单点故障( 相互备份 )
1.主机
   (1)安装memcached
             yum install memcached -y
   (2)所分配内存大小
         free -m
             vim /etc/sysconfig/memcached
               CACHESIZE="30"
   (3)启动服务 /etc/init.d/memcached start
   (4)端口     netstat -antlp | grep 11211
2.副机
   (1)安装memcached
             yum install memcached -y
   (2)所分配内存大小
         free -m
             vim /etc/sysconfig/memcached
               CACHESIZE="30"
   (3)启动服务 /etc/init.d/memcached start
   (4)端口     netstat -antlp | grep 11211
3.测试是否可以存数据
    如果没有telnet 需要安装 yum install -y telnet
   telnet localhost 11211
   (1)查看 stats
   (2)添加 add user 0 0 6
              westos
              STORED
   (3)获取   get user
              VALUE user 0 6
              westos
              END
   (4)设置   set user 0 0 6
              redhat
              STORED
   (5)删除   delete user
              DELETED

memcached （ 存放tomcat的数据 ） 交叉存储