Rebuilding the node-red snap in a device focused way with additional node-red modules

While there is a node-red snap in the snap store (to be found at https://snapcraft.io/node-red with the source at https://github.com/dceejay/nodered.snap) it does not really allow you to do a lot with it on i.e. a Raspberry Pi if you want to read sensor data that does not actually come in via the network …

The snap is missing all essential interfaces that could be used for any sensor access (gpio, i2c, Bluetooth, spi or serial-port) and it does not even come with basics like hardware-observe, system-observe or mount-observe to get any systemic info from the device it runs on.

While the missing interfaces are indeed a problem, there is the fact that strict snap packages need to be self contained and hardly have any ability to dynamically compile any software …. Now, if you know nodejs and npm (or yarn or gyp) you know that additional node modules often need to compile back-end code and libraries when you add them to your nodejs install. Technically it is actually possible to make “npm install” work but it is indeed hard to predict what a user may want to install in her installation so you would also have to ship all possible build systems (gcc, perl, python, you name it)
plus all possible development libraries any of the added modules could ever require …

That way you might technically end up with a full OS inside the snap package. Not really a desirable thing to do (beyond the fact that this would even with the high compression snap packages use end up in a gigabytes big snap).

So lets take a look at whats there already in the upstream snapcraft.yaml we can find a line like the following:

npm install --prefix $SNAPCRAFT_PART_INSTALL/lib node-red node-red-node-ping node-red-node-random node-red-node-rbe node-red-node-serialport

This is actually great, so we can just append any modules we need to that line …

Now, as noted above, while there are many node-red modules that will simply work this way, many that are interesting for us to access sensor data will need additional libs that we will need to include in the snap as well …

In Snapcraft you can easily add a dependency via simply adding a new part to the snapcraft.yaml, so lets do this with an example:

Lets add the node-red-node-pi-gpio module, lets also break up the above long line into two and use a variable that we can append more modules to:

DEFAULT_MODULES="npm node-red node-red-node-ping node-red-node-random node-red-node-rbe \
                 node-red-node-serialport node-red-node-pi-gpio"
npm install --prefix $SNAPCRAFT_PART_INSTALL/lib $DEFAULT_MODULES

So this should get us the GPIO support for the Pi into node-red …

But ! Reading the module documentation shows that this module is actually a front-end to the RPi.GPIO python module, so we need the snap to ship this too … luckily snapcraft has an easy to use python plugin that can pip install anything you need. We will add a new part above the node-red part:

parts:
...
  sensor-libs:
    plugin: python
    python-version: python2
    python-packages:
      - RPi.GPIO
  node-red:
    ...
    after: [ sensor-libs ]

Now Snapcraft will pull in the python RPi.GPIO module before it builds node-red (see the “after:” statement i added) and node-red will find the required RPi.GPIO lib when compiling the node-red-node-pi-gpio node module. This will get us all the bits and pieces to have GPIO support inside the node-red application …

Snap packages are running confined, this means they can not see anything of the system we do not allow it to via an interface connection. Remember that i said above the upstream snap is lacking some such interfaces ? So lets better add them to the “apps:” section of our snap (the pi-gpio node module wants to access /dev/gpiomem as well as the gpio device-node itself, so we make sure both these plugs are available to the app):

apps:
  node-red:
    command: bin/startNR
    daemon: simple
    restart-condition: on-failure
    plugs:
      ...
      - gpio
      - gpio-memory-control

And this is it, we have added GPIO support to the node-red snap source, if we re-build the snap, install it on an Ubuntu Core device and do a:

snap connect node-red:gpio-memory-control
snap connect node-red:gpio pi:bcm-gpio-4

We will be able to use node-red flows using this GPIO (for other GPIOs you indeed need to connect to the pi:bcm-gpio-* of your choice … (the mapping for Ubuntu Core follows https://pinout.xyz/ )

I have been collecting a good bunch of possible modules in a forked snap that can be found at https://github.com/ogra1/nodered-snap a binary of this is at https://snapcraft.io/node-red-rpi and i plan a series of more node-red centric posts the next days telling you how to wire things up, with example flows and some deeper insight how to make your node-red snap talk to all the Raspberry Pi interfaces, from i2c to Bluetooth.

Stay tuned !

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