为什么很多人用keepalived来实现redis故障转移

目前，Redis还没有一个类似于MySQL Proxy或Oracle RAC的官方HA方案。
Redis作者有一个名为Redis Sentinel的计划 ，据称将会有监控，报警和自动故障转移三大功能，非常不错。
但可惜的是短期内恐怕还不能开发完成。
因此，如何在出现故障时自动转移是一个需要解决的问题。
通过对网上一些资料的搜索，有建议采用HAProxy或Keepalived来实现的，事实上如果是做Failover而非负载均衡的话，Keepalived的效率肯定是超过HAProxy的，所以我决定采用Keepalived的方案。
环境介绍：
Master: 10.6.1.143
Slave: 10.6.1.144
Virtural IP Address (VIP): 10.6.1.200
设计思路：
当 Master 与 Slave 均运作正常时, Master负责服务，Slave负责Standby；
当 Master 挂掉，Slave 正常时, Slave接管服务，同时关闭主从复制功能；
当 Master 恢复正常，则从Slave同步数据，同步数据之后关闭主从复制功能，恢复Master身份，于此同时Slave等待Master同步数据完成之后，恢复Slave身份。
然后依次循环。
需要注意的是，这样做需要在Master与Slave上都开启本地化策略，否则在互相自动切换的过程中，未开启本地化的一方会将另一方的数据清空，造成数据完全丢失。
下面，是具体的实施步骤：
在Master和Slave上安装Keepalived
$ sudo apt-get install keepalived
修改Master和Slave的/etc/hosts文件
$ sudo vim /etc/hosts
127.0.0.1	localhost
10.6.1.143	redis
10.6.1.144	redis-slave

默认安装完成keepalived之后是没有配置文件的，因此我们需要手动创建：
首先，在Master上创建如下配置文件：
$ sudo vim /etc/keepalived/keepalived.conf
vrrp_script chk_redis { 
                script "/etc/keepalived/scripts/redis_check.sh"   ###监控脚本 
                interval 2                                        ###监控时间 
} 
vrrp_instance VI_1 { 
        state MASTER                            ###设置为MASTER
        interface eth0                          ###监控网卡    
        virtual_router_id 51
        priority 101                            ###权重值
        authentication { 
                     auth_type PASS             ###加密 
                     auth_pass redis            ###密码 
        } 
        track_script { 
                chk_redis                       ###执行上面定义的chk_redis
        } 
        virtual_ipaddress { 
             10.6.1.200                         ###VIP 
        }
        notify_master /etc/keepalived/scripts/redis_master.sh
        notify_backup /etc/keepalived/scripts/redis_backup.sh
        notify_fault  /etc/keepalived/scripts/redis_fault.sh
        notify_stop   /etc/keepalived/scripts/redis_stop.sh 
} 

然后，在Slave上创建如下配置文件：
$ sudo vim /etc/keepalived/keepalived.conf
vrrp_script chk_redis { 
                script "/etc/keepalived/scripts/redis_check.sh"   ###监控脚本 
                interval 2                                        ###监控时间 
} 
vrrp_instance VI_1 { 
        state BACKUP                                ###设置为BACKUP 
        interface eth0                              ###监控网卡
        virtual_router_id 51 
        priority 100                                ###比MASTRE权重值低 
        authentication { 
                     auth_type PASS 
                     auth_pass redis                ###密码与MASTRE相同
        } 
        track_script { 
                chk_redis                       ###执行上面定义的chk_redis
        } 
        virtual_ipaddress { 
             10.6.1.200                         ###VIP 
        } 
        notify_master /etc/keepalived/scripts/redis_master.sh
        notify_backup /etc/keepalived/scripts/redis_backup.sh
        notify_fault  /etc/keepalived/scripts/redis_fault.sh
        notify_stop   /etc/keepalived/scripts/redis_stop.sh 
}

在Master和Slave上创建监控Redis的脚本
$ sudo mkdir /etc/keepalived/scripts
$ sudo vim /etc/keepalived/scripts/redis_check.sh
#!/bin/bash

ALIVE=`/opt/redis/bin/redis-cli PING`
if [ "$ALIVE" == "PONG" ]; then
  echo $ALIVE
  exit 0
else
  echo $ALIVE
  exit 1
fi

编写以下负责运作的关键脚本：
notify_master /etc/keepalived/scripts/redis_master.sh
notify_backup /etc/keepalived/scripts/redis_backup.sh
notify_fault /etc/keepalived/scripts/redis_fault.sh
notify_stop /etc/keepalived/scripts/redis_stop.sh
因为Keepalived在转换状态时会依照状态来呼叫：
当进入Master状态时会呼叫notify_master
当进入Backup状态时会呼叫notify_backup
当发现异常情况时进入Fault状态呼叫notify_fault
当Keepalived程序终止时则呼叫notify_stop
首先，在Redis Master上创建notity_master与notify_backup脚本：
$ sudo vim /etc/keepalived/scripts/redis_master.sh
#!/bin/bash

