Massively multiplayer online games (MMOGs)

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Massively multiplayer online games offer an immersive experience whereby very large numbers of users interact through the Internet with a persistent virtual world. Leading examples of such games include Sony’s EverQuest II and EVE Online from the Finnish
company CCP Games. Such worlds have increased significantly in sophistication and now include, complex playing arenas (for example EVE, Online consists of a universe with over 5,000 star systems) and multifarious social and economic systems. The number of players is also rising, with systems able to support over 50,000 simultaneous
online players (and the total number of players perhaps ten times this figure). The engineering of MMOGs represents a major challenge for distributed systems technologies, particularly because of the need for fast response times to preserve the user experience of the game. Other challenges include the real-time propagation of events to the many players and maintaining a consistent view of the shared world. This, therefore, provides an excellent example of the challenges facing modern distributed systems designers.
A number of solutions have been proposed for the design of massively multiplayer online games:
Perhaps surprisingly, the largest online game, EVE Online, utilizes a client-server an architecture where a single copy of the state of the world is maintained on a  
centralized server and accessed by client programs running on players’ consoles or other devices. To support large numbers of clients, the server is a complex the entity in its own right consisting of a cluster architecture featuring hundreds of computer nodes (this client-server approach is discussed in more detail in Section 1.4 and cluster approaches are discussed in Section 1.3.4). The centralized architecture helps significantly in terms of the management of the virtual world and the single copy also eases consistency concerns. The goal is then to ensure fast response through optimizing network protocols and ensuring a rapid response to incoming events. To support this, the load is partitioned by allocating individual ‘star systems’ to particular computers within the cluster, with highly loaded star systems having their own dedicated computer and others sharing a computer.

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Incoming events are directed to the right computers within the cluster by keeping track of movement of players between star systems.
Other MMOGs adopt more distributed architectures where the universe is partitioned across a (potentially very large) number of servers that may also be geographically distributed. Users are then dynamically allocated a particular server based on current usage patterns and also the network delays to the server (based on geographical proximity for example). This style of architecture, which
is adopted by EverQuest, is naturally extensible by adding new servers.
Most commercial systems adopt one of the two models presented above, but researchers are also now looking at more radical architectures that are not based on client-server principles but rather adopt completely decentralized approaches based on peer-to-peer technology where every participant contributes resources (storage and processing) to accommodate the game.


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