Drools - Frequently Used Terms



The heart of the Rules Engine where you specify conditions (if ‘a’ then ‘b’).


Facts are the data on which the rules will act upon. From Java perspective, Facts are the POJO (Plain Old Java Object).


A Knowledge Session in Drools is the core component to fire the rules. It is the knowledge session that holds all the rules and other resources. A Knowledge Session is created from the KnowledgeBase.

For the rules engine to work, Facts are inserted into the session and when a condition is met, the subsequent rule gets fired. A Session is of two types −

  • Stateless Knowledge Session
  • Stateful Knowledge Session


It’s a logical concept. The agenda is the logical place where activations are waiting to be fired.


Activations are the then part of the rule. Activations are placed in the agenda where the appropriate rule is fired.