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   Fallacies of Validation ... RE: [xml-dev] Are people really using Identi

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Hi Folks,

From reading yesterday's messages, I feel like the real issues are coming
out.  And the real issues, I perceive, are in the various fallacies with
validation.  Below I provide a start at listing the fallacies.  Your help in
elaborating these is needed.

Fallacies of Validation

1. Fallacy of "THE Schema"

2. Fallacy of Schema Locality

3. Fallacy of Requisite Validation

Let's examine each of these fallacies.

1. Fallacy of "THE Schema"

This fallacy was identified by Michael Kay last week:

> ... there's no harm in using XML Schema to check data 
> against the business rules, so long as you realize this 
> is *an* XML Schema, not *the* XML Schema. We need to stop 
> thinking that there can only be one schema.

Yesterday Len Bullard made a similar statement:

> ... most fundamental errors are ... to consider only a single schema.

and at another point Len states:

> ... fall into the trap of thinking of THE schema and not 
> recognizing the system as a declarative ecosystem of schemas 
> and schema components.

Both Michael and Len are stating that in a system there should be numerous
schemas.  This is a big mindshift for me.  I admit being trapped into
thinking that there should be a single schema.

It would be very useful if we could have a simple example that shows how
several schemas might be employed, rather than a single schema.  Could
someone provide an example?  

Len, I like the term you used, "declarative ecosystem".  Could you elaborate
upon what this means?

2. Fallacy of Schema Locality

Yesterday Len also identified this fallacy:

> ... most fundamental errors are to consider schemas only at the external
system junctions ...

Len notes that many people think that validation should occur at a certain
place in the system, namely, at the outermost edges of the system.  (Len, I
assume this to mean the user-interface?)  Len argues that validation can
rightfully be done at many locations in a system.  Len, perhaps some more
words on this fallacy would be in order?

3. Fallacy of Requisite Validation

Yesterday Michael Kay made a very compelling statement with regards to
whether validation should be done at all in certain situations.  Michael was
responding to the example of an online service validating a user's address.
Here's what Michael said about the online service's insistence on validating
the user's address:

> The strategy (validating the user's address) assumes that  
> you know better than your customers what constitutes a  
> valid address. Let's face it, you don't, and you never  
> will. A much better strategy is to let them (the user) express   
> their address in their own terms. After all, that's what they  
> do in old-fashioned paper correspondence, and it seems 
> to work quite well.

Michael argues very effectively that in this situation it makes no sense to
do any validation at all!

I have not yet read all of yesterday's postings, so I may have missed some
other fallacies.  If you know of any fallacies that I missed, would you
please send them along?  

Also, if you have comments on the fallacies identified above, please send
them along.  Note: examples are much needed!



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