traffic server freshness_limit试用
ats的官方文档说，当没有Expires或max-age头的情况下，ats会根据date, last_modified以及heuristic_lm_factor计算过期时间。ats中相关配置如下：
CONFIG proxy.config.http.cache.heuristic_min_lifetime INT 3600
CONFIG proxy.config.http.cache.heuristic_max_lifetime INT 86400
CONFIG proxy.config.http.cache.heuristic_lm_factor FLOAT 0.100000
然后在源站创建文件pnow.jpg, 发起请求：
# wget -S --header "Host:test.ats.com" http://127.0.0.1/pnow.jpg?t1 -P /tmp/
--2013-04-02 22:17:09--http://127.0.0.1/pnow.jpg?t1
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response...
HTTP/1.0 200 OK
Server: ATS/3.2.4
Date: Tue, 02 Apr 2013 14:17:09 GMT
Content-Type: image/jpeg
Content-Length: 483632
Last-Modified: Tue, 02 Apr 2013 14:00:00 GMT
Accept-Ranges: bytes
Age: 0
Connection: keep-alive
Length: 483632 (472K)
Saving to: `/tmp/pnow.jpg?t1.6'
100%[===================================================================================>] 483,632 --.-K/s in 0.002s
然而无论多少次请求，Age始终是0，源站有回源记录。
本次测试中，Last-Modified时间是 Tue, 02 Apr 2013 14:00:00 GMT, 请求date在这之后10~15分钟期间，根据freshness_limit = ( date - last_modified ) * 0.10的公式，在1~1.5分钟期间应该是fresh的，为什么实测看到的是没有缓存而回源了呢？
ats初学者，求解答。
找到原因了，修改proxy.config.http.cache.required_headers值为0, 原先为2, 只缓存带expires和max-age头的对象。
# wget -S --header "Host:test.ats.com" http://127.0.0.1/pnow.jpg?t14 -P /tmp/
--2013-04-02 22:48:38--http://127.0.0.1/pnow.jpg?t14
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response...
HTTP/1.0 200 OK
Server: ATS/3.2.4
Date: Tue, 02 Apr 2013 14:41:37 GMT
Content-Type: image/jpeg
Content-Length: 483632
Last-Modified: Tue, 02 Apr 2013 14:30:00 GMT
Accept-Ranges: bytes
Age: 421
Connection: keep-alive
Length: 483632 (472K)
Saving to: `/tmp/pnow.jpg?t14.175'
100%[===================================================================================>] 483,632 --.-K/s in 0.002s
2013-04-02 22:48:38 (288 MB/s) - `/tmp/pnow.jpg?t14.175' saved
但依照上述Date, Last-modified, 我计算出的缓存时间是(14:41:37 - 14:30:30) * 0.1 = 1 分钟， 为什么现在已经421s了还是fresh的？
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