[rules-users] Instatiating Global Variables

[Edson Tirelli]

   Joshua

   Globals in drools are designed to be a way for applications to expose
time-constant values to rules, and also a way to make application call backs
available to rules (like service methods, etc). It is not designed to be a
data sharing mechanism between rules, as we believe if you need to reason
over data, the data should be inserted into the working memory as facts.

   Having said that, I agree that for the use case you are describing, the
management of globals is not straight forward. For instance, I do believe we
need to work around technical limitations to make globals available to
functions. What I am not so sure is if we can or should do is allow rules to
change global values and cause the engine to re-test constraints using the
globals, since in this case, globals would behave exactly like facts, and as
so, why not use facts instead of globals?

   Regards,
      Edson



2007/11/2, Joshua Undesser <jundesse at cdmtech.com>:
>
>
> I am having a few issues with Global Variables and was wondering if
> someone
> could possibly explain them to me.....
>
> I am coming from a CLIPS/JESS background where globals are very straight
> forward.  ie (global ?*FAMILY_NAME* = "SMITH") and you can use it
> everywhere
> after that.   You can declare them anywhere you want(ie outside/inside
> rules, inside a function call), use them whenever you want (within the
> LHS/RHS or in a function) and modify them whenever you want (although you
> shouldn't).
>
> In Drools however, they seem overly complicated.
>
> For instance.....I can't declare and initialize a global in the same line.
> IE you can't do this....  global String FAMILY_NAME = "SMITH";
> Instead you have to just declare the global as.....global String
> FAMILY_NAME.
>
> Then after it's declared it can be set.    Although for the life of me,
> setting it is not straight forward either.  In a different thread on this
> list someone suggested doing the following....ie making sure the salience
> number is the highest of any rule forcing it to always fire first....
>
> rule "Initialize Rule" salience 100
>
> when
>         eval(FAMILY_NAME == null);
> then
>         WorkingMemory wm = drools.getWorkingMemory();
>         String familyName = "SMITH";
>         wm.setGlobal("FAMILY_NAME", familyName);
>         System.out.println("FAMILY_NAME =  " + FAMILY_NAME);
> end
>
> this does not seem to set the Global Variable value.    The result
> is...FAMILY_NAME = null.
>
> and any other rule that I have in place that utilizes this global doesn't
> fire.
>
> ie
> rule "First Rule to Fire"
>         when
>                 eval(FAMILY_NAME != null)
>         then
>                 .....
> end
>
> This never fires....
>
>
>
> However, if through my java application (not in a DRL file), I call
>
>
> ......code.....
> workingMemory.setGlobal("FAMILY_NAME", familyName);
> fireAllRules();
> ....more code....
>
> then my "Firest Rule To Fire" rule fires as expected.
>
>
> There has to be something I am not fully understanding here because it
> doesn't seem right that I can't create and set the Global all in one step,
> or the fact that I can't set the variable value in a rule, but only
> through
> a java app.
>
> I also don't understand why the Globals can't be used directly by
> functions
> and why they have to be passed in as a variable to the function.  If the
> function and the Global are defined within a specific working memory, then
> it should be available to all rules and functions within that same working
> memory.  Similar to Java Class Member Data.   The member data is available
> anywhere in that class.
>
> If anyone can help shed some light on the subject I would be very
> grateful!
>
>
> Joshua
>
>
> --
> View this message in context:
> http://www.nabble.com/Instatiating-Global-Variables-tf4740897.html#a13556883
> Sent from the drools - user mailing list archive at Nabble.com.
>
> _______________________________________________
> rules-users mailing list
> rules-users at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/rules-users
>



-- 
  Edson Tirelli
  Software Engineer - JBoss Rules Core Developer
  Office: +55 11 3529-6000
  Mobile: +55 11 9287-5646
  JBoss, a division of Red Hat @ www.jboss.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.jboss.org/pipermail/rules-users/attachments/20071105/625fe3c6/attachment.html