[rules-users] Constants?

[Ansgar Konermann]

We use dedicated Java classes named after the kind of constant they
represent, like Slack in your case or RetirementAge, LegalAgeThreshold.
Most of the time, they contain a single property "value".

This makes it easy to use Drools patterns to match them, plus we get rather
strong type checking during rules compilation AKA packaging. We only need
around a dozen constants in our rules and their values rarely change (like
once every few months). The amount and type of constants are even more
static: add/remove one constant per year on average.

Best regards

Ansgar
Am 18.02.2013 20:52 schrieb "Willem van Asperen" <willem at van.asperen.org>:

>  Hi All,
>
> I have a bunch of rules that evaluate using some constant variable (say
> "slackTicks"). Currently, in my rules, I write down the value of this
> "slack" each and every time I use it, for example:
>
> TransferSchedule( tick > currentTick - 10d )
>
> where 10 is the slack.
>
> A solution would be to insert constant facts:
>
> Constant( key = "slack", $slackTicks : value )
> TransferSchedule( tick > currentTick - $slackTicks )
>
> Or inject a global properties object:
>
> global Properties constants
>
> $slackTicks : Double.valueOf(constants.getPropertyValue("model.slack",
> "0"))
>
> Or, the last one I can think of: a public final static on the fact class:
>
> TransferSchedule( tick > currentTick - TransferSchedule.SLACK_TICKS )
>
> These all cross the boundary between Java and Drools. In my Java code I
> have to insert the Constant facts or inject the global. When my drools
> developers think of some new ways to create the rules, I run the risk of
> having to go in and modify the .properties file or, worse, change the Java
> code.
>
> To be able to define a "constant" would be a clean solution:
>
> declare constant double SLACK_TICKS = 10;
>
> ...
>  TransferSchedule( tick > currentTick - SLACK_TICKS )
> ...
>
> This separates the rules from the Java code and give the rules developer
> all freedom to do whatever they want to make constant.
>
> Ah, just thought of another way to do it. You can insert the facts from
> within Drools itself:
>
> declare Constant
>     key : String
>     value : Object
> end
>
> rule initialize
>     salience 100000
>     when
>         // always
>     then
>         insert( new Constant( "slack", 10d ) );
>         //... other constants go here
> end
>
> I will still have to "capture" the constants that I need in every rule
> with a Constants( key == "...", $c : value ) premise, though... And you
> would need to add some more Constant classes to cater to contain type
> safety...
>
> What's your thought? What strategy do you use?
>
> Regards,
> Willem
>
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