friend Alex started to make package 2 days after you. And this is his deps/0 (he took data from Hex - no place for mistake!)

defp deps do
  [{:bobs_lib, "= 0.2.2",
    :ex_doc, github: "elixir-lang/ex_doc"}]

What would be with Mix, if it’s try to resolve mix deps.get in project with yours and Alex’s packages?

Yes… you already know the answer.

So. Please. Be careful with your deps versioning. To be familiar with them - read here

But let’s return to Alex’s code:

defp deps do
  [{:bobs_lib, "= 0.2.2",
    :ex_doc, github: "elixir-lang/ex_doc"}]

Does he really needs this :ex_doc package in release? It’s just a junk in production. It can increase deployment package size. But can’t increase anything else.

So, we can use it just in deployment with:

{:ex_doc, "~> 0.x.y", only: :dev}

Why don’t to do that, if we can? Just be patient to your package users - don’t force them to pack all junk in there releases!

Going deeper. Hold on: production is coming.

Today the most common way to generate releases - Distillery. This is easy to use, up-to-date, and almost silver-bullet library, that helps with making a candy from your Elixir application.

If you think that using Distillery has no sense, your users may think different. They will try to build there applications, fail because of your package, and come to your Github’s Issues tab. Ciao free time!

Of course, we can avoid this nerves.

Distillery doesn’t know your Erlang deps!

As you know, you can use Erlang modules in your code simply calling them via atom. For example :crypto.ec_curves, :observer.start or :calendar.local_time. While doing this operation, you don’t list Erlang modules as deps in your deps/0 function. And this is the anthill!

Distillery doesn’t check the code of your application trying to find Erlang module calls. It just consolidates your deps from deps and :applications list. So your modules :crypto and :calendar would not be in release. Will application work? Of course not. What is the solution to this problem?

Well, you should explicitly put all this Erlang deps in your :applications list. Even if you knows that they are not startable. This hack is dirty. But it works. And your package also works in production.

If you know better solution - use it and notify me in comments, so I can fix this article

Release doesn’t have Mix application!

Forget about custom Mix tasks and all functions from Mix module in your code. They gonna fail in production. There are some alternatives that you can read about in Distillery docs.

Your config.exs compiles into release!

Yes, it is. You can’t change configuration after you released your program. For example, you have different machines for :dev, :build, :test and :prod purposes. They all have different configuration. But after you build the release on your build machine… fairy-tale goes bad. Config now is nailed to your release. So… what to do?

Well, obviously programmers are clever people. And you know how to deal with that problem, aren’t you? Environment variables! And have all problems gone?… No

How do you read environments? Via System.get_env? But values in config.exs are evaluated during the build time. And System.get_env will be derived for your current environments! The same with Application.get_env! What is the solution?

For first, don’t use System.get_env or Application.get_env in your config files. There current values will be nailed.

Other clever people decided - we will use tuples for system variables like this:

config :my_application,
  my_key: {:system, "ENV_VAR_NAME"}

Thus, this tuple can be evaluated in runtime. Of course, you can configure some keys in old style way.

You can ask - but how is this connected to my package, if I don’t want to use and don’t use environment? Well, look here:

config :my_application,
  my_key: {:system, "ENV_VAR_NAME"}

config :your_library,
  your_key: {:system, "ENV_YOUR_KEY_NAME"}

What will be the result of Application.get_env({:system, "ENV_YOUR_KEY_NAME"}) in release? Yes… you know the answer.

So… what to do? Get prepared!. Because this approach has backward compatibility - implement it in your package! And if once somebody very clever will try to use your package as a nunchaku - it will work! Here is the package to help you (probably you can find some more with this functionality or create one by yourself).

And read about config in releases here.


Well, my dear alchemist. I hope now you are ready to enter the Hex.pm hall of fame! And don’t loose your free time removing curses from users of your package!

Read this article before each mix hex.publish execution.

Comment this article to help me keep it up-to date.

Special thanks to @noma4i for pointing the area of the problem. Big ups.

Dmitry Rubinstein

Elixir Developer, Architect and Evangelist. Has more then 5 years in Elixir production development, and half a dozen Hex packages under maintaining