Re: Mixing OO and DB

"Brian Selzer" <brian_at_selzer-software.com> writes:
> "Patrick May" <pjm_at_spe.com> wrote in message news:m2wso2vb5i.fsf_at_spe.com...
>> We seem to be in agreement that different specific schemas can
>> provide access to the same underlying information. That suggests
>> that your statement that ". . . the schema is an integral part of
>> the application specification, and it cannot be decoupled . . ."
>> needs clarification. Would you agree with the formulation "The
>> logical schema is an integral part of the application
>> specification."? By "logical schema" I mean the implementation
>> independent set of data that supports the application.
>>
>> If you agree with this, the second half of your claim
>> ". . . and it cannot be decoupled" is clearly incorrect because the
>> application implementation deals with the physical schema. Since
>> you agree that multiple different physical schemas are possible,
>> decoupling the application from any particular set of those is both
>> possible and good design.
>
> No, it isn't incorrect. Whenever a schema evolves, views are often
> used to provide backward compatibility for existing applications.
> An application need not concern itself with whether it is accessing
> a view or a table; therefore it is not necessarily tied to any
> physical schema.

     The fact that views can be used actually demonstrates that the application can be decoupled from the schema. You are suggesting using views to do so. That's one possible mechanism. OO languages provide others.

     Even when views are used, the application should be decoupled from the schema because the two models are often very different. Applications can organize information in ways other than the relational model.

>> Further, the view that the schema is integral to the
>> application is very data centric. Different applications may need
>> the data in different forms, not all of which are relational.
>> There is therefore a need to translate between the data structures,
>> which is another good reason to decouple the application from the
>> specific physical schema being used.
>
> If by data centric you mean that the information that is to be and
> can be recorded must be specified before even considering how that
> information may behave, then I agree: it is a data centric view.

     It is also possible to define a system in terms of behavior and only decide on a particular data representation once those behaviors are designed. In practice, both approaches are typically used.

> But again, you're laboring under the delusion that an application
> must be tied to a physical schema.

     I'm under no such delusion. An application implementation that uses data from a relational databas must deal with a particular schema. Since the two change for different reasons and since the models are often different, decoupling them is good design.

>>>> Even if an application uses the database system's
>>>> capabilities to implement some application functionality, there
>>>> are still changes to the underlying schema that do not, or should
>>>> not, require change to the applications that use that schema.
>>>> This is why approaches like dependency inversion are useful. The
>>>> application depends on an interface and the combination of
>>>> database schema and any logic running in the database implements
>>>> that interface. Either can change without impacting the other.
>>>
>>> The database contains that which can be manipulated. An
>>> application is that which does the manipulating. These are
>>> completely different species of functionality.
>>
>> The database sometimes contains some of that which can be
>> manipulated.
>
> I don't follow you. Please elaborate.

     Not all data used by an application needs to be in the database.

Regards,

Patrick

                       | systems design and implementation.
          pjm_at_spe.com  | (C++, Java, Common Lisp, Jini, middleware, SOA)