W3C home > Mailing lists > Public > http-caching-historical@w3.org > February 1996

Re: why no-cache is not reload

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 20 Feb 96 18:52:11 PST
Message-Id: <9602210252.AA10854@acetes.pa.dec.com>
To: "Roy T. Fielding" <fielding@avron.ICS.UCI.EDU>
Cc: http-caching@pa.dec.com
I wrote:
    > (1) Semantics: the HTTP protocol needs to support these cases
    > in request messages:
    > 
    > 	(Ajeff) I want a firsthand response from the origin server.
    > 
    > 	(Bjeff) I want a response that has been validated with the
    > 	origin server after I made this request.
    > 
    > 	(Cjeff) I want to validate my own cached response with
    > 	the origin server.
    > 	
    > 	(Djeff) I want to validate my own cached response, but I'll
    > 	accept a non-authoritative answer (i.e., from a cache
    > 	that has a still-fresh copy with a matching validator)
    > 	
    > 	(Ejeff) I want a fresh response.
    > 
    > Note that I've arranged these in roughly decreasing order of "paranoia".
    
Roy wrote:
    The way you phrased the above sentences would lead you down the wrong
    track in trying to determine the actual semantics.  Try this:
    
	(Aroy) I want a new copy from the origin server.
    
	(Broy) I want a response from the origin server
	    based on the parameters of this request.
    
	(Croy) I want a new copy from the next inbound cache or origin.
    
	(Droy) I want a response from the next inbound cache or origin
	    based on the parameters of this request.
    
	(Eroy) I want whatever is currently available.
    
Note that I have relabelled these ("Ajeff" and "Aroy") because
in some cases there is a correspondence between my letters and
Roy's letters, and in other cases there is not.

Roy has pointed out some things that I had missing, but has managed
to omit some stuff that I think is important.  So let's try again
(and I'll use a disjoint set of names);

	(Ax) I want a new copy from the origin server.
		(Same as Aroy, similar to Ajeff)

	(Bx) I want a response that is either from a cache or
	from the origin server, based on the parameters of this
	request, that is either a new copy from the origin server,
	or one that the origin server considers valid.
		(Bjeff, restated to make it clearer, and I
		think what Roy meant by Broy if the client
		doest not have a cache entry)

	(Cx) I want to validate my own cached response with
	the origin server.  I don't care what any intervening
	cache has.
		(Cjeff, restated to make it clearer, and I
		think what Roy meant by Broy if the client
		has a cache entry)
	
	(Dx) I want to validate my own cached response, but I'll
	accept either a non-authoritative answer (i.e., from a cache
	that has a still-fresh copy with a matching validator) or
	an authoritative response from the origin server, whichever
	comes first.
		(Djeff, restated to make it clearer, and I think
		what Roy meant by Droy if the client has a cache
		entry)

	(Ex) I don't have a cache entry locally, so I want a fresh
	response (either a non-authoritative but fresh response
	from an inbound cache, or an authoritative response from
	the origin server), whichever comes first.
		(Ejeff, restated to make it clearer, and I think
		what Roy meant by Croy if the client does not
		have a cache entry)

	(Fx) I want whatever is currently available, and I'm
	willing to settle for a stale response if that's the best
	I can have.  I don't have a local cache entry.
		(Eroy, I think.  I think this depends on our
		fundamental disagreement about semantic transparency)

	(Gx) I want whatever is currently available, and I'm
	willing to settle for a stale response if that's the best
	I can have.  I have a local cache entry, so if what
	you have is what I already have, don't send it again.
		(new)

Roy, does this cover the space?  I'm NOT asking you to rearrange
things again, I'm asking whether I left anything out.

If it does cover the space, feel free to rearrange things, but
then please do not leave anything out when you do that!

