一般来说，负载均衡就是将客户端的请求分流给后端的各个真实服务器，达到负载均衡的目的。还有一种方式是用两台服务器，一台作为主服务器(Master)，另一台作为热备份(Hot Standby)，请求全部分给主服务器，在主服务器当机时，立即切换到备份服务器，以提高系统的整体可靠性。 
负载均衡的设置 
Apache可以应对上面这两种需求。先来讨论一下如何做负载均衡。首先需要启用Apache的几个模块： 

 程序代码 
LoadModule proxy_module modules/mod_proxy.so 
LoadModule proxy_balancer_module modules/mod_proxy_balancer.so 
LoadModule proxy_http_module modules/mod_proxy_http.so 
mod_proxy提供代理服务器功能，mod_proxy_balancer提供负载均衡功能， mod_proxy_http让代理服务器能支持HTTP协议。如果把mod_proxy_http换成其他协议模块（如mod_proxy_ftp），或许能支持其他协议的负载均衡，有兴趣的朋友可以自己尝试一下。 
然后要添加以下配置： 

 程序代码 

ProxyRequests Off 
 <Proxy balancer://mycluster> 
     BalancerMember http://node-a.myserver.com:8080  
     BalancerMember http://node-b.myserver.com:8080  
 </Proxy>


ProxyPass / balancer://mycluster 
# 警告：以下这段配置仅用于调试，绝不要添加到生产环境中！！！ 
<Location /balancer-manager> 
    SetHandler balancer-manager 
    order Deny,Allow 
    Deny from all 
    Allow from localhost 
</Location>  
从上面的 ProxyRequests Off 这条可以看出，实际上负载均衡器就是一个反向代理，只不过它的代理转发地址不是某台具体的服务器，而是一个 balancer:// 协议： 
ProxyPass / balancer://mycluster协议地址可以随便定义。然后，在<Proxy>段中设置该balancer协议的内容即可。 BalancerMember指令可以添加负载均衡组中的真实服务器地址。 
下面那段<Location /balancer-manager>是用来监视负载均衡的工作情况的，调试时可以加上（生产环境中禁止使用！），然后访问 http://localhost/balancer-manager/  即可看到负载均衡的工作状况。 
OK，改完之后重启服务器，访问你的Apache所在服务器的地址，即可看到负载均衡的效果了。打开 balancer-manager 的界面，可以看到请求是平均分配的。 
如果不想平均分配怎么办？给 BalancerMember 加上 loadfactor 参数即可，取值范围为1-100。比如你有三台服务器，负载分配比例为 7:2:1，只需这样设置： 
程序代码 

ProxyRequests Off 
 <Proxy balancer://mycluster> 
     BalancerMember http://node-a.myserver.com:8080  loadfactor=7 
     BalancerMember http://node-b.myserver.com:8080  loadfactor=2 
     BalancerMember http://node-c.myserver.com:8080  loadfactor=1 
 </Proxy> 
 ProxyPass / balancer://mycluster


默认情况下，负载均衡会尽量让各个服务器接受的请求次数满足预设的比例。如果要改变算法，可以使用 lbmethod 属性。如： 

 程序代码 

ProxyRequests Off 
 <Proxy balancer://mycluster> 
     BalancerMember http://node-a.myserver.com:8080  loadfactor=7 
     BalancerMember http://node-b.myserver.com:8080  loadfactor=2 
     BalancerMember http://node-c.myserver.com:8080  loadfactor=1 
 </Proxy> 
 ProxyPass / balancer://mycluster 
 ProxySet lbmethod=bytraffic 
 lbmethod可能的取值有： 
 lbmethod=byrequests 按照请求次数均衡(默认)  
 lbmethod=bytraffic 按照流量均衡  
 lbmethod=bybusyness 按照繁忙程度均衡(总是分配给活跃请求数最少的服务器)


各种算法的原理请参见Apache的文档。 
热备份(Hot Standby) 
热备份的实现很简单，只需添加 status=+H 属性，就可以把某台服务器指定为备份服务器： 

 程序代码 

ProxyRequests Off 
 <Proxy balancer://mycluster> 
     BalancerMember http://node-a.myserver.com:8080  
     BalancerMember http://node-b.myserver.com:8080  status=+H 
 </Proxy> 
 ProxyPass / balancer://mycluster



