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

[Brian Smith]

Chris McDonough wrote:
> I have applications that do detect the difference between a 
> GET and a HEAD (they do slightly less work if the request is 
> a HEAD request), so I suspect this is not a totally 
> reasonable thing to add to the spec.

Yes, of course. In order to avoid doing unnecessary work for a HEAD
request, the extra work needs to be transferred to the response
iterable; for a HEAD request, the gateway would skip the iterable except
for its close() method, and so all the extra work is skipped as well.

> Maybe instead the middleware that does what you're describing should
be changed 
> instead to deal with HEAD requests.

I agree. But, this problem is often overlooked by middleware, which
indicates that we at least need an explanation of the problem in the
specification. But, when the middleware are corrected, then applications
like yours will only work efficiently if they transfer the extra work
they do for GET (vs. HEAD) requests to the response iterable.

> In general, I don't think is (or should be) any guarantee 
> that an arbitrary middleware stack will work with an 
> arbitrary application.  Although that would be nice in 
> theory, I suspect it would require a very complex protocol 
> (more complex than what WSGI requires now).

That is exactly what WSGI is designed for. There is no point to having a
standard if there is no interoperability amongst compliant components.

- Brian