On to (2) Terminology:

I originally had
> 	Ajeff = Reload
> 	Bjeff = Revalidate
> 	Cjeff = Specific revalidate
> 	Djeff = Conditional method
> 	Ejeff = Normal method
Roy proposed
        Aroy = Reload
        Broy = Refresh             (whether or not it is valid is immaterial)
        Croy = Refresh from origin (Revalidate is okay, but less descriptive)
        Droy = Conditional GET
        Eroy = GET

Equating Eroy with specific syntax is precisely what I was trying to
avoid (describing these cases in terms of how one of us thinks they
ought to be implemented in HTTP/1.1) because it forces a particular
syntax on us.  Which is precisely what Roy is trying to do, I guess,
but I'm not going to follow that lead.

As for Roy's comment on Broy ("whether or not it is valid is immaterial"):
what is the point of asking the origin server for a response if the
result isn't "valid"?  This is based on my definition for "valid"

   valid           A cached entity is valid, with respect to a given
                   request at a given time, if it is exactly what the
                   origin server would return in response to that
                   request at that time.

(Roy's definition, offered on 2 Feb 1996, is 5 times as long but
specifically includes my definition as one alternative.  So if
it isn't valid by Roy's definition, it cannot be valid by mine
either.)

Let me try again:

	Ax = Reload
	Bx = Revalidate
	Cx = Specific Revalidate
	Dx = Conditional method
	Ex = Unconditional method
	Fx = Unconditional method, stale responses allowed
	Gx = Conditional method, stale responses allowed

For Ax through Ex, I'm asserting that stale responses are not
allowed.  Even Roy has to agree with that on Ax through Cx,
because these are defined as contacting the origin server.
And I hope Roy can agree with me that it makes sense to allow
the user (the ultimate ruler, in Roy's model) to decide whether
or not a stale response should be accepted, so this means that
Dx and Ex are just as important as Fx and Gx.

One could insert the word "answerable-from-cache" in front of the
the names of Dx-Gx, but I won't bloat this terminology like that.

(3) Syntax:

Because Roy started by redefining what I meant by (B), (C), etc.,
he then had to take issue with what I wrote as his syntax for these.
Which makes it pointless for me to quibble with his response, since
it's at cross purposes with what I was asking him.

Again, if I understand Roy's spec correctly, this is how he would
implement Ax-Gx:

	Ax = Reload
 	    GET /home.html HTTP/1.1
 	    Cache-control: no-cache

[Roy agrees already]

	Bx = Revalidate
 	    GET /home.html HTTP/1.1
 	    Cache-control: max-age=0

Bx cannot include a validator, because of the way I defined it:
the client does not have a validator to offer.  If your syntax
cannot express case Bx, then you have a problem to fix.

	Cx = Specific Revalidate
            GET /home.html HTTP/1.1
            <some-conditional-header>: <cache-validation condition>
            Cache-control: no-cache

[Roy agrees already]

Now we get into a touchy area.  Roy writes:
    I'm not sure why you have it that HTTP will only transfer fresh
    entities by default.
This is precisely our argument on transparency.  I've already argued
repeatedly that it is wrong (in the technical/moral/ethical sense)
for a cache to return a response that it knows is stale to a user
that expects transparency.  I suppose Roy doesn't expect that users
(by default) want transparency.

    If you change that default, then you also
    need a new syntax for obtaining the old default.

... just keep reading, Roy!

	Dx = Conditional method
            GET /home.html HTTP/1.1
            <some-conditional-header>: <cache-validation condition>

	Ex = Unconditional method
            GET /home.html HTTP/1.1

In my view, these are saying "I want transparency" by default.

	Fx = Unconditional method, stale responses allowed
            GET /home.html HTTP/1.1
	    Cache-control: stale-max=630000

	Gx = Conditional method, stale responses allowed
            GET /home.html HTTP/1.1
            <some-conditional-header>: <cache-validation condition>
	    Cache-control: stale-max=630000

These are what you asked for ("a new syntax for obtaining the old
default") except that here I have aribitrarily limited the duration
of staleness to 7 days.  I suppose you could instead send
stale-max=1576800000, which is about 50 years (ignoring leap years)
and is representable in a signed 32-bit field.  I trust this is
sufficient for your purposes?