REDISCLI="/opt/redis/bin/redis-cli"
LOGFILE="/var/log/keepalived-redis-state.log"

echo "[master]" >> $LOGFILE
date >> $LOGFILE
echo "Being master...." >> $LOGFILE 2>&1

echo "Run SLAVEOF cmd ..." >> $LOGFILE
$REDISCLI SLAVEOF 10.6.1.144 6379 >> $LOGFILE  2>&1
sleep 10 #延迟10秒以后待数据同步完成后再取消同步状态

echo "Run SLAVEOF NO ONE cmd ..." >> $LOGFILE
$REDISCLI SLAVEOF NO ONE >> $LOGFILE 2>&1

$ sudo vim /etc/keepalived/scripts/redis_backup.sh
#!/bin/bash

REDISCLI="/opt/redis/bin/redis-cli"
LOGFILE="/var/log/keepalived-redis-state.log"

echo "[backup]" >> $LOGFILE
date >> $LOGFILE
echo "Being slave...." >> $LOGFILE 2>&1

sleep 15 #延迟15秒待数据被对方同步完成之后再切换主从角色
echo "Run SLAVEOF cmd ..." >> $LOGFILE
$REDISCLI SLAVEOF 10.6.1.144 6379 >> $LOGFILE  2>&1

接着，在Redis Slave上创建notity_master与notify_backup脚本：
$ sudo vim /etc/keepalived/scripts/redis_master.sh
#!/bin/bash

REDISCLI="/opt/redis/bin/redis-cli"
LOGFILE="/var/log/keepalived-redis-state.log"

echo "[master]" >> $LOGFILE
date >> $LOGFILE
echo "Being master...." >> $LOGFILE 2>&1

echo "Run SLAVEOF cmd ..." >> $LOGFILE
$REDISCLI SLAVEOF 10.6.1.143 6379 >> $LOGFILE  2>&1
sleep 10 #延迟10秒以后待数据同步完成后再取消同步状态

echo "Run SLAVEOF NO ONE cmd ..." >> $LOGFILE
$REDISCLI SLAVEOF NO ONE >> $LOGFILE 2>&1

$ sudo vim /etc/keepalived/scripts/redis_backup.sh
#!/bin/bash

REDISCLI="/opt/redis/bin/redis-cli"
LOGFILE="/var/log/keepalived-redis-state.log"

echo "[backup]" >> $LOGFILE
date >> $LOGFILE
echo "Being slave...." >> $LOGFILE 2>&1

sleep 15 #延迟15秒待数据被对方同步完成之后再切换主从角色
echo "Run SLAVEOF cmd ..." >> $LOGFILE
$REDISCLI SLAVEOF 10.6.1.143 6379 >> $LOGFILE  2>&1

然后在Master与Slave创建如下相同的脚本：
$ sudo vim /etc/keepalived/scripts/redis_fault.sh
#!/bin/bash

LOGFILE=/var/log/keepalived-redis-state.log

echo "[fault]" >> $LOGFILE
date >> $LOGFILE

$ sudo vim /etc/keepalived/scripts/redis_stop.sh
#!/bin/bash

LOGFILE=/var/log/keepalived-redis-state.log

echo "[stop]" >> $LOGFILE
date >> $LOGFILE

给脚本都加上可执行权限：
$ sudo chmod +x /etc/keepalived/scripts/*.sh
脚本创建完成以后，我们开始按照如下流程进行测试：
1.启动Master上的Redis
$ sudo /etc/init.d/redis start
2.启动Slave上的Redis
$ sudo /etc/init.d/redis start
3.启动Master上的Keepalived
$ sudo /etc/init.d/keepalived start
4.启动Slave上的Keepalived
$ sudo /etc/init.d/keepalived start
5.尝试通过VIP连接Redis:
$ redis-cli -h 10.6.1.200 INFO
连接成功，Slave也连接上来了。
role:master
slave0:10.6.1.144,6379,online
6.尝试插入一些数据：
$ redis-cli -h 10.6.1.200 SET Hello Redis
OK
从VIP读取数据
$ redis-cli -h 10.6.1.200 GET Hello
"Redis"
从Master读取数据
$ redis-cli -h 10.6.1.143 GET Hello
"Redis"
从Slave读取数据
$ redis-cli -h 10.6.1.144 GET Hello
"Redis"
下面，模拟故障产生：
将Master上的Redis进程杀死：
$ sudo killall -9 redis-server
查看Master上的Keepalived日志
$ tailf /var/log/keepalived-redis-state.log
[fault]
Thu Sep 27 08:29:01 CST 2012
同时Slave上的日志显示：
$ tailf /var/log/keepalived-redis-state.log
[master]
Fri Sep 28 14:14:09 CST 2012
Being master....
Run SLAVEOF cmd ...
OK
Run SLAVEOF NO ONE cmd ...
OK
然后我们可以发现，Slave已经接管服务，并且担任Master的角色了。
$ redis-cli -h 10.6.1.200 INFO
$ redis-cli -h 10.6.1.144 INFO
role:master
然后我们恢复Master的Redis进程
$ sudo /etc/init.d/redis start
查看Master上的Keepalived日志
$ tailf /var/log/keepalived-redis-state.log
[master]
Thu Sep 27 08:31:33 CST 2012
Being master....
Run SLAVEOF cmd ...
OK
Run SLAVEOF NO ONE cmd ...
OK
同时Slave上的日志显示：
$ tailf /var/log/keepalived-redis-state.log
[backup]
Fri Sep 28 14:16:37 CST 2012
Being slave....
Run SLAVEOF cmd ...
OK
可以发现目前的Master已经再次恢复了Master的角色，故障切换以及自动恢复都成功了。
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