[rules-users] Constants?

[Willem van Asperen]

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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