# Weak ETags and on-the-fly gzipping **URL:** \ **Category:** NGINX\ **Created:** [June 15, 2013, 2:59pm UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590 "2013-06-15T14:59:12Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Sadaf\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@Sadaf\_N](https://www.ruby-forum.com/u/Sadaf_N)\ **Post date:** [June 15, 2013, 2:59pm UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/1 "2013-06-15T14:59:12Z")
I am serving dynamic requests behind the Nginx HTTP server. HTTP requests are mostly from mobile HTTP clients. That’s is why I care about two things. 1. Do not send the same representation twice: Use ETag caching mechanism. 2. Make better us of available bandwidth: Use Accept-Encoding HTTP compression. Today I noticed that these two don’t match. My application sets the ETag header in the response, but when I set the ‘Accept-Encoding: gzip’ in the request header, nginx clears the ETag header when gzipping on-the-fly. I understand why this is: If two responses have the same ETag, their bodies should be byte-for-byte comparable. When the content is gzipped, and actually modified, this is of course not the case. The gzipped response is not equal to the non-gzipped response. That’s why there are two ETag validation mechanisms. A strong validation, used in case of byte-for-byte comparable responses, and a weak validation, to indicate semantic equivalency. In the case of weak validation an ETag would look like: ETag: W/“123456789” Why is Nginx stripping weak ETag validators in its gzip filter? ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [June 16, 2013, 4:08am UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/2 "2013-06-16T04:08:58Z")
Hello! On Sat, Jun 15, 2013 at 02:58:33PM +0200, Matthijs L. wrote: > ETag: W/“123456789” > > Why is Nginx stripping weak ETag validators in its gzip filter? Just stripping ETags are easier than converting them to weak etags (and implementing weak etags support various places). On the other hand, relevant caching functionality is still here with Last-Modified cache validator. – Maxim D. > **[nginx: donation](http://nginx.org/en/donation.html)** ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [June 16, 2013, 11:40am UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/3 "2013-06-16T11:40:30Z")
Hi, thanks for answering so quickly. You are right, we should not go so far as to implement a conversion from strong to weak ETags. And alo, Last-Modified header could be preferable. Wen dynamic content is generated, setting a Last-Modified header requires inside knowledge from the app server. ETags are much simpler, as it’s often implemented as an md5 hash of the response body. A (Ruby) app server can easily add an ETag to all responses. Setting a Last-Modified header is different per request, depending on what dynamic resource is generated. So I would not say that the Last-Modified header covers all cases that ETag does. Instead of stripping all ETags, I would propose to only strip strong ETags. According to the spec: [HTTP/1.1: Caching in HTTP](http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.3) a weak ETag is perfectly fine for validating semantic equivalence. Just not for range requests. So by adding something like: if (is\_strong\_etag(r)) { ngx\_http\_clear\_etag(r) } We would allow applications to set weak Etags for caching. ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [October 18, 2013, 11:13pm UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/4 "2013-10-18T23:13:21Z")
Please someone implement weak etags or a “do\_not\_strip\_etags” options for this, it’s a big hit on our performance / page response time and we cannot simply replace that with Last-Modified. Posted at Nginx Forum: > **[Re: Weak ETags and on-the-fly gzipping](https://forum.nginx.org/read.php?2,240120,243845#msg-243845)** > > I am serving dynamic requests behind the Nginx HTTP server. HTTP requests are mostly from mobile HTTP clients. That's is why I care about two things. 1. Do not send the same representation twice: Use ETag caching mechanism. 2. Make better us of... ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [October 18, 2013, 11:20pm UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/5 "2013-10-18T23:20:22Z")
