[RFE] Multi-stage updates

[krnowak]

Current situation

Currently however old version of Flatcar is running, it's always updated to the latest version directly. This requires us to make sure that such an update is feasible, even for ancient versions of Flatcar.

Impact

We may want to implement a feature that requires a migration to happen during an update, which by itself may have some requirements not met by really old versions of Flatcar. Currently such features can't be implemented or need to be implemented in a way that increases maintenance burden.

Ideal future situation

Really old versions are updated eventually to the latest version by doing a series of updates to intermediate versions of Flatcar. This would allow us to implement a feature with a cutoff version, where Flatcar instances with a version lower than the cutoff will need to update to the cutoff version before being updated to the newest one.

Implementation options

This is something to implement on the Nebraska side. Syncing with other Nebraska instances needs to be thought out too.

Additional information

Citing @pothos from our matrix channel:

My first thought was it would be a property of the channel, but no, it should be a property of the package
And for it to work with syncing downstream Nebraska servers, it would have to be an extra Omaha annotation that regular clients ignore.

[till]

@krnowak Maybe instead of building something, adding this to the documentation would suffice? @pothos mentioned this ticket when I reported that I couldn't update. Thinking about it now, I think I hit this before but did not remember yesterday when I opened the ticket.

The work around to set GROUP=lts and to update to the LTS, and then resetting to GROUP=stable worked nicely.

Adding this here would be great:
https://www.flatcar.org/docs/latest/setup/releases/update-strategies/

[pothos]

Thanks for the idea, I've opened a PR: flatcar/flatcar-website#320