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AppSignal for Elixir doesn’t automatically instrument mix tasks, but you can set up manual instrumentation and error handling using custom instrumentation.

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Starting and stopping AppSignal

Since Mix tasks don’t automatically start applications, you’ll need to explicitly start the :appsignal application in your task: 
Application.ensure_all_started(:appsignal)
To make sure AppSignal has enough time to flush data to its agent, call Appsignal.Nif.stop/0 at the end of your task, which blocks until the extension has had enough time to flush the sent payloads to the agent before stopping.

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Catching errors

To wrap errors in your task to be sent to AppSignal, add a rescue block to catch errors from your code. Pass the captured exception to Appsignal.send_error/3 to report it to AppSignal. Add an after clause to your rescue block to call Appsignal.Nif.stop/0. 
defmodule Mix.Tasks.Rescue do
  use Mix.Task

  def run(_) do
    {:ok, _} = Application.ensure_all_started(:appsignal)

    raise "rescue!"
  catch
    kind, reason ->
      stack = __STACKTRACE__
      Appsignal.send_error(kind, reason, stack)
      reraise(reason, stack)
  after
    Appsignal.Nif.stop
    :timer.sleep(35_000) # For one-off containers
  end
end
Note: A sleep of 35 seconds was added to the end of the Mix task. This is recommended for tasks that are run on one-off containers. When the task completes the process stops. The Appsignal.Nif.stop/0 function call flushes all the transaction data currently in the AppSignal extension to our agent. There it is transmitted on a 30 second interval. The one-off container may have exited before that time. To ensure the data is still transmitted, it needs to wait for 35 seconds.

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Measuring performance

To measure performance in your task, start and stop a transaction manually and add events using the instrumentation helpers, like described in the custom helper instrumentation. 
defmodule Mix.Tasks.Instrument do
  use Mix.Task

  def run(_) do
    {:ok, _} = Application.ensure_all_started(:appsignal)

    Appsignal.instrument(fn span ->
      span
      |> Appsignal.Span.set_namespace("background_job")
      |> Appsignal.Span.set_name("Mix.Tasks.Instrument")

      Appsignal.instrument(fn span ->
        Appsignal.Span.set_name("task.instrumented")
        :timer.sleep(1_000)
      end)
    end)

    Appsignal.Nif.stop()
    :timer.sleep(35_000) # For one-off containers
  end
end