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注意：HTTP_X_FORWARDED_FOR如果經(jīng)過(guò)了多個(gè)中間代理服務(wù)器，有何能是逗號(hào)分割的多個(gè)地址，
比如：200.28.7.155,200.10.225.77 unknown,219.101.137.3
因此在很多舊的數(shù)據(jù)庫(kù)設(shè)計(jì)中（比如BBS）往往用來(lái)記錄客戶(hù)端地址的字段被設(shè)置成20個(gè)字節(jié)就顯得過(guò)小了。
經(jīng)常見(jiàn)到類(lèi)似以下的錯(cuò)誤信息：
Microsoft JET Database Engine 錯(cuò)誤 '80040e57'
字段太小而不能接受所要添加的數(shù)據(jù)的數(shù)量。試著插入或粘貼較少的數(shù)據(jù)。
/inc/char.asp，行236
原因就是在設(shè)計(jì)客戶(hù)端訪問(wèn)地址時(shí)，相關(guān)用戶(hù)IP字段大小最好要設(shè)計(jì)到50個(gè)字節(jié)以上，當(dāng)然經(jīng)過(guò)3層以上代理的幾率也非常小。
如何檢查目前站點(diǎn)頁(yè)面的可緩存性（Cacheablility）呢？可以參考以下2個(gè)站點(diǎn)上的工具：
http://www.ircache.net/cgi-bin/cacheability.py
附：SQUID性能測(cè)試試驗(yàn)
phpMan.php是一個(gè)基于php的man page server，每個(gè)man
page需要調(diào)用后臺(tái)的man命令和很多頁(yè)面格式化工具，系統(tǒng)負(fù)載比較高，提供了Cache
Friendly的URL，以下是針對(duì)同樣的頁(yè)面的性能測(cè)試資料：
測(cè)試環(huán)境：Redhat 8 on Cyrix 266 / 192M Mem
測(cè)試程序：使用apache的ab(apache benchmark)：
測(cè)試條件：請(qǐng)求50次，并發(fā)50個(gè)連接
測(cè)試項(xiàng)目：直接通過(guò)apache 1.3 (80端口) vs squid 2.5(8000端口：加速80端口)
測(cè)試1：無(wú)CACHE的80端口動(dòng)態(tài)輸出：
ab -n 100 -c 10 http://www.chedong.com:81/phpMan.php/man/kill/1
This is ApacheBench, Version 1.3d <$Revision: 1.2 $> apache-1.3
Copyright (c) 1996 Adam Twiss, Zeus Technology Ltd,
http://www.zeustech.net/
Copyright (c) 1998-2001 The Apache Group, http://www.apache.org/
Benchmarking localhost (be patient).....done
Server Software:
Apache/1.3.23
Server Hostname: localhost
Server
Port:
80
Document Path:
/phpMan.php/man/kill/1
Document Length: 4655 bytes
Concurrency Level: 5
Time taken for tests: 63.164 seconds
Complete requests: 50
Failed requests: 0
Broken pipe errors: 0
Total transferred: 245900 bytes
HTML transferred: 232750 bytes
Requests per second: 0.79 [#/sec] (mean)
Time per request: 6316.40 [ms]
(mean)
Time per request: 1263.28 [ms]
(mean, across all concurrent requests)
Transfer rate:
3.89 [Kbytes/sec] received
Connnection Times (ms)
min mean[+/-sd] median max
Connect: 0
29 106.1 0 553
Processing: 2942 6016
1845.4 6227 10796
Waiting:
2941 5999 1850.7 6226 10795
Total:
2942 6045 1825.9 6227 10796
Percentage of the requests served within a certain time (ms)
50% 6227
66% 7069
75% 7190
80% 7474
90% 8195
95% 8898
98% 9721
99% 10796
100% 10796 (last request)
測(cè)試2：SQUID緩存輸出
/home/apache/bin/ab -n50 -c5
"http://localhost:8000/phpMan.php/man/kill/1"
This is ApacheBench, Version 1.3d <$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)
結(jié)論：No Cache / Cache = 6045 / 84 = 70
結(jié)論：對(duì)于可能被緩存請(qǐng)求的頁(yè)面，服務(wù)器速度可以有2個(gè)數(shù)量級(jí)的提高，因?yàn)镾QUID是把緩存頁(yè)面放在內(nèi)存里的（因此幾乎沒(méi)有硬盤(pán)I/O操作）。
小節(jié)：
大訪問(wèn)量的網(wǎng)站應(yīng)盡可能將動(dòng)態(tài)網(wǎng)頁(yè)生成靜態(tài)頁(yè)面作為緩存發(fā)布，甚至對(duì)于搜索引擎這樣的動(dòng)態(tài)應(yīng)用來(lái)說(shuō)，緩存機(jī)制也是非常非常重要的。
在動(dòng)態(tài)頁(yè)面中利用HTTP Header定義緩存更新策略。
利用緩存服務(wù)器獲得額外的配置和安全性
日志非常重要：SQUID日志缺省不支持COMBINED日志，但對(duì)于需要REFERER日志的這個(gè)補(bǔ)丁非常重要：http://www.squid-cache.org/mail-archive/squid-dev/200301/0164.html
參考資料：
HTTP代理緩存
http://vancouver-webpages.com/proxy.html
可緩存的頁(yè)面設(shè)計(jì)
http://linux.oreillynet.com/pub/a/linux/2002/02/28/cachefriendly.html
運(yùn)用ASP.NET的輸出緩沖來(lái)存儲(chǔ)動(dòng)態(tài)頁(yè)面 - 開(kāi)發(fā)者 - ZDNet China
http://www.zdnet.com.cn/developer/tech/story/0,2000081602,39110239-2,00.htm
相關(guān)RFC文檔：
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
可緩存性檢查
http://www.web-caching.com/cacheability.html
緩存設(shè)計(jì)要素
http://vancouver-webpages.com/CacheNow/detail.html
ZOPE上的幾篇使用APACHE MOD_PROXY MOD_GZIP加速的文檔
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
本文鏈接：http://www.95time.cn/tech/program/2004/1803.asp 
出處：CheDong
責(zé)任編輯：cjj
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