Is it possible to never shutdown linux?

Assuming hardware failure is not a factor, and the requirement of being able to update periodically, is it possible to never shutdown Linux?

I typically do a full reboot after updates, especially kernel updates, but is there a way to keep my machine on and still do these? People always hear about incredible up-time, but how is that really possible if you must reboot after major updates.

Maybe a different run level? But then how would the kernel update kick in?

Server/Box "uptime" is an illusion. Unless your objective is to have incredible uptime in order to prove some kind of point then I wouldn't focus on it.

What matters is service availability. If you need a service to be available all the time then it might be useful to improve individual system uptime or it may well be simpler and more cost effective to create a cluster, for example, than to try and take the availability of a commodity server from 99% to 99.999%

A standard linux is not designed to do this. It tries to minimze the reboots, but on some updates, like switching the kernel, reboot may be required.

There are projects that aim to also avoid this. See http://www.ksplice.com/ - now Oracle.

For live patching there is pannus, see http://lists.linux-foundation.org/pipermail/live_patching/attachments/20050725/14611ec4/050726pannus_latest.pdf