There has been significant research effort into peer-to-peer (P2P) massively multi-user virtual environments (MMVEs). A number of architectures have been proposed to implement the P2P approach; however, the development of fully distributed MMVEs has met with a number of challenges. In this work, we address one of the key remaining challenges of state consistency and persistency in P2P MMVEs. Having reviewed state management and persistency architectures currently receiving research attention, we have identified deficiencies such as lack of load balancing, responsiveness, and scalability. To address these deficiencies, we present Pithos—a reliable, responsive, secure, load-balanced, and scalable distributed storage system, suited to P2P MMVEs. Pithos is designed specifically for P2P MMVEs, and we show that it improves the reliability and responsiveness of storage architectures as compared to existing P2P state persistency architectures. Pithos is implemented as an OverSim simulation running on the OMNeT++ network simulation framework. It is evaluated using up to 10,400 peers, with realistic latency profiles, with up to 15.8 million storage and retrieval requests that are generated to store a total of 2.4 million objects. Each peer in Pithos uses a maximum of 1,950Bps bandwidth to achieve 99.98% storage reliability, while the most reliable overlay storage configuration tested only achieved 93.65% reliability, using 2,182Bps bandwidth. Pithos is also more responsive than overlay storage, with an average responsiveness of 0.192s, compared with the average overlay responsiveness of 1.4s when retrieving objects from storage.

Recently, there has been significant research focus on Peer-to-Peer (P2P) Massively Multiplayer Online Games (MMOGs). A number of architectures have been presented in the literature to implement the P2P approach. One aspect that has not received sufficient attention in these architectures is state persistency in P2P MMOGs. This survey presents an overview of the current challenges present in P2P MMOGs, followed by an overview of classic state consistency models used in C/S MMOGs. The survey then classifies the state persistency techniques currently used in P2P MMOGs into super peer storage, overlay storage, hybrid storage, and distance-based storage. Key characteristics, namely scalability, fairness, reliability, responsiveness, and security are then defined. Each state persistency technique is evaluated according to these characteristics and recommendations are then made of possible future areas of research into the different storage types.