View Source Plug.Telemetry (Plug v1.15.3)

A plug to instrument the pipeline with :telemetry events.

When plugged, the event prefix is a required option:

plug Plug.Telemetry, event_prefix: [:my, :plug]

In the example above, two events will be emitted:

  • [:my, :plug, :start] - emitted when the plug is invoked. The event carries the system_time as measurement. The metadata is the whole Plug.Conn under the :conn key and any leftover options given to the plug under :options.

  • [:my, :plug, :stop] - emitted right before the response is sent. The event carries a single measurement, :duration, which is the monotonic time difference between the stop and start events. It has the same metadata as the start event, except the connection has been updated.

Note this plug measures the time between its invocation until a response is sent. The :stop event is not guaranteed to be emitted in all error cases, so this Plug cannot be used as a Telemetry span.

Time unit

The :duration measurements are presented in the :native time unit. You can read more about it in the docs for System.convert_time_unit/3.


defmodule InstrumentedPlug do
  use Plug.Router

  plug :match
  plug Plug.Telemetry, event_prefix: [:my, :plug]
  plug Plug.Parsers, parsers: [:urlencoded, :multipart]
  plug :dispatch

  get "/" do
    send_resp(conn, 200, "Hello, world!")

In this example, the stop event's duration includes the time it takes to parse the request, dispatch it to the correct handler, and execute the handler. The events are not emitted for requests not matching any handlers, since the plug is placed after the match plug.