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Re: New document on "Simple hit-metering for HTTP"

From: Koen Holtman <koen@win.tue.nl>
Date: Sun, 4 Aug 1996 15:11:51 +0200 (MET DST)
Message-Id: <199608041311.PAA04819@wsooti04.win.tue.nl>
To: Jeffrey Mogul <mogul@pa.dec.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Jeffrey Mogul:
>
[...]
>Our goal was NOT to solve the general problem of collecting demographic
>information; it was to reduce the incentive for origin servers to
>defeat caching merely so that they could collect simple hit-counts, of
>a sort that the caches could just as easily collect for them.
[...]
>You can find a copy at
>
>    http://ftp.digital.com/~mogul/draft-ietf-http-hit-metering-00.txt
[...]

I have several comments.

1. Cascaded proxy caches.

At first glance, there seem to be counting problems in a cascaded
proxy cache situation.  If we have the arrangement

   origin server ---- proxy 1 ------ proxy 2 ---- user agent

and the user agent requests and uncached page, section 5.1 seems to
say that both proxy 1 and proxy 2 must set the use count to 1 when
relaying the page.  This results in a count of 2 being reported in the
end, though the page is only viewed once.  It seems like there needs
to be a special case for proxy 1: a proxy should not count if it is
relaying the response to another proxy.  (Under HTTP/1.1, the Via:
header in the request would tell you that you are talking to another
proxy.)

2. Number of unconditional GETs = number of times read???

You argue that the number of unconditional GETs, rather than the total
number of GETs, more accurately reflects the number of times a page is
read.  I don't know if this is true; I would like to see section 4
discuss user agents on shared machines, and situations in which user
agent disk caches are disabled entirely because there is a central
proxy (like on our local sun cluster).

3. A `hit' being an *un*conditional GET

In the current (classic) meaning of the word,

  1 hit-classic = 1 request on an origin server.

Your draft defines a new kind of hit:

  1 hit-new = 1 200/203/206 response returned to a user agent.

Now, if I am an origin server which uses cache busting, and if most
caches play by the rules, then for my server I will measure:

  hit-new < hit-classic .

Assuming that I get payed by the hit, I have absolutely no incentive
to start measuring hit-news instead of hit-classics.  To stop using
the cache-busting based hit-classics would be economic suicide.

So even if hit-new is a better metric than hit-classic, I fear it
won't be effective at reducing cache busting.

The nicest solution to this problem seems to be for proxies to count
both hit-new and a second metric:

  1 touch-new = 1 response returned to a user agent

for which it is guaranteed that

   hit-classic <= touch-new .

(Note: `hit' and `touch' would *not* be my proposals for adequate
names for these metrics.)

4. Interaction with Vary

I don't like the extra complexity and inefficiency introduced by the
Vary counting rules in section 3. (See second-to-last sentence of
Section 5.1.)

I think the proposal would be better if the Vary special case were
removed entirely.

5. Overhead in proxy efficiency

I'm wondering if the counting mechanisms in the draft won't cause an
unacceptable overhead for high-performance cache implementations.  I
think we definitely need the opinions of proxy cache implementers on
this issue.

One possible hit counting alternative, post-processing proxy logfiles
and delivering the results to the servers, seems to have less
overhead.

6. Max-uses mechanism

The max-uses mechanism seems to be a way for origin servers to specify
an upper bound to the inaccuracy of their information.  

But to allocate max-uses values to proxies an efficient way, an origin
server seems to have to keeping per-proxy database of
`max-use-qouta-use-speed' (last two paragraphs of Section 2), which
adds some overhead to every request.  Reading these paragraphs, the
goal of the max-uses allocation heuristics seem to be to ensure that
all counts are reported `soon enough'.

It seems that a max-time-to-wait-before-reporting-hits mechanism, can
achieve the same goal without the same computational overhead in
origin servers.  This mechanism would also eliminate the need for
implementing difficult max-use distribution heuristics in proxy
caches: a cache could simply subtract the age of the response from
the max-time value.

Even better, we *already have* a
max-time-to-wait-before-reporting-hits mechanism in the form of
cache-control: max-age.

I conclude that the max-use mechanism is unnecessary and propose that
it is removed, and that a section about using cache-control: max-age
is added.

7. Hit-counts for 302 responses

Section 7 talks about hit counts of 302 responses, but the definitions
in section 5.1 do not allow such counting.  This can be easily fixed
by rewriting the definitions, they should probably enumerate the 2xx
and 3xx class response codes which should *not* be counted (as
hit-news).

8. How big is the cache busting problem anyway?

About a year ago, I tried to measure cache busting for the web content
accessed through our local proxy.  Contrary to my expectations, I
could not find any definite signs of it.  I could find several
resources and even whole servers which never sent Last-Modified
headers, but I accounted this to bad CGI programming more than to
`malicious' intent.

Now, a lot can happen in a year, and maybe the cache busting sites
which did exist a year ago were not sites which would get accessed
(often) from a Dutch university.  But I would like to see some
statistics/stories to indicate how big the cache busting problem
really is, since cache busting (not improving your site through better
statistics) seems to be the sole motivation the draft has for
introducing the counting mechanisms at all.


Koen.
Received on Sunday, 4 August 1996 06:17:58 EDT

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