This paper proposes the use of repetitive broadcast as a way of augmenting the memory hierarchy of clients in an asymmetric communication environment. We describe a new technique called \Broadcast Disks" for structuring the broadcast in a way that provides improved performance for non-uniformly accessed data. The Broadcast Disk superimposes multiple disks spinning at di erent speeds on a single broadcast channel | in e ect creating an arbitrarily ne-grained memory hierarchy. In addition to proposing and de ning the mechanism, a main result of this work is that exploiting the potential of the broadcast structure requires a re-evaluation of basic cache management policies. We examine several \pure" cache management policies and develop and measure implementable approximations to these policies. These results and others are presented in a set of simulation studies that substantiates the basic idea and develops some of the intuitions required to design a particular broadcast program.

Currently, a variety of information retrieval systems are available to potential users. These services are provided by commercial enterprises (such as Dow Jones [6] and The Source [7]), while others are research efforts (the Boston Community Information System [S]). While in many cases these systems are accessed from personal computers, typically no advantage is taken of the computing resources of those machines (such as local processing and storage). In this paper we explore the possibility of using the user's local storage capabilities to cache data at the user's site. This would improve the response time of user queries albeit at the cost of incurring the overhead required in maintaining multiple copies. In order to reduce this overhead it may be appropriate to allow copies to diverge in a controlled fashion. This would not only make caching less costly, but would also make it possible to propagate updates to the copies more efficiently, for example, when the system is lightly loaded, when communication tariffs are lower, or by batching updates together. Just as importantly, it also makes it possible to access the copies even when the communication lines or the central site are down. Thus, we introduce the notion of quasi-copies, which embodies the ideas sketched above. We also define the types of deviations that seem useful, and discuss the available implementation strategies.

No abstract

Mobile computers and wireless networks are emerging technologies that will soon be available to a wide variety of computer users. Unlike earlier generations of laptop computers, the new generation of mobile computers can be an integrated part of a distributed computing environment, one in which users change physical location frequently. The result is a new computing paradigm, nomadic computing . This paradigm will affect the design of much of our current systems software, including that of database systems. This paper discusses in some detail the impact of nomadic computing on a number of traditional database system concepts. In particular, we point out how the reliance on short-lived batteries changes the cost assumptions underlying query processing. In these systems, power consumption competes with resource utilization in the definition of cost metrics. We also discuss how the likelihood of temporary disconnection forces consideration of alternative transaction processing protocols. The limited screen space of mobile computers along with the advent of pen-based computing provides new opportunities and new constraints on database interfaces and languages. Lastly, we believe that the movement of computers and data among networks potentially belonging to distinct, autonomous organizations creates serious security problems.

No abstract