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Yes, you can use the inbuilt H2 database. I'm haven't used it, so I don't know how to query it.

Check this for the docs:

http://www.jboss.org/jbpm/documentation

User Guide:

http://docs.jboss.org/jbpm/v5.3/userguide/

API:

http://docs.jboss.org/jbpm/v5.3/javadocs

Dig into the examples to clarify some of your doubts:

http://sourceforge.net/projects/jbpm/files/jBPM%205/jbpm-5.3.0.Final/ -----> check jbpm-5.3.0.Final-examples.zip