We present NetTraveler, a database middleware system for WANs that is designed to efficiently run queries over sites that are either mobile clients or enterprise servers. NetTraveler is designed to cope with dynamic WANs where data sources go off-line, change location, have limited power capabilities, and form ad-hoc federations of sites. We present how NetTraveler can continue running a query when the client posing the query leaves, and then delivers the query results when the client returns. We present an implementation experience of NetTraveler using Web services. We validate our design and implementation choices with a preliminary performance study of NetTraveler. This study shows that NetTraveler can complete more queries per unit of time than middleware systems that cannot gracefully recover from contingencies during query execution caused by a client leaving the system. This study also shows that NetTraveler can handle a larger query load than solutions based on previous approaches.