[Web-SIG] HEAD requests, WSGI gateways, and middleware

[Graham Dumpleton]

On 25/01/2008, Ian Bicking <ianb at colorstudy.com> wrote:
> Brian Smith wrote:
> > Graham Dumpleton wrote:
> >> The issue here is that Apache has its own output filtering
> >> system where filters can set headers based on the actual
> >> content. Because of this, any output filter must always
> >> receive the response content regardless of whether the
> >> request is a GET or HEAD. If an application handler tries to
> >> optimise things and not return the content, then these output
> >> filters may generate different headers for a HEAD request
> >> than a GET request, thereby violating the requirement that
> >> they should actually be the same.
> >>
> >> Note that response content is still thrown away for a HEAD
> >> request, it is just done at the very last moment after all
> >> Apache output filters have processed the data.
> >
> > Right, that is exactly what I am saying. In Apache's documentation, it
> > says that every handler should include the response entity for HEAD
> > requests, so that output filters can process the output. However, there
> > is nothing in PEP 333 that talks about this behavior.
>
> Unlike Apache there are no output filters in WSGI;

Well, the concept of output filters does exist in WSGI, they are just
called something different. ;-)

Anyway, the end result is the same, it is just that how they are
modeled in the worlds of Apache and WSGI at the interface level is
different.

Graham

> all middleware gets
> to adjust the request as well as the response.  So middleware that can't
> handle a real HEAD request has an opportunity to turn it into a GET
> request.  I don't see why PEP 333 needs to talk about this, to me it
> seems straight forward enough in a WSGI context, and PEP 333 can't cover
> every possible bug someone might introduce into their middleware.
>
>    Ian
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