> Accessing the value of a header doesn't necessarily mean that the
response varies based up on, for example it might simply be accessed for
storage in informational logs.

True, probably a way to access headers without marking them as used would
be required - maybe something like request.headers.get(XYZ,
vary_response=False).

However, right now people are commonly forgetting to patch Vary, which
leads to problems with caching. This way - this won't happen ever again;
but in some cases, we might make caching less efficient than possible,
because somebody used request.headers[XYZ] and not request.headers.get(XYZ,
vary_response=False). Given these two cases - I feel that working correctly
is more important than perfectly-efficient caching - but opinions here may
differ.

> Additionally, Request.headers is not the only way to access the values
now, Request.META has not been removed. I don't believe any of Django's
internal header lookups have been changed to use Request.headers and it's
unlike third party packages or applications will ever all be moved.

It's not, but we can assume that all this code uses patch_vary_headers
correctly, so we don't need to track it here. It's mostly about new code,
that's going to be written with request.headers, so that it will work
correctly without worrying about the Vary header.

> Anyway I'm pretty sure you can write such a middleware yourself,
replacing Request.headers with a proxy object (and maybe Request.META too),
then adding 'Vary' on the way out based upon accessed keys, at least as a
proof of concept. If it gets some usage that would prove it could be
valuable to add to Django itself.

I guess it isn't something that people are going to be looking for, it's
always easier to add that another patch_vary_headers invocation than to add
a new package, so the usage won't be high; but I'll probably do so, at
least for my own usage. I'm already using it in one project, and I need it
in others.

On Fri, Jan 25, 2019 at 4:56 PM Adam Johnson <m...@adamj.eu> wrote:

>
> On Fri, 25 Jan 2019 at 14:46, Linus Lewandowski <
> linus.lewandow...@netguru.pl> wrote:
>
>> Right now, it's hard to report Vary header correctly. Headers might get
>> accessed in many different places, like middlewares, subroutines (which
>> can't use patch_vary_headers as they don't have access to the response
>> object), etc - and all those cases should be reflected in the Vary header,
>> or something might get cached incorrectly.
>>
>> However, thanks to the newly added request.headers property (see
>> https://code.djangoproject.com/ticket/20147 and
>> https://github.com/django/django/commit/4fc35a9c3efdc9154efce28cb23cb84f8834517e),
>> we now have a single place which is used to access request headers. We can
>> track which ones were accessed, and then set Vary header automatically, for
>> example in a middleware.
>>
>> What do you think about:
>> 1) adding some code to track accessed headers in request.headers,
>> 2) adding a new middleware (or expanding an existing one), that sets the
>> Vary header based on 1),
>> 3) deprecating patch_vary_headers function
>> and vary_on_headers/vary_on_cookie decorators and recommending to use
>> request.headers instead?
>>
>> Thanks,
>> Linus
>>
>> PS. This is a follow-up to the
>> https://code.djangoproject.com/ticket/28533 ticket.
>>
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-- 
Regards:
Linus Lewandowski
Senior Python Developer
https://netguru.co

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