A network simulation system comprises a request processor and a simulation engine. The request processor receives node data indicative of resource levels of nodes of the network, and relationship data indicative of relationship parameters of a plurality of relationships between the nodes. The simulation engine outputs a stream of event data based on the relationship data. Respective events are generated by: determining source and destination nodes, and corresponding relationship parameters for the relationship between them; determining an event time based on a rate parameter of the corresponding relationship parameters; determining a transfer value; and if the transfer value does not exceed a current resource level of the source node, modifying current resource levels of the source and destination nodes according to the transfer value, and generating, at the event time, event data comprising a timestamp, a source node identifier of the source node, a destination node identifier of the destination node, and the transfer value.