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Compose web applications with functions
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lib/plug/telemetry.ex
defmodule Plug.Telemetry do
@moduledoc """
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 a single measurement, `:time`, which is the
monotonic time in native units at the moment the event is emitted.
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 request 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.
After the Plug is added, please be sure to add
[:telemetry](https://github.com/beam-telemetry/telemetry) as
project dependency.
Note that this plug measures only the time between its invocation and
the rest of the plug pipeline - this can be used to exclude some plugs
from measurement.
## Time unit
Both `:time` and `:duration` measurements are presented in the `:native`
time unit. You can read more about it in the docs for `System.convert_time_unit/3`.
## Example
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!")
end
end
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.
"""
@behaviour Plug
@impl true
def init(opts) do
{event_prefix, opts} = Keyword.pop(opts, :event_prefix)
unless event_prefix do
raise ArgumentError, ":event_prefix is required"
end
ensure_valid_event_prefix!(event_prefix)
start_event = event_prefix ++ [:start]
stop_event = event_prefix ++ [:stop]
{start_event, stop_event, opts}
end
@impl true
def call(conn, {start_event, stop_event, opts}) do
start_time = System.monotonic_time()
:telemetry.execute(start_event, %{time: start_time}, %{conn: conn, options: opts})
Plug.Conn.register_before_send(conn, fn conn ->
duration = System.monotonic_time() - start_time
:telemetry.execute(stop_event, %{duration: duration}, %{conn: conn, options: opts})
conn
end)
end
defp ensure_valid_event_prefix!(event_prefix) do
if is_list(event_prefix) && Enum.all?(event_prefix, &is_atom/1) do
:ok
else
raise ArgumentError,
"expected :event_prefix to be a list of atoms, got: #{inspect(event_prefix)}"
end
end
end