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Game Event Synchronization

nodes based consistency events

Definition: Event synchronization in online games deals with the maintenance of game state consistency across several nodes.

Many gaming architectural solutions devised for the support of Massively Multiplayer Online Games are based on the idea of resorting to different nodes, distributed throughout the network, that manage a redundant game state. Indeed, it is well known that these approaches, which replicate the game state across multiple hosts, may ensure high levels of game availability and fault-tolerance. It is also widely accepted that the maintenance of game state consistency across several nodes results as a crucial issue to be addressed. According to this approach, in fact, game events generated by players during a gaming session must be locally processed by all nodes to guarantee that the game computation is kept consistent with those of all other nodes in the game system. This ensures that all game participants will perceive the same game evolution. Several studies have been performed on synchronization models for online games . The most common approach to keep synchronized and consistent the state of an event-based distributed game is based on the idea of guaranteeing that all nodes receive the same information needed to correctly update the replicated game state (reliable event delivery) as well as process this information according to a unique order (typically a timestamp-based order). This, in fact, ensures the consistency of the replicated game state. Unfortunately, it is well known that the use of globally ordered, reliable transmission strategies may result in performance degradation if smart synchronization solutions are not utilized.

Along this line, there exist novel mechanisms that adopt a “time bucket”-based approach. In essence, these schemes delay and order events (before being processed) for a time that should be long enough to prevent disordering. In other words, the responsiveness of the application is voluntarily decreased with the aim of eliminating inconsistencies. The problem with all these schemes is mainly concerned with the size of the time bucket, which should be large enough to permit the reception of all game events generated by players within their related bucket without jeopardizing the interactivity degree. Thus, alternative solutions are needed for the support of online games deployed over very dynamic and mutable networks.

There is an emerging need for mechanisms that try to speed up the game event processing activity with the aim of guaranteeing an acceptable interactivity degree among players while maintaining game consistency.

In line with this, several synchronization schemes have been recently presented which adopt optimistic approaches that process game events as soon as they are received and adopt “detect-and-correct” strategies able to rollback the computation when incorrect and inconsistent game states are computed.

As an additional alternative, schemes exist that exploit the semantics of the game to relax reliability and ordering delivery requirements in order to speed up the event delivery and the event processing activities, thus guaranteeing an augmented interactivity degree to users without loosing game consistency.

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