# Proxy cache

**URL:** <https://www.ruby-forum.com/t/proxy-cache/168313>\
**Category:** NGINX\
**Created:** [May 19, 2009, 3:04pm UTC](https://www.ruby-forum.com/t/proxy-cache/168313 "2009-05-19T15:04:46Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![MichaSSSSSJaszczyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@MichaSSSSSJaszczyk](https://www.ruby-forum.com/u/MichaSSSSSJaszczyk)\
**Post date:** [May 19, 2009, 3:04pm UTC](https://www.ruby-forum.com/t/proxy-cache/168313/1 "2009-05-19T15:04:46Z")

</div>

Hey,

I need a little bit of help in understanding how proxy cache works in  
Nginx. I’ve already managed to set up the cache and it works. Now I’m  
tweaking it and I’m having some trouble with the proxy\_cache\_valid  
directive. How does it work exactly? In some recent changelog I found  
that the cache now respects “Expires” and “Cache-Control” headers. How  
does this relate to proxy\_cache\_valid? Imagine the following  
scenarios:

1. proxy\_cache\_valid is set to 1h and upstream’s response is valid for  
2h.
2. proxy\_cache\_valid is set to 2h and upstream’s response is valid for  
1h.
3. proxy\_cache\_valid is set to 1h and upstream’s response doesn’t  
contain any caching hints.

For how long will Nginx cache the response in each of the above  
scenarios?

Does Nginx respect “Pragma: no-cache” header?

Which “Cache-Control” header directives (see  
[HTTP/1.1: Header Field Definitions](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9)) does  
Nginx respect?

What happens if the client sends “If-Modified-Since” header?

How does Nginx deal with “Vary” header?

That’s a lot of questions, sorry for that :). Answer to any of them  
will be appreciated :). If someone has the time to answer, I’ll try to  
spend some time and update the Wiki.

Regards,

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<div class="post-metadata">

**Author:** ![MichaSSSSSJaszczyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@MichaSSSSSJaszczyk](https://www.ruby-forum.com/u/MichaSSSSSJaszczyk)\
**Post date:** [May 19, 2009, 3:25pm UTC](https://www.ruby-forum.com/t/proxy-cache/168313/2 "2009-05-19T15:25:18Z")

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On Tue, May 19, 2009 at 02:57:23PM +0200, Micha?? Jaszczyk wrote:

> 1. proxy\_cache\_valid is set to 1h and upstream’s response is valid for 2h.  
> nginx will cache for 2h.

> 1. proxy\_cache\_valid is set to 2h and upstream’s response is valid for 1h.

nginx will cache for 1h.

> 1. proxy\_cache\_valid is set to 1h and upstream’s response doesn’t  
> contain any caching hints.

nginx will cache for 1h.

> For how long will Nginx cache the response in each of the above scenarios?
> 
> Does Nginx respect “Pragma: no-cache” header?

No. I belive no one browser respects this header, since it’s client side  
header only.

> Which “Cache-Control” header directives (see  
> [HTTP/1.1: Header Field Definitions](http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9)) does  
> Nginx respect?

“no-cache”, “max-age=0”, “max-age=NNN”.  
Probably “no-store” should be added.

> What happens if the client sends “If-Modified-Since” header?

This header does not send to backend, however, nginx may responses 304.

> How does Nginx deal with “Vary” header?

Not at all.

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<div class="post-metadata">

**Author:** ![MichaSSSSSJaszczyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@MichaSSSSSJaszczyk](https://www.ruby-forum.com/u/MichaSSSSSJaszczyk)\
**Post date:** [May 19, 2009, 3:48pm UTC](https://www.ruby-forum.com/t/proxy-cache/168313/3 "2009-05-19T15:48:16Z")

</div>

> nginx will cache for 1h.

This just means that upstream’s caching instructions are more  
important than proxy\_cache\_valid, right?

> > What happens if the client sends “If-Modified-Since” header?
> 
> This header does not send to backend, however, nginx may responses 304.

I’m not sure if I understand. Let’s imagine some scenarios (I-M-S  
stands for If-Modified-Since, L-M stands for Last-Modified):

1. Client sends I-M-S, Nginx has response in cache, cached response  
has L-M \< I-M-S, Nginx sends 304.

2. Client sends I-M-S, Nginx has response in cache, cached response  
has L-M \> I-M-S, Nginx serves response from cache.

3. Client sends I-M-S, Nginx doesn’t have response in cache, sends  
request to upstream, upstream’s L-M \< I-M-S, Nginx stores response in  
cache and sends 304.

4. Client sends I-M-S, Nginx doesn’t have response in cache, sends  
request to upstream, upstream’s L-M \> I-M-S, Nginx stores response in  
cache and sends response from upstream.

Am I right?

Regards,

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<div class="post-metadata">

**Author:** ![MichaSSSSSJaszczyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@MichaSSSSSJaszczyk](https://www.ruby-forum.com/u/MichaSSSSSJaszczyk)\
**Post date:** [November 4, 2010, 3:23pm UTC](https://www.ruby-forum.com/t/proxy-cache/168313/4 "2010-11-04T15:23:17Z")

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So,

proxy\_ignore\_headers X-Accel-Expires Expires Cache-Control;

Does work, but i lost quite some time until i realised that nginx sets  
an expiration time at the moment when a response is cached, restarting  
nginx afterwards with other configuration settings won’t change  
anything.

So, whenever your playing with proxy\_ignore\_headers, proxy\_cache\_valid  
remember to empty your cache dir 😉

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<div class="post-metadata">

**Author:** ![MichaSSSSSJaszczyk](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a3d4f5/32.png) [@MichaSSSSSJaszczyk](https://www.ruby-forum.com/u/MichaSSSSSJaszczyk)\
**Post date:** [May 19, 2009, 3:54pm UTC](https://www.ruby-forum.com/t/proxy-cache/168313/5 "2009-05-19T15:54:43Z")

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On Tue, May 19, 2009 at 03:39:59PM +0200, Micha?? Jaszczyk wrote:

> > nginx will cache for 1h.
> 
> This just means that upstream’s caching instructions are more  
> important than proxy\_cache\_valid, right?

Yes, the order is

X-Accel-Expires  
Expires/Cache-Control  
proxy\_cache\_valid

However, you may ignore the headers using

proxy\_ignore\_headers X-Accel-Expires Expires Cache-Control;

> > > What happens if the client sends “If-Modified-Since” header?
> > 
> > This header does not send to backend, however, nginx may responses 304.
> 
> I’m not sure if I understand. Let’s imagine some scenarios (I-M-S  
> stands for If-Modified-Since, L-M stands for Last-Modified):
> 
> 1. Client sends I-M-S, Nginx has response in cache, cached response  
> has L-M \< I-M-S, Nginx sends 304.

Yes. Small note, by default nginx sends 304 only if L-M == I-M-S.  
Controlled by if\_modified\_since [off|exact|before] directive

> 1. Client sends I-M-S, Nginx has response in cache, cached response  
> has L-M \> I-M-S, Nginx serves response from cache.

Yes.

> 1. Client sends I-M-S, Nginx doesn’t have response in cache, sends  
> request to upstream, upstream’s L-M \< I-M-S, Nginx stores response in  
> cache and sends 304.

Yes.

> 1. Client sends I-M-S, Nginx doesn’t have response in cache, sends  
> request to upstream, upstream’s L-M \> I-M-S, Nginx stores response in  
> cache and sends response from upstream.

Yes.