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Re: Injective Quality (Was: Re: URIs quack like a duck)

From: Clark C. Evans <cce@clarkevans.com>
Date: Tue, 30 May 2000 14:14:46 -0400 (EDT)
To: Tim Berners-Lee <timbl@w3.org>
cc: "Clark C. Evans" <cce@clarkevans.com>, michaelm@netsol.com, abrahams@acm.org, xml-uri@w3.org
Message-ID: <Pine.LNX.4.10.10005301309231.28088-100000@clarkevans.com>
On Tue, 30 May 2000, Tim Berners-Lee wrote:
> >
> >  "If the name X and the name Y are different,
> >   then we know the resource identified by X is
> >   different from the resource identified by Y"
> 
> I disagree.

I am not talking about URIs in general, rather I
am referring to URI* = { subset of URI that make
acceptable namespace names }  

The specification clearly states a "uniqueness" 
requirement.  Allowing any-old-URI will not
meet this requirement.  The subset below 
does meet this requirement.

  URI' = { URI such that injective quality holds }
       = { URI such that there are no aliases    }

> In fact, I would say that you can't make a system 
> which scales globally in a decentralized way
> with that tautology.

I agree.  However, the java package naming 
scheme has scaled globally by bootstrapping
from a centralized naming system, DNS.  

The java package naming scheme also has the 
injective property... if someone changes the
package name, even slightly, people expect that,
in some manner, the code identified is different.

A slight change of an http URL by dropping a 
"www." does not generate the same expectation
of change; as aliases are common with URLs.

> >The problem with URLs is that, in general, they
> >lack this injective quality.  In other words,
> >I can find N distinct URLs that identify
> >(resolve to) the same resource.
> 
> That is life in the big city.   The problem is with the
> big city, noteth URIs.   The goot news is that you
> can do everything you need to do using
> 
>      u1 == u2 ==> R1 == R2
> 
> as I said in previous messages.

Without the uniqueness constraint,  R1 == R2 
implies u1 == u2,  one cannot acertain if the 
two URI given identify the same resource.

I am not asking to change the way URIs work;
but rather, I am trying to describe the
subset of URIs that will operate as needed
for the namespace mechanism to retain the
essential uniqueness property required.

Best,

Clark
Received on Tuesday, 30 May 2000 14:10:32 UTC

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