从 balancer-manager 界面中可以看到，请求总是流向 node-a ，一旦node-a挂掉， Apache会检测到错误并把请求分流给 node-b。Apache会每隔几分钟检测一下 node-a 的状况，如果node-a恢复，就继续使用node-a。

--------------------------------------------------------------------------------------------------------------------

配合play!的配置轻松完成负载均衡

Set-up a front-end HTTP server

You can easily deploy your application as a stand-alone server by setting the application HTTP port to 80 :

%production.http.port=80

But if you plan to host several applications in the same server or load balance several instances of your application for scalability or fault tolerance, you can use a front-end HTTP server.

Note that using a front-end HTTP server will never give you better performance than using Play server directly!

Set-up with lighttpd

This example shows you how to configure lighttpd as a front-end web server. Note that you can do the same with Apache, but if you only need virtual hosting or load balancing, lighttpd is a very good choice and much easier to configure!

The /etc/lighttpd/lighttpd.conf file should define things like this:

server.modules = (
      "mod_access",
      "mod_proxy",
      "mod_accesslog" 
)
...
$HTTP["host"] =~ "www.myapp.com" {
    proxy.balance = "round-robin" proxy.server = ( "/" =>
        ( ( "host" => "127.0.0.1", "port" => 9000 ) ) )
}
 
$HTTP["host"] =~ "www.loadbalancedapp.com" {
    proxy.balance = "round-robin" proxy.server = ( "/" => ( 
          ( "host" => "127.0.0.1", "port" => 9000 ), 
          ( "host" => "127.0.0.1", "port" => 9001 ) ) 
    )
}

Set-up with Apache

The example below shows a simple set-up with Apache httpd server running in front of a standard Play configuration. 

LoadModule proxy_module modules/mod_proxy.so
...
<VirtualHost *:80>
  ProxyPreserveHost On
  ServerName www.loadbalancedapp.com
  ProxyPass / http://127.0.0.1:9000/
  ProxyPassReverse / http://127.0.0.1:9000/
</VirtualHost>

Apache as a front proxy to allow transparent upgrade of your application

The basic idea is to run 2 Play instances of your web application and let the front-end proxy load-balance them. In case one is not available, it will forward all the requests to the available one.

Let’s start the same Play application two times: one on port 9999 and one on port 9998.

Copy the application 2 times and edit the application.conf in the conf directory to change the port numbers.

For each web application directory:

play start mysuperwebapp

Now, let’s configure our Apache web server to have a load balancer.

In Apache, I have the following configuration:

如何需要监听90端口，需要在外部配上

listen 90
<VirtualHost localhost:90>

即可，如是默认是80端口，则可按照下面的配置进行

<VirtualHost mysuperwebapp.com:80>
  ServerName mysuperwebapp.com
  <Location /balancer-manager>
    SetHandler balancer-manager
    Order Deny,Allow
    Deny from all
    Allow from .mysuperwebapp.com
  </Location>
  <Proxy balancer://mycluster>
    BalancerMember http://localhost:9999
    BalancerMember http://localhost:9998 status=+H
  </Proxy>
  <Proxy *>
    Order Allow,Deny
    Allow From All
  </Proxy>
  ProxyPreserveHost On
  ProxyPass /balancer-manager !
  ProxyPass / balancer://mycluster/
  ProxyPassReverse / http://localhost:9999/
  ProxyPassReverse / http://localhost:9998/
</VirtualHost>

The important part is balancer://mycluster. This declares a load balancer. The +H option means that the second Play application is on stand-by. But you can also instruct it to load-balance.

Every time you want to upgrade mysuperwebapp, here is what you need to do:

play stop mysuperwebapp1

The load-balancer then forwards everything to mysuperwebapp2. In the meantime update mysuperwebapp1. Once you are done:

play start mysuperwebapp1

You can now safely update mysuperwebapp2.

Apache also provides a way to view the status of your cluster. Simply point your browser to /balancer-manager to view the current status of your clusters.

Because Play is completely stateless you don’t have to manage sessions between the 2 clusters. You can actually easily scale to more than 2 Play instances.