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I've noticed that Stackoverflow.com does not respond with an ETag header when queried through either a HEAD or GET method (assuming I am correct of course).

Why?

RSS aggregators (such as Google Reader) make good use of this header resulting in less useless traffic on your servers.

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ETag isn't necessary if Last-Modified is present –  Jeff Atwood Oct 6 '09 at 1:28
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@jeff: but in the case of SO, neither are present. –  jldupont Oct 6 '09 at 1:47
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Good suggestion: Last-Modified is present now where it matters -- on search engine hits to questions –  Jeff Atwood Nov 9 '09 at 5:03

2 Answers 2

up vote 6 down vote accepted

RSS aggregators (such as Google Reader) make good use of this header resulting in less useless traffic on your servers.

The tag should be present on RSS feeds, along with Last-Modified.

UPDATE: Well, I added this to the /question pages for search bots. We now respect If-Modified-Since on question retrievals for bots, and we return Last-Modified for bots.

Request:

GET /questions/1696044/how-to-delete-a-row-in-a-sqlite-database-table HTTP/1.1
User-Agent: Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)

Response:

HTTP/1.1 200 OK
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Encoding: gzip
Expires: Sun, 08 Nov 2009 15:23:33 GMT
Last-Modified: Sun, 08 Nov 2009 15:03:18 GMT
Vary: Accept-Encoding
Server: Microsoft-IIS/7.0
Date: Sun, 08 Nov 2009 15:23:32 GMT
Content-Length: 9309

UPDATE 2: It's working! I see 304s in the log from Google. Which is a nice break from the 2+GB of data Google pulls down from us every day. You see, the Google giveth, and the Google taketh away...

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and for the record, eTag is 100% redundant if you have last-modified and if-modified-since. It's great on checksummy files like say images, not so great on .. everything else. –  Jeff Atwood Nov 9 '09 at 4:51

Probably something to do with this: http://blog.stackoverflow.com/2009/08/a-few-speed-improvements/

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so, if I understood correctly, SO is advocating an ETag-less site? That can't be good when users aggregate their RSS feed... of course it depends how many folks do this ;-) –  jldupont Oct 6 '09 at 1:01
    
I advocate etags for statically generated files. –  Jeff Atwood Nov 9 '09 at 4:51

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