telemetry_metrics_statsd

defmodule TelemetryMetricsStatsd do
  @moduledoc """
  `Telemetry.Metrics` reporter for StatsD-compatible metric servers.

  To use it, start the reporter with the `start_link/1` function, providing it a list of
  `Telemetry.Metrics` metric definitions:

      import Telemetry.Metrics

      TelemetryMetricsStatsd.start_link(
        metrics: [
          counter("http.request.count"),
          sum("http.request.payload_size"),
          last_value("vm.memory.total")
        ]
      )

  > Note that in the real project the reporter should be started under a supervisor, e.g. the main
  > supervisor of your application.

  By default the reporter sends metrics to 127.0.0.1:8125 - both hostname and port number can be
  configured using the `:host` and `:port` options.

  Note that the reporter doesn't aggregate metrics in-process - it sends metric updates to StatsD
  whenever a relevant Telemetry event is emitted.

  ## Translation between Telemetry.Metrics and StatsD

  In this section we walk through how the Telemetry.Metrics metric definitions are mapped to StatsD
  metrics and their types at runtime.

  Telemetry.Metrics metric names are translated as follows:
    * if the metric name was provided as a string, e.g. `"http.request.count"`, it is sent to
      StatsD server as-is
    * if the metric name was provided as a list of atoms, e.g. `[:http, :request, :count]`, it is
      first converted to a string by joiging the segments with dots. In this example, the StatsD
      metric name would be `"http.request.count"` as well

  Since there are multiple implementations of StatsD and each of them provides slightly different
  set of features, other aspects of metric translation are controlled by the formatters.
  The formatter can be selected using the `:formatter` option. Currently only two formats are
  supported - `:standard` and `:datadog`.

  The following table shows how `Telemetry.Metrics` metrics map to standard StatsD metrics:

  | Telemetry.Metrics | StatsD |
  |-------------------|--------|
  | `last_value`      | `gauge` |
  | `counter`         | `counter` |
  | `sum`             | `gauge` or `counter` |
  | `summary`         | `timer` |
  | `distribution`    | `timer` |

  [DataDog](https://docs.datadoghq.com/developers/metrics/types/?tab=count#metric-types) provides a richer
  set of metric types:

  | Telemetry.Metrics | DogStatsD |
  |-------------------|-----------|
  | `last_value`      | `gauge` |
  | `counter`         | `counter` |
  | `sum`             | `gauge` or `counter` |
  | `summary`         | `histogram`  |
  | `distribution`    | `distribution` |

  ### The standard StatsD formatter

  The `:standard` formatter is compatible with the
  [Etsy implementation](https://github.com/statsd/statsd/blob/master/docs/metric_types.md) of StatsD.
  Since this particular implementation doesn't support explicit tags, tag values are appended as
  consecutive segments of the metric name. For example, given the definition

      counter("db.query.count", tags: [:table, :operation])

  and the event

      :telemetry.execute([:db, :query], %{}, %{table: "users", operation: "select"})

  the StatsD metric name would be `"db.query.count.users.select"`. Note that the tag values are
  appended to the base metric name in the order they were declared in the metric definition.

  Another important aspect of the standard formatter is that all measurements are converted to
  integers, i.e. no floats are ever sent to the StatsD daemon.

  Now to the metric types!

  #### Counter

  Telemetry.Metrics counter is simply represented as a StatsD counter. Each event the metric is
  based on increments the counter by 1. To be more concrete, given the metric definition

      counter("http.request.count")

  and the event

      :telemetry.execute([:http, :request], %{duration: 120})

  the following line would be send to StatsD

      "http.request.count:1|c"

  Note that the counter was bumped by 1, regardless of the measurements included in the event
  (careful reader will notice that the `:count` measurement we chose for the metric wasn't present
  in the map of measurements at all!). Such behaviour conforms to the specification of counter as
  defined by `Telemetry.Metrics` package - a counter should be incremented by 1 every time a given
  event is dispatched.

  #### Last value

  Last value metric is represented as a StatsD gauge, whose values are always set to the value
  of the measurement from the most recent event. With the following metric definition

      last_value("vm.memory.total")

  and the event

      :telemetry.execute([:vm, :memory], %{total: 1024})

  the following metric update would be send to StatsD

      "vm.memory.total:1024|g"

  #### Sum

  Sum metric is also represented as a gauge - the difference is that it always changes relatively
  and is never set to an absolute value. Given metric definition below

      sum("http.request.payload_size")

  and the event

      :telemetry.execute([:http, :request], %{payload_size: 1076})

  the following line would be send to StatsD

      "http.request.count:+1076|g"

  When the measurement is negative, the StatsD gauge is decreased accordingly.

  When the `report_as: :counter` reporter option is passed, the sum metric is reported as
  a counter and increased with the value provided. Only positive values are allowed, negative
  measurements are discarded and logged.

