Title: Message



 
In my opinion, if we refer 
to interfaces (or languages/syntaxes) rather than, generically, to "information 
systems", the distinction between evolution and extension becomes clearer and 
more useful.
 
My 
concept of "extensible" interface is that the interface supports certain 
rigorously-defined extensions to it with the following 
property:
 
------------------
Given:
 
- an implementation that exposes the base 
(non-extended) interface B;
- an implementation that exposes an extended 
interface E1;

- an implementation that exposes another extended 
interface E2,
 
a 
program written to use interface B is guaranteed to work (as expected) when 
using any of these three implementations, "thinking" that it is using an 
implementation of interface B.
------------------
 
The 
same concept of extensibility would also apply to 
languages/syntaxes.
 
On the 
other hand, if an interface or a language "evolves", one cannot expect that the 
condition above is satisfied (although it could be).
 
In 
other words, while I expect to be able to use an extended interface as if it 
were the corresponding base interface, I don't expect to be able to use an 
evolved interface as if it were the corresponding unevolved interface.  
Same thing for a language/syntax.
 
I 
think "evolvability" in this context means simply that an interface or a 
language can be modified easily, inexpensively, and nicely, in order to 
meet new requirements - without implying that a user can ignore that there has 
been a modification.
 
If we 
refer to a system, I would say that it is possible for a system to *evolve* 
while some of its interfaces are simply *extended* (while others 
evolve).
 
Alessandro Triglia
 

  
  -----Original Message-----
From: Bullard, Claude 
  L (Len) [mailto:len.bullard@i...] 
Sent: Wednesday, June 
  02, 2004 09:23
To: 'Roger L. Costello'; 
  xml-dev@l...
Subject: RE: [xml-dev] [OT] Difference between 
  an extensible versus an evolvable information system?


  If 
  you define evolution as the accretion of new features, sure.  If you mean 
  adding new 
  instances of those features, no.  In any layered system, one must 
  be careful to specify 
  which layer is evolving vs one that is merely 
  accreting.
   
  If 
  you define evolution as 'change' one *might* say this is evolution.  
  Amazon added categories 
  so 
  the addition of the categorical function is evolution (a new feature) but 
  adding categories 
  is 
  extensibility.  
   
  In a 
  biological system, evolution is a feature that is inheritable.  Comparing 
  that to Amazon, 
  those are not evolution unless adding categories or search to Amazon 
  adds it to any 
  descendant of Amazon or any system derived from Amazon.   If 
  Amazon cannot have 
  decendants, it cannot evolve.   In this sense, evolution is 
  an observable process of 
  populations as they adapt to their environment, shape their 
  environment, and then 
  adapt to those changes (mediated feedback).    So my 
  question to you is, what 
  is 
  the equivalent of Amazon.com genes?  Adding categories could mean the 
  
  information is evolving, but Amazon evolved only when the categorical 
  function 
  was 
  added, and then it accretes categories.
   
  My position would be that XML evolution is in the addition or 
  deletion of features 
  of a 
  schema (at any metalevel one cares to work) because it defines the system. 
  
   
  So 
  one might look at how instances acquire new elements and attributes that are 
  
  then 
  added to their schemas as definitions.  For example, 
  aggregation:  why would 
  one 
  relax constraints such that composite documents become 
  homogenous? 
  One 
  might do that if working on one product in isolation and fits HTML elements 
  into 
  say 
  SVG applications.
   
  Is 
  XML an evolution of SGML or simply an adaptation (it lost features, so 
  
  maybe it is devolution)?
   
  len
  
    
From: Roger L. Costello 
    [mailto:costello@m...]


    
    Hi 
    Folks,
     
    I am interested 
    in hearing your thoughts on the differences between an information system 
    that is extensible versus an information system that is 
    evolvable.
     
    For example, 
    suppose that Amazon.com gives users the ability to do a keyword 
    search.  Further, suppose that Amazon empowers its users to create new 
    keywords (and map the new keywords to information at the Web site).  Is 
    this support for new keywords an example of extensibility or 
    evolution?  By adding new keywords has Amazon merely been extended, or has it 
    evolved?  
     
    Let's take another example.  
    Suppose that Amazon gives users the ability to search by book category 
    (e.g., Fiction, Non-Fiction, etc.)  Further, suppose that Amazon 
    empowers its users to create new categories (and map the categories to 
    information at the Web site).  Is this support for new categories an 
    example of extensibility or evolution?  By adding new categories has 
    Amazon merely been extended, or has it 
    evolved?
     
    If adding new keywords and adding new 
    categories are merely examples of extension, then can you give an example 
    of evolution?   
    /Roger