Re: Declarative constraints in practical terms

I think you're missing the point. A database is a knowledge repository, not an application. It is the foundation upon which applications are built. If the foundation is weak, then the building will crumble. The purpose of the Relational Model isn't to build applications, but rather to provide a sound footing. The Principles of Normalization and Orthogonal Design strengthen the foundation by eliminating redundancy. Integrity constraints seal the foundation so that inconsistencies cannot leach their way into the foundation, causing it to crack.

Comments inline.

"dawn" <dawnwolthuis_at_gmail.com> wrote in message news:1140555731.823399.160020_at_g44g2000cwa.googlegroups.com...
> This is related to a recent exchange related to declarative
> constraints. I know I've worked on it before, but I am still perplexed
> on this one. Here are two common options for handling constraints
> (business rules):
>
> a) declarative constraints: SQL declarations enforced by a DBMS engine
> b) metadata + code: metadata specifications plus custom-build
> validation/constraint functions or services written in a general
> purpose language
>
> Some differences I can see are
>
> 1) Declarative constraints are coded in the SQL sublanguage while with
> metadata + code, the metadata declarations are simpler, being in the
> form of name=value pairs (e.g. maxValue=100, valTable=MyTable) but
> proprietary code is written.
>
> 2) The functions combining the data and constraints in the declarative
> case are in well-tested (over time, at least) engines written by a
> database vendor, otherwise often less generic, more specific, functions
> written by whomever whatever team is writing validation routines for
> software services. .
>
> 3) The functions for validation of data can be used anywhere within an
> application if packaged outside of the dbms, otherwise most of them
> need to be coded at least twice -- once for the dbms and once for use
> in a UI or web service.

But now it's up to the application(s) to maintain integrity, which is a recipe for disaster. It is the database's responsibility to maintain integrity. If the database contains redundant information, then it's up to every application to make sure that every instance is updated at the same time so that queries can be relied upon. If constraints aren't declared on the objects in the database, then it's up to every application to ensure that garbage doesn't make it into the database. If you delegate the database's responsibility to the applications, then all it takes is one poorly written or malicious application to introduce garbage into the database--making the results of all applications suspect.

>
> 4) The declarative constraints use the RM, so they work with the
> restrictions of the RM, including 1NF. Because they are written in
> SQL, they use a 3VL whereas using metadata + code, 2VL and non-1NF are
> the norm.

NULLs and 3VL are optional. I prefer to avoid them. For every relational database that uses NULLs, there exists an equivalent relational database that does not.

I would argue that 1NF is a good thing. Sets are already covered, because relations are sets. Bags and lists can also be easily mapped onto relations. Once mapped and indexed, queries are easier and faster.

>
> 5) In the case of declarative constraints, they are necessarily
> employed by any application writing to the database. In the case of
> metadata + code, each organization must determine whether and how to
> technically enforce business rules for all applications or enforce them
> through standards and QA approaches.
>
> 6) If there are local constraints (constraints for one application and
> not another) then these are either declared in SQL as local constraints
> (something I've read about, but never used) or, often, coded only in
> the individual applications using metadata + code. So, local
> constraints are often handled the same but in one case local and global
> constraints are coded in the same language, typically by the same team.

How can there be constraints for one application and not another? Either the information in the database is consistent or it is not. If you don't enforce all constraints all of the time, then garbage can get into the database! Once garbage gets in, every query is suspect.

>
> I lean toward not duplicating constraints, coding and maintaining them
> in multiple places and languages, but I understand that someone else
> might choose the other strategy. Whatever choice, it doesn't look
> obvious to me that declarative constraints are better as I gather it
> appears to many others.
>

I think that it is more important to keep garbage out of the database. Again, if the foundation is weak, then the building will fall. An additional layer of protection is a good thing. Rollbacks are expensive, so it's better to scrub the data before it's introduced to the database.

> This relates to the fact that the RM is not sufficient for writing
> software (as mentioned in my current blog entry that I'll again boldly
> advertise as being at http://www.tincat-group.com/mewsings ) and coding
> constraints using the RM doesn't seem like it can get you all the way
> there. So if you go that route, you end up duplicating your work, both
> up front and for all maintenance.

I read your blog. I disagree. It's a simple matter to persist object state in a relational database. It also makes it much easier to use the information for other purposes, such as reporting.

>
> Is there a way to get the best of both worlds on this one? This issue
> is really bothering me, so thanks in advance for any help you can give
> me to gain a better understanding and apologies for bringing it up
> again. --dawn
>
Received on Wed Feb 22 2006 - 09:09:14 CET