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From Craig L Russell <Craig.Russ...@Sun.COM>
Subject Re: SCO and FCO treatment of String, Locale, etc.
Date Mon, 24 Jul 2006 06:18:13 GMT
Hi Ilan,

On Jul 24, 2006, at 12:10 AM, Ilan Kirsh wrote:

> Hi Craig,
>
> Probably this discussion is mainly theoretical because most JDO  
> implementations do not support system immutable types as FCO, and  
> even in implementations that do support this, the default is to use  
> SCO, and this default is rarely changed (if at all). Maybe the only  
> application that tries to use this optional feature is the JDOTCK,  
> in which some immutable type fields are defined with embedded=false  
> in the metadata.
>
> In my opinion the suggested fix in [1], at least for the fields  
> that are defined with embedded=false is the right solution.

I had not understood that the tck tests in question defined  
embedded=false. Changing this to "default" to allow the  
implementation to choose would then be better. I'll take another look  
at the test cases.
>
> Unfortunately the suggested fix to the specification might be  
> insufficient because it will cause conflicts in many other places.  
> For instance, what should return JDOHelper.getPersistenceManager 
> (field) when a FCO immutable type field is shared by two different  
> PersistenceManagers?

This is not portable behavior in any case. The specification could  
define this to return null always for these cases. These instances  
should not be associated with a PersistenceManager.

> What should happen when such a field value is passed as a query  
> argument (according to the spec: "If a persistent instance  
> associated with another PersistenceManager is passed as a  
> parameter, JDOUserException is thrown during execute()"), etc.

Since there is no ambiguity as to the semantics of passing such a  
reference (there is no PersistenceManager that owns the instances) no  
exception needs to be thrown.
>
> The most logical solution IMO is to treat such FCO instances as any  
> other FCO, and a user that chooses to use them with embedded=false  
> (again, very rare) will have to use clone.

It might be better to treat such FCO instances as special cases that  
are not owned by a PersistenceManager, but are shared among all  
PersistenceManagers. This treatment is consistent with what at least  
one object database does (having a special database representation  
for fixed precision integers etc.)

Thanks,

Craig
>
> Regards,
>
> Ilan
>
>   ----- Original Message -----
>   From: Craig L Russell
>   To: JDO Expert Group ; Apache JDO project
>   Sent: Monday, July 24, 2006 7:01 AM
>   Subject: SCO and FCO treatment of String, Locale, etc.
>
>
>   Javadogs,
>
>
>   An issue has been raised [1] with regard to storage of instances  
> of immutable system object classes, e.g. Integer, String, Locale.  
> The specification calls for users of the API to not depend on  
> whether these instances are stored as FCO or SCO.
>
>
>   The intent of the specification is that applications should not  
> depend on whether the instances are stored by themselves as  
> persistent instances or stored as embedded within the domain class  
> instances. Furthermore, the application should not depend on FCO  
> behavior: uniquing of instances in the same PersistenceManager for  
> the same instance. Finally, the application should not depend on  
> guaranteeing SCO behavior: copying the instance upon commit and  
> guaranteeing a distinct instance in memory upon reinstantiation.
>
>
>   The rationale of this wording was specifically to allow an  
> implementation of JDO to either store instances embedded in the  
> "containing" domain class instance or as a nullable reference to a  
> persistence-capable System class in the datastore, as is done by  
> some object databases.
>
>
>   The intent was not to impose restrictions on the application's  
> use of the identical instance as the value of multiple fields of  
> domain classes. A literal interpretation of this part of the  
> specification would require users to guarantee that the same  
> instance of any of the classes was never contained in instances  
> managed by multiple PersistenceManagers. The effect of this  
> interpretation is to require cloning for each instance of Integer,  
> String, Locale, etc. used in domain classes.
>
>
>   I believe that this interpretation should be disallowed by the  
> specification, as it imposes a great burden on users. I would like  
> the Expert Group to discuss this issue and straw proposal.
>
>
>   <spec 6.4.3>
>   Immutable Object Class types
>   JDO implementations must support fields that reference instances  
> of immutable object
>   classes, and may choose to support these instances as SCOs or FCOs:
>   •package java.lang: Boolean, Character, Byte, Short, Integer, Long,
>   Float, Double, and String;
>   •package java.util: Locale, Currency.
>   •package java.math: BigDecimal, BigInteger.
>   Portable JDO applications must not depend on whether instances of  
> these classes are treat-
>   ed as SCOs or FCOs.
>   </spec 6.4.3>
>
>
>   <proposed 6.4.3>
>   Immutable Object Class types
>   JDO implementations must support fields that reference instances  
> of immutable object
>   classes, and may choose to support these instances as SCOs or FCOs:
>   •package java.lang: Boolean, Character, Byte, Short, Integer, Long,
>   Float, Double, and String;
>   •package java.util: Locale, Currency.
>   •package java.math: BigDecimal, BigInteger.
>   Portable JDO applications must not depend on SCO or FCO uniquing  
> behavior, nor on the storage mechanism in the datastore. Portable  
> applications may use the same instance of these classes as field  
> values in any persistence-capable class instance.
>   </proposed 6.4.3>
>
>
>   [1]http://issues.apache.org/jira/browse/JDO-397?page=all
>
>
>   Craig Russell
>   Architect, Sun Java Enterprise System http://java.sun.com/ 
> products/jdo
>   408 276-5638 mailto:Craig.Russell@sun.com
>   P.S. A good JDO? O, Gasp!
>

Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!


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