测试环境：
192.168.1.1 (squid) , 192.168.1.2 (nginx proxy) , 192.168.1.3 (nginx web)
squid 作为最外层的代理；nginx proxy 作为第二层代理，通过upstream访问后台；nginx web 真正的数据后台web server。
测试开始：
$curl -I -H host:blog.lazybug.me http://192.168.1.1/test.html HTTP/1.0 200 OK Server: nginx/0.8.32 Date: Mon, 08 Feb 2010 18:11:48 GMT Content-Type: text/html; charset=GBK Expires: Mon, 08 Feb 2010 18:12:58 GMT Cache-Control: max-age=70 X-Cache: MISS from proxy.lazybug.me X-Cache-Lookup: HIT from proxy.lazybug.me:80 Via: 192.168.1.3.nginx, 1.1 proxy.lazybug.me:80 (squid/2.7.STABLE7) Connection: close
此时， user -> squid -> nginx proxy -> nginx web
此时，这个页面已经被缓存住，往后的访问都是TCP_MEM_HIT的了。等到页面过期后，我把nginx web 后台弄挂，我在这里是直接把nginx proxy的upstream给改了，改为 192.168.1.4。此时，访问nginx proxy
$curl -I -H host:blog.lazybug.me http://192.168.1.2/test.html HTTP/1.1 502 Bad Gateway Server: nginx/0.8.32 Date: Mon, 08 Feb 2010 18:28:06 GMT Content-Type: text/html Content-Length: 173 Connection: keep-alive
因为根本没有192.168.1.4这个ip，肯定bad gateway了。
但此时访问squid
$curl -I -H host:blog.lazybug.me http://192.168.1.1/test.html HTTP/1.0 200 OK Server: nginx/0.8.32 Date: Mon, 08 Feb 2010 18:11:48 GMT Content-Type: text/html; charset=GBK Expires: Mon, 08 Feb 2010 18:12:58 GMT Cache-Control: max-age=70 Age: 1004 Content-Length: 0 X-Cache: HIT from proxy.lazybug.me X-Cache-Lookup: HIT from proxy.lazybug.me:80 Via: 192.168.51.3.nginx, 1.1 proxy.lazybug.me:80 (squid/2.7.STABLE7) Connection: close
squid 仍然返回200，不过并不是立刻返回，而是经过了几秒才返回，因为此时squid认为请求页面过期，正在尝试去后端取新数据，但此时，后端返回一个502 bad gateway，那它的工作机制就是，返回旧的缓存数据给用户。
看一下squid的access.log，写着TCP_REFRESH_FAIL_HIT。查一下手册，如下：
An expired copy of the requested object was in the cache. Squid attempted to make an If-Modified-Since request, but it failed. The old (stale) object was delivered to the client.
如果 squid 没有缓存到页面，而出现502，squid 会不会返回200呢？？测试下
清缓存：
squidclient -p 80 -m purge http://blog.lazybug.me/test.html
然后再次访问：
$curl -I -H host:blog.lazybug.me http://192.168.1.1/test.html HTTP/1.0 502 Bad Gateway Server: nginx/0.8.32 Date: Tue, 09 Feb 2010 18:17:13 GMT Content-Type: text/html Content-Length: 173 X-Cache: MISS from proxy.lazybug.me X-Cache-Lookup: HIT from proxy.lazybug.me:80 Via: 1.1 proxy.lazybug.me:80 (squid/2.7.STABLE7) Connection: close
呵呵，不出所料。
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结论：squid 作为前端代理，而后端502的话，如果squid已经缓存住的页面，那squid就返回用户旧的数据，如果squid没有缓存住的话，很遗憾，还是会502的。
