User Interfaces

Phoenix web interfaces

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.

Choosing a project structure

Although Nerves supports umbrella projects, the preferred project structure is to simply use separate Mix projects, side-by-side with path dependencies between them, in the same source code repository. We call this a "poncho project" structure. For the reasoning behind this, please see the original blog post describing poncho projects.

Using a poncho project structure

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

mkdir my_app && cd my_app
mix nerves.new my_app_firmware
mix phx.new my_app_ui --no-ecto --no-webpack

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_firmware/mix.exs

# ...
  defp deps do
    [
      # Dependencies for all targets
      {:my_app_ui, path: "../my_app_ui"},
      {:nerves, "~> 1.4", runtime: false},
      # ...
    ]
  end
# ...

If we're using the poncho project structure, we can skip ahead to the section where we configure networking.

Using an umbrella project structure

If we would rather use the umbrella project structure instead, we can do so as follows:

mix new my_app --umbrella
cd my_app/apps
mix nerves.new my_app_firmware
mix phx.new my_app_ui --no-ecto --no-webpack

Then, we add the Phoenix my_app_ui project to the my_app_firmware project as a dependency using the in_umbrella option instead of the path option:

# apps/my_app_firmware/mix.exs

# ...
  defp deps do
    [
      # Dependencies for all targets
      {:my_app_ui, in_umbrella: true},
      {:nerves, "~> 1.4", runtime: false},
      # ...
    ]
  end
# ...
Specifying configuration order

By default when you use the umbrella project style, the top-level configuration loads the sub-project configurations in lexicographic order:

# my_app/config/config.exs

use Mix.Config

import_config "../apps/*/config/config.exs"

This can cause problems, depending on the names of your sub-projects, because it is likely that we will want to override certain device-specific settings in the my_app_firmware config. We can solve this by specifying the order in which the config files get imported:

# my_app/config/config.exs

use Mix.Config

import_config "../apps/my_app_ui/config/config.exs"
import_config "../apps/my_app_firmware/config/config.exs"

Configure networking

By default, the my_app_firmware project will include the nerves_pack dependency, which simplifies the network setup and configuration process. At runtime, nerves_pack will detect all available interfaces that have not been configured and apply defaults for usb* and eth* interfaces.

For eth* interfaces, the device attempts to connect to the network with DHCP using ipv4 addressing.

For usb* interfaces, it uses [vintage_net_direct] to run a simple DHCP server on the device and assign the host an IP address over a USB cable.

If you want to use some other network configuration, such as wired or wireless Ethernet, please refer to the [nerves_pack documentation] and the underlying [vintage_net documentation] as needed.

Configure Phoenix

In order to deploy the my_app_ui Phoenix-based project along with the Nerves-based 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_ui configuration won't be applied automatically, so we should either import it from there or duplicate the required configuration.

Assuming that we're using the poncho project structure, our configuration might look like this:

# my_app_firmware/config/config.exs

use Mix.Config

# When we deploy to a device, we use the "prod" configuration:
import_config "../../my_app_ui/config/config.exs"
import_config "../../my_app_ui/config/prod.exs"

config :my_app_ui, MyAppUiWeb.Endpoint,
  # Nerves root filesystem is read-only, so disable the code reloader
  code_reloader: false,
  http: [port: 80],
  # Use compile-time Mix config instead of runtime environment variables
  load_from_system_env: false,
  # Start the server since we're running in a release instead of through `mix`
  server: true,
  url: [host: "nerves.local", port: 80]

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 or umbrella project.

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

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

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

cd my_app_firmware
export MIX_TARGET=rpi3
mix deps.get
mix firmware
# (Connect the SD card)
mix firmware.burn