The NSDistributedNotificationCenter provides a versatile yet simple mechanism for objects in different processes to communicate effectively while knowing very little about each others' internals.
A distributed notification center acts much like a normal notification center, but it handles notifications on a machine-wide (or local network wide) basis rather than just notifications within a single process. Objects are able to register themselves as observers for particular notification names and objects, and they will then receive notifications (including optional user information consisting of a dictionary of property-list objects) as they are posted.

Since posting of distributed notifications involves inter-process (and sometimes inter-host) communication, it is fundamentally slower than normal notifications, and should be used relatively sparingly. In order to help with this, the NSDistributedNotificationCenter provides a notion of 'suspension', whereby a center can be suspended causing notifications for observers in the process where the center was suspended to cease receiving notifications. Observers can specify how notifications are to be handled in this case (queued or discarded) and posters can specify that particular notifications are to be delivered immediately irrespective of suspension.

Distributed notifications are mediated by a server process which handles all notifications for a particular center type. In GNUstep this process is the gdnc tool, and when started without special options, a gdnc process acts as the local centre for the host it is running on. When started with the GSNetwork user default set to YES, the gdnc tool acts as a local network wide server (you should only run one copy of gdnc like this on your LAN).
The gdnc process should be started at machine boot time, but GNUstep will attempt to start it automatically if it can't find it.

MacOS-X currently defines only a notification center for the local host. GNUstep also defines a local network center which can be used from multiple hosts. By default the system sends this to any gdnc process it can find which is configured as a network-wide server, but the GDNCHost user default may be used to specify a particular host to be contacted... this may be of use where you wish to have logically separate clusters of machines on a shared LAN.