Drools - EventListener - jboss

Question

We are planning to use Drools/JBoss BRMS 6 for business rules management. Our plan is to write rules using the workbench, deploy the rules package in multiple Execution Servers and allow applications to access the Rules package by making calls to the REST API. We do not have any Java wrappers or custom classes in between the calling applications and the rules package.
I am trying to incorporate some logging into the rules engine. I understand that there are EventListener interfaces that can be implemented.
Please would you provide some information/guidance on how to implement Listeners in our kind of set up? Where will I create and store the Java classes/methods that would implement Event Listeners?
How can a calling application insert an Event Listener into the session? Will it be part of the xml/json payload?
Thanks

Answer 1

1. Where to implement the listeners?
The listeners must be obviously implemented in Java. One simple place I found to put those implementation is in a separate maven project. After all, a project in the kie-workbench is a maven project itself. So you can create a separate project (outside the kie-workbench) implement the listeners you want to and then add this new project as a dependency in your kie-workbench's project (check the documentation on how to do that).
The only problem I found with this approach is that once you defined the dependency between your projects, the kie-workbench will scan every single class of it and of any other dependency it has. Check this link for more information.
So, if your listener project doesn't have too many dependencies, you should be good to go. Please note that, in theory, you could add any kie/drools dependency you have in your listener project as <scope>provided</scope>.
2. How can I configure these listeners?
A trick that I always use is to have what I call a "configuration" rule to do this kind of job.
A "Configuration" rule is a rule without LHS (and, if you are distrustful, a high salience). This kind of rules are guaranteed to be executed only once. Just make sure that you call a fireAllRules() before the first interaction with the kie-server, or that the first interaction always starts with a fireAllRules command.
Your configuration rule could look like this:
/**
Configures the session's listeners.
**/
rule "[SUB-CONFIG] Listeners Configuration"
salience 1000
when
then
((org.drools.impl.StatefulKnowledgeSessionImpl)kcontext.getKnowledgeRuntime()).addEventListener(new MyWorkingMemoryEventListener());
((org.drools.impl.StatefulKnowledgeSessionImpl)kcontext.getKnowledgeRuntime()).addEventListener(new MyAgendaEventListener());
end
You can place this rule in your kie-server project.
Hope it helps,