[rules-users] Abstraction between rules and data model?
Horváth Péter Gergely
h.peter at mailbox.hu
Mon Jun 2 08:59:20 EDT 2014
Hi Davide,
Thanks, I just wanted to get a grasp of the time frame we could expect
(whether it is a month or half a year etc.) :)
A bit more about our use case: we have a *massive* old system, with
basically all of the logic embedded into Java. We are in the process of
refactoring and moving up-to-date Java technologies. We would like to
introduce a rule engine so as to extract logic from the core of the
application. Since we need to re-structure the whole back-end and
persistence layer, we face a challenge with rules: as the object model
changes, the rules have to change also, which simply would not be
manageable. (Imagine something really crazy/complex here: 6-8-... levels of
object hierarchy, grown most of the time organically during the last 15
years.) Deferring the introduction of the rule engine until the data model
classes reach the final, ideal state is not something we can now afford,
the rule engine and data model refactoring projects should be done in
parallel, without the one paralyzing the other one...
Take the following as an example (from the manual). Say, you have Applicant
class, with age and name fields:
public class Applicant {
private String name;
private int age;
// getters/setters
}
Then, you could have a simple rule, requiring, that name must be supplied:
rule "Supplied a proper name"
when
Applicant( name == null)
$a : Application()
then
output.println("### RULE MATCH: Invalid application: No name
specified!");
$a.setValid( false );
end
But there is an issue: every single time you refactor / rename any field in
your domain model, the rules are no longer valid. That is why we have a
problem: we should start developing rules against a relatively fluid object
model. Say Applicant.name is renamed to Applicant.fullName, then the
existing rules are no longer valid. That is why I would like to have some
sort of abstraction, since the rules do not really need to know the
*exact *details
of the underlying class.
As a demonstration of my idea, I managed to hack together a custom
parameterizable operator, that calls the method where the annotation
contains the same parameter as in the rule. With this approach, changes to
the field names would no longer affect the rules. Again, I have to
highlight, I don't think this should necessarily be implemented in the core
engine. *What I am looking for is some kind of extension point allowing us
to hook into the expression evaluation part of Drools*. (Instead of a
custom operator, something, where we can access the object and the
expression being resolved so that we can implement our custom logic for
returning the value of the expression)
The rule now looks like this - note the custom satisfiesRule operation. The
"Name of this Applicant" expression now abstracts away the name of the
actual field (the second parameter is the operator for the test, please
ignore it for now):
rule "Supplied a proper name"
agenda-group "evaluate-application"
auto-focus true
when
Applicant( this satisfiesRule[ "Name of this Applicant" , "is equal
to" ] null )
$a : Application()
then
output.println("### RULE MATCH: Invalid application: No name
specified!");
$a.setValid( false );
end
The name is simply mapped by a custom annotation:
public class Applicant {
private String name;
private int age;
@MyBusinessExpression("Name of this Applicant")
public String getName() {
return name;
}
// getters/setters
}
I think having a level of abstraction between the rules and the data model
would not only be useful for de-coupling, but also can make rule authoring
easier for the business users. The custom operator (please see attachment)
is a kind of hacky workaround, but demonstrates how something similar could
be achieved at run time (without using DSL or any other rule
transformation).
Please let me know what you think. :)
Cheers,
Peter
2014-06-02 11:54 GMT+02:00 Davide Sottara <dsotty at gmail.com>:
> I can't guarantee a public date.. as a community member, I work on a
> "best effort" basis...
> I'll try to do it before the end of the month, though.
> For now, as a workaround, I would create derived getter/setter pairs that
> expose the desired
> computations. A concrete example of what you are trying to do exactly
> would also be helpful,
> feel free to contact me privately if you can't share your code here
> Best
> Davide
>
>
> On 05/28/2014 10:08 AM, Péter Gergely, Horváth wrote:
>
> Thanks for the explanation, I was a bit confused because of the
> terminology; "virtual" is not mentioned in the docs. ;)
>
> Is there any plan for the public release of Trait property binding to a
> nested path? We would definitely need something like that in our
> environment. Or do you see any way we could hook into the property look-up
> mechanism? Based on what I know, I don't see any official extension point
> for that.
