Rule Example: Determine whether an Airline Passenger is Suitable for Upgrade

In this example, we're determining whether a given airline passenger is eligible to have their coach seat upgraded to a first-class seat. In order to be eligible, a passenger must:

be in economy class now and either
hold a Gold card or
hold a Silver card and
their carry-on luggage must be less than or equal to 15.0 pounds.

In order to determine this, we must compare a passenger's facts with our rule.

The Rule

We fetch the rule from our RuleBase, which, for simplicity's sake, is merely a text file with a .rul extension. It's contents are in Reverse Polish Notation (to handle precedence):

# Rule establishing when an airline passenger may be
# allowed to upgrade his/her seat.

passengerIsEconomy IS true      
passengerIsGoldCardHolder IS true
passengerIsSilverCardHolder IS true 
OR
AND
passengerCarryOnBaggageAllowance EQUALS 15.0        
passengerCarryOnBaggageWeight EQUALS 10.0
LESSTHANOREQUALTO       
AND

The Rule Context:

We get facts pertaining to a Rule from our FactBase, which again is simply a text file. Note the difference between a sql.con file and a txt.con file.

# Format of this file is:
# Rule_Element_Type|Rule_Element_Name|SQL_Statement|Expected_Return_Data_Type_from_SQL_Query

p|passengerIsEconomy|SELECT if(((SELECT p.class FROM passengers p WHERE p.id = ? ) = 'economy'), 1, 0);|boolean
p|passengerIsGoldCardHolder|SELECT if(((SELECT p.card_type FROM passengers p WHERE p.id = ? ) = 'gold'), 1, 0);|boolean
p|passengerIsSilverCardHolder|SELECT if(((SELECT p.card_type FROM passengers p WHERE p.id = ? ) = 'silver'), 1, 0);|boolean
v|passengerCarryOnBaggageWeight|SELECT b.weight FROM passengers p, baggage b WHERE b.passenger_id = p.id and p.id = ?;|double
v|passengerCarryOnBaggageAllowance|SELECT v.value FROM variables v WHERE v.name = 'passengerCarryOnBaggageAllowance';|double

The Resulting Proposition:

Proposition statement = ( ( passengerCarryOnBaggageWeight <= passengerCarryOnBaggageAllowance ) AND ( ( passengerIsSilverCardHolder OR passengerIsGoldCardHolder ) AND passengerIsEconomy ) ), value = TRUE

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