prom_ex

defmodule Mix.Tasks.PromEx.Create do
  @moduledoc """
  This will generate a PromEx config module in the appropriate place
  """

  use Mix.Task

  alias Mix.Shell.IO

  @impl true
  def run(args) do
    # Compile the project
    Mix.Task.run("compile")

    # Get CLI args
    otp_app =
      args
      |> parse_options()
      |> Map.get_lazy(:otp_app, fn ->
        Mix.Project.config()
        |> Keyword.get(:app)
        |> Atom.to_string()
      end)

    # Generate relevant path info
    project_root = File.cwd!()
    path = Path.join([project_root, "lib", otp_app, "prom_ex.ex"])
    dirname = Path.dirname(path)

    unless File.exists?(dirname) do
      raise "Required directory path #{dirname} does not exist. " <>
              "Be sure that the --otp-app argument or that you Mix project file is correct."
    end

    write_file =
      if File.exists?(path) do
        IO.yes?("File already exists at #{path}. Overwrite?")
      else
        true
      end

    if write_file do
      # Write out the config file
      create_config_file(path, otp_app)
      IO.info("Successfully wrote out #{path}")

      first_line = "| Be sure to follow the @moduledoc instructions in #{Macro.camelize(otp_app)}.PromEx |"
      line_length = String.length(first_line) - 2
      second_line = "| to complete the PromEx setup process" <> String.duplicate(" ", line_length - 37) <> "|"
      divider = "+" <> String.duplicate("-", line_length) <> "+"

      IO.info(Enum.join(["", divider, first_line, second_line, divider], "\n"))
    else
      IO.info("Did not write file out to #{path}")
    end
  end

  defp parse_options(args) do
    cli_options = [otp_app: :string]
    cli_aliases = [o: :otp_app]

    args
    |> OptionParser.parse(aliases: cli_aliases, strict: cli_options)
    |> case do
      {options, _remaining_args, [] = _errors} ->
        Map.new(options)

      {_options, _remaining_args, errors} ->
        raise "Invalid CLI args were provided: #{inspect(errors)}"
    end
  end

  defp create_config_file(path, otp_app) do
    module_name = Macro.camelize(otp_app)
    assigns = [module_name: module_name, otp_app: otp_app]

    module_template =
      prom_ex_module_template()
      |> EEx.eval_string(assigns: assigns)

    path
    |> File.write!(module_template)
  end

  defp prom_ex_module_template do
    """
    defmodule <%= @module_name %>.PromEx do
      @moduledoc \"\"\"
      Be sure to add the following to finish setting up PromEx:

      1. Update your configuration (config.exs, dev.exs, prod.exs, releases.exs, etc) to
      configure the necessary bit of PromEx. Be sure to check out `PromEx.Config` for
      more details regarding configuring PromEx:
      ```
        config :<%= @otp_app %>, <%= @module_name %>.PromEx,
          manual_metrics_start_delay: :no_delay,
          drop_metrics_groups: [],
          grafana: :disabled,
          metrics_server: :disabled
      ```

      2. Add this module to your application supervision tree. It should be one of the first
         things that is started so that no Telemetry events are missed. For example, if PromEx
         is started after your Repo module, you will miss Ecto's init events and the dashbaords
         will be missing some data points:
      ```
      def start(_type, _args) do
        children = [
          <%= @module_name %>.PromEx,

          ...
        ]

        ...
      end
      ```

      3. Update your `endpoint.ex` file to expose your metrics (or configure a standalone
      server using the `:metrics_server` config options). Be sure to put this plug before
      your `Plug.Telemetry` entry so that you can avoid having calls to your `/metrics`
      endpoint create their own metrics and logs which can pollute your logs/metrics given
      that Prometheus will scrape at a regular interval and that can get noisy:
      ```
      defmodule <%= @module_name %>Web.Endpoint do
        use Phoenix.Endpoint, otp_app: :<%= @otp_app %>

        ...

        plug PromEx.Plug, prom_ex_module: <%= @module_name %>.PromEx

        ...
      end
      ```
      \"\"\"

      use PromEx, otp_app: :<%= @otp_app %>

      @impl true
      def plugins do
        [
          # PromEx built in plugins
          PromEx.Plugins.Application,
          PromEx.Plugins.Beam,
          {PromEx.Plugins.Phoenix, router: <%= @module_name %>Web.Router},
          PromEx.Plugins.Ecto

          # Add your own PromEx metrics plugins
          # <%= @module_name %>.Users.PromEx
        ]
      end

      @impl true
      def dashboards do
        [
          # PromEx built in dashboard definitions. Remove dashboards that you do not need
          {:prom_ex, "application.json"},
          {:prom_ex, "beam.json"},
          {:prom_ex, "phoenix.json"},
          {:prom_ex, "ecto.json"}

          # Add your dashboard definitions here with the format: {:otp_app, "path_in_priv"}
          # {:<%= @otp_app %>, "/grafana_dashboards/user_metrics.json"}
        ]
      end
    end
    """
  end
end