Esper is a stream processing and analytics platform that markets itself as “pure-play CEP (Complex Event Processing)”. In fact, it was one of the first CEP engines introduced to the market and, at that time, was the only one available under an open-source license. That said, while an open-source license is available for end users, commercial licenses are required for OEMs. There are also two premium versions available: EsperHA (High Availability) and Esper Enterprise Edition, that include various features, including (as you would expect) high availability, a graphical user interface, enterprise-level scalability, and commercial support.
Author/s: Daniel Howard