>
> My only idea would be using some Java proxy voodoo-magic to wrap objects
> before they are inserted to the session, but my gut feeling is that it
> would be a way to debug hell...
>
> What do you think?
>
> Cheers,
> Peter
>
>
>
> 2014-05-27 19:42 GMT+02:00 Davide Sottara <dsotty at gmail.com>:
>
>> Consider that a trait is an interface applied to some class. In the
>> context of the pair:
>> A "hard" field is a property (get/set) exposed by the interface AND the
>> underlying class
>> A "soft" (or "virtual") field is a property exposed by the interface BUT
>> NOT by the underlying class
>> A "hidden" field is a field of the underlying class NOT exposed by the
>> interface
>>
>> Hard and Soft fields can be accessed using the interface, hidden fields
>> are accessible using the map-like
>> construct fields[ "fieldName" ].
>>
>> This said,
>> the mapping is by default done using the property name and (then) the
>> property type.
>> However, this mapping can be decoupled using the annotation @Alias() on
>> either the class OR the trait.
>> E.g.
>> declare Core
>> name : String @Alias( "any-Id-or-even-an-IRI-here" )
>> end
>>
>> declare trait SomeTrait
>> label : String @Alias( "..." ) // if two "aliases" match, this will
>> be considered a hard field
>> end
>>
>> The "accessor", i.e. the ability to bind a trait property to a (possibly
>> deeply) nested path is what I'm working
>> on these days, I have the same requirement from another urgent use case
>>
>> For the time being, you can probably create a "shortcut" accessor pair in
>> your implementation class,
>> to execute the complex expression, and @Alias it to the trait field.
>>
>> Please let me know if you find any issue/bugs and any feature request you
>> may have!
>> Best
>> Davide
>>
>>
>>
>> On 05/27/2014 07:57 AM, Horváth Péter Gergely wrote:
>>
>> Hi Davide,
>>
>> Drools trait functionality is one of the powerful concepts which makes
>> Drools a good candidate for the project. So keep up the good work! :)
>> However I'm not sure if its current level of flexibility would be
>> sufficient for our use case. I've checked the documentation, but haven't
>> really found the term virtual field -- could you please elaborate on this?
>>
>> Do you think we could somehow hook into the evaluation of the aliases
>> or the "fields" Map? Sometimes you would need slightly more than merely
>> aliasing fields to something else; e.g. calculating values for the purpose
>> of rule processing or extracting a value from a more complex object tree
>> etc. Citing the example -- GoldenCustomer( fields[ "age" ] > 18 ) -- being
>> able to get a reference to the target object and the field map expression
>> "age" would be quite close to what I imagined. Our custom code could then
>> perform the appropriate translation and return the requested value, hiding
>> the fact whether "age" is an actual field in the Customer object
>> itself/retrieved from an encapsulated complex object e.g. replacing
>> expression "customer.personalInformation.birthData.age"/calculated on the
>> flight.
>>
>> What do you think?
>>
>> Cheers,
>> Peter
>>
>>
>>
>> 2014-05-26 17:58 GMT+02:00 Davide Sottara <dsotty at gmail.com>:
>>
>>> We are working on the trait framework for cases like this. Essentially,
>>> it allows to use
>>> interfaces when writing rules AND to inject the interfaces dynamically
>>> at runtime,
>>> at the instance level. It relies on transparent proxies which wrap the
>>> data classes
>>> and implement the required interfaces. A simple field aliasing mechanism
>>> is provided
>>> (work in progress). For more complex transformations, "virtual" fields
>>> can be added.
>>> See section 7.7.8 of the manual for more details and let me know if it
>>> can help
>>> with your use case.
