Take a Photo (Raspberry Pi)

CM3

This lambda captures a still frame using a Raspberry Pi camera on a remote edge device. The On Prem CLI is used to demonstrate manually triggering the lambda and taking delivery of the image using a remote desktop.

graph LR;

cli --> control_plane;
control_plane <-- tunnel --> agent;
agent --> camera;

subgraph user_edge[User Edge]
cli[CLI];
end

subgraph cloud[Cloud <small>api.on-prem.net</small>]
control_plane[Control Plane];
end

subgraph device_edge[Device Edge]
agent[Agent];
camera[Camera];
end

Note that manually triggering a lambda is unusual in that it requires device connectivity to the control plane. A more typical scenario is where Lambdas and their Lambda Trigger control loops run autonomously at the device edge, regardless of the device's connectivity to the control plane.

Register the lambda

$ onprem generate xid
clut5qm56a1d39be96j0

# capture_image_rpi.yaml
id: clut5qm56a1d39be96j0
kind: Lambda
name: capture_image_rpi
description: >
  Capture a still frame from a Raspberry Pi camera.
runAt:
  deviceId: ci2fabp32ckvhk1g9qe0
scriptContentType: text/x-lua
script: >
  local M = {}

  function M.handler(event, context)
    local filename = os.tmpname()
    os.execute('rpicam-jpeg --nopreview -o ' .. filename)
    local file = io.open(filename, 'rb')
    local event = {
      data = file:read('*a'),
    }
    io.remove(file)
    return event
  end

  return M

$ onprem apply capture_image_rpi.yaml

It will now show up in the cloud console.

Cloud Console

Invoke it

The CLI can display the returned event as JSON, which is inefficient:

$ onprem run lambda clut5qm56a1d39be96j0
{"data":[...much raw data...]}

But it can also pluck the data field out of the returned JSON, which is efficient and does not involve JSON parsing. This is because agents return event data fields separately for efficient transport encoding.

If the returned event contains any other fields, they are displayed to stdout while other things are written to stderr.

$ onprem run lambda clut5qm56a1d39be96j0 --event-data-to-file out.jpeg > event.json
Wrote event[data] to out.jpeg (940.9K)

$ cat event.json
{}

$ open out.jpeg

Photo