There take this patch and just apply it 🙂 — nginx-1.3.8/src/http/modules/ngx\_http\_gzip\_filter\_module.c 2012-07-07 17:22:27.000000000 -0400 +++ nginx-1.3.8-weak-etags-shorter/src/http/modules/ngx\_http\_gzip\_filter\_module.c2012-11-21 17:05:12.758389000 -0500 @@ -306,7 +306,15 @@ ngx\_http\_clear\_content\_length(r); ngx\_http\_clear\_accept\_ranges(r); - ngx\_http\_clear\_etag(r); - - /\* Clear etags unless they’re marked as weak (prefixed with ‘W/’) \*/ - h = r-\>headers\_out.etag; - if (h && !(h-\>value.len \>= 3 && - h-\>value.data[0] == ‘W’ && - h-\>value.data[1] == ‘/’ && - h-\>value.data[2] == ‘"’)) { - ngx\_http\_clear\_etag(r); - } return ngx\_http\_next\_header\_filter(r); } Posted at Nginx Forum: > **[Re: Weak ETags and on-the-fly gzipping](https://forum.nginx.org/read.php?2,240120,243846#msg-243846)** > > I am serving dynamic requests behind the Nginx HTTP server. HTTP requests are mostly from mobile HTTP clients. That's is why I care about two things. 1. Do not send the same representation twice: Use ETag caching mechanism. 2. Make better us of... ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [May 17, 2016, 4:06pm UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/6 "2016-05-17T16:06:21Z")
Is there anyway i can override this from nginx config file? I need weak etag to be untouched by nginx ☹ Posted at Nginx Forum: > **[Re: Weak ETags and on-the-fly gzipping](https://forum.nginx.org/read.php?2,240120,266944#msg-266944)** > > I am serving dynamic requests behind the Nginx HTTP server. HTTP requests are mostly from mobile HTTP clients. That's is why I care about two things. 1. Do not send the same representation twice: Use ETag caching mechanism. 2. Make better us of... ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [May 17, 2016, 5:00pm UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/7 "2016-05-17T17:00:41Z")
Hello! On Tue, May 17, 2016 at 10:05:53AM -0400, neoelit wrote: > Is there anyway i can override this from nginx config file? > I need weak etag to be untouched by nginx ☹ Weak ETags are supported and used since nginx 1.7.3 as released on 08 Jul 2014, quote from [http://nginx.org/en/CHANGES:](http://nginx.org/en/CHANGES:) ``` *) Feature: weak entity tags are now preserved on response modifications, and strong ones are changed to weak. ``` You are writing to an ancient thread from 2013. If you are still using an older version, it should be a good idea to upgrade. – Maxim D. > **[nginx](http://nginx.org/)** ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [May 18, 2016, 6:42am UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/8 "2016-05-18T06:42:02Z")
Thanks for the quick reply. 🙂 Yeah so nginx shouldn’t be the issue. I’m trying to figure why If-None-Match header not reaching rails on passenger-standalone. Since it has nginx version 1.8+ this should be the issue. [https://groups.google.com/forum/#!topic/phusion-passenger/eZgw3TqrfSI](https://groups.google.com/forum/#!topic/phusion-passenger/eZgw3TqrfSI) Posted at Nginx Forum: > **[Re: Weak ETags and on-the-fly gzipping](https://forum.nginx.org/read.php?2,240120,266970#msg-266970)** > > I am serving dynamic requests behind the Nginx HTTP server. HTTP requests are mostly from mobile HTTP clients. That's is why I care about two things. 1. Do not send the same representation twice: Use ETag caching mechanism. 2. Make better us of... ---
**Author:** ![mlangenberg](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@mlangenberg](https://www.ruby-forum.com/u/mlangenberg)\ **Post date:** [May 18, 2016, 6:43am UTC](https://www.ruby-forum.com/t/weak-etags-and-on-the-fly-gzipping/245590/9 "2016-05-18T06:43:47Z")
## neoelit Wrote: > Thanks for the quick reply. 🙂 > Yeah so nginx shouldn’t be the issue. I’m trying to figure why > If-None-Match header not reaching rails on passenger-standalone. Since > it has nginx version 1.8+ this should be the issue. > I mean this shouldn’t be the issue 😛 > [Redirecting to Google Groups](https://groups.google.com/forum/#!topic/phusion-passenger/eZgw3TqrfSI) Posted at Nginx Forum: [https://forum.nginx.org/read.php?2,240120,266971#msg-266971](https://forum.nginx.org/read.php?2,240120,266971#msg-266971)