4.13. Activations, Agenda and Conflict Sets Example

In this example, a cashflow calculation system is featured. These are the three classes implemented: 
public class CashFlow {
    private Date   date;
    private double amount;
    private int    type;
    long           accountNo;
    // getter and setter methods here
}

public class Account {
    private long   accountNo;
    private double balance;
    // getter and setter methods here
}

public AccountPeriod {
    private Date start;
    private Date end;
    // getter and setter methods here
}
Two rules can be used to determine the debit and credit for that quarter and update the Account balance. The two rules below constrain the cashflows for an account for a given time period. Notice the "&&" which use short cut syntax to avoid repeating the field name twice. 
rule "increase balance for credits"
when
  ap : AccountPeriod()
  acc : Account( $accountNo : accountNo )
  CashFlow( type == CREDIT,
            accountNo == $accountNo,
            date >= ap.start && <= ap.end,
            $amount : amount )
then
  acc.balance  += $amount;
end
 
rule "decrease balance for debits" 
when 
  ap : AccountPeriod() 
  acc : Account( $accountNo : accountNo ) 
  CashFlow( type == DEBIT, 
            accountNo == $accountNo,
            date >= ap.start && <= ap.end, 
            $amount : amount ) 
then 
  acc.balance -= $amount; 
end
If the AccountPeriod is set to the first quarter we constrain the rule "increase balance for credits" to fire on two rows of data and "decrease balance for debits" to act on one row of data.
The data is matched during the insertion stage and only fires after fireAllRules() is called. Meanwhile, the rule plus its matched data is placed on the Agenda and referred to as an Activation. The Agenda is a table of Activations that are able to fire and have their consequences executed, as soon as fireAllRules() is called. Activations on the Agenda are executed in turn. Notice that the order of execution so far is considered arbitrary.
After all of the above activations are fired, the account has a balance of -25.
If the AccountPeriod is updated to the second quarter, we have just a single matched row of data, and thus just a single Activation on the Agenda.
The firing of that Activation results in a balance of 25.
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