Next I tried to propose an alternative syntax. I've already had to
use it for Fx and Gx, because Roy's syntax cannot distinguish
between Dx and Gx or between Ex and Fx.  "Cache-control: max-age"
simply doesn't make this distinction; it makes a different one.

For Ax, where Roy would send
	GET /home.html HTTP/1.1
	Cache-control: no-cache

I would send
	GET /home.html HTTP/1.1
	Cache-control: reload

but that is just a spelling difference.

For Bx, where Roy would send (I think)
	GET /home.html HTTP/1.1
	Cache-control: max-age=0
I would send
	GET /home.html HTTP/1.1
	Cache-control: revalidate
which is close to being a spelling difference, if one is careful
about comparing the age of a response that has been received
within the past second. 

For Cx, where Roy would send
	GET /home.html HTTP/1.1
	<some-conditional-header>: <cache-validation condition>
	Cache-control: no-cache
I would send
	GET /home.html HTTP/1.1
	<some-conditional-header>: <cache-validation condition>
	Cache-control: revalidate
which I think is also basically a spelling difference.

Roy writes that my syntax for Ax
introduces an unnecessary contradiction with

	GET /home.html HTTP/1.1
	Cache-control: reload
	If-Modified-Since: Thu, 15 Feb 1996 15:05:20 GMT

by which I believe he means that (in my scheme) one could also write
Cx as
	GET /home.html HTTP/1.1
	<some-conditional-header>: <cache-validation condition>
	Cache-control: reload
which is true, I suppose, but I fail to see the harm.  Either that,
or we could make this a "protocol error", in the same way that
a request of the form
	GET /home.html HTTP/1.1
	Cache-control: private="Date"
	Cache-control: no-cache
is also internally contradictory.

Roy objects to my syntax for Bx
	GET /home.html HTTP/1.1
	Cache-control: revalidate
for two reasons:
    No, it introduces a contradiction if no validation information is
    provided to go along with the revalidate directive.
Not at all.  I want the client to be able to say "I don't have
this in my local cache, so I don't have a validator to check, but
I want *you* Dr. Cache to check your validator."  I would use this
if I was stuck with a proxy run by someone who thought it was cool
to ignore Expires: dates.

    Furthermore, it places syntax significant to an origin server in a
    place that origin servers should never need to look.
Huh?  This isn't significant to the origin server.  When the
intermediary cache that has a cache entry (if there is one) makes
its validation request to the origin server, it adds its own validator
to that request.  The origin server sees the validator (If-Valid: or
I-M-S), and can ignore the "Cache-control: revalidate".  Any cache
further along the inbound chain, however, is forced to pass the request
along to the origin server for validation.

Roy says my syntax for Cx
	GET /home.html HTTP/1.1
	If-Modified-Since: Thu, 15 Feb 1996 15:05:20 GMT
	Cache-control: revalidate
is "Redundant", apparently because he though this was the same
case as Dx ... but it isn't.  They both mean "I have this already
in my cache, is it OK?", but Cx means "and check with the origin
server" while Dx means "tell me what you know yourself, Dr. Cache".

300+ lines later, I've worn out my fingers (and probably my welcome),
but I wanted to try to get this as clear as possible.

To be honest, with the addition of "Cache-control: stale-max=NNN"
and a requirement that all other requests implicitly require
non-stale responses, the only different here between me and Roy
is syntax.  Which is why I hope that Roy agrees with me up
through the point in this message where I introduce "Cache-control:
reload", because then we have agreement on all of the important
stuff.  And if he doesn't agree with me to that point, there's
no point in arguing about syntax.

-Jeff
Received on Wednesday, 21 February 1996 03:09:09 UTC

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