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A Mobility-Transparent Model for Consistency
$\mathcal{P}$$e$$\mathcal{S}$$t$$\mathcal{O}$
$<$http://www.diku.dk/distlab/amigos/pesto.html$>$

Michael G. Sørensen
{mgsj@diku.dk}
$<$http://www.diku.dk/students/mgsj/$>$

Master Thesis
Department of Computer Science
University of Copenhagen
DIKU
$<$http://www.diku.dk/$>$

Written Work no. 96-3-7

Not all those who wander are lost;
- J.R.R. Tolkien (Lord of the Rings)

This HTML version: April 23-27, 2000
Last modified 18.04.2002

Abstract:

The design, implementation, and evaluation of a mobility-transparent model for consistency is presented.

A distributed file system with support for mobile computing has been designed and implemented. The system enables applications to utilize any desired level of optimism or pessimism and to adapt their behaviour according to different communication characteristics or user demands. Guidelines for extending the system with a transactional facility are also given.

The distributed file system is based on a model that uses time as a consistency measure. Using such a scheme the applications can relax their consistency requirements as the quality of communication decreases in order to achieve higher availability or reduce cost, and strengthen them again when suited.

The implementation has introduced a small, but acceptable, overhead. The system has some minor flaws, but it is my belief that the implementation has proven the feasibility of the system.

"The design of a worldwide, fully transparent distributed file system for simultaneous use by millions of mobile and frequently disconnected users is left as an exercise for the reader."

- Andrew S. Tanenbaum (Distributed Operating Systems)

Keywords: mobile computing, distributed file systems, client/server, communication, adaptation, availability, consistency, file sharing semantics, replica control strategies, optimism, pessimism, caching, read and write operations, locks, conflict detection and resolution, transactions.

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