Multi-server MMO Middleware: Unlocked

Massively multiplayer online (MMO) games consist of huge worlds populated by thousands of clients, far beyond the ability of a single server to maintain. This naturally leads to the division of the game world into many partitions, each controlled by a single server. To provide players with the illusion of a single large world, some MMO systems divide their game world across servers and synchronize all nearby activity between them. To accomplish and synchronize player movement and interaction between servers, a system of inter-server locks are employed. These locks introduce considerable software complexity and network traffic overhead. We discuss the practical problems of such locking systems, and present an alternate approach. A lockless middleware design is described which implements sequential consistency using inter-server remote writes. This approach provides consistency across all clients and servers, allowing for seamless movement of clients within an arbitrarily large game world. We compare the lockless architecture to the traditional locking design, and analyze its performance.