A tale of two directories: implementing distributed shared objects in Java']Java

A directory service keeps track of the location and status of mobile objects in a distributed system. This paper describes our experience implementing two distributed directory protocols as part of the Aleph toolkit, a distributed shared object system implemented in Java. One protocol is a conventional home-based protocol, in which a fixed node keeps track of the object's location and status. The other is a novel Arrow protocol, based on a simple path-reversal algorithm. We were surprised to discover that the Arrow protocol outperformed the home protocol, sometimes substantially, across a range of system sizes, This paper describes a series of experiments testing whether the discrepancy is due to an artifact of the Java run-time system (such as differences in thread management or object serialization costs), or whether it is something inherent in the protocols themselves, In the end, we use insights gained from these experimental results to design a new directory protocol that combines advantages of both. Copyright (C) 2000 John Whey & Sons, Ltd.