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Hot code-reloading

By default, apps are restarted when new versions are deployed. This is to make it easy for people to deploy apps quickly with quick setup.

To enable

  • Set the deploy_type variable to upgrade in your project's playbooks/vars/main.yml file
  • Everytime you deploy, update the app's version in the project's mix.exs

Automatic versioning using git

For hot code-reloading, the app's version needs to be updated in mix.exs for every deploy. That can get repeatitive, since hobby projects are updated & deployed frequently. We've got a workaround for that. We'll use automatic versioning based on git commit SHAs. Technical details are explained in this blog post.

  • Change the following in mix.exs
def project do
  [app: :hello_phoenix,
   version: "1.4.1",
   elixir: "~> 1.0",
   ...

to look like the following

def project do
  {result, _exit_code} = System.cmd("git", ["rev-parse", "HEAD"])

  # We'll truncate the commit SHA to 7 chars. Feel free to change
  git_sha = String.slice(result, 0, 7)

  [app: :hello_phoenix,
   version: "1.4.1-#{git_sha}",
   elixir: "~> 1.0",
   ...

That just changes the version to use the git commit SHA.

###Update

Using commit SHA will not work with distillery.

The issue with git_sha is that it’s not ordered. The next version is assumed to have a number that is greater than the previous one by trivial alphanumeric sort. With sha that’s not the case.

You might use date for that purpose. E. g.

git log -1 --date=raw --format=%cd
#⇒ 1535467693 +0200 # seconds since epoch

Now let’s elixir it:

{epoch, _} = System.cmd("git", ~w|log -1 --date=raw --format=%cd|)
[sec, tz] =
  epoch
  |> String.split(~r/\s+/, trim: true)
  |> Enum.map(&String.to_integer/1)
#⇒ [1527769224, 200]
sec + tz * 36 # * 60 * 60 / 100
#⇒ 1527776424

The number above is always growing.

Sidenote: use inplace binary pattern match instead of String.slice/3 whenever possible:

{<<git_sha::binary-size(8), _rest::binary>>, _exit_code} =
  System.cmd("git", ["rev-parse", "HEAD"])
#⇒ {"556c53987eb55c82ffb6925f9f56eae5de01c119\n", 0}
git_sha
#⇒ "556c5398"

So in your application's mix.exs file

  defp versions do
    {epoch, _} = System.cmd("git", ~w|log -1 --date=raw --format=%cd|)
    [sec, tz] =
      epoch
      |> String.split(~r/\s+/, trim: true)
      |> Enum.map(&String.to_integer/1)
    sec + tz * 36
  end

and do this way

def project do
  [app: :hello_phoenix,
   version: "1.4.1-#{versions()}",
   elixir: "~> 1.0",
   ...

You might do whatever you want, the date is constantly growing with time, unlike sha, so it’s applicable for your needs. Also you might use commit number, or whatever constantly growing.