defmodule Extatus do
  @moduledoc """
  Extatus is an application that reports metrics to Prometheus via the HTTP
  endpoint `/metrics` from an instrumented `GenServer`.

  ## Small Example

  The following is an uninstrumented `GenServer` that tracks its uptime.

  ```
  defmodule Uninstrumented do
    use GenServer

    def start_link do
      GenServer.start_link(__MODULE__, nil)
    end

    def get_uptime(pid) do
      GenServer.call(pid, :uptime)
    end

    def init(_) do
      start_time = :os.system_time(:seconds)
      {:ok, seconds}
    end

    def handle_call(:uptime, _from, start_time) do
      uptime = :os.system_time(:seconds) - start_time
      {:reply, uptime, start_time}
    end
  end
  ```

  If we would want to track the uptime of this process in Prometheus with
  `Extatus`, we would need:
    1. `use Extatus.Process` behaviour.
    2. Declare a gauge metric to track the uptime e.g. a metric called `:uptime`
      with a label for the module name.
    3. Implement the function `get_name/1` that receives the `GenServer` process
      state and returns the name of the process. This name must be unique.
    4. Implement the function `report/1` that receives the `GenServer` process
      state. In this function, you can report the metrics.
    5. Start the `Extatus` watchdog for your process in the `init/1` function by
      adding the function `add_extatus_watchdog/0` (included by
      `use Extatus.Process`).

  ```
  defmodule Instrumented do
    use GenServer
    use Extatus.Process # Extatus behaviour

    def start_link do
      GenServer.start_link(__MODULE__, nil)
    end

    # Metric declaration
    defmetrics do
      gauge :uptime do
        label :module
        help "Uptime gauge"
      end
    end

    # Name of the process. This must be unique.
    def get_name(_state) do
      {:ok, Atom.to_string(__MODULE__)}
    end

    # Report function
    def report(start_time) do
      uptime = :os.system_time(:seconds) - start_time
      Gauge.set(:uptime, [module: Atom.to_string(__MODULE__)], uptime)
    end

    def init(_) do
      :ok = add_extatus_watchdog() # Add extatus watchdog
      {:ok, :os.system_time(:seconds)}
    end
  end
  ```

  The HTTP `/metric` endpoint is implemented in `:cowboy` and the output is
  provided by the library `:prometheus_ex`. If you start this process, you will
  see the metric `:uptime` being reported.

  Additionally, for every instrumented `GenServer` process, extatus reports the
  metric `:extatus_process_activity` (gauge). This metric indicates that a
  process is up (2), down (0) or idle (1) depending on its value.

  `Extatus` uses `Yggdrasil` to report the status of the processes in the
  following channel:

  ```elixir
  %Yggdrasil.Channel{name: :extatus}
  ```

  This can be used to get the updates on the current state of the processes in
  a subscriber e.g:

  ```elixir
  iex> chan = %Yggdrasil.Channel{name: :extatus}
  iex> Yggdrasil.subscribe(chan)
  iex> flush()
  {:Y_CONNECTED, (...)}
  iex> {:ok, _} = Instrumented.start_link()
  {:ok, #PID<0.603.0>}
  iex> flush()
  {:Y_EVENT, _, %Extatus.Message{name: "instrumented_process", state: :up}}
  ```

  ## Configuration

  The following are the configuration arguments available:

    - `:timeout` - Frequency the handlers get the metrics from the processes.
    - `:port` - Port where cowboy is listening to request coming from
    Prometheus.
    - `:prometheus_registry` - Prometheus registry. By default is `:default`.

  e.g:

  ```elixir
  config :extatus,
    timeout: 5000,
    port: 1337,
    prometheus_registry: :test
  ```

  """
  use Application

  alias Extatus.Settings
  alias Extatus.Generator


  @doc """
  Starts a status handler for the provided `module` and `pid`. Links to
  calling process. This is to avoid having processes without Extatus handlers.
  """
  @spec set(module, pid) :: Supervisor.on_start_child
  def set(module, pid) do
    {:ok, watchdog} = Generator.start_handler(Generator, module, pid)
    true = Process.link(watchdog)
    {:ok, watchdog}
  end

  ###################
  # Application start

  @doc false
  def start(_type, _args) do
    import Supervisor.Spec, warn: false

    children = get_children()

    opts = [strategy: :one_for_one, name: Extatus.Supervisor]
    Supervisor.start_link(children, opts)
  end

  @doc false
  def get_children, do: run_sandbox() |> get_children()

  @doc false
  def run_sandbox do
    Settings.extatus_counter_mod() == Extatus.Sandbox.Counter or
    Settings.extatus_gauge_mod() == Extatus.Sandbox.Gauge or
    Settings.extatus_histogram_mod() == Extatus.Sandbox.Histogram or
    Settings.extatus_summary_mod() == Extatus.Sandbox.Summary
  end

  @doc false
  def get_children(true) do
    import Supervisor.Spec, warn: false
    [
      worker(Extatus.Sandbox.Metric, []),
      worker(Extatus.Server, [[name: Extatus.Server]]),
      supervisor(Generator, [[name: Generator]])
    ]
  end
  def get_children(false) do
    import Supervisor.Spec, warn: false
    [
      worker(Extatus.Server, [[name: Extatus.Server]]),
      supervisor(Generator, [[name: Generator]])
    ]
  end
end