I'll use a university's library system to explain my use case. Students register in the library system and provide their profile: gender, age, department, previously completed courses, currectly registered courses, books already borrowed, etc. Each book in the library system will define some borrowing rules based on students' profile, for example, a textbook for the computer algorithm can only be borrowed by students currently registered with that class; another textbook may only be borrowed by students in the math department; there could also be rules such that students can only borrow 2 computer networking book at most. As a result of the borrowing rules, when a student searches/browses in the library system, he will only see the books that can be borrowed by him. So, the requirement really comes down to the line of efficiently generating the list of books that a student is eligible to borrow. Here is how I vision the design using Drools - each book will have a rule with a few field constraints on the student profile as LHS, the RHS of the book rule simply adds the book id to a global result list, then all the book rules are loaded into a RuleBase. When a student searches/browsers the library system, a stateless session is created from the RuleBase and the student's profile is asserted as the fact, then every book that the student can borrow will fire its book rule and you get the complete list of books that the students can borrow in the global result list. A few assumptions: the library will handle millions of books; I don't expect the book rule be too complicated, 3 simple field constraints for each rule on average at the most; the number of students that the system needs to handle is in the range of 100K, so the load is fairly heavy. My questions are: how much memory will Drools take if loaded with a million book rules? How fast will it be for all those million rules to fire? If Drools is the right fit, I'd like to hear some best practices in designing such a system from you experienced users. Thanks.