Central to all ICOS operations will be a comprehensive and continuously updated metadata database that describes all ICOS data objects - including sensor data, aggregated data products, observation station information and measurement protocols. This database will be the backbone of the ICOS cataloguing service, serving the data discovery functionalities of the Carbon Portal, and supporting the long-term repository archiving. The data object metadata database (DOMDB) design must be flexible in order to both handle (merge) the various ICOS-internal metadata schemas, as well as allowing efficient interfacing with other data portals and cataloguing services.