View Source PromEx.Plugins.PlugRouter (PromEx v1.10.0)

This plugin captures HTTP request metrics emitted by Plug.Router and Plug.Telemetry.

This plugin is heavily inspired on plugin Plug.Cowboy, and exposes the following metric group:

• :plug_router_http_event_metrics

Plugin options

• routers: Required This is a list with the full module names of your Routers (e.g MyAppWeb.Router). Metrics produced by routers not in this list will be discarded.

• event_prefix: Required, allows you to set the event prefix defined in your Plug.Telemetry configuration:

• duration_unit: This is an OPTIONAL option and is a Telemetry.Metrics.time_unit(). It can be one of: :second | :millisecond | :microsecond | :nanosecond. It is :millisecond by default.

defmodule WebApp.Router do
  use Plug.Router

  plug PromEx.Plug, prom_ex_module: WebApp.PromEx, path: "/metrics"
  plug Plug.Telemetry, event_prefix: [:webapp, :router]
  ...
end

With the above configuration, this plugin will subscribe to [:webapp, :router, :stop] telemetry events produced by Plug.Telemetry. These events will be fired before the response is actually sent, therefore this plugin will be able to export response body size metrics, since the Plug.Conn struct in the metadata of the telemetry measurement still contains the response body.

However, Plug.Telemetry does not use :telemetry.span/3, which means the :stop event might not always be fired (eg. if the process handling the request crashes). For this reason, this PromEx plugin also subscribes to [:plug, :router_dispatch, :exception] telemetry events fired by Plug.Router, which are fired within a :telemetry.span/3 call.

Unfortunately, we cannot safely rely on [:plug, :router_dispatch, :stop] events produced by Plug.Router since these are fired after the response is sent (as opposed to Plug.Telemetry :stop events). As a consequence, the response body is no longer available in the Plug.Conn struct attached to the telemetry measurement metadata.

• metric_prefix: This option is OPTIONAL and is used to override the default metric prefix of [otp_app, :prom_ex, :plug_router]. If this changes you will also want to set plug_router_metric_prefix in your dashboard_assigns to the snakecase version of your prefix, the default plug_router_metric_prefix is {otp_app}_prom_ex_plug_router.
• ignore_routes: This option is OPTIONAL and is used to ignore certain paths.

To use plugin in your application, add the following to your PromEx module:

defmodule WebApp.PromEx do
  use PromEx, otp_app: :web_app

  alias PromEx.Plugins

  @impl true
  def plugins do
    [
      ...
      {Plugins.PlugRouter,
        event_prefix: [:webapp, :router], metric_prefix: [:prom_ex, :router], routers: [WebApp.Router]}
    ]
  end

  @impl true
  def dashboard_assigns do
    [
      datasource_id: "...",
      plug_router_metric_prefix: "prom_ex_router"
    ]
  end

  @impl true
  def dashboards do
    [
      ...
      {:prom_ex, "plug_router.json"}
    ]
  end
end

To ignore certain paths, pass a list of routes using the :ignore_routes option

defmodule WebApp.PromEx do
  use PromEx, otp_app: :web_app

  @impl true
  def plugins do
    [
      ...
      {PromEx.Plugins.PlugRouter,
        event_prefix: [:webapp, :router], metric_prefix: [:prom_ex, :router], routers: [WebApp.Router],
        ignore_routes: ["/metrics"]}
    ]
  end

  @impl true
  def dashboards do
    [
      ...
      {:prom_ex, "plug_router.json"}
    ]
  end
end