What updates require a reboot ?

SUSE Linux Enterprise Server (any)

After applying updates it is unsure if the system needs to be rebooted or not.

It depends of what kind of updates have been installed. If it is unsure if a reboot is required or not we strongly recommend to reboot to be on the safe side.

zypper does try to notice prior the updates applied that this updates do require a reboot. Following message will be printed prior confirming updates to be applied

The following packages require system reboot

Depending on the zypper version, after the upgrade there is a message.

Since the last system boot core libraries or services have been updated.
Reboot is suggested to ensure that your system benefits from these updates.

In both cases a reboot is needed. If the updates do only affect any running program following message is printed after update

There are running programs which still use files and libraries deleted or updated by recent upgrades. 
They should be restarted to benefit from the latest updates. Run 'zypper ps -s' to list these programs.

In that case the administrator needs to restart the affected programs/services. A list of services that need to be restarted may be obtained by executing:

zypper ps -sss

If the programs/services are not restarted, it is possible that unexpected behaviour occurs sometime after the update, which could include programs/services to fail suddenly. While libraries are loaded into memory this will not affect the program/service itself immediately after update, but there could be a plugin system or similar that does result in a additional library is loaded which was changed during the update and a related already loaded library does miss some newly added or removed symbols etc. To avoid this, it must be taken care to restart the affected programs/services.

In doubt, a reboot after a patch action is recommended.