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nginx与apache对于If-Modified-Since实现的不同

在HTTP 1.1中,If-Modified-Since与Last-Modified互成一对，用来判断浏览器cache是否已经有效，对此，HTTP 1.0 RFC1945对此有如下说明:

The If-Modified-Since request-header field is used with the GET  method to make it conditional: if the requested resource has not been modified since the time specified in this field, a copy of the   resource will not be returned from the server; instead, a 304 (not  modified) response will be returned without any Entity-Body.

无意中测试发现nginx与apache对此有不同的算法:

APACHE:

（1）直接发送请求,返回200,Last-Modified: Mon, 26 Apr 2010 13:22:17 GMT

[root@test ~]# curl -I http://www.pengyao.org/test.html

HTTP/1.1 200 OK
Date: Mon, 26 Apr 2010 14:59:09 GMT
Server: Apache/1.3.41 (Unix)
Last-Modified: Mon, 26 Apr 2010 13:22:17 GMT
ETag: “92c027-897-4bd59389″
Accept-Ranges: bytes
Content-Length: 2199
Content-Type: text/plain
(2)指定与Last-Modified时间相同的If-Modified-Since 发送GET请求,返回304

[root@test ~]# curl -I -G -H “If-Modified-Since: Mon, 26 Apr 2010 13:22:17 GMT”  http://www.pengyao.org/test.html

HTTP/1.1 304 Not Modified
Date: Mon, 26 Apr 2010 15:02:06 GMT
Server: Apache/1.3.41 (Unix)
ETag: “92c027-897-4bd59389″

(3)调后If-Modified-Since 1小时，再次发送GET请求，返回依然为304
[root@test ~]# curl -I -G -H “If-Modified-Since: Mon, 26 Apr 2010 14:22:17 GMT”  http://www.pengyao.org/test.html

HTTP/1.1 304 Not Modified
Date: Mon, 26 Apr 2010 15:05:02 GMT
Server: Apache/1.3.41 (Unix)
ETag: “92c027-897-4bd59389″

说明Apache 在判断浏览器cache是否过期时，依据从If-Modified-Since开始，文件Last-Modified是否修改过来判断的,与RFC1945中对If-Modified-Since描述吻合.

那么再看下nginx对于此的测试结果:

(1)直接发送请求,返回200,Last-Modified: Wed, 21 Apr 2010 13:14:21 GMT
[root@test pengyao.org]# curl -I http://www.pengyao.org/index.html
HTTP/1.1 200 OK
Server: nginx/0.7.61
Date: Mon, 26 Apr 2010 15:18:29 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 323
Last-Modified: Wed, 21 Apr 2010 13:14:21 GMT
Connection: keep-alive
Accept-Ranges: bytes

(2)指定与Last-Modified时间相同的If-Modified-Since 发送GET请求,返回304

[root@test pengyao.org]# curl -I -G -H “If-Modified-Since: Wed, 21 Apr 2010 13:14:21 GMT” http://www.pengyao.org/index.html
HTTP/1.1 304 Not Modified
Server: nginx/0.7.61
Date: Mon, 26 Apr 2010 15:20:45 GMT
Last-Modified: Wed, 21 Apr 2010 13:14:21 GMT
Connection: keep-alive

(3)调后If-Modified-Since 1小时，再次发送GET请求，发现返回的结果为200,与Apache不同

[root@test pengyao.org]# curl -I -G -H “If-Modified-Since: Wed, 21 Apr 2010 14:14:21 GMT” http://www.pengyao.org/index.html
HTTP/1.1 200 OK
Server: nginx/0.7.61
Date: Mon, 26 Apr 2010 15:21:19 GMT
Content-Type: text/html; charset=UTF-8
Content-Length: 323
Last-Modified: Wed, 21 Apr 2010 13:14:21 GMT
Connection: keep-alive
Accept-Ranges: bytes
由此可以判断出，nginx在判断浏览器cache是否过期时，判断If-Modified-Since与Last-Modified是否匹配，如果不匹配，则认为cache过期，返回200重新下载.

要说更喜欢哪种算法，个人更倾向于nginx的这种，因为在对文件更新前，本人习惯于对需要修改的文件进行备份(保留时间戳)，一旦测试出现问题，及时的回滚，使用nginx的这种算法就能保障回滚前后浏览器cache失效，而Apache的这种策略导致回滚到之前的时间戳的话(Last-Modified)，本地由于进行了测试，本地cache文件的If-Modified-Since时间一般比回滚后的Last-Modified新，这样本地的cache认为依然有效，返回304,而实际上文件已经不是最新的。当然，Apache的这样设计完全遵守RFC,个人喜好不代表真实需求.

如果你想对If-Modified-Since了解更多，可以直接访问RFC 1945:http://www.w3.org/Protocols/rfc1945/rfc1945

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