I think we can easy add a validation framework to drools. So that facts are validated on assertion, invalid facts are auto retracted. First we need to add support for sub accessors - we can make a compromise here and only allow access to immutables and primitives, then we don't have to  worry about chaning values - this way we can add it to the existing engine, without many changes.


    So with the sub accessors now allowed we could write a validation system as below. This checks that the name is less than 15 chars and that the location is in a valid location. The beauty of this is that it will leverage existing indexes, so it can validate the location very quickly.


    rule "Validate Person"
            $person  : Person(name.length < 15, $location :location)
            Location(country  =="UK", name == $location )    


    So any asserted facts go into an invalid list, they are only removed when they valid thus matching the rule that. However this is quite verbose, what  we need is a special validation dsl. This validation dsl would be essential for expressing ontology validations:


    validate Person
                name.length < 15
                location  == Location( countr == "UK" ).name


    So here I have added a  new type  of constraint - "location  == Location( countr == "UK" ).name" standard field constraints go inside the Location pattern, we allow t he .name to  show that it is joinedwith the location field. further we could allow bound variables to also be used in the Location pattern, I  think it would  be  a good idea to auto bind variables in this, so that variabels are automatically available for each field - probably done using a naming convention of just adding a $.


    You could even take this a step further and have the validation information embedded in the java code as attributes.