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Re: 3 XML Design Principles

• To: "Roger L. Costello" <costello@m...>
• Subject: Re: 3 XML Design Principles
• From: Robert Koberg <rob@k...>
• Date: Sat, 29 Jan 2005 09:45:22 -0800
• Cc: "'XML Developers List'" <xml-dev@l...>
• In-reply-to: <200501291720.j0THK3517843@s...>
• References: <200501291720.j0THK3517843@s...>
• User-agent: Mozilla Thunderbird 1.0 (Macintosh/20041206)

First let me say you must have the coolest job :)

But you are totally ignoring a way to reference some unique 'thing.' You 
can define something once and reference it. So I would say nesting is 
good but define references to things that can go more than one place.

best,
-Rob


Roger L. Costello wrote:
> Hi Folks,
> 
> Below I propose a few XML design principles.  I am interested in hearing 
> your thoughts on them, i.e., do you agree or disagree with them?
> 
> Which is Better Design?
> 
> Suppose that I have data about a grape vineyard.  Below I show two lots 
> on the vineyard, and a picker on one of the lots.  I show two ways of 
> designing the data.  Which design is better?
> 
> *Version #1
> *
> <Lot id="1">
>       <ripe-grapes>4</ripe-grapes>
>       <Picker id="John">
>             <metabolism>2</metabolism>
>             <grape-wealth>20</grape-wealth>
>       </Picker>
> </Lot>
> <Lot id="2">
>       <ripe-grapes>3</ripe-grapes>
> </Lot>
> 
> *Version #2
> *
> <Lot id="1">
>       <ripe-grapes>4</ripe-grapes>
> </Lot>
> <Lot id="2">
>       <ripe-grapes>3</ripe-grapes>
> </Lot>
> <Picker id="John" locatedOn="1">
>       <metabolism>2</metabolism>
>       <grape-wealth>20</grape-wealth>
> </Picker>
> 
> Suppose that I want an application to move the Picker to Lot 2.  With 
> which version would it be easier to do this? 
> 
> The above two versions both have two components:
> 
> 1. A Lot component
> 2. A Picker component
> 
> Both versions are *modular*, i.e., both represent the Picker located on 
> Lot 1.
> 
> However, the two versions differ in three respects:
> 
> - Implicit relationship versus explicit relationship.
> - Tight coupling versus loose coupling.
> - Nest (hierarchical) versus flat data.
> 
> Implicit versus Explicit Relationships
> 
> *Version #1*
> 
> What is the relationship between the Lot and the Picker?
> 
> <Lot id="1">
>       <Picker id="John">
>             <metabolism>2</metabolism>
>             <grape-wealth>20</grape-wealth>
>       </Picker>
> </Lot>
> 
> You might state that the relationship is:
> 
>       "The Lot *contains* the Picker"
> 
> Another person might state that the relationship is:
> 
>       "The Lot *has a* Picker"
> 
> /The relationship between the Lot and the Picker is implicit.  Implicit 
> relationships are bad because one person may interpret the relationship 
> in one way, another person may interpret the relationship in another way./
> 
> *Version #2*
> 
> What is the relationship between the Lot and the Picker?
> 
> <Lot id="1">
>       <ripe-grapes>4</ripe-grapes>
> </Lot>
> <Picker id="John" locatedOn="1">
>       <metabolism>2</metabolism>
>       <grape-wealth>20</grape-wealth>
> </Picker>
> 
> Clearly the relationship is:
> 
>       "The Picker is locatedOn the Lot"
> 
> /The relationship (locatedOn) is explicitly specified in the instance 
> document!
> /
> XML Design Principle #1
> 
> Implicit relationships are bad.  They are subject to misinterpretation.
> 
> Explicit relationships are good.  They are not subject to misinterpretation.
> 
> Tight Coupling vs Loose Coupling
> 
> Compare the two versions with respect to how loosely connected the Lot 
> and Picker components are:
> 
> *Version #1
> *
> <Lot id="1">
>       <Picker id="John">
>             <metabolism>2</metabolism>
>             <grape-wealth>20</grape-wealth>
>       </Picker>
> </Lot>
> 
> The Picker is nested (buried) within the Lot ... /Tight coupling!/
> 
> *Version #2
> *
> <Lot id="1">
>       <ripe-grapes>4</ripe-grapes>
> </Lot>
> <Picker id="John" locatedOn="1">
>       <metabolism>2</metabolism>
>       <grape-wealth>20</grape-wealth>
> </Picker>
> 
> The Lot and Picker components are physically completely separate.  The 
> only connection between them is the pointer from the Picker to the Lot 
> (the locatedOn attribute) ./.. Loose coupling!
> /
> XML Design Principle #2
> 
> Tight coupling is bad.  It makes processing difficult.
> 
> Example: moving the Picker from Lot 1 to Lot 2 in the tightly coupled 
> version is difficult in terms of the amount of code needed, memory 
> needed, and processing time needed.
> 
> Loose coupling is good.  It makes processing easy.
> 
> Example: moving the Picker from Lot 1 to Lot 2 in the loosely coupled 
> version is easy - just change the value of locatedOn.
> 
> Nested (hierarchical) vs Flat Data
> 
> Compare these two versions with respect to the physical placement of the 
> Lot and Picker components:
> 
> *Version #1
> *
> <Lot id="1">
>       <Picker id="John">
>             <metabolism>2</metabolism>
>             <grape-wealth>20</grape-wealth>
>       </Picker>
> </Lot>
> 
> There is a "box within a box" ... /It is nested (hierarchical) data/
> 
> *Version #2*
> 
> <Lot id="1">
>       <ripe-grapes>4</ripe-grapes>
> </Lot>
> <Picker id="John" locatedOn="1">
>       <metabolism>2</metabolism>
>       <grape-wealth>20</grape-wealth>
> </Picker>
> 
> There are two separate "boxes" ... /It is flat data/
> 
> XML Design Principle #3
> 
> Minimize the amount of nesting you use.
> 
> Nested data is tightly coupled and uses implicit relationships, both of 
> which are bad.
> 
> Flat data is good data!
> 
> Flat data is loosely coupled and promotes the use of explicit 
> relationships, both of which are good.
> 
> Comments?  /Roger
> 

• References:
  • 3 XML Design Principles
    • From: "Roger L. Costello" <costello@m...>

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