1. A method, comprising: receiving, by a processing device of a first event processing node, an event data item comprising a state variable reflecting a change of a system state; applying an event processing rule to the event data item, by evaluating a conditional statement comprising a value of the state variable, to generate an event processing command; producing a command identifier identifying the event processing command; attempting to store the command identifier in a hash table employed for storing identifiers of executed commands, wherein the hash table is to reject duplicate command identifiers; responsive to failing to insert the command identifier into the hash table, discarding the event processing command; and processing a plurality of event data items reflecting a plurality of historic events, in order to synchronize a state of the first event processing node with a state of a second event processing node.