Are “Disks in the Air” Just Pie in the Sky?

Mobile computers and wireless networks are emerging technologies which will soon make ubiquitous computing a reality. In the wireless environment, mobile clients may often be disconnected from stationary server machines or may have only a low-bandwidth channel for sending messages to servers. This environment raises new challenges for the support of database applications for three reasons: 1) the limited storage capacities of mobile machines, 2) the inability to accurately predict the future data needs of many data-intensive applications, and 3) the need to provide clients with new or updated data values in order to ensure consistent data access. One way (perhaps the only way) to address these challenges is to provide the stationary server machines with a relatively high bandwidth channel over which to broadcast portions of the database. Wireless networks are but one compelling example of the more general class of asymmetric cornmunication environments, which also includes CATV networks and information distribution services. We have proposed a mechanism called Broadcast Disks to provide database access in asymmetric communication environments. By appropriate organization of the broadcast program, an arbitrarlly fine-grained memory hierarchy can be created. Such a hierarchy raises fundamental new issues for pre-fetching and client cache management. In this short paper we present a brief overview of asymmetric environments, our novel approach to organizing broadcast disks and some initial simulation results.