CEP with DB4O
Esper is an open-source CEP engine written entirely in Java and fully embeddable into any Java process - custom, JEE, ESB, BPM, etc. It recently has reached version 2.0, is backed by EsperTech under a professional open source / dual license business model, and triggers a growing interest in the Java and .Net community as well with NEsper, its full .Net/C# implementation. Both can be downloaded from esper.codehaus.org with complete examples and documentation.
The Esper engine works somewhat like a database turned upside-down. Instead of storing the data and running queries against stored data, Esper enables applications to store queries (or statements) and run the data through them. Response from the Esper engine is thus real-time when conditions occur that match queries. The execution model is thus continuous rather than only when a query is submitted.
One of the most attractive features in Esper is that is easy to formulate continuous queries: Esper provides an SQL-like language extended to handle event streams and patterns, an Event Processing Language (EPL). For developers more inclined to program against an API, a continuous query object model API is also available.