Re: URIs / URLs

Pierre-Antoine Champin <> wrote:

> Though, URLs are frequently used to identify resources, either the one
> they locate or another one.
> This makes them tricky and unreliable as identifiers.

I'm a bit confused -- what would be a resource that they located but didn't

>> I don't see any justification for the claim
>> that namespaces are disjoint from HTTP resources.
> Because it can not be transported over that protocol (bor any protocol:
> it is an abstract thing).
> A specification of it can, indeed.

But neither can the abstract concept of a "document", but a rendering of it
can. Remember content negotiation?
> The system works better when you know what exactly is identified by an
> identifier.
> Here, is it the decscriton or the described thing ??
> There is often no mean to know.
> Note that the description and the described thing are *distinct*, hence
> they need disctinct identifier.

Yes, this is an interesting problem. How does a URI which upon HTTP GET
returns description turn into a URI which identifies a concept? This really
needs to be clarified.

>>   "URLs are transient
>>   That means they may become invalid after a certain period
>>   of time [9]. "
>> That's a fact of life in a distributed system. URNs may
>> become invalid after a period of time too.
> There is no reason why an URN would becom invalid once defined.
> It may becom obsolete, unused, but one thing is for sure :
> it will never be re-defined (according to URNs requirements).

Why not? According to URI guidelines, URIs should not be redefined either,
but they are. As it says:
    "There are no reasons at all in theory to change URIs but millions in

> The point we wanted to raise is that locating is not the same as
> identifying, and identifying a description is not the same as
> identifying the described thing. And doing thos approximations must be
> done, if ever, with awareness.

Such misunderstanding of URIs bothers me (I can begin to understand DanC's
pain...). Sure, URIs return bits, I don't think anything else would travel
well over the Internet. But that doesn't mean that the URI identifies those
bits! Please remember that URIs represent resources, and resources are much
more than a plain, old document.

[ Aaron Swartz | | ]

Received on Wednesday, 11 April 2001 20:47:22 UTC