Re: IIS 6.0 Not Caching Images

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From: Joe Rattz (joerattz_at_yahoo.com)
Date: 08/05/04 

Date: Thu, 5 Aug 2004 09:15:29 -0400

Based on what I see in the log (many 200's for the same URL/image), I must
be seeing #2.

> 2. /GPCStoreFront/Images/watermark.gif is not requested with the right
> "If-None-Match: <ETag>" header so IIS responds with 200, and it could be
> coming out of the kernel mode cache or not.

But, I don't know what you are talking about with this. This is an ASP.NET
web form with an image. What is determining the header for it?

"David Wang [Msft]" <someone@online.microsoft.com> wrote in message
news:u84%23T0oeEHA.248@TK2MSFTNGP12.phx.gbl...
> You're not exactly wrong, but you're making some assumptions that may not
be
> valid.
>
> In any case, I looked at this, and here is what is going on:
> 1. There is a known bug where if a 204/304 is fulfilled from the kernel
> response cache, HTTP.SYS won't log it
> 2. If 204/304 is fulfilled by usermode, IIS will cause HTTP.SYS to log it
> 3. However, the right HTTP communication is happening between
client/server
> in those cases -- if you send the right ETag, a 304 will be returned
> regardless if it is fulfilled from the kernel response cache or not.
>
> I can show with #2 and #3 that IIS6 is caching images -- it's just the log
> that is wrong in the kernel cache hit case. I do not think you're seeing
#1,
> though, because your log pattern does not suggest it.
>
> Finally, in order for you to track down what is going on, you need to know
> when a request is fulfilled by the kernel mode response cache and when a
> request is cached in kernel mode (in addition to using WFetch to make the
> proper requests or use NetMon as proof).
> 1. You will need to look at PerfMon for "Web Service Cache\Kernel cache:
> Kernel URI Hits" for if a request is a kernel mode cache hit or not
> 2. You will need to make three consecutive requests to the same
static-file
> URL within 10 seconds to make it cache in the kernel. Without programming,
> you cannot evict an item from the cache, so "NET STOP HTTP /y & NET START
> W3SVC" is the easiest way to flush the caches.
>
>
> So, what I think is happening in your case is either:
> 1. /GPCStoreFront/Images/watermark.gif is already cached in the kernel
> response cache and returning 304s -- which aren't logged
> 2. /GPCStoreFront/Images/watermark.gif is not requested with the right
> "If-None-Match: <ETag>" header so IIS responds with 200, and it could be
> coming out of the kernel mode cache or not.
>
> If #1 was happening, then you would see only a couple log entries for
> /GPCStoreFront/Images/watermark.gif and then no mention of this URL for a
> very long time (until the entry expired from the kernel mode cache).
>
> If #2 was happening, then you will repeatedly see log entries for
> /GPCStoreFront/Images/watermark.gif with 200 . It may be cached on the
> server or not -- you need perfmon to determine.
>
> I think you are seeing #2. In any case, you should now have enough info
to
> determine for yourself what is going on.
>
> --
> //David
> IIS
> This posting is provided "AS IS" with no warranties, and confers no
rights.
> //
> "Joe Rattz" <joerattz@yahoo.com> wrote in message
> news:OtjB%23HieEHA.592@TK2MSFTNGP11.phx.gbl...
> > A 304 "Not Modified" does NOT indicate that the server cached the
> response.
>
> I am not referring to the server caching, I am referring to the client
> browser caching.
>
> I would expect that when a client first requests an image, the status code
> would be a 200 because the server has no way of knowing the client has it.
> But, all subsequent requests for the same image I would expect to see 304
> for.
>
> Here is a sample from my log. These two records are 19 seconds apart and
> are from the same session:
>
> 2004-08-04 12:41:03 10.28.98.130 GET /GPCStoreFront/Images/watermark.gif -
> 80 - 10.28.11.180
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> LR+1.1.4322) 200 0 64
> 2004-08-04 12:41:22 10.28.98.130 GET /GPCStoreFront/Images/watermark.gif -
> 80 - 10.28.11.180
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> LR+1.1.4322) 200 0 0
>
> It seems to me that the second record's status should have been a 304,
