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Re: Issue 39: proposed example for varying the etag based on conneg

From: Jamie Lokier <jamie@shareable.org>
Date: Tue, 6 Apr 2010 16:22:46 +0100
To: Julian Reschke <julian.reschke@gmx.de>
Cc: Yves Lafon <ylafon@w3.org>, nathan@webr3.org, HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <20100406152246.GB29236@shareable.org>
Julian Reschke wrote:
> On 06.04.2010 16:49, Jamie Lokier wrote:
> >...
> >So back to an earlier query: When *would* you use weak etag
> >equivalence for different representations?  If never, do weak etags
> >have any purpose at all?  What would you use weak etags for?  Because
> >if you only use the same weak etag when representations are identical,
> >you should be using strong etags instead for that.
> >...
> 
> For instance, the way you construct your representation may not 
> guarantee binary identity, although the underlying resource is the same 
> (think an XML database with a generic XML serializer, affecting, for 
> instance, attribute order).

Well, Yves Lafon said: 
>> No reason to forbid it, but if you rely on weak etag equivalence for
>> different representations, expect it will break.

Changing attribute order *is* making a different representation.

Thus Julian's idea of when to use the same weak etags is recommended
against by Yves.

I happen to agree with Julian(*), but I'm really interested in when
Yves thinks it is appropriate to use same weak etags, given the
statement that it's unsafe to use them for different representations.

-- Jamie

ps. (*) Why my pov matches Julian's:

Instead of changing attribute order, what about different output
encodings at serialization time?  Note that the *input* XML database
might not even *have* a character encoding.  I think that is in the
same category as attribute order, because the underlying XML resource,
right down to the individual characters, is unchanged, although it's
clearly not binary identical.

If the underlying resource is the same for different character
encodings (Content-Type charset=), in what way is compression
(Content-Encoding) semantically different?  I think it isn't.

-- Jamie
Received on Tuesday, 6 April 2010 15:23:14 GMT

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