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Note: Support for custom namespaces was added in version 1.3.0 of the AppSignal for Elixir package.
The AppSignal for Elixir package integrates with Plug. To set up the integration, please follow our installation guide. This page describes how to set up AppSignal in a Plug application, and how to add instrumentation for events within requests. For more information about custom instrumentation, read the Elixir instrumentation documentation. More information can be found in the AppSignal Hex package documentation.

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Installation

The AppSignal instrumentation for Plug is part of a separate package, which depends on the primary appsignal package. Add the appsignal_plug package to your mix.exs file. 
# mix.exs
defmodule AppsignalPlugExample.MixProject do
  # ...

  defp deps do
    [
      {:plug, "~> 1.0"}, # Or plug_cowboy ~> 2.0
      {:appsignal_plug, "~> 2.0"}
      # ...
    ]
  end
end
If you’re already using our appsignal_phoenix package, it’s not necessary to add the appsignal_plug package as it’s already a dependency of the appsignal_phoenix package. See also our Phoenix integration guide.

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Incoming HTTP requests

We’ll start out with a small Plug app that accepts GET requests to / and returns a welcome message. To start logging HTTP requests in this app, we’ll use the Appsignal.Plug module. 
defmodule AppsignalPlugExample do
  use Plug.Router
  use Appsignal.Plug # <- Add this

  plug :match
  plug :dispatch

  get "/" do
    send_resp(conn, 200, "Welcome")
  end
end
This will create a transaction for every HTTP request which is performed on the endpoint.

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Custom instrumentation

Although Appsignal.Plug will start transactions for you, it won’t instrument events in your app just yet. To do that, we need to add some custom instrumentation. This example app looks like the one we used before, but it has a slow function (aptly named slow/0) we’d like to add instrumentation for. To do that, we need to use the Appsignal.instrument/2-3 helper in our called function: 
defmodule AppsignalPlugExample do
  use Plug.Router

  plug :match
  plug :dispatch

  get "/" do
    slow()
    send_resp(conn, 200, "Welcome")
  end

  defp slow do
    Appsignal.instrument("slow", fn ->
      :timer.sleep(1000)
    end)
  end

  use Appsignal.Plug
end
This will add an event for the slow/0 function to the current transaction whenever it’s called. For more information about custom instrumentation, read the Elixir instrumentation documentation.

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Instrumentation for included Plugs

Exceptions in included Plugs are automatically caught by AppSignal, but performance samples need to be set up manually using the custom instrumentation helpers or decorators.

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Plug instrumentation with decorators

To add instrumentation to Plugs, use the Appsignal.instrument/2 function: 
defmodule SlowPlugWithDecorators do
  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _) do
    Appsignal.instrument("SlowPlugWithDecorators", fn ->
      :timer.sleep(1000)
      conn
    end)
  end
end