Catalogues that provide uuids for metadata (for example GeoNetwork and a CSW server) can be harvested several times without having to take care about metadata overlap. This allows the possibility to perform a thematic search and a metadata belonging to multiple searches is harvested only once and not duplicated.
This mechanism allows the GeoNetwork harvesting type to be combined with other GeoNetwork nodes to perform hierarchical harvesting. This way a metadata can be harvested from several nodes. For example, consider this scenario:
Node (A) has created metadata (a)
Node (B) harvests (a) from (A)
Node (C) harvests (a) from (B)
Node (D) harvests from both (A), (B) and (C)
In this scenario, Node (D) will get the same metadata (a) from all 3 nodes (A), (B), (C). The metadata will flow to (D) following 3 different paths but thanks to its uuid only one copy will be stored. When (a) will be changed in (A), a new version will flow to (D) but, thanks to the change date, the copy in (D) will be updated with the most recent version.