Running Ecto migrations in a release

By Jake Morrison in DevOps on Sun 30 June 2019

In a dev or test environment, we execute the mix ecto.migrate command to run database migrations. When running from a release, however, the mix command is not available. Instead, we need to execute from code.

We do this by defining a helper function which we can then call from the release startup script:

/srv/foo/current/bin/foo eval ""

The main point is that we need to initialize the database connection information the same way as your application normally does, e.g. by loading a runtime file like /etc/foo/config.toml or setting environment variables. If you use TOML configuration files, you will need to install the package toml_config_provider from Hex.

Following is a working example. Where you see CHANGEME, use the name of your project.

defmodule Foo.ReleaseTasks.Migrate do
  @moduledoc "Mix task to run Ecto database migrations"

  # CHANGEME: Name of app as used by Application.get_env
  @app :foo
  # CHANGEME: Name of app repo module
  @repo_module Foo.Repo

  def run(args \\ [])

  def run(_args) do
    ext_name = @app |> to_string |> String.replace("_", "-")
    config_dir = Path.join("/etc", ext_name)

    # Read config.exs if present
    config_exs = Path.join(config_dir, "config.exs")
    app_env =
      case File.exists?(config_exs) do
        true ->
          IO.puts("==> Loading config file #{config_exs}")

        _ ->

    # Read TOML config if present (requires `toml_config_provider` package)
    config_toml = Path.join(config_dir, "config.toml")
    app_env =
      case File.exists?(config_toml) do
        true ->
          IO.puts("==> Loading config file #{config_toml}")
          TomlConfigProvider.load(config, config_toml)

        _ ->

    Application.put_env(@app, @repo_module, app_env[@app][@repo_module])

    # Start requisite apps
    IO.puts("==> Starting applications..")

    for app <- [:crypto, :ssl, :postgrex, :ecto, :ecto_sql] do
      {:ok, res} = Application.ensure_all_started(app)
      IO.puts("==> Started #{app}: #{inspect(res)}")

    # Start repo
    IO.puts("==> Starting repo")
    {:ok, _pid} = apply(@repo_module, :start_link, [[pool_size: 2, log: :info, log_sql: true]])

    # Run migrations for the repo
    IO.puts("==> Running migrations")
    priv_dir = Application.app_dir(@app, "priv")
    migrations_dir = Path.join([priv_dir, "repo", "migrations"])

    opts = [all: true]
    config = apply(@repo_module, :config, [])
    pool = config[:pool]

    if function_exported?(pool, :unboxed_run, 2) do
      pool.unboxed_run(@repo_module, fn ->, migrations_dir, :up, opts)
    else, migrations_dir, :up, opts)

    # Shut down