Normally when a patch is applied the Oracle services must be shutdown. This means a downtime and a maintenance window must be open while the operation takes place. The DBA must ensure that the process will be successful on the first attempt otherwise there it must be a fall back procedure in place.
Among the high availability features provided by 11g, Oracle introduced the Hot patching concept. Hot patching allows the DBA to install, enable, and disable a patch online without disruption to Oracle services. Hot patches don't require instance shutdown, and they are installed with the traditional OPatch tool. This tool can detect conflicts between hot patches.
Not all patches in 11g can be installed in Hot patch mode. First you must find out if the patch supports the hot patch apply feature. You can use the following command to determine if this mode is allowed:
opatch query -is_online_patch <PatchLocation>
or
opatch query <PatchLocation> -all
The patches...