There are two critical services controlling update and reboot in CoreOS. They are
Update-engine.service takes care of periodically checking for updates from the appropriate release channel specified. A default check for update is done 10 minutes after reboot or at one-hour intervals.
The following output shows you the status of update-engine.service:
The release channel is specified in
/etc/coreos/update.conf. In the following node, the release channel is selected as stable. The release channel is derived from
Update-engine.service takes care ...