No revalidation when using stale-while-revalidate

[Adam Volek]

Hi,

On 24. 07. 20 4:33, Maxim Dounin wrote:
 > As long as the response returned isn't cacheable (either
 > as specified in the response Cache-Control / Expires
 > headers, or per proxy_cache_valid), nginx won't put
 > the response into cache and will continue serving previously
 > cached response till stale-while-revalidate timeout expires.
 >
 > Most likely "specific status code" in your tests in fact means
 > responses returned by your upstream server without Cache-Control
 > headers, and hence not cached by nginx.

This is not the case as far as I can tell. In our tests, the upstream 
server was set up to send these two responses, the 204 first, and then 
then the 404:

HTTP/1.1 204 No Content
Date: Fri, 24 Jul 2020 11:32:33 GMT
Connection: keep-alive
cache-control: max-age=5, stale-while-revalidate=10

HTTP/1.1 404 Not Found
Date: Fri, 24 Jul 2020 11:32:35 GMT
Content-Type: text/plain
Connection: close
cache-control: max-age=5, stale-while-revalidate=10

In this scenario, nginx returns fresh 204 for five seconds and then it 
returns stale 204 for ten seconds even though it's attempting 
revalidation according to access log at the upstream server. If we send 
the following 410 response instead of 404 however, nginx behaves as we 
would expect: it returns the fresh 204 for five seconds, then it 
revalidates it almost instantly and starts returning the fresh 410:

HTTP/1.1 410 Gone
Date: Fri, 24 Jul 2020 11:41:56 GMT
Content-Type: text/plain
Connection: close
cache-control: max-age=5, stale-while-revalidate=10

Adam Volek

On 24. 07. 20 4:33, Maxim Dounin wrote:
> Hello!
> 
> On Thu, Jul 23, 2020 at 08:06:36PM +0200, Adam Volek wrote:
> 
>> We're running into some strange behaviour with the
>> stale-while-revalidate extension of the cache-control header
>> when using nginx as a reverse proxy. When
>> there is a stale response in the cache with nonzero
>> stale-while-revalidate time, nginx attempts revalidation but
>> seems to ignore the upstream answer if it
>> has specific status code, such as 404 or 500, and server a stale
>> response to the client. Other response codes such as 200 or 410
>> don't trigger this
>> behaviour. Is this intended, and if so, is there a way to
>> configure nginx to treat 404 as any other response?
> 
> As long as the response returned isn't cacheable (either
> as specified in the response Cache-Control / Expires
> headers, or per proxy_cache_valid), nginx won't put
> the response into cache and will continue serving previously
> cached response till stale-while-revalidate timeout expires.
> 
> Most likely "specific status code" in your tests in fact means
> responses returned by your upstream server without Cache-Control
> headers, and hence not cached by nginx.
> 
>> I understand that this behaviour might be desirable in some
>> situations (especially for the responses with 5xx status codes),
>> but in our case, if the
>> upstream return a 404 response, we would want the cache to start
>> returning it as well as soon as the revalidation is finished.
> 
> As long as the above analysis is correct, the solution is simple:
> make sure responses you want nginx to cache are cacheable, that
> is, make sure appropriate headers are present or configure
> proxy_cache_valid (http://nginx.org/r/proxy_cache_valid).
>