Disseminating Updates on Broadcast Disks

Lately there has been increasing interest in the use of data dissemination as a means for delivering data from servers to clients in both wired and wireless environments. Using data dissemination, the transfer of data is initiated by servers, resulting in a reversal of the traditional relationship between clients and servers. In previous papers, we have proposed Broadcast Disks as a model for structuring the repetitive transmission of data in a broadcast medium. Broadcast Disks are intended for use in environments where, for either physical or application-dependent reasons, there is asymmetry in the communication capacity between clients and servers. Examples of such environments include wireless networks with mobile clients, cable and direct satellite broadcast, and information dispersal applications. Our initial studies of Broadcast Disks focused on the performance of the mechanism when the data being broadcast did not change. In this paper, we extend those results to incorporate the impact of updates. We first propose several alternative models for updates and examine the fundamental tradeoff that arises between the currency of data and performance. We then propose and analyze mechanisms for implementing these various models. The performance results show that, even in a model where updates must be transmitted immediately, the performance of the Broadcast Disks technique can be made quite robust through the use of simple techniques for propagating and prefetching data items.