>>> Best,
>>> Davide
>>>
>>> On 05/26/2014 09:55 AM, Wolfgang Laun wrote:
>>> > Even a relatively sophisticated transformation would be easier to
>>> implement
>>> > and most certainly safer from changes in the unstable Drools API than
>>> some
>>> > hook-and-intercept mechanism built into Drools.
>>> >
>>> > Notice that violent structural departure of the model the BUs see from
>>> what
>>> > you call "persistence model" might make it impossible for the BUs to
>>> come
>>> > up with rules that can be transformed to match the other model at all;
>>> > if it is possible, rules might still incur a heavy performance penalty.
>>> >
>>> > It is (IMHO) a myth that "Rules" is a foolproof way of establishing
>>> > business logic
>>> > independent from the data model and application environment with which
>>> > this logic should be able to cooperate. As long as everything is kept
>>> in the
>>> > abstract (i.e., formulated in terms of mathematics) it will look
>>> good, but
>>> > any implementation may throw a spanner in the works, or worse.
>>> >
>>> > -W
>>> >
>>> > On 26/05/2014, Péter Gergely, Horváth <h.peter at mailbox.hu> wrote:
>>> >> Hi Wolfgang,
>>> >>
>>> >> Thank you for your input. You are right that some of the cases could
>>> simply
>>> >> be covered by regexp-replace, but I'm afraid, not all of them.
>>> Interfaces
>>> >> could also help, but we have a requirement that the business rules
>>> should
>>> >> not be tightly coupled to the underlying persistence model. (I
>>> understand
>>> >> that some might say this is not ideal, but that is our current
>>> situation)
>>> >>
>>> >> I am wondering whether it is possible to hook into Drools engine and
>>> >> intercept field value reference expression evaluations in run time
>>> (e.g if
>>> >> "foo.bars" is used in an expression, we could return "foo.barList") ?
>>> By
>>> >> injecting some custom code, we could make the necessary decisions and
>>> >> extract the proper value from an object. Unfortunately these parts of
>>> >> Drools are pretty much undocumented.
>>> >>
>>> >> Regards,
>>> >> Peter
>>> >>
>>> >>
>>> >>
>>> >> 2014-05-26 13:57 GMT+02:00 Wolfgang Laun <wolfgang.laun at gmail.com>:
>>> >>
>>> >>> What you describe can be done with /bin/sed.
>>> >>>
>>> >>> Notice that the DSL processor doesn't require you to translate entire
>>> >>> patterns; there is a mechanism for translating "keywords", which is
>>> >>> just arbitrary tokens to whatever replacement text.
>>> >>>
>>> >>> If a "bar" must be translated to a "barList" in the context of a
>>> class
>>> >>> "Foo" but not in any other context, a more sophisticated translation
>>> >>> is required in any case (with /bin/sed still being sufficient if
>>> >>> patterns aren't split across lines).
>>> >>>
>>> >>> Some say that good design makes use of Interfaces, which leaves room
>>> >>> for actual implementations being changed as long as the interfaces
>>> are
>>> >>> implemented. Here, note that rules can be written against interface
>>> >>> types.
>>> >>>
>>> >>> -W
>>> >>>
>>> >>>
>>> >>> On 26/05/2014, Péter Gergely, Horváth <h.peter at mailbox.hu> wrote:
>>> >>>> Hi All,
>>> >>>>
>>> >>>> We are evaluating Drools 6 for our use case, and face challenges
>>> where
>>> >>>> we
>>> >>>> would need some ideas from more experienced users of Drools.
>>> >>>>
>>> >>>> We have an application with a massive code base and a large number
>>> of
>>> >>> model
>>> >>>> (entity) classes. We are in the process of moving away from
>>> inherited
>>> >>>> legacy technologies and refactoring the old code base. As a part of
>>> >>>> this
>>> >>>> work we would like extract some of the hard-coded business logic to
>>> >>>> external rules, that is why we are looking at Drools as a potential
>>> >>>> solution.
