ClusterAPI and Systemd-sysext

[t-lo]

Discussed and presented on in the September 25, 2024 ClusterAPI meeting (https://docs.google.com/document/d/1GgFbaYs-H6J5HSQ6a7n4aKpk0nDLE2hgG2NSOM9YIRw/edit#heading=h.s6d5g3hqxxzt).

1. systemd-sysext are a distro-independent and vendor-independent way of shipping Kubernetes for clusterAPI. While it doesn't have much traction with CAPI providers at this time, it is supported by a wide range of distros and with recent changes, has become feasible for general purpose distros like Ubuntu (systemd 256 and above). Sysexts allow using stock distro images on vendor clouds, reducing CAPI operators' maintenance load. (no custom-built, self-hosted images required)
   1. Sysexts are easy to adapt to non-systemd distros as they use basic Linux mechanisms ("glorified overlayfs mounts").
2. systemd-sysupdate is a complementary service that allows integration of atomic in-place updates of Kubernetes. It is supported on a wide range of distros and too uses basic mechanisms like HTTPS endpoints, index files, and semver matching. It uses symlinks for staging / applying updates; roll-back is possible by simply sym-linking the previous release. Sysupdate is very easy to integrate with Kubernetes reboot managers like kured.

A presentation (with live demos) on the above is here: https://www.youtube.com/watch?v=nSd_S-W-G9Y

[t-lo]

This topic was originally discussed here: #11029. Branching this off into a separate discussion so as to not dilute #11029's scope.