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Engagement is meant to be for a long period of time in MMOGs with users spending several months or even years playing and improving their character. As these games are designed to run continuously for years it is expected that the hardware infrastructure will at some point inevitably fail, be forced to shut down or otherwise go offline. In such situations it is crucial that the game can be restored to the last correct state it was in. Data replication therefore becomes of paramount importance.
Replication is the process of sharing data to ensure consistency between redundant resources, in order to improve reliability, fault tolerance, and accessibility. It could be data replication if the same data is stored on multiple storage devices or computation replication if the same computing task is executed many times. Current MMOGs are in favour of computation replication due to the issues of bandwidth and network latency. For example, in Russell et al.’s (2008) solution a server send both the initial state of a given object and deterministic code to simulate the object over time to all interested clients, so that clients can independently update the area of interest consistently with updates computed by the server. Computation replication results in efficient use of network bandwidth and the power of multi-core processors of game platforms.
Traditionally if the game state is dynamically stored during game play, it could be restored upon restarting a crashed or failed server. This is usually a relatively simple task where a complete snapshot of the world taken immediately before shutting down the server can be re-issued. Difficulties however arise when the termination is unpredictable, such as in the case of a server crash. Ideally, there should be no difference between the stored states and the actual game states in memory at the time of failure however this may not be realistic due to scalability and efficiency reasons. In this context the major requirements for replication in MMOGs are (Zhang et al., 2008):