>>> >>>>
>>> >>>> What we would like to have is some kind of abstraction or mapping
>>> >>>> between
>>> >>>> actual entities and rules the business users can define so that
>>> they do
>>> >>> not
>>> >>>> have to know the _exact_ details of the data model (field names,
>>> >>>> precise
>>> >>>> relations etc). This would be important for us so that we can
>>> refactor
>>> >>> the
>>> >>>> old model classes without affecting business rules; also it would
>>> make
>>> >>> life
>>> >>>> easier for the business users. While IDE support might make
>>> refactoring
>>> >>>> easier, we definitely want to have a separation between rules and
>>> >>> entities.
>>> >>>> Given our situation, writing and maintaining "stable"
>>> wrapper/adapter
>>> >>>> classes for the sole purpose of rule processing is out of question.
>>> I
>>> >>> have
>>> >>>> checked the documentation of Drools DSL support and for me it seems
>>> to
>>> >>>> be
>>> >>>> overkill for our use case: we do not really need a custom language,
>>> but
>>> >>>> simply an abstraction between rules and the data model classes.
>>> >>>>
>>> >>>> What I could imagine is a piece of code, (a custom property
>>> resolver? -
>>> >>> no
>>> >>>> sure how it is called) which maps property expressions to actual
>>> >>> properties
>>> >>>> based on a custom annotation on the entity class or something like
>>> >>>> that,
>>> >>> so
>>> >>>> that a rule containing "Foo.bars" expression does not have to change
>>> >>>> even
>>> >>>> if we decide to rename "Foo.bars" to "Foo.barList" in the model
>>> >>>> classes.
>>> >>>> (This was just a simple example of a potential use cases)
>>> >>>>
>>> >>>> Could you please share your thoughts on this topic and point me into
>>> >>>> the
>>> >>>> right direction?
>>> >>>>
>>> >>>> Thanks,
>>> >>>> Peter
>>> >>>>
>>> >>> _______________________________________________
>>> >>> rules-users mailing list
>>> >>> rules-users at lists.jboss.org
>>> >>> https://lists.jboss.org/mailman/listinfo/rules-users
>>> >>>
>>> > _______________________________________________
>>> > rules-users mailing list
>>> > rules-users at lists.jboss.org
>>> > https://lists.jboss.org/mailman/listinfo/rules-users
>>> >
>>>
>>> _______________________________________________
>>> rules-users mailing list
>>> rules-users at lists.jboss.org
>>> https://lists.jboss.org/mailman/listinfo/rules-users
>>>
>>
>>
>>
>> _______________________________________________
>> rules-users mailing listrules-users at lists.jboss.orghttps://lists.jboss.org/mailman/listinfo/rules-users
>>
>>
>>
>> _______________________________________________
>> rules-users mailing list
>> rules-users at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-users
>>
>
>
>
> _______________________________________________
> rules-users mailing listrules-users at lists.jboss.orghttps://lists.jboss.org/mailman/listinfo/rules-users
>
>
>
> _______________________________________________
> rules-users mailing list
> rules-users at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/rules-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.jboss.org/pipermail/rules-users/attachments/20140602/dc5fae73/attachment-0001.html
-------------- next part --------------
A non-text attachment was scrubbed...
Name: BusinessFieldEvaluatorDefinition.java
Type: application/octet-stream
Size: 8427 bytes
Desc: not available
Url : http://lists.jboss.org/pipermail/rules-users/attachments/20140602/dc5fae73/attachment-0002.obj
-------------- next part --------------
A non-text attachment was scrubbed...
Name: MyBusinessExpression.java
Type: application/octet-stream
Size: 328 bytes
Desc: not available
Url : http://lists.jboss.org/pipermail/rules-users/attachments/20140602/dc5fae73/attachment-0003.obj
More information about the rules-users
mailing list