Bøger af Henry F. Korth

The rapid development of wireless digital communication technology has cre- ated capabilities that software systems are only beginning to exploit. The falling cost of both communication and of mobile computing devices (laptop computers, hand-held computers, etc. ) is making wireless computing affordable not only to business users but also to consumers. Mobile computing is not a "e;scaled-down"e; version of the established and we- studied field of distributed computing. The nature of wireless communication media and the mobility of computers combine to create fundamentally new problems in networking, operating systems, and information systems. Further- more, many of the applications envisioned for mobile computing place novel demands on software systems. Although mobile computing is still in its infancy, some basic concepts have been identified and several seminal experimental systems developed. This book includes a set of contributed papers that describe these concepts and sys- tems. Other papers describe applications that are currently being deployed and tested. The first chapter offers an introduction to the field of mobile computing, a survey of technical issues, and a summary of the papers that comprise sub- sequent chapters. We have chosen to reprint several key papers that appeared previously in conference proceedings. Many of the papers in this book are be- ing published here for the first time. Of these new papers, some are expanded versions of papers first presented at the NSF-sponsored Mobidata Workshop on Mobile and Wireless Information Systems, held at Rutgers University on Oct 31 and Nov 1, 1994.

Transaction processing is an established technique for the concurrent and fault- tolerant access of persistent data. While this technique has been successful in standard database systems, factors such as time-critical applications, emerg- ing technologies, and a re-examination of existing systems suggest that the performance, functionality and applicability of transactions may be substan- tially enhanced if temporal considerations are taken into account. That is, transactions should not only execute in a "e;legal"e; (i.e., logically correct) man- ner, but they should meet certain constraints with regard to their invocation and completion times. Typically, these logical and temporal constraints are application-dependent, and we address some fundamental issues for the man- agement of transactions in the presence of such constraints. Our model for transaction-processing is based on extensions to established mod- els, and we briefly outline how logical and temporal constraints may be ex- pressed in it. For scheduling the transactions, we describe how legal schedules differ from one another in terms of meeting the temporal constraints. Exist- ing scheduling mechanisms do not differentiate among legal schedules, and are thereby inadequate with regard to meeting temporal constraints. This provides the basis for seeking scheduling strategies that attempt to meet the temporal constraints while continuing to produce legal schedules.

For scheduling the transactions, we describe how legal schedules differ from one another in terms of meeting the temporal constraints. Exist ing scheduling mechanisms do not differentiate among legal schedules, and are thereby inadequate with regard to meeting temporal constraints.