5.5. Rule
Figure 5.8. rule
A rule specifies that "when" a particular set of conditions occur, specified in the Left Hand Side (LHS), then do this, which is specified as a list of actions in the Right Hand Side (RHS). A common question from users is "why use when instead of if". "when" was chosen over "if" because "if" is normally part of a procedural execution flow, where at a specific point in time it checks the condition. Where as "when" indicates it's not tied to a specific evaluation sequence or point in time, at any time during the life time of the engine "when" this occurs, do that. Rule
A rule must have a name, and be a unique name for the rule package. If you define a rule twice in the same DRL it produce an error while loading. If you add a DRL that has includes a rule name already in the package, it will replace the previous rule. If a rule name is to have spaces, then it will need to be in double quotes (its best to always use double quotes).
Attributes are optional, and are described below (they are best kept as one per line).
The LHS of the rule follows the "when" keyword (ideally on a new line), similarly the RHS follows the "then" keyword (ideally on a newline). The rule is terminated by the keyword "end". Rules cannot be nested of course.
rule "<name>"
<attribute>*
when
<conditional element>*
then
<action>*
end
Example 5.2. Rule Syntax Overview Example
rule "Approve if not rejected"
salience -100
agenda-group "approval"
when
not Rejection()
p : Policy(approved == false, policyState:status)
exists Driver(age > 25)
Process(status == policyState)
then
log("APPROVED: due to no objections.");
p.setApproved(true);
end
Example 5.3. A rule example
5.5.1. Rule Attributes
Rule attributes provide a declarative way to influence the behavior of the rule, some are quite simple, while others are part of complex sub systems; such as ruleflow. To get the most from Drools you should make sure you have a proper understanding of each attribute.
Figure 5.9. rule attributes
5.5.1.1. no-loop
default value : false
type : Boolean
When the Rule's consequence modifies a fact it may cause the Rule to activate again, causing recursion. Setting no-loop to true means the attempt to create the Activation for the current set of data will be ignored.
5.5.1.2. lock-on-active
default value : false
type : Boolean
when a ruleflow-group becomes active or an agenda-group receives the focus any rules that ahve lock-on-active set to try cannot place activations onto the agenda, the rules are matched and the resulting activations discarded. This is a stronger version of no-loop. It's idealy for calculation rules where you have a number of rules that will modify a fact and you don't want any rule re-matching and firing. In summary fire these currently active rules and only these rules, no matter how the data changes, do not allow any more activations for the rules with the attribute set to true. When the ruleflow-group is no longer active or agenda-group loses the focus those rules with lock-on-active set to true can once again add activations onto the agenda.
5.5.1.3. salience
default value : 0
type : integer
Each rule has a salience attribute that can be assigned an Integer number, defaults to zero, the Integer and can be negative or positive. Salience is a form of priority where rules with higher salience values are given higher priority when ordered in the Activation queue.
5.5.1.4. agenda-group
default value : MAIN
type : String
Agenda group's allow the user to partition the Agenda providing more execution control. Only rules in the focus group are allowed to fire.
5.5.1.5. auto-focus
default value false
type : Boolean
When a rule is activated if the auto-focus value is true and the Rule's agenda-group does not have focus then it is given focus, allowing the rule to potentially fire.
5.5.1.6. activation-group
default value : N/A
type : String
Rules that belong to the same named activation-group will only fire exclusively. In other words, the first rule in an activation-group to fire will cancel the other rules activations (stop them from firing). The Activation group attribute is any string, as long as the string is identical for all the rules you need to be in the one group.
NOTE: this used to be called Xor group, but technically its not quite an Xor, but you may hear people mention Xor group, just swap that term in your mind with activation-group.
5.5.1.7. dialect
default value : as specified by the package
type : String
possible values: "java" or "mvel"
The dialect species the language to be used for any code expressions in the LHS or the RHS code block. Currently two dialects are available, Java and MVEL. While the dialect can be specified at the package level, this attribute allows the package definition to be overridden.
5.5.1.8. date-effective
default value : N/A
type : String, which contains a Date/Time definition
A rule can only activate if the current date and time is after date-effective attribute.
5.5.1.9. date-exptires
default value : N/A
type : String, which contains a Date/Time definition
A rule cannot activate if the current date and time is after date-expires attribute.
5.5.1.10. duration
default value : no default value
type : long
The duration dictates that the rule will fire after a specified duration, if it is still true.
rule "my rule"
salience 42
agenda-group "number 1"
when ...