10:16 a.m.
Stephen,
Really glad to see you picking up this stuff.
Let me clarify something: "templates" were an experimental feature in
Drools 4.0, and so far, will continue to be experimental in Drools 5.0 as we
lack the manpower to move it ahead ourselves. If you are willing to move
this ahead, we can provide all assistance/guidance you may need.
Having said that, lets split the template construct into concept and
syntax. The syntax is being completely removed in Drools 5 and replaced by
the TypeDeclaration syntax. The type declaration concept is more abstract
than template (or should I say a super set?). Type declaration in drools 5
has 2 purposes: 1st it is a way to assign metadata to types, 2nd it is a way
to declare/generate underlying structures for facts.
So talking about the first usage, as it is today, you can have an
existing fact type (e.g. a class named StockTick) and assign metadata for
it. For instance, saying it should be given "event semantics" in the system:
declare StockTick
@role( event )
end
Talking about the second usage, the idea is that you can declare types
that may have any underlying implementation would best fit you. So far, the
underlying implementation that we have working and tested is POJO
generation.
declare Address
line1 : String
line2 : String
end
declare Person
name : String
age : int
address : Address
end
Now, it is planned, though not implemented/wired/tested yet, that when
you declare a type, you may inform the engine what is its underlying format
and the engine will know how to handle it:
declare Address
@format( template )
line1 : String
line2 : String
end
Idea is that you could have any arbitrary underlying format for facts,
like XML, JMS Messages, Database Records, CSV records, etc.
Anyway, that is the vision. If you look at the PackageBuilder.java, line
602, processTypeDeclaration() method, that is where the compilation/wiring
process starts. We probably need to extract that method and create a
separate API for it. The code there is not tested except for POJOs. If you
look at the line 631, it has an "if" from the first idea we had, where you
would use "template" construct to create a template and then a
"declare"
construct to assign metadata to it. We dropped this idea, since a unified
approach is better, but that code is still there:
// is it a POJO or a template?
String templateName = typeDescr.getMetaAttribute(
TypeDeclaration.ATTR_TEMPLATE );
Also, just for the sake of completeness, the final vision is to allow
both initialization expressions and attribute validation in the type
declaration. For instance:
declare Person
name : String( length < 30 ) @key
age : int( value >= 0 && < 150 ) = 0
addresses : Map<String, Address> = {"home" : null,
"business" : null}
end
There are a few more bits like automatic creation of facts, but that is
subject for another time.
There is a lot to do, but it can be easily split into an incremental
approach. If you want to contribute working on this, it will be a pleasure
to support you.
Cheers,
Edson
2008/8/27 Stephen Kestle <stephen.kestle(a)orionhealth.com>
I believe I have it... "declare" is meant to be used for
easy generation
of java classes, so that rules can instantiate objects and use them as
such. For dynamic data models, "template" is a better fit, as getField etc
are geared towards map-like data structures.
I believe that I could have :
template "Address"
String line1
String line2
end
template "Person"
String name
int age
Object address
end
rule "Test"
when
add: Address(line1 == "address")
person: Person(address == add)
then
...
Is this correct?
Cheers
Stephen Kestle wrote:
I'm creating a type hierarchy using TypeDeclarations based on an
ontological model (analagous to an xml schema for the purposes of this
post).
If I declare a drl file to have the following declarations
declare Address
line1 : String
line2 : String
end
declare Person
name : String
age : int
address : Address
end
it will create Java Pojo classes for me. However, I had it all working
using fact templates, except for the address item. Is it possible to insert
fact implementations based on templates, while declaring TypeDeclarations in
the drl? (I now go to look for how to specify the format = TEMPLATE for the
drl)
I say this because I'm looking up values using id strings - e.g. the exact
same way that FactTemplate based Facts does accessing. I don't want to have
to convert all my String ids to reflectively call the set methods on the
created Pojo.
Hopefully this is enough info for you 5.0 developers - I'm still trying to
wrap my head around it all.
Cheers
Stephen
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