Esper
Esper is a Complex Event Processing Engine(CPE) that is written in both C#.N.E as well as Java.
It is used in a situation when we have a few thousand events occurring simultaneously but we have to perform operations only on a few of them. It is also used for observing the relations between the events and tracking -- tracing the events with the help of EPL (Event Processing Language).
Live Scenario
Suppose your service is rendering numerous events (around 10,000 in a second) and you have to separate the events that start with the letter ‘X’. Then, Esper is the best way to work with it. It basically reduces the complexity of the events.