Using Erlang Distribution to test hardware

Not just for distributed networks

Iterating quickly over code changes when working with new hardware can be tricky. Nerves has built-in mechanisms that can be helpful, but what if there were an even faster, more efficient way?

The problem

Imagine you are writing code that targets a specific piece of hardware. For this basic example, lets assume you have a button attached to a pin on a Nerves powered device.

You want to iterate quickly over your code. It’s early days and you don’t know exactly what your modules will look like. You know you need to turn the LED on but are unsure of how to do this right now.

Solution 1: The “intuitive” solution

This will be one way to accomplish what you are after. I will outline basic steps below:

1) write code

2) mix firmware

3) mix firmware.burn or mix firmware.push

4) (only if used firmware.burn) insert SDCard + power

5) (only if used firmware.push) wait for reboot

6) test changes

7) back to step 1

Pros to the “intuitive” solution

  1. It is relatively quick.

On my machine building + pushing firmware takes about 1.5 minutes all in all, then waiting for boot is another minute or so depending on the complexity of the application. Say 2 minutes to start to finish. Note this is the same for one single character change, or an entire rewrite of your module/application.

  1. It is simple.

This is about the easiest thing one could understand. Change code, push code, reboot. No complexity added to code.

Cons to the “intuitive” solution

1) it could be quicker.

2 minutes seems fast to push an update, until you are in a position where you are doing tons of iteration. Waiting 2 minutes to test out a one line change is a ton of time spent waiting. Relevant XKCD Comic.

2) it’s inefficient.

You only changed one line? why should you have to wait for an entire update AND a reboot for that?

Solution 2: The “Raspbian” Solution

This is another common way to bootstrap an application or feature. Basically, you spin up a version of Raspbian or similar distro on your device such as plain Debian on Beagle based boards. I’ll outline the steps to this one:

1) Install OS.

2) Connect to OS via ssh, keyboard+mouse, UART etc.

3) configure/update OS (one time only if you keep the SDCard handy).

4) get code onto OS.

5) edit code.

6) test code.

7) back to step 5.

Pros of the “Raspbian” Solution

1) After initial setup it can be quick

After you’ve got your OS up and running and configured, iteration is pretty fast, as long as you can make changes quickly.

2) It’s good for testing hardware

A lot of sensors/hats/hardware has libraries that expect you to be using the standard OS for your device. Libraries are often written in languages such as Python, and may require porting to Elixir to work with Nerves easily. Testing on the standard OS is a good way to test out hardware and check which dependencies might be needed for Nerves.

Cons of the “Raspbian” Solution

1) It’s awkward to setup

Getting up and running can be easy, but bloated OS makes it take a long time, you need to know OS tools to get connected.

2) It’s awkward to use

Even after getting up and running, using this solution is hard. You need to be connected directly to the device, to write code and test it. Most of these devices aren’t quite powerful enough to run a web browser, or you aren’t using a desktop environment at all, so you need a way of reading docs on a different machine, or loading them a different way. Copying examples from a web browser is a no-go if not using SSH or UART. If using SSH or UART, you won’t have a visual text editor.

Solution 3: The “Erlang” Solution

Since Nerves is built on top of Erlang and OTP, we have the ability to get the best of both worlds. By design Erlang has the following built into the language:

  • Hot code reloading
  • Remote code loading

This solves both of the issues with the “intuitive” solution. Code can be reloaded in real time without pushing a new update, and without a reboot. Your iteration is now only limited to how fast it can be tested. I’ll outline the steps to this one:

1) build/push firmware. Same as steps 1-3 of the “Intuitive” solution. but it only needs to be done once.

2) Edit code.

3) load code.

4) test code.

5) back to step 2.

Pros of the “Erlang” Solution

1) It’s Instant

Code can be reloaded and tested as fast as you can type it. (well almost) Reloading an OTP app takes about 3 seconds for my machine.

2) It keeps you on the “host” machine as much as possible

When implemented properly, you will be pressed to find a reason to actually connect directly to your device to investigate things.

Cons of the “Erlang” Solution

1) It’s more complex than other solutions

This solution requires you to actively write code in a way that can work this way. It’s not that bad once you get the hang of it.

2) It’s not compatible with all libraries.

Since it requires code to be written in a particular way, this solution has some issues working with some libraries. This turns out to not really be a huge problem in practice however.

