When testing my lp:~mvo/software-center/no-restfulclient-for-what-is-available/ branch I noticed that the etags header is not always returned:
$ curl -I https://software-center.ubuntu.com/apps/en/ubuntu/maverick/i386/
HTTP/1.1 200 OK
Date: Wed, 22 Sep 2010 15:46:34 GMT
Server: Apache/2.2.14 (Ubuntu)
ETag: "4278440f25214c811b8c6b176645e944"
Expires: Wed, 22 Sep 2010 15:48:07 GMT
Cache-Control: max-age=600
Last-Modified: Wed, 22 Sep 2010 15:38:07 GMT
Content-Type: application/json
Via: 1.1 software-center.ubuntu.com
$ curl -I https://software-center.ubuntu.com/apps/en/ubuntu/maverick/i386/
HTTP/1.1 200 OK
Date: Wed, 22 Sep 2010 15:46:39 GMT
Server: Apache/2.2.14 (Ubuntu)
Content-Type: application/json
Via: 1.1 software-center.ubuntu.com
It seems to be a bit random, maybe 1 out of 3 or 1 out of 4 times.
Hi, this was due to the interaction of two django middlewares, CommonMiddleware and UpdateCacheMidd
This was compounded by the fact that, having two app servers, the header was some times cached in one app server but not in the other.
I've changed the USE_ETAGS setting to True to have the CommonMiddleware always calculate the ETags header for every request, which solves the issue locally. We should test again once this config change has been deployed.