A great article. A couple of open source projects for CEP/event processing.

1. Esper (http://www.espertech.com) has been around for awhile. This is the CEP engine a lot of people are familiar with.

2. Norikra (http://norikra.github.io/) is a schema-less event processing engine, often used with Fluentd (https://www.fluentd.org) as the data collector (Fluentd can do stream processing as well).

3. Apache Storm (https://storm.apache.org) has been popular in the Hadoop community, often with Kafka as the event source.