  WorldGRID is an intercontinental testbed spanning Europe and the US
integrating architecturally different Grid implementations based on the Globus
toolkit. The WorldGRID testbed has been successfully demonstrated during the
WorldGRID demos at SuperComputing 2002 (Baltimore) and IST2002 (Copenhagen)
where real HEP application jobs were transparently submitted from US and Europe
using "native" mechanisms and run where resources were available, independently
of their location. To monitor the behavior and performance of such testbed and
spot problems as soon as they arise, DataTAG has developed the EDT-Monitor tool
based on the Nagios package that allows for Virtual Organization centric views
of the Grid through dynamic geographical maps. The tool has been used to spot
several problems during the WorldGRID operations, such as malfunctioning
Resource Brokers or Information Servers, sites not correctly configured, job
dispatching problems, etc. In this paper we give an overview of the package,
its features and scalability solutions and we report on the experience acquired
and the benefit that a GRID operation center would gain from such a tool.
