On the network impact of dynamic server selection

Abstract

Widespread replication of information can ameliorate the problem of server overloading but raises the allied question of server selection. Clients may be assigned to a replica in a static manner or they may choose among replicas based on client-initiated measurements. The latter technique, called dynamic server selection (DSS), can provide significantly improved response time to users when compared with static server assignment policies (for example, based on network distance in hops). In the first part of this paper we demonstrate the idea of DSS using experiments performed in the Internet. We compare a range of policies for DSS and show that obtaining additional information about servers and paths in the Internet before choosing a server improves response time significantly. The best policy we examine adopts a strategy of never adding more than 1% additional traffic to the network, and is still able to provide nearly all the benefits of the most expensive policies. While these results suggest that DSS is beneficial from the network user's standpoint, the system-wide effects of DSS schemes should also be closely examined. In the second part of this paper we use large-scale simulation to study the system-wide network impact of dynamic server selection. We use a simulated network of over 100 hosts that allows local-area effects to be distinguished from wide-area effects within traffic patterns. In this environment we compare DSS with static server selection schemes and confirm that client benefits remain even when many use DSS simultaneously. Importantly, we also show that DSS confers system-wide benefits from the network standpoint, as compared to static server selection. First, overall data traffic volume in the network is reduced, since DSS tends to diminish network congestion. Second, traffic distribution improves – traffic is shifted from the backbone to regional and local networks.