Entity Density Scalability of Multiplayer Online Games via Replication - based Parallelization : A Case Study of Quake

Massively Multiplayer Online Games (MMOGs) are an increasingly popular class of real-time interactive distributed applications that require scalable network architectures and parallelization approaches. While games of the role-playing genre already allow thousands of users to concurrently participate in a single game session, there are important genres, in particular action and strategy games, which have not been scaled to the massively multiplayer realm so far. These games have hard requirements in terms of scalability, in particular regarding density: many players tend to congregate in small locations. In this paper, we provide a comprehensive analysis of different scalability dimensions for online games and describe our novel approach of game state replication for scaling the density of players. The practical impact of our work is demonstrated by porting the popular action game QFusion, based on the famous Quake 2, onto our proxy-server system architecture using the replication approach. We discuss our solutions for maintaining data consistency of distributed replicas of the game state, as well as our enhancement of the game implementation in order to support multi-server replication. The experiments with the ported QFusion demonstrate its high responsiveness and show that our approach allows to almost triple the maximum number of simultaneous players on four servers as compared with a single-server version.