  Given the metric definition

      sum("kafka.consume.batch_size", reporter_options: [report_as: :counter])

  and the event

      :telemetry.execute([:kafka, :consume], %{batch_size: 200})

  the following would be sent to StatsD

      "kafka.consume.batch_size:200|c"

  #### Summary

  The summary is simply represented as a StatsD timer, since it should generate statistics about
  gathered measurements. Given the metric definition below

      summary("http.request.duration")

  and the event

      :telemetry.execute([:http, :request], %{duration: 120})

  the following line would be send to StatsD

      "http.request.duration:120|ms"

  #### Distribution

  There is no metric in original StatsD implementation equivalent to Telemetry.Metrics distribution.
  However, histograms can be enabled for selected timer metrics in the
  [StatsD daemon configuration](https://github.com/statsd/statsd/blob/master/docs/metric_types.md#timing).
  Because of that, the distribution is also reported as a timer. For example, given the following metric
  definition

      distribution("http.request.duration")

  and the event

      :telemetry.execute([:http, :request], %{duration: 120})

  the following line would be send to StatsD

      "http.request.duration:120|ms"

  ### The DataDog formatter

  The DataDog formatter is compatible with [DogStatsD](https://docs.datadoghq.com/developers/dogstatsd/),
  the DataDog StatsD service bundled with its agent.

  #### Tags

  The main difference from the standard formatter is that DataDog supports explicit tagging in its
  protocol. Using the same example as with the standard formatter, given the following definition

      counter("db.query.count", tags: [:table, :operation])

  and the event

      :telemetry.execute([:db, :query], %{}, %{table: "users", operation: "select"})

  the metric update packet sent to StatsD would be `db.query.count:1|c|#table:users,operation:select`.

  #### Metric types

  There is no difference in how the counter, last value, and sum metrics are handled between
  the standard and DataDog formatters.

  The summary metric is reported as [DataDog
  histogram](https://docs.datadoghq.com/developers/metrics/types/?tab=histogram), as that is the
  metric that provides a set of statistics about gathered measurments on the DataDog side.

  The distribution is flushed as [DataDog
  distribution](https://docs.datadoghq.com/developers/metrics/types/?tab=distribution) metric, which
  provides statistically correct aggregations of data gathered from multiple services or DogStatsD
  agents.

  Also note that DataDog allows measurements to be floats, that's why no rounding is performed when
  formatting the metric.

  ## Global tags

  The library provides an option to specify a set of global tag values, which are available to all
  metrics running under the reporter.

  For example, if you're running your application in multiple deployment environment (staging, production,
  etc.), you might set the environment as a global tag:

      TelemetryMetricsStatsd.start_link(
        metrics: [
          counter("http.request.count", tags: [:env])
          ],
          global_tags: [env: "prod"]
      )

  Note that if the global tag is to be sent with the metric, the metric needs to have it listed under the
  `:tags` option, just like any other tag.

  Also, if the same key is configured as a global tag and emitted as a part of event metadata or returned
  by the `:tag_values` function, the metadata/`:tag_values` take precedence and override the global tag
  value.

  ## Prefixing metric names

  Sometimes it's convenient to prefix all metric names with particular value, to group them by the
  name of the service, the host, or something else. You can use `:prefix` option to provide a prefix
  which will be prepended to all metrics published by the reporter (regardless of the formatter used).

  ## Maximum datagram size

  Metrics are sent to StatsD over UDP, so it's important that the size of the datagram does not
  exceed the Maximum Transmission Unit, or MTU, of the link, so that no data is lost on the way.
  By default the reporter will break up the datagrams at 512 bytes, but this is configurable via
  the `:mtu` option.

  ## Sampling data

  It's not always convenient to capture every piece of data, such as in the case of high-traffic
  applications. In those cases, you may want to capture a "sample" of the data. You can do this
  by passing `[sampling_rate: <rate>]` as an option to `:reporter_options`, where `rate` is a
  value between 0.0 and 1.0. The default `:sampling_rate` is 1.0, which means that all
  the measurements are being captured.

  ### Example

      TelemetryMetricsStatsd.start_link(
        metrics: [
          counter("http.request.count"),
          summary("http.request.duration", reporter_options: [sampling_rate: 0.1]),
          distribution("http.request.duration", reporter_options: [sampling_rate: 0.1])
        ]
      )

  In this example, we are capturing 100% of the measurements for the counter, but only 10% for both
  summary and distribution.
  """

  use GenServer

  require Logger

  alias Telemetry.Metrics
  alias TelemetryMetricsStatsd.{EventHandler, UDP}

  @type prefix :: String.t() | atom() | nil
  @type host :: String.t() | :inet.ip_address()
  @type option ::
          {:port, :inet.port_number()}
          | {:host, host()}
          | {:metrics, [Metrics.t()]}
          | {:mtu, non_neg_integer()}
          | {:prefix, prefix()}
          | {:formatter, :standard | :datadog}
          | {:global_tags, Keyword.t()}
  @type options :: [option]

  @default_port 8125
  @default_mtu 512
  @default_formatter :standard

  @doc """
  Reporter's child spec.

