I have a web server which upon restarting fails to render our main site. I think this is to do with updates applied upon rebooting.
Am I able to restart Ubuntu without applying updates?
After you did
apt-get upgrade or something similar and it upgraded some packages, there's no easy way to make the old versions of files survive a reboot. You can play with packages and restore their old versions; this technically doesn't bring back the old files themselves, it creates new files with identical content.
"Applying updates" during a reboot is not what you may think it is.
Linux and inode-based filesystems work in a way that allows replacing any file with its new version, while the old version is still in use. The old version is available to any process that had opened it before the update occurred, until the process closes the file. If no other path (hardlink) leads to the old file, its content will only be kept within the filesystem until it's no longer in use, although the filesystem itself provides no path to the old content anymore (but OS may).
For this reason updates (or, in term of
apt-get: upgrades) can be done from within the running system. This is common. Compare to the Windows way, where some files are locked unless the OS is not yet (or no longer) fully running. In Linux there's no need to reboot if you just want to replace a file within a filesystem that is smart enough.
Note "replacing a file within a filesystem" is something different than "making processes use the new file".
Making processes use new files requires them to open descriptors leading to the new versions. I don't know any safe way to forcefully "remap" a descriptor, unless the process cooperates and reopens the file (e.g. it tracks the filename like
tail --follow=name, or it reopens on demand). Additionally the executable itself may have been updated.
Usually the best way to "switch" a daemon to new files is to restart it. Some upgrade scripts restart some daemons. On the other hand restarting only some parts of the running OS may be cumbersome or impossible (see How Linux servers update their kernel without rebooting).
That's why you may get "system restart required". Note there's no updating during the reboot. What happens is the old versions become "not in use" during the shutdown phase, the filesystem then knows it can treat them as nonexistent and reuse the space. Restart is just a firm way to ensure everything uses the updated files that are already there.
In most Ubuntu configurations, updates are not applied by default upon rebooting (as opposed to Windows). Unless you have specifically installed and configured automatic update software, Ubuntu will not automatically update. There is likely some other error with your web server configuration.