Re: Mixing OO and DB

From: Patrick May <pjm_at_spe.com>
Date: Sun, 16 Mar 2008 13:20:09 -0400
Message-ID: <m2wso2vb5i.fsf_at_spe.com>

"Brian Selzer" <brian_at_selzer-software.com> writes:

> "Patrick May" <pjm_at_spe.com> wrote in message news:m2wso9i9be.fsf_at_spe.com...


>>>>     In any case, we're getting a little far afield from the

>>>> original question.  In enterprise systems, denormalization for

>>>> performance does take place.  This is just one of several reasons

>>>> for decoupling the application logic from the database schema.

>>>

>>> I don't agree with this. You're equating the database schema with
>>> the database implementation.
>>
>> Not at all. I don't see where you get that from what I wrote.
>>
>>> The schema specifies what information is to be and can be recorded.
>>
>> Yes.
>>
>>> As such the schema is an integral part of the application
>>> specification, and it cannot be decoupled
>>
>> No. One schema can support multiple applications, and often
>> does in enterprise environments. One application can be supported
>> by different schemas -- there is not one and only one way to
>> represent the information required by the application.

>
> I'll buy that a schema can be part of multiple applications, but
> that there can be multiple ways to represent the same information
> does not alter what information is to be and can be recorded.

     First, please excuse the delay in replying.  I was busy

destroying the world by replacing relational databases with in-memory distributed object repositories. ;-)

     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.

     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.

>> 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 agree that they are different species of functionality. That's another good reason (or a restatement of one of my reasons) for decoupling them.

Sincerely,

Patrick

---

S P Engineering, Inc. | Large scale, mission-critical, distributed OO

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

Received on Sun Mar 16 2008 - 18:20:09 CET