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Phoenix web interface

The Phoenix web framework makes an excellent companion to Nerves-based devices that need to serve content over HTTP directly from the device. For example, a device with no display might provide administration and configuration interfaces intended to be accessed from a computer or mobile device. Hello Phoenix is an example of structuring a project as described here.

Choosing a project structure

There are two commonly used project structures for Nerves-based devices that uses Phoenix web interface:

Although Nerves supports both, the preferred project structure is what we call poncho project structure. For the reasoning behind this, please see the original blog post describing poncho projects. The following steps assume that we use the poncho project structure.

Using the poncho project structure, we simply use separate Mix projects, side-by-side with path dependencies between them, in the same source code repository.

Create a poncho project

First, we generate the two new Mix projects in a containing directory:

# Create a container directory called "my_app"
mkdir my_app && cd my_app

# Create a Nerves firmware project called "my_app_firmware"
mix nerves.new my_app_firmware

# Create a Phoenix 1.6 UI project called "my_app_ui"
mix phx.new my_app_ui --no-ecto --no-mailer

Now, we add the Phoenix-based my_app_ui project to the my_app_firmware project as a dependency, because we want to use the my_app_firmware project as a deployment wrapper around the my_app_ui project.

# my_app/my_app_firmware/mix.exs

# ...
  defp deps do
    [
      # Dependencies for all targets
      {:nerves, "~> 1.7.0", runtime: false},
      # ...
      {:my_app_ui, path: "../my_app_ui", targets: @all_targets, env: Mix.env()},
      # ...
    ]
  end
# ...

We need a few adjustments to the UI project's mix.exs. By default when MIX_ENV is dev, the default Phoenix setup runs:

phoenix_live_reload to reload code changes
esbuild to rebuild assets as needed
tailwindcss to optimize css (Phoenix 1.7+)

This doesn't work on target device, so we need to limit it to only run on the host:

# my_app/my_app_ui/mix.exs

  defp deps do
    [
      {:phoenix, "~> 1.6.0"},
      # ...
      {:phoenix_live_reload, "~> 1.2", only: :dev && Mix.target() == :host},
      # ...
      {:esbuild, "~> 0.5", runtime: Mix.env() == :dev && Mix.target() == :host},
      {:tailwind, "~> 0.1.8", runtime: Mix.env() == :dev && Mix.target() == :host},
      # ...
    ]
  end

Configure networking

Refer to the Connecting to Nerves Target page.

Configure Phoenix

In order to deploy the my_app/my_app_ui Phoenix-based project along with the Nerves-based my_app/my_app_firmware project, we need to configure our Phoenix.Endpoint using appropriate settings for deployment on an embedded device. If we're using a poncho project structure, we'll need to keep in mind that the my_app/my_app_ui configuration won't be applied automatically, so we should either import it from there or duplicate the required configuration.

Our configuration might look like this (as of Phoenix 1.6.2):

# my_app_/my_app_firmware/config/target.exs

config :my_app_ui, MyAppUiWeb.Endpoint,
  url: [host: "nerves.local"],
  http: [port: 80],
  cache_static_manifest: "priv/static/cache_manifest.json",
  secret_key_base: "HEY05EB1dFVSu6KykKHuS4rQPQzSHv4F7mGVB/gnDLrIu75wE/ytBXy2TaL3A6RA",
  live_view: [signing_salt: "AAAABjEyERMkxgDh"],
  check_origin: false,
  render_errors: [view: MyAppUiWeb.ErrorView, accepts: ~w(html json), layout: false],
  pubsub_server: Ui.PubSub,
  # Start the server since we're running in a release instead of through `mix`
  server: true,
  # Nerves root filesystem is read-only, so disable the code reloader
  code_reloader: false

# Use Jason for JSON parsing in Phoenix
config :phoenix, :json_library, Jason

There we have it! A Phoenix-based web application is now ready to run on our Nerves-based embedded device. By separating the Phoenix-based project from the Nerves-based project, we enable teams to work on the core functionality and user interface code even without having physical hardware. We also minimize the hardware/software integration effort by managing both the core software and the firmware deployment infrastructure in a single poncho project.

Develop the UI

When developing the UI, we can simply run the Phoenix server from the my_app_ui project directory:

cd path/to/my_app_ui
iex -S mix phx.server

Deploy the firmware

First we build our assets in the my_app_ui project directory and prepare them for deployment to the firmware:

cd path/to/my_app_ui

# We want to build assets on our host machine.
export MIX_TARGET=host
export MIX_ENV=dev

# This needs to be repeated when you change dependencies for the UI.
mix deps.get

# This needs to be repeated when you change JS or CSS files.
mix assets.deploy

When it's time to deploy firmware to our hardware, we can do it from the my_app_firmware project directory:

cd path/to/my_app_firmware

# Specify our target device.
export MIX_TARGET=rpi3
export MIX_ENV=dev

mix deps.get
mix firmware
# (Connect the SD card)
mix firmware.burn

Scenic

Scenic is 2D UI framework written in Elixir that's designed with embedded systems in mind and works well with Nerves on screens like the Raspberry Pi Touch Display or HDMI connected screens.

Helpful links:

eInk displays

Some initial work has been done to support eInk displays like the Pimoroni Inky pHAT and wHAT models. Look at the :inky repo for more info.

OLED

Basic work has been done to support small OLED screens with the SSD1306 chip which are usually smaller screens a few inches wide. More info in the :oled docs

Web Kiosks

Several companies have integrated web browsers with Nerves for kiosk applications. Unfortunately, public repositories are currently unmaintained. If this is of interest to you, take a look at the repos in the Nerves Web Kiosks org and consider helping maintain them.

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