Rewrite大全之DNS 负载均衡

DNS 负载均衡
一种成熟的基于DNS的负载均衡方法是使用http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html的lbnamed程序，它是一个Perl 5程序，带有若干辅助工具，实现了真正的基于DNS的负载均衡。

代理吞吐循环(Proxy Throughput Round-Robin)
这是一个使用mod_rewrite及其代理吞吐特性的方法。首先，在DNS记录中，将www0.foo.com固定为www.foo.com，如下：

代码:
www IN CNAME www0.foo.com.

其次，将www0.foo.com转换为一个专职代理服务器，即，由这个机器把所有到来的URL通过内部代理分散到另外5个服务器(www1-www5)。为此，必须建立一个规则集，对所有URL调用一个负载均衡脚本lb.pl。

代码:
RewriteEngine on
RewriteMap lb prg:/path/to/lb.pl
RewriteRule ^/(. )$ $ [P,L]

以下是lb.pl:

代码:
#!/path/to/perl
##
## lb.pl — load balancing script
##

$| = 1;

$name = “www”; # the hostname base
$first = 1; # the first server (not 0 here, because 0 is myself)
$last = 5; # the last server in the round-robin
$domain = “foo.dom”; # the domainname

$cnt = 0;
while () {
$cnt = (($cnt 1) % ($last 1-$first));
$server = sprintf(“%s%d.%s”, $name, $cnt $first, $domain);
print “http://$server/$_”;
}

##EOF##

最后的说明：这样有用吗？www0.foo.com似乎也会超载呀？答案是：没错，它的确会超载，但是它超载的仅仅是简单的代理吞吐请求！所有诸如SSI、CGI、ePerl等等的处理完全是由其他机器完成的，这个才是要点。
硬件/TCP循环
还有一个硬件解决方案。Cisco有一个叫LocalDirector的东西，实现了TCP/IP层的负载均衡，事实上，它是一个位于网站集群前端的电路级网关。如果你有足够资金而且的确需要高性能的解决方案，那么可以用这个。

反向代理
说明:
…
方案:
代码:
##
## apache-rproxy.conf — Apache configuration for Reverse Proxy Usage
##

# server type
ServerType standalone
Listen 8000
MinSpareServers 16
StartServers 16
MaxSpareServers 16
MaxClients 16
MaxRequestsPerChild 100

# server operation parameters
KeepAlive on
MaxKeepAliveRequests 100
KeepAliveTimeout 15
Timeout 400
IdentityCheck off
HostnameLookups off

# paths to runtime files
PidFile /path/to/apache-rproxy.pid
LockFile /path/to/apache-rproxy.lock
ErrorLog /path/to/apache-rproxy.elog
CustomLog /path/to/apache-rproxy.dlog “%t %h -> %e URL: %U”

# unused paths
ServerRoot /tmp
DocumentRoot /tmp
CacheRoot /tmp
RewriteLog /dev/null
TransferLog /dev/null
TypesConfig /dev/null
AccessConfig /dev/null
ResourceConfig /dev/null

# speed up and secure processing

Options -FollowSymLinks -SymLinksIfOwnerMatch
AllowOverride None

# the status page for monitoring the reverse proxy

SetHandler server-status

# enable the URL rewriting engine
RewriteEngine on
RewriteLogLevel 0

# define a rewriting map with value-lists where
# mod_rewrite randomly chooses a particular value
RewriteMap server rnd:/path/to/apache-rproxy.conf-servers

# make sure the status page is handled locally
# and make sure no one uses our proxy except ourself
RewriteRule ^/apache-rproxy-status.* – [L]
RewriteRule ^(http|ftp)://.* – [F]

# now choose the possible servers for particular URL types
RewriteRule ^/(.*.(cgi|shtml))$ to://$/ [S=1]
RewriteRule ^/(.*)$ to://$/

# and delegate the generated URL by passing it
# through the proxy module
RewriteRule ^to://([^/] )/(.*) http:/// [E=SERVER:,P,L]

# and make really sure all other stuff is forbidden
# when it should survive the above rules…
RewriteRule .* – [F]

# enable the Proxy module without caching
ProxyRequests on
NoCache *

# setup URL reverse mapping for redirect reponses
ProxyPassReverse / http://www1.foo.dom/
ProxyPassReverse / http://www2.foo.dom/
ProxyPassReverse / http://www3.foo.dom/
ProxyPassReverse / http://www4.foo.dom/
ProxyPassReverse / http://www5.foo.dom/
ProxyPassReverse / http://www6.foo.dom/

##
## apache-rproxy.conf-servers — Apache/mod_rewrite selection table
##

# list of backend servers which serve static
# pages (HTML files and Images, etc.)
static www1.foo.dom|www2.foo.dom|www3.foo.dom|www4.foo.dom

# list of backend servers which serve dynamically
# generated page (CGI programs or mod_perl scripts)
dynamic www5.foo.dom|www6.foo.dom