Disable GenServer in test environment

Questions / Help
1

So I have this common OTP application, starting up several (mostly Phoenix related) children. One of them is a GenServer I created to poll an external TCP socket every second. This is something I don’t want to do in the test environment. Is the following snippet the “common” way to disable this within my application?

defmodule Example.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  @impl true
  def start(_type, _args) do
    children = [
      # Start the Ecto repository
      Example.Repo,
      # Start the TCP poller
      poller_spec(),
      # Start the Telemetry supervisor
      ExampleWeb.Telemetry,
      # Start the PubSub system
      {Phoenix.PubSub, name: Example.PubSub},
      # Start the Endpoint (http/https)
      ExampleWeb.Endpoint
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Example.Supervisor]
    Supervisor.start_link(children, opts)
  end

  # Tell Phoenix to update the endpoint configuration
  # whenever the application is updated.
  @impl true
  def config_change(changed, _new, removed) do
    ExampleWeb.Endpoint.config_change(changed, removed)
    :ok
  end

  defp poller_spec() do
    if Mix.env() == :test
      %{id: Example.Poller, start: {Function, :identity, [:ignore]}}
    else
      {Example.Poller, host: "example.com"}
    end
  end
end

I searched a lot on Github to find such a pattern (found this one in Dashbit’s Bytepack repo), but there aren’t that many open-source Phoenix examples to peek at. Maybe it is super obvious thing to implement if you have more GenServer experience :sweat_smile:

2

You can have environment-specific children, but you need to make sure your app always knows what environment it’s running in. Since Mix.env() isn’t available at runtime (in a release), you’ll need to make sure the right value gets compiled into your app’s configuration. So first, in config/config.exs, add:

# If you're using Elixir 1.13+
config :example, :environment, config_env()
# For older versions of Elixir
config :example, :environment, Mix.env()

Then, in your application.ex file:

defmodule Example.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  @impl true
  def start(_type, _args) do
    children = 
      Application.get_env(:example, :environment)
      |> children()

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: Example.Supervisor]
    Supervisor.start_link(children, opts)
  end

  defp children(:test) do
    [
      Example.Repo,
      poller_spec(),
      ExampleWeb.Telemetry,
      {Phoenix.PubSub, name: Example.PubSub},
      ExampleWeb.Endpoint
    ]
  end

  defp children(_) do
    [
      Example.Repo,
      poller_spec(),
      ExampleWeb.Telemetry,
      {Phoenix.PubSub, name: Example.PubSub},
      ExampleWeb.Endpoint,
      Example.TheGenServerThatShouldNotStartInTest
    ]
  end
end

Of course, you can eliminate some redundancy:

defp children(:common) do
    [
      Example.Repo,
      poller_spec(),
      ExampleWeb.Telemetry,
      {Phoenix.PubSub, name: Example.PubSub},
      ExampleWeb.Endpoint
    ]
end

defp children(:test), do: children(:common)
defp children(_), do: children(:common) ++ [Example.TheGenServerThatShouldNotStartInTest]

You just need to make sure that your children/1 returns the right stuff to start for the given environment.

3

Thanks, that helped me getting the logic implemented! In the end, I’ve added an additional configuration value that can be set to false in the test environment.

defp poller_spec() do
  if Application.get_env(:example, :poller_enabled, true)
    {Example.Poller, host: "example.com"}
  else
    %{id: Example.Poller, start: {Function, :identity, [:ignore]}}
  end
end
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4

Another scenario which is a bit difficult to figure out; what about the seeds file and any GenServer that doesn’t need to be started? How do I know in the Application if it is fully starting or only the seeds are run.

5

If I understand the question correctly, you can control your app’s startup using any conditional logic you want. For example, you could have the list of children be nothing but [Example.Repo] if an environment variable called REPO_ONLY is set. You’d then set that environment variable when running your seeds (e.g., REPO_ONLY=true mix run priv/repo/seeds.exs).

This is just a broad example. In the end, your list of child processes to start is just that – a list. You can generate it however you see fit.

6

In the end I found an even clearer way to prevent the poller from running. By checking if the Phoenix server is about to start as well, I conditionally add the poller spec to the list of children.

defp poller_spec() do
  if Application.get_env(:example, :poller_enabled, true) 
      && Phoenix.Endpoint.server?(:example, ExampleWeb.Endpoint)
    {Example.Poller, host: "example.com"}
  else
    %{id: Example.Poller, start: {Function, :identity, [:ignore]}}
  end
end
1 Like