With the development of mobile technology, mobile virtual worlds have attracted massive users. To improve scalability, a peer-to-peer virtual world provides the solution to accommodate more users without increasing hardware investment. In mobile settings, however, existing P2P solutions are not applicable due to the unreliability of mobile devices and the instability of mobile networks. To address the issue, a novel infrastructure model, called Virtual Net, is proposed to provide fault-tolerance in managing user content and object state. In this paper, the key problem, namely object state update, is resolved to maintain state consistency and high interaction responsiveness. This work is important in implementing a scalable mobile virtual world.
Deep Dive into Virtual Net: a Decentralized Architecture for Interaction in Mobile Virtual Worlds.
With the development of mobile technology, mobile virtual worlds have attracted massive users. To improve scalability, a peer-to-peer virtual world provides the solution to accommodate more users without increasing hardware investment. In mobile settings, however, existing P2P solutions are not applicable due to the unreliability of mobile devices and the instability of mobile networks. To address the issue, a novel infrastructure model, called Virtual Net, is proposed to provide fault-tolerance in managing user content and object state. In this paper, the key problem, namely object state update, is resolved to maintain state consistency and high interaction responsiveness. This work is important in implementing a scalable mobile virtual world.
Virtual worlds, including multiplayer online games and virtual social worlds, allow users to inhabit in virtual environments, create their own content, and interact with each other. Mobile virtual worlds allow users to access the simulated environments through mobile devices, achieving the possibility to play anywhere. Mobile virtual worlds have gained large attraction from the development of mobile devices. They have become an important market and revenue source for the game industry, and attracted a large number of users. For example, Fortnite has earned $1,996,917 gross daily revenue 1 and reported 3.4 million concurrent players 2 in 2018. The success and expansion of mobile virtual worlds raise new challenges in infrastructure development, one of them is the scalability problem. In virtual worlds, interaction is implemented by sending events to servers for processing and receiving updates from the servers for rendering and state synchronization. With the increase of concurrent online users, more computing load is imposed on game infrastructures. Servers have to process and respond to more client requests within a short period for high responsiveness. Also, network bandwidth consumption is increased to pack multiple game states in an update. For scaling, more computing resources have to be invested. Otherwise, user experience will be affected.
Peer-to-peer (P2P) virtual worlds, firstly introduced in [1], explore the possibility of running a virtual world without a central server. In P2P virtual worlds, user devices run both the client program and server program for event handling and state update. Thus, computing resources naturally scale along with the change of user population. Mobile applications, however, have different characteristics with respect to their desktop counterparts. One outstanding issue is client failure. Compared to desktop PCs, mobile devices are more prone to failure, due to, for example, battery depletion or application conflict. Moreover, the access to mobile networks, such are MANETs and VANETs, are also unstable. Client unreliability may cause content loss or state inconsistency, if user content and object state are not properly saved or backed up before failure. Yet, existing P2P virtual worlds do not concern the peer device unreliability problem [3]. Thus, they cannot be directly applied in mobile settings.
In this paper, a Virtual Net model is proposed to address the client unreliability problem for mobile P2P virtual worlds. The model utilizes the cloud-fog structure, but totally decentralized. To avoid content loss, the cloud layer stores user contents for content persistency. The fog layer caches object states for client recovery and maintains state consistency. The separation of content storage and state caching can improve responsiveness, since operations direct on P2P storage have more communication overhead [21]. Based on the P2P content storage, a content addressing scheme is devised, which can facilitate content integrity check.
To avoid reinventing the wheel, this paper mainly focuses on the state update problem to maintain object state consistency. At the fog layer, object states are replicated on several nodes for fault-tolerance. Thus, all replicas must maintain the same state in event handling so that interaction can be performed within a consistent shared environment. Yet, the requirement of high responsiveness in virtual world interaction makes the problem difficult. To attack the difficulty, an opportunistic approach, called fast event delivery, is proposed. Based on the approach, a virtual world interaction model is then designed. In short, the main contributions of the paper are listed as follows.
- A new P2P cloud-fog structure, called Virtual Net model, is proposed to resolve the client unreliability problem, which can provide fault-tolerance in playing a mobile virtual world. 2. A fast event delivery approach is proposed to both maintain replica state consistency and high responsiveness in the process of handling user events. 3. A new virtual world interaction model is designed to achieve game state consistency and high responsiveness when interacting with different neighbors.
The remainder of the paper is organized as follows. The related works are introduced in Section 2. The overall Virtual Net model is described in Section 3. Section 4 studies the state update problem in detail. Based on the solution of the problem, the virtual world interaction model is provided in Section 5 with neighbor change management. The correctness of the solution is proved in Section 6. Section 7 and Section 8 evaluate the performance through theoretical analysis and experiments. Section 9 concludes the paper.
Mobile P2P virtual worlds combine the characteristics of mobile virtual world and P2P virtual world problems. Due to the lack of study in this field, the related work in P2P virtual worlds and cloud-fog mobile applications are surveyed to shape the distinct char
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