A resource can be added manually to a catalogue service by creating or uploading its metadata, but metadata can also be added by automated crawling of other catalogues.
Work on this concept is done in the following link:
Identify main components of manual metadata creation software tools
Demonstrate harvesting and crawling mechanisms for automated metadata collection
Apply harvesting using GeoNetwork Open Source tool
Demonstrate publishing in some popular SDI (NSDI) portals like INSPIRE and GOS geoportals
In progress (GI-N2K)