IIRC, there were some rather positive statements during the evoluion of 5.x that a session is thread-safe. -W ---------- Forwarded message ---------- From: Davide Sottara <dsotty(a)gmail.com&gt; Date: Wed, 12 Feb 2014 19:12:32 +0100 Subject: Re: [rules-users] ArrayIndexOutOfBoundsException under high events load in drools CEP To: rules-users(a)lists.jboss.org I can see a few independent issues here. Drools 5.x was not designed to be thread-safe in general: there are some known critical races that may arise when multiple threads are inserting facts in parallel. Many have been patched in 5.6, but the real solution to the problem will be introduced in the 6.x series.