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Re: PROPOSAL: Weak Validator definition [i101]

From: Henrik Nordstrom <henrik@henriknordstrom.net>
Date: Tue, 18 Mar 2008 15:12:25 +0100
To: Robert Siemer <Robert.Siemer-httpwg@backsla.sh>
Cc: Werner Baumann <werner.baumann@onlinehome.de>, ietf-http-wg@w3.org
Message-Id: <1205849545.18425.50.camel@HenrikLaptop>

On Sun, 2008-03-16 at 16:23 +0100, Robert Siemer wrote:

> To see no useful weak etag implementations within the static file 
> serving code among common servers does not surprise me at all. - How 
> should they know about semantic equivalence?
> 
> I still don't know why this mecanism has to be an illusion. 

It's an illusion only because the meaning of rough semantic equivalence
(or no significant change in semantics) isn't defined in technical terms
and means different things to different people, and can not be enforced
by the protocol. 

But this does not make weak validators a useless feature by any means.
It's a very interesting aspect of HTTP. Just means we need to get the
language cleaned up so people do not get so confused on what the specs
really means.

In the spec conditions using weak etags is placed pretty much equal to
conditions based on Last-Modified which has no semantic guarantee at
all, even if one MAY deduce some strength from modification time when
some time has passed.

Weak validators (and weak etags in specific) is a fuzzy feature of the
protocol which can be used for many interesting applications. But the
only "semantic equivalence" that can really be counted on is that a
strong ETag guarantees equality down to the octet, and that a weak etag
(or a condition based on only Last-Modified) signals that the object
most likely have not changed meaning in any significant manner.

The problem with "semantic equivalence" is a language thing, defining
"semantic equivalence". What the spec means to say is basically this
(from "Entity tags")

   A "weak entity tag," indicated by the "W/" prefix, MAY be shared by
   two entities of a resource only if the entities are equivalent and
   could be substituted for each other with no significant change in
   semantics. A weak entity tag can only be used for weak comparison.

or this from "Weak and strong validators"

   However, there might be cases when a server prefers to change the
   validator only on semantically significant changes, and not when
   insignificant aspects of the entity change. A validator that does not
   always change when the resource changes is a "weak validator."

not strict "semantic equivalence" as it mistakenly says in other
sections or paragraps.

What has got people upset about weak etags is the (in my opinion) valid
assumption taken by Apache and a few other implementations that if an
object changes twice in the same second it's most likely not a
significant change in semantics. Which isn't a bad assumption, but also
one which can not be guaranteed to 100%, and because it can not be
guaranteed it must not be used, right? (sure... never heard of a weak
condition which can not be guaranteed have you..)

The world is full of such things which fall into the "most likely"
category, and if we did not make use of such assumptions when it makes
sense then most things would take a grinding halt, not only HTTP..

Regards
Henrik
Received on Tuesday, 18 March 2008 14:14:11 GMT

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