I’m looking for a breakdown of the minimal requirements for a kubelet implementation. Something like sequence diagrams/descriptions and APIs.
I’m looking to write a minimal kubelet I can run on a reasonably capable microcontroller so that app binaries can be loaded and managed from an existing cluster (the container engine would actually flash to a connected microcontroller and restart). I’ve been looking through the kubelet code and there’s a lot to follow so any starting points would be helpful.
A related question, does a kubelet need to run gRPC or can it fall back to a RESTful api? (there’s no existing gRPC I can run on the micro but there is nanopb and existing https APIs)
This probably won't be a full answer, however there are some details that will help you.
First I'll start with related question about using gRPC and/or REST API.
Based on the kubelet code there is a new server creation part to handle HTTP requests. Taking this into account, we can consider kubelet gets requests to its HTTPS endpoint.
Also indirectly seen from kubelet authentication/authorization documentation, there are details only about HTTPS endpoint.
Moving to an API part. It's still not documented properly so the best way to find some information is to look into code, e.g. about endpoints
Last part is this useful page where a lot of information about kubelet API is gathered