towerops/lib/towerops_web/telemetry.ex

defmodule ToweropsWeb.Telemetry do
  @moduledoc false
  use Supervisor

  import Telemetry.Metrics

  def start_link(arg) do
    Supervisor.start_link(__MODULE__, arg, name: __MODULE__)
  end

  @impl true
  def init(_arg) do
    # Attach telemetry handlers for logging request failures
    _ =
      :telemetry.attach(
        "towerops-router-exception",
        [:phoenix, :router_dispatch, :exception],
        &__MODULE__.handle_router_exception/4,
        nil
      )

    _ =
      :telemetry.attach(
        "towerops-endpoint-stop",
        [:phoenix, :endpoint, :stop],
        &__MODULE__.handle_endpoint_stop/4,
        nil
      )

    children = [
      # Telemetry poller will execute the given period measurements
      # every 10_000ms. Learn more here: https://hexdocs.pm/telemetry_metrics
      {:telemetry_poller, measurements: periodic_measurements(), period: 10_000}
      # Add reporters as children of your supervision tree.
      # {Telemetry.Metrics.ConsoleReporter, metrics: metrics()}
    ]

    Supervisor.init(children, strategy: :one_for_one)
  end

  def metrics do
    [
      # Phoenix Metrics
      summary("phoenix.endpoint.start.system_time",
        unit: {:native, :millisecond}
      ),
      summary("phoenix.endpoint.stop.duration",
        unit: {:native, :millisecond}
      ),
      summary("phoenix.router_dispatch.start.system_time",
        tags: [:route],
        unit: {:native, :millisecond}
      ),
      summary("phoenix.router_dispatch.exception.duration",
        tags: [:route],
        unit: {:native, :millisecond}
      ),
      summary("phoenix.router_dispatch.stop.duration",
        tags: [:route],
        unit: {:native, :millisecond}
      ),
      summary("phoenix.socket_connected.duration",
        unit: {:native, :millisecond}
      ),
      sum("phoenix.socket_drain.count"),
      summary("phoenix.channel_joined.duration",
        unit: {:native, :millisecond}
      ),
      summary("phoenix.channel_handled_in.duration",
        tags: [:event],
        unit: {:native, :millisecond}
      ),

      # Database Metrics
      summary("towerops.repo.query.total_time",
        unit: {:native, :millisecond},
        description: "The sum of the other measurements"
      ),
      summary("towerops.repo.query.decode_time",
        unit: {:native, :millisecond},
        description: "The time spent decoding the data received from the database"
      ),
      summary("towerops.repo.query.query_time",
        unit: {:native, :millisecond},
        description: "The time spent executing the query"
      ),
      summary("towerops.repo.query.queue_time",
        unit: {:native, :millisecond},
        description: "The time spent waiting for a database connection"
      ),
      summary("towerops.repo.query.idle_time",
        unit: {:native, :millisecond},
        description: "The time the connection spent waiting before being checked out for the query"
      ),

      # VM Metrics
      summary("vm.memory.total", unit: {:byte, :kilobyte}),
      summary("vm.total_run_queue_lengths.total"),
      summary("vm.total_run_queue_lengths.cpu"),
      summary("vm.total_run_queue_lengths.io")
    ]
  end

  defp periodic_measurements do
    [
      # A module, function and arguments to be invoked periodically.
      # This function must call :telemetry.execute/3 and a metric must be added above.
      # {ToweropsWeb, :count_users, []}
    ]
  end

  # Telemetry handler for router exceptions
  def handle_router_exception(_event, _measurements, metadata, _config) do
    require Logger

    Logger.error(
      "Router exception on #{metadata.plug} #{metadata.conn.method} #{metadata.conn.request_path}",
      kind: metadata.kind,
      reason: metadata.reason,
      stacktrace: metadata.stacktrace,
      request_id: metadata.conn.assigns[:request_id]
    )
  end

  # Telemetry handler for endpoint stop events (log slow requests and errors)
  def handle_endpoint_stop(_event, measurements, metadata, _config) do
    require Logger

    duration_ms = System.convert_time_unit(measurements.duration, :native, :millisecond)

    # Log slow requests (over 5 seconds)
    if duration_ms > 5_000 do
      Logger.warning(
        "Slow request: #{metadata.conn.method} #{metadata.conn.request_path} took #{duration_ms}ms",
        request_id: metadata.conn.assigns[:request_id],
        duration_ms: duration_ms
      )
    end

    # Log requests with non-2xx status codes
    status = metadata.conn.status

    if status >= 500 do
      Logger.error(
        "Server error: #{metadata.conn.method} #{metadata.conn.request_path} returned #{status}",
        request_id: metadata.conn.assigns[:request_id],
        status: status,
        duration_ms: duration_ms
      )
    end
  end
end