Forwarders vs. centralized server

Abstract

The Internet has allowed the creation of huge amounts of data located on many sites. Performing complex operations on some data requires that the data be transferred first to the machine on which the operations are to be executed, which may require a non-negligible amount of bandwidth and may seriously limit performance if it is the bottleneck. However, instead of moving the data to the code, it is possible to move the code to the data, and perform all the operations locally. This simple idea has led to a new paradigm called code-mobility: a mobile object --- sometimes called an agent --- is given a list of destinations and a series of operations to perform on each one of them. The agent will visit all of the destinations, perform the requested operations and possibly pass the result on to another object. Any mobility mechanism must first provide a way to migrate code from one host to another. It must also ensure that any communication following a migration will not be impaired by it, namely that two objects should still be able to communicate even if one of them has migrated. Such a mechanism is referred to as a location mechanism since it often relies on the knowledge of the location of the objects to ensure communications. Two location mechanisms are widely used: the first one uses a centralized server whereas the second one relies on special objects called forwarders. This paper evaluates and compares the performance of an existing implementation of these approaches in terms of cost of communication in presence of migration. Based on a Markov chain analysis, we will construct and solve two mathematical models, one for each mechanism and will use them to evaluate the cost of location. For the purpose of validation, we have developed for each mechanism a benchmark that uses ProActive [2], a Java library that provides all the necessary primitives for code mobility. Experiments conducted on a LAN and on a MAN have validated both models and have shown that the location server always performs better than the forwarders. Using our analytical models we will nevertheless identify situations where the opposite conclusion holds. However, under most operational conditions location servers will perform better than forwarders.