  This function allows you to start the reporter under a supervisor like this:

      children = [
        {TelemetryMetricsStatsd, options}
      ]

  See `start_link/1` for a list of available options.
  """
  @spec child_spec(options) :: Supervisor.child_spec()
  def child_spec(options) do
    %{id: __MODULE__, start: {__MODULE__, :start_link, [options]}}
  end

  @doc """
  Starts a reporter and links it to the calling process.

  The available options are:
  * `:metrics` - a list of Telemetry.Metrics metric definitions which will be published by the
    reporter
  * `:host` - hostname or IP address of the StatsD server. Defaults to `{127, 0, 0, 1}`. Keep
    in mind Erlang's UDP implementation looks up the hostname each time it sends a packet.
    Furthermore, telemetry handlers are blocking. For latency-critical applications, it is best
    to use an IP here (or resolve it on startup).
  * `:port` - port number of the StatsD server. Defaults to `8125`.
  * `:formatter` - determines the format of the metrics sent to the target server. Can be either
    `:standard` or `:datadog`. Defaults to `:standard`.
  * `:prefix` - a prefix prepended to the name of each metric published by the reporter. Defaults
    to `nil`.
  * `:mtu` - Maximum Transmission Unit of the link between your application and the StatsD server in
    bytes. This value should not be greater than the actual MTU since this could lead to the data loss
    when the metrics are published. Defaults to `512`.
  * `:global_tags` - Additional default tag values to be sent along with every published metric. These
    can be overriden by tags sent via the `:telemetry.execute` call.

  You can read more about all the options in the `TelemetryMetricsStatsd` module documentation.

  ## Example

      import Telemetry.Metrics

      TelemetryMetricsStatsd.start_link(
        metrics: [
          counter("http.request.count"),
          sum("http.request.payload_size"),
          last_value("vm.memory.total")
        ],
        prefix: "my-service"
      )
  """
  @spec start_link(options) :: GenServer.on_start()
  def start_link(options) do
    config =
      options
      |> Enum.into(%{})
      |> Map.put_new(:host, {127, 0, 0, 1})
      |> Map.update!(:host, fn host ->
        if(is_binary(host), do: to_charlist(host), else: host)
      end)
      |> Map.put_new(:port, @default_port)
      |> Map.put_new(:mtu, @default_mtu)
      |> Map.put_new(:prefix, nil)
      |> Map.put_new(:formatter, @default_formatter)
      |> Map.update!(:formatter, &validate_and_translate_formatter/1)
      |> Map.put_new(:global_tags, Keyword.new())

    GenServer.start_link(__MODULE__, config)
  end

  @doc false
  @spec get_udp(pid()) :: UDP.t()
  def get_udp(reporter) do
    GenServer.call(reporter, :get_udp)
  end

  @doc false
  @spec udp_error(pid(), UDP.t(), reason :: term) :: :ok
  def udp_error(reporter, udp, reason) do
    GenServer.cast(reporter, {:udp_error, udp, reason})
  end

  @impl true
  def init(config) do
    metrics = Map.fetch!(config, :metrics)

    case UDP.open(config.host, config.port) do
      {:ok, udp} ->
        Process.flag(:trap_exit, true)

        handler_ids =
          EventHandler.attach(
            metrics,
            self(),
            config.mtu,
            config.prefix,
            config.formatter,
            config.global_tags
          )

        {:ok, %{udp: udp, handler_ids: handler_ids, host: config.host, port: config.port}}

      {:error, reason} ->
        {:error, {:udp_open_failed, reason}}
    end
  end

  @impl true
  def handle_call(:get_udp, _from, state) do
    {:reply, state.udp, state}
  end

  @impl true
  def handle_cast({:udp_error, udp, reason}, %{udp: udp} = state) do
    Logger.error("Failed to publish metrics over UDP: #{inspect(reason)}")

    case UDP.open(state.host, state.port) do
      {:ok, udp} ->
        {:noreply, %{state | udp: udp}}

      {:error, reason} ->
        Logger.error("Failed to reopen UDP socket: #{inspect(reason)}")
        {:stop, {:udp_open_failed, reason}, state}
    end
  end

  def handle_cast({:udp_error, _, _}, state) do
    {:noreply, state}
  end

  @impl true
  def handle_info({:EXIT, _pid, reason}, state) do
    {:stop, reason, state}
  end

  @impl true
  def terminate(_reason, state) do
    EventHandler.detach(state.handler_ids)

    :ok
  end

  defp validate_and_translate_formatter(:standard), do: TelemetryMetricsStatsd.Formatter.Standard
  defp validate_and_translate_formatter(:datadog), do: TelemetryMetricsStatsd.Formatter.Datadog

  defp validate_and_translate_formatter(_),
    do: raise(ArgumentError, ":formatter needs to be either :standard or :datadog")
end