注意：HTTP_X_FORWARDED_FOR如果经过了多个中间代理服务器，有何能是逗号分割的多个地址，
比如：200.28.7.155,200.10.225.77 unknown,219.101.137.3
因此在很多旧的数据库设计中（比如BBS）往往用来记录客户端地址的字段被设置成20个字节就显得过小了。
经常见到类似以下的错误信息：

Microsoft JET Database Engine 错误 '80040e57'
字段太小而不能接受所要添加的数据的数量。试着插入或粘贴较少的数据。
/inc/char.asp，行236

　　原因就是在设计客户端访问地址时，相关用户IP字段大小最好要设计到50个字节以上，当然经过3层以上代理的几率也非常小。
如何检查目前站点页面的可缓存性（Cacheablility）呢？可以参考以下2个站点上的工具：
http://www.ircache.net/cgi-bin/cacheability.py

附：SQUID性能测试试验
phpMan.php是一个基于php的man page server，每个man
page需要调用后台的man命令和很多页面格式化工具，系统负载比较高，提供了Cache
Friendly的URL，以下是针对同样的页面的性能测试资料：
测试环境：Redhat 8 on Cyrix 266 / 192M Mem
测试程序：使用apache的ab(apache benchmark)：
测试条件：请求50次，并发50个连接
测试项目：直接通过apache 1.3 (80端口) vs squid 2.5(8000端口：加速80端口)

测试1：无CACHE的80端口动态输出：
ab -n 100 -c 10 http://www.chedong.com:81/phpMan.php/man/kill/1
This is ApacheBench, Version 1.3d <$Revision: 1.2 $> apache-1.3
Copyright (c) 1996 Adam Twiss, Zeus Technology Ltd,
http://www.zeustech.net/
Copyright (c) 1998-2001 The Apache Group, http://www.apache.org/

Benchmarking localhost (be patient).....done
Server Software:
Apache/1.3.23
Server Hostname: localhost
Server
Port:
80

Document Path:
/phpMan.php/man/kill/1
Document Length: 4655 bytes

Concurrency Level: 5
Time taken for tests: 63.164 seconds
Complete requests: 50
Failed requests: 0
Broken pipe errors: 0
Total transferred: 245900 bytes
HTML transferred: 232750 bytes
Requests per second: 0.79 [#/sec] (mean)
Time per request: 6316.40 [ms]
(mean)
Time per request: 1263.28 [ms]
(mean, across all concurrent requests)
Transfer rate:
3.89 [Kbytes/sec] received

Connnection Times (ms)

min mean[+/-sd] median max
Connect: 0
29 106.1 0 553
Processing: 2942 6016
1845.4 6227 10796

Waiting:
2941 5999 1850.7 6226 10795

Total:
2942 6045 1825.9 6227 10796

Percentage of the requests served within a certain time (ms)
50% 6227
66% 7069
75% 7190
80% 7474
90% 8195
95% 8898
98% 9721
99% 10796
100% 10796 (last request)

测试2：SQUID缓存输出
/home/apache/bin/ab -n50 -c5
"http://localhost:8000/phpMan.php/man/kill/1"
This is ApacheBench, Version 1.3d <$Revision: 1.2 $> apache-1.3
Copyright (c) 1996 Adam Twiss, Zeus Technology Ltd,
http://www.zeustech.net/
Copyright (c) 1998-2001 The Apache Group, http://www.apache.org/

Benchmarking localhost (be patient).....done
Server Software:
Apache/1.3.23
Server Hostname: localhost
Server
Port:
8000

Document Path:
/phpMan.php/man/kill/1
Document Length: 4655 bytes

Concurrency Level: 5
Time taken for tests: 4.265 seconds
Complete requests: 50
Failed requests: 0
Broken pipe errors: 0
Total transferred: 248043 bytes
HTML transferred: 232750 bytes
Requests per second: 11.72 [#/sec] (mean)
Time per request: 426.50 [ms] (mean)
Time per request: 85.30 [ms] (mean,
across all concurrent requests)
Transfer rate:
58.16 [Kbytes/sec] received

Connnection Times (ms)

min mean[+/-sd] median max
Connect:
0 1
9.5 0 68
Processing:
7 83 537.4
7 3808

Waiting:
5 81 529.1
6 3748

Total:
7 84 547.0
7 3876

Percentage of the requests served within a certain time (ms)
50% 7
66% 7
75% 7
80% 7
90% 7
95% 7
98% 8
99% 3876
100% 3876 (last request)

结论：No Cache / Cache = 6045 / 84 = 70
结论：对于可能被缓存请求的页面，服务器速度可以有2个数量级的提高，因为SQUID是把缓存页面放在内存里的（因此几乎没有硬盘I/O操作）。

小节：

大访问量的网站应尽可能将动态网页生成静态页面作为缓存发布，甚至对于搜索引擎这样的动态应用来说，缓存机制也是非常非常重要的。

在动态页面中利用HTTP Header定义缓存更新策略。

利用缓存服务器获得额外的配置和安全性

日志非常重要：SQUID日志缺省不支持COMBINED日志，但对于需要REFERER日志的这个补丁非常重要：http://www.squid-cache.org/mail-archive/squid-dev/200301/0164.html

参考资料：
HTTP代理缓存
http://vancouver-webpages.com/proxy.html

可缓存的页面设计
http://linux.oreillynet.com/pub/a/linux/2002/02/28/cachefriendly.html
运用ASP.NET的输出缓冲来存储动态页面 - 开发者 - ZDNet China
http://www.zdnet.com.cn/developer/tech/story/0,2000081602,39110239-2,00.htm
相关RFC文档：

RFC
2616:

section
13 (Caching)
section
14.9 (Cache-Control header)
section
14.21 (Expires header)
section
14.32 (Pragma: no-cache) is important if you are interacting with
HTTP/1.0 caches
section
14.29 (Last-Modified) is the most common validation method
section
3.11 (Entity Tags) covers the extra validation method

可缓存性检查
http://www.web-caching.com/cacheability.html
缓存设计要素
http://vancouver-webpages.com/CacheNow/detail.html

ZOPE上的几篇使用APACHE MOD_PROXY MOD_GZIP加速的文档
http://www.zope.org/Members/anser/apache_zserver/
http://www.zope.org/Members/softsign/ZServer_and_Apache_mod_gzip
http://www.zope.org/Members/rbeer/caching

本文链接：http://www.blueidea.com/tech/program/2004/1803.asp　

出处：CheDong
责任编辑：cjj

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