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From Andrus Adamchik <and...@objectstyle.org>
Subject DO/PO design
Date Mon, 30 Nov 2009 10:09:27 GMT
Another possible "lightweight" design that would allow to reconcile  
DO's and PO's is to use Object[] instead of either HashMap or ivars to  
store values, with indexes in array corresponding to ObjAttribute  
names sorted in alphabetic order. Will probably have its own quirks I  
guess (e.g. how do we define the array size for a generic object that  
can be mapped to different entities), however it has some serious  
benefits:

* memory use very close to ivars
* access speed very close to ivars
* can be used with Faults
* can be used with generic objects

Andrus


On Nov 24, 2009, at 9:47 PM, Andrus Adamchik wrote:

> True, but they are still user facing collections (or more generally  
> valueholders, as this also works for to-one), that get inflated  
> internally as needed. DO faults are singletons of this type:
>
> http://cayenne.apache.org/doc/api/org/apache/cayenne/Fault.html
>
> So PO lazy relationship resolving goes like this:
>
> 1. on select, init lazy property with a "hollow" ValueHolder
> 2. on ValueHolder value access, inflate ValueHolder with the DB data
>
> For DO it has an extra step:
>
> 1. on select, init lazy property with a singleton Fault of a type  
> corresponding to the relationship semantics
> 2. on property access, *replace* Fault with either a hollow  
> collection or a target DO (which can be hollow or resolved from cache)
> 3. on collection or hollow DO value access, inflate them.
>
> The difference may be subtle, but still essential. First, in DO case  
> there's no to-one ValueHolder. Target object is attached directly.  
> Also the user never sees the Fault object itself, so we can replace  
> it with another object when needed. Among other things this allows a  
> major memory optimization, as Fault instances are singletons shared  
> by all DOs.
>
> So all in all, DO design seems more sound (and quite possibly that  
> whatever memory we save on using ivars in PO, is getting lost to  
> extra and more eager ValueHolders).
>
> Andrus
>
>
> On Nov 24, 2009, at 9:28 PM, Andrey Razumovsky wrote:
>
>> Actually, I don't understand :) PO arc properties are all faults  
>> (classes
>> PersistentObject*)
>>
>> 2009/11/24 Andrus Adamchik <andrus@objectstyle.org>
>>
>>>
>>> On Nov 24, 2009, at 5:14 PM, Andrey Razumovsky wrote:
>>>
>>>
>>>>>
>>>>> So ... another attempt at abstract analysis (or an attempt at  
>>>>> unwinding
>>>>> my
>>>>> memory...) IIRC the main difference between DO and PO is fault  
>>>>> handling -
>>>>> we
>>>>> can store a Fault instance in a map, but not in a typed ivar. So  
>>>>> PO's
>>>>> have
>>>>> no concepts of faults at all, and the lifecycle is different. DO  
>>>>> approach
>>>>> is
>>>>> more lazy (it won't try to create even a placeholder collection  
>>>>> until the
>>>>> property is accessed), and I prefer it to PO's. Wonder if when we
>>>>> reconcile
>>>>> that somehow, the rest will fall into place on its own??
>>>>>
>>>>> (E.g. for PO's, we generate extra boolean "fault" properties for  
>>>>> each
>>>>> relationship, that are checked on every property access?)
>>>>>
>>>>>
>>>> Nice idea, need to think about it. But anyways, this is separate  
>>>> task
>>>>
>>>
>>> May or may not be a separate task. But definitely bigger in scope.
>>>
>>> Andrus
>>>
>>>
>>
>>
>> -- 
>> Andrey
>
>


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