So this is simply a GET request with the If-Modified-Since header set correct? -Casey On Tuesday 27 August 2002 09:56 am, Steve Lay wrote:

We've been having some difficulty with image-performance on one of our servers and have traced the problem to the Content-Length header returned by Zope as part of a 304 response. We're running Zope 2.5.1 through Apache 1.3.26 and pcgi.

When a request is made to the server with an appropriate Is-Modified-Since header the code in lib/python/OFS/Image.py sets the Content-Length to the length of the image data but returns an empty string. My reading of the HTTP/1.1 spec suggests this is an error, see quote below:

"If a Content-Length header field (section 14.13) is present, its decimal value in OCTETs represents both the entity-length and the transfer-length. The Content-Length header field MUST NOT be sent if these two lengths are different".

So, we've patched our server by commenting out the line that sets the Content-Length and our problem goes away. Is this a bug in 2.5.1 or have I misunderstood something? I'm slightly concerned because this bit of code is commented as follows:

# Set header values since apache caching will return Content-Length # of 0 in response if size is not set here

I can confirm that this is true, but don't understand why this should be deemed a problem - no data is being returned.

For those interested, our problem is that we have a firewall (well known brand I think) that seems to cause the connections to hang for a about 10s each time a 304 request for an image from one of our Zope servers passes through it. As a result, graphic pages are fast the first time you hit them (due to 200 responses) but deadly slow from then on!

Before I go patching my other servers, I thought I'd post here in case it was a known problem and/or I've done something stupid.

Steve

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Steve Lay wrote:

We've been having some difficulty with image-performance on one of our servers and have traced the problem to the Content-Length header returned by Zope as part of a 304 response. We're running Zope 2.5.1 through Apache 1.3.26 and pcgi.

When a request is made to the server with an appropriate Is-Modified-Since header the code in lib/python/OFS/Image.py sets the Content-Length to the length of the image data but returns an empty string. My reading of the HTTP/1.1 spec suggests this is an error, see quote below:

"If a Content-Length header field (section 14.13) is present, its decimal value in OCTETs represents both the entity-length and the transfer-length. The Content-Length header field MUST NOT be sent if these two lengths are different".

So, we've patched our server by commenting out the line that sets the Content-Length and our problem goes away. Is this a bug in 2.5.1 or have I misunderstood something? I'm slightly concerned because this bit of code is commented as follows: [...]

Steve

Steve, maybe you don't need to patch all zopes. I know it's not a clean solution, but you should be able to use apache to rewrite the offending headers. You should take a look at the header and rewrite directives, I'm quite sure that it's possible to fix up whatever zopes seems to be doing inappropriate for your firewall. cheers, oliver