Esper Esper is a 100% Java complex events (CEP) / event stream processing (ESP) engine - A .NET version also is available! Esper is a 100% Java complex events (CEP) / event stream processing (ESP) engine for grouping, aggregation, sorting, filtering, merging and joining of event streams and logical and temporal event correlation via patterns. NEsper for .NET is the C# engine for event-driven applications using CEP/ESP.