Re: Integrating Django with a legacy but active database

On Thu, Jun 3, 2010 at 7:48 PM, kakarukeys <kakarukeys@gmail.com> wrote:
> On Jun 3, 7:23 pm, Russell Keith-Magee <russ...@keith-magee.com>
> wrote:
>> On Thu, Jun 3, 2010 at 6:31 PM, kakarukeys <kakaruk...@gmail.com> wrote:
>> > I am developing a django web app which does some web publishing
>> > (pushing some data to website). My customer is already using
>> > ExpressionEngine 2.0 for publishing, hopes that I can reuse the CMS
>> > for any publishing purpose. They does not wish to redevelop the web
>> > publishing platform in Django.
>>
>> > I'm asking if it is safe to...
>>
>> > use the technique documented in
>> >http://docs.djangoproject.com/en/dev/howto/legacy-databases/
>>
>> > to create models and do publishing using the models. So we will have
>> > two web apps being able to access / query the database at the same
>> > time. Will it cause issues like data corruption, etc?
>>
>> It should be entirely safe. You'll have the same transaction and
>> consistency issues that exist whenever you have two clients attached
>> to the same database, but that's just due to have two clients talking
>> to the same database -- you would get the same problems if you had two
>> EE users attached simultaneously.
>>
>> The only other potential risk is if you have large amounts of data
>> consistency logic implemented in user code under Expression Engine. In
>> this case, you will need to duplicate this logic on the Django side,
>> and there's the risk that you might introduce errors as a result of
>> inconsistencies between the two implementations.
>>
>> Yours,
>> Russ Magee %-)
>
> Hi,
>
> Thanks. I find what you said rather abstract for me to understand.
> Could you give a simple example scenario where that might happen
> (referring to the "potential risk")?

Let's say you have a model with an integer field, but for some
business-logic reason, the integer must be greater than 10. If this
constraint is imposed at the database level, then you will never be
able to save a model with a value of 5 for that field. However, if the
constraint is imposed at the code level, you need to implement the
validation logic twice -- once under EE, and once under Django.

This means it's easy to inadvertently introduce errors; for example,
you could implement x > 10 on EE, but x>= 10 under Django. This means
that your Django client will be able to save a value of 10, but the
same value will raise a validation error under EE. Identifying and
testing for this sort of consistency is hard to do; so if you have a
lot of data validation constraints like this, it's worth considering
as a potential risk.

Yours,
Russ Magee %-)

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