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Clara offers forward-chaining rules in Clojure with Java interoperability. The project page gives an overview; here we look at Clara's approach to simplify expression of business logic.
Two things pop out as Clojure's strengths: the dramatic reduction of our systems' state space, and flexibility to model logic and program flow as simply as possible. Excellent Clojure libraries have emerged to make solving many classes of programs simpler, ranging from web development (ring, hiccup, compojure), to asynchronous program flow (core.async), to logic programming (core.logic), and many others.
This library is targeted at problems that lend themselves to forward-chaining rules. As we get better at managing state and program flow, our biggest source of complexity comes from what I call logical knots: arbitrary business logic with complex dependencies on other logic and data, with frequent changes that require deep dives into that logic. Even a functional approach to such problems can become unwieldy, with hierarchies of function calls too deep to easily reason about. Evolving business needs may require some function deep in the stack to suddenly require new input, forcing changes to multiple consumers.
These are good cases for a rules approach. Instead of adding and updating functions for new business logic, we express logic independently as rules that run against a working memory of our data. We can reason about units of logic independently and apply updates without cascading changes.
Here we look at a simple example using Clara to simplify business logic. Readers familiar with existing rules engines like Jess or Drools may be interested in the more involved sensors example.
Imagine a retail setting where discounts and promotions come and go based on arbitrary criteria. We might have a special where anyone who buys a gizmo gets a free lunch, and express it simply:
(defrule free-lunch-with-gizmo "Anyone who purchases a gizmo gets a free lunch." [Purchase (= item :gizmo)] => (insert! (->Promotion :free-lunch-with-gizmo :lunch)))
In rules engines terms, Purchase is an example of a fact. Clara generally (but not exclusively) represents facts as simple Clojure records. We detect a gizmo purchase in our working memory, and insert the presence of a promotion for a free lunch.
Now suppose our business declares August as "free widget month" for orders over $200. Rather than updating some "get-promotions" function to include this as well, we simply add another rule to our rulebase:
(defrule free-widget-month "All purchases over $200 in August get a free widget." [Order (and (= month :august) (> total 200))] => (insert! (->Promotion :free-widget-month :widget)))
Note the constraints on facts are simply Clojure s-expressions that have access to the corresponding fact's fields. Furthermore, the right-hand side (the part of the rule after the => symbol) is also simply an s-expression.
Users of the system can then query working memory for arbitrary facts. Here's a query that gets all of our promotions:
(defquery get-promotions "Query to find promotions for the purchase."  [?promotion <- Promotion])
More sophisticated rules and queries will use bound variables to join facts and select pertinent subsets. In this case we simply want the Promotion facts, so this query finds them and binds them to the ?promotion variable.
Consuming logic can then query the working memory for Promotion facts, returning a list of all promotions for display. Here's an example of how this could be invoked:
(-> (mk-session 'clara.examples.shopping) ; Load the rules. (insert (->Customer :vip) (->Order 2013 :march 20) (->Purchase 20 :gizmo) (->Purchase 120 :widget)) ; Insert some facts. (fire-rules) (query get-promotions))
This loads the set of rules and queries defined in the clara.examples.shopping namespace, inserts some facts, fires all rules, and executes a query to get the promotions. The return value is a sequence containing items that matched the query.
Of course, this simple example doesn't demonstrate some of the more powerful aspects of a rules engine, including:
- Rule logic and constraints can be arbitrarily complicated, composed with boolean operators.
- Values in rules can be bound to variables, which the underlying engine will unify with other values.
- Accumulators support reasoning over collections of facts, using Clojure reducers.
The sensors example shows some of these features in action.
There is also an example using the Java API, which also demonstrates the use of Java Beans as facts.
The Rules Engine
See the architecture overview for details on Clara's design.
- The engine typically deals with collections of facts rather than individual items for efficiency.
- Accumulators similar to Jess and Drools allows users to write logic in terms of arbitrary collections of facts.
- All facts must be immutable. This simplifies the underlying system and makes it possible to distribute facts and processing over multiple machines and processes.