> thereby preventing the server from shipping the image data back the second
> time. Am I wrong here?
>
> Thanks.
>
>
>
> > "David Wang [Msft]" <someone@online.microsoft.com> wrote in message
> news:uwPfZwceEHA.556@tk2msftngp13.phx.gbl...
> > I think you are mis-interpreting what is going on. The log file is not
> the
> > correct place to determine whether IIS is caching or not. You observe
> > caching behavior by watching performance counters (perfmon) or
empirically
> > clocking request round-trip times over a large enough sample size.
> >
> > A 304 "Not Modified" does NOT indicate that the server cached the
> response.
> > It simply indicates that the client asked the server "I'm retrieving
this
> > URL, but I already have a copy of it with the following signature. If
> this
> > resource matches that signature, then don't send it again -- I already
> have
> > it. Otherwise, please give me the resource".
> >
> > Thus, a server answers 304 to such questions to tell the client "yup,
> you've
> > already got the resource. I'm not sending it to you again". Otherwise,
> the
> > server will return 200 with the updated resource as entity body.
> >
> > Clearly, a 304 (telling the client whether it already has the resource
or
> > not) is completely orthogonal to whether a response is cached or not.
If
> > IIS served out of the cache, you'd see a 200 response with no round-trip
> to
> > the slow hard drive. If it didn't serve out of the cache (but also sent
> no
> > entity body), then you'd see a 304.
> >
> > As for what is going on with IIS6 -- IIS6 caches some responses in
kernel
> > mode, and such cached responses are returned as 200 OK. Non-kernel-mode
> > cached URLs still follow the 304 semantic and has its own user-mode
based
> > caching mechanism.
> >
> > Finally, it may be possible that you have caching disabled (many things
> will
> > disable many types of caching) -- but without looking at performance
> > counters, it is not possible to confirm.
> >
> > --
> > //David
> > IIS
> > This posting is provided "AS IS" with no warranties, and confers no
> rights.
> > //
> > "Joe Rattz" <joerattz@yahoo.com> wrote in message
> > news:%23GV099WeEHA.1652@TK2MSFTNGP09.phx.gbl...
> > I just noticed a difference between my development box running IIS 5.0
and
> > my production server running IIS 6.0 that seems detrimental to
> performance.
> > In IIS 5, my images appear to be getting cached in that the server is
> > returning an http status code (sc-status from the log) of 304. Here is
a
> > sample from the log:
> >
> > 14:15:32 127.0.0.1 GET /GPCStoreFront/Images/Collapse.gif 304
> > 14:15:32 127.0.0.1 GET /GPCStorefront/Images/questionmark.gif 304
> > 14:15:32 127.0.0.1 GET /GPCStoreFront/Images/Expand.gif 304
> > 14:15:32 127.0.0.1 GET /GPCStorefront/Images/newuser.gif 304
> > 14:15:32 127.0.0.1 GET /GPCStorefront/Images/addtofavorites.jpg 304
> >
> > But, in IIS 6, I am getting 200 for images:
> >
> > 2004-08-03 15:26:53 10.28.98.130 GET
/GPCStoreFront/Images/Collapse.gif -
> > 80 - 10.28.11.180
> >
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> > LR+1.1.4322) 200 0 0
> > 2004-08-03 15:26:53 10.28.98.130 GET
> > /GPCStoreFront/Images/questionmark.gif - 80 - 10.28.11.180
> >
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> > LR+1.1.4322) 200 0 0
> > 2004-08-03 15:26:53 10.28.98.130 GET /GPCStoreFront/Images/Expand.gif -
> 80 -
> > 10.28.11.180
> >
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> > LR+1.1.4322) 200 0 0
> > 2004-08-03 15:26:53 10.28.98.130 GET /GPCStorefront/Images/newuser.gif -
> > 80 - 10.28.11.180
> >
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> > LR+1.1.4322) 200 0 0
> > 2004-08-03 15:26:53 10.28.98.130 GET
> > /GPCStoreFront/Images/addtofavorites.jpg - 80 - 10.28.11.180
> >
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> > LR+1.1.4322) 200 0 0
> > 2004-08-03 15:26:53 10.28.98.130 GET /GPCStoreFront/GPCStoreFront.css -
> 80 -
> > 10.28.11.180
> >
>
Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.0;+.NET+CLR+1.0.3705;+.NET+C
> > LR+1.1.4322) 200 0 0
> >
> > CSS files are behaving the same way.
> >
> > Can someone tell me why my images (and CSS file) are not getting cached?
> >
> > Thanks.
> >
> >
> >
>
>
>