Optimistic database-driven distributed real-time simulation (05F-SIW-031)

In this paper we present an optimistic synchronization protocol for distributed real-time simulations that uses a database as communication and storage mechanism. Each node in the simulation is also a database node and communication in the simulation is done by storing and reading to the database. The underlying replication protocol in the database then makes sure that all updates are propagated. The progress in the simulation is optimistic, i.e., each node tries to simulate as far ahead as possible without waiting for input from any other node. Since the simulations are said to be real-time we must guarantee that no events can be delivered too early nor too late. Also, recovery of a node must be done within predictable time due to the real-time constraints. Since all updates in the simulation are done through transactions we have a well-defined foundation for recovery and we show how the recovery can be done deterministically. For the simulation to function (and keep deadlines) during network partitions we allow local commits in the database. This requires that all data required on a specific node must be reachable from that node, i.e., no remote accesses should be needed. However, allowing local commits may introduce conflicting updates. These conflicts are detected and solved predictably.