3) It only works for Erlang/Elixir code.

This solution will not work for data that winds up in the priv directory, such as C ports/nifs, eex templates, etc.

Implementing the “Erlang” Solution

Implementing this solution isn’t as simple as adding a dependency. It requires you to write your code in a way that is compatible. I’m assuming you already have an application scaffolded with mix nerves.new and it already has nerves_init_gadget configured and working properly.

Looking back at our problem, let’s start with toggling an LED.

Add {:elixir_ale, "~> 1.2"} to your deps (not just your target deps), and push firmware to your device.

Consulting the gpio docs we can see that we need to call GPIO.start_link/3 to start a GPIO connection, but that will start a pid locally on your host machine if you try it. But turns out Erlang has the solution built in:

defmodule MyApp.LEDs do
  @moduledoc """
  Control some LEDs
  """

  alias ElixirALE.GPIO

  def toggle(pin, time) do
    {:ok, pid} = start_gpio(pin, :output)
    :ok = GPIO.write(pid, 1)
    Process.sleep(time)
    :ok = GPIO.write(pid, 0)
    GPIO.release(pid)
  end

  case Mix.Project.config()[:target] do
    "host" ->
      defp start_gpio(pin, direction, opts \\ []) do
        # :"my_app@nerves.local" is the node name of your Nerves
        # device. See `nerves_init_gadget` docs for more info.
        :rpc.call(:"my_app@nerves.local", GPIO, :start_link, [pin, direction, opts])
      end
    _ ->
      defp start_gpio(pin, direction, opts \\ []) do
        GPIO.start_link(pin, direction, opts)
      end
  end
end

and on our host machine if we try it out:

iex -S mix
MyApp.LEDs.toggle(16, 100)
** (MatchError) no match of right hand side value: {:badrpc, :nodedown}
    (remote_io) lib/mix/my_app/leds.ex:9: MyApp.LEDs.toggle/2

What happened? Well Our host machine isn’t connected to our device, so we get an error saying :nodedown. We will have to setup distribution on the host machine only. In application.ex we should have a scaffolded function:

  # ...
  def children("host") do
  # ...

Lets add something to that function:

  def children("host") do
    {:ok, _} = Node.start(:host_machine)
    # This cookie can be found in `rel/vm.args`
    Node.set_cookie(:lyclzfysmqcpj5xwxwqnlkvvlda2ukrbswalcfqm45jgikhza65xzechthr7qd7r)
    true = Node.connect(:"my_app@nerves.local")
    []
  end

This starts distribution and should connect to your device. Now on your host machine try it again:

iex -S mix
Erlang/OTP 21 [erts-10.0.4] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Interactive Elixir (1.7.3) - press Ctrl+C to exit (type h() ENTER for help)
iex(host_machine@hostname.lan)1> MyApp.LEDs.toggle(13, 1000)
:ok

You should see your led turn on and back off a second later. And the best part? You didn’t need to do any firmware building, burning or pushing. Want to try it again with a different amount of time? Just change the file, recompile and try again.

So now your LED flashes for exactly how long you want, and you’re ready to push it out? Well you could build firmware and push it, but we can do this a bit faster.

Create a new file in your project called lib/mix/tasks/firmware.reload.ex

defmodule Mix.Tasks.Firmware.Reload do
  use Mix.Task

  def run(_) do
    node_name = :"my_app@nerves.local"
    {:ok, _} = Node.start(:host_machine)
    # This cookie can be found in `rel/vm.args`
    Node.set_cookie(:lyclzfysmqcpj5xwxwqnlkvvlda2ukrbswalcfqm45jgikhza65xzechthr7qd7r)
    true = Node.connect(node_name)
    Application.load(:my_app)
    {:ok, my_app_mods} = :application.get_key(:my_app, :modules)
    for module <- my_app_mods do
      {:ok, [{^node_name, :loaded, ^module}]} = IEx.Helpers.nl([node_name], module)
    end
  end
end

This will reload your module in real time, and allow you to test it on the device without need for a reboot.

Conclusion

There is not always exactly one solution for iterating code on real hardware. Erlang’s built in mechanisms can allow us to iterate over code in a quick and easy way in many circumstances. I’ve recently adopted this method and along with proper mocking and contracts it has proved to be an incredibly valuable tool for writing Nerves applications quickly and efficiently.

comments powered by Disqus