I'm facing difficulties with finding the best CEP product for our problem. We need a distributed CEP solution with shared memory. The main reason for distribution isn't speeding up the process, but having a fallback in case of hardware or software problems on nodes. Because of that, all nodes should keep their own copy of the event-history.
Some less important requirements to the CEP product are: - Open source is a big pre. - It should run on a Linux system. - Running in a Java environment would be nice.
Which CEP products are recommended?
A number of commercial non-open source products employ a distributed data grid to store the stateful event processing data in a fault-tolerant manner. My personal experience is with TIBCO BusinessEvents, which internally uses TIBCO ActiveSpaces. Other products claim do similar things, e.g., Oracle Event Processing uses Oracle Coherence.
Open source solutions, I wouldn't be aware that any of them offers functionality like this out of the box. With the right skills you might be able to use them in conjunction with a data grid (I've seen people try to use Drools Fusion together with infinispan), but there are quite a number of complexities that you need think about that a pre-integrated product would take care of for you (transaction boundaries, data access, keeping track of changes, data modeling).
An alternative you might consider if performance doesn't dictate a distributed/load-balanced setup could be to just run a hot standby, i.e., two engines performing the same CEP logic, but only one engine (the active one) actually triggering outgoing actions. The hot-standby engine would be just evaluating the CEP logic to have the data in its memory ready to take over in case of failure but not trigger outgoing actions as long as the other engine is running.