MHA自动切换流程

MHA的全名叫做mysql-master-ha，配置后可以在10-30秒内完成master自动切换，切换过程如下：

1. 检测master的状态，方法是一秒一次“ SELECT 1 As Value”，发现没有响应后会重复3次检查，如果还没有响应，shutdown并再重复一次SELECT 1 As Value确认master关闭

2. 确认SSH到master所在的机器是否可达

3. 给出消息：Connecting to a master server failed，并开始读取配置文件masterha_default.conf和app1.conf

4. 确认复制切换模式: [info] GTID failover mode = 1

5. 报告整个架构中的机器存活情况

Fri Jul  1 13:35:33 2016 - [info] Dead Servers:
Fri Jul  1 13:35:33 2016 - [info]   192.168.118.63(192.168.118.63:3306)
Fri Jul  1 13:35:33 2016 - [info] Alive Servers:
Fri Jul  1 13:35:33 2016 - [info]   192.168.118.62(192.168.118.62:3306)
Fri Jul  1 13:35:33 2016 - [info]   192.168.118.64(192.168.118.64:3306)

6. 检查存活的实例版本、GTID开启情况、是否开启read_only以及复制过滤情况

Fri Jul  1 13:35:33 2016 - [info] Alive Slaves:
Fri Jul  1 13:35:33 2016 - [info]   192.168.118.62(192.168.118.62:3306)  Version=5.6.28-log (oldest major version between slaves) log-bin:enabled
Fri Jul  1 13:35:33 2016 - [info]     GTID ON
Fri Jul  1 13:35:33 2016 - [info]     Replicating from 192.168.118.63(192.168.118.63:3306)
Fri Jul  1 13:35:33 2016 - [info]     Primary candidate for the new Master (candidate_master is set)
Fri Jul  1 13:35:33 2016 - [info]   192.168.118.64(192.168.118.64:3306)  Version=5.6.28-log (oldest major version between slaves) log-bin:enabled
Fri Jul  1 13:35:33 2016 - [info]     GTID ON
Fri Jul  1 13:35:33 2016 - [info]     Replicating from 192.168.118.63(192.168.118.63:3306)
Fri Jul  1 13:35:33 2016 - [info]     Primary candidate for the new Master (candidate_master is set)
Fri Jul  1 13:35:33 2016 - [info] Checking slave configurations..
Fri Jul  1 13:35:33 2016 - [info]  read_only=1 is not set on slave 192.168.118.62(192.168.118.62:3306).
Fri Jul  1 13:35:33 2016 - [info]  read_only=1 is not set on slave 192.168.118.64(192.168.118.64:3306).
Fri Jul  1 13:35:33 2016 - [info] Checking replication filtering settings..
Fri Jul  1 13:35:33 2016 - [info]  Replication filtering check ok.

7. 接下来就是在GTID复制基础上的切换过程

(1) 配置检查阶段，具体检查如下

[info] ** Phase 1: Configuration Check Phase completed.

检查项目如下：

 Query     SELECT @@global.server_id As Value
 Query     SELECT VERSION() AS Value                              #如果是GTID模式，版本不得小于5.6，如果是普通模式，版本不得小于5.0.45
  Query     SELECT @@global.gtid_mode As Value                     #MHA0.56版本开始支持GTID，之前的版本不支持
 7  Query     SHOW GLOBAL VARIABLES LIKE 'log_bin'                   #binlog必须开启
   Query     SHOW MASTER STATUS
  Query     SELECT @@global.datadir AS Value
  Query     SELECT @@global.slave_parallel_workers AS Value        #确定slave是不是多线程并行复制，这个参数的影响还没整明白，再研究下
  Query     SHOW SLAVE STATUS
 Query     SELECT @@global.read_only As Value                     #确定read_only的设置，如果要转为新的master，这个值要设为0
  Query     SELECT @@global.relay_log_purge As Value               #确定relay_log是否可自动删除，默认是可以
  Query     SELECT @@global.relay_log_info_repository AS Value     #确定relay_log是以file还是table格式存放的，默认是file
  Query     SELECT @@global.datadir AS Value                       #确定数据存放位置
 7  Query              SELECT @@global.relay_log_info_file AS Value                                                   #确定relay_log的文件名，为后面slave之间的relay_log应用做准备

备注：(1)默认情况下，从服务器上的中继日志在SQL线程执行完后会被自动删除的。但是这些中继日志在恢复其他从服务器时候可能会被用到，因此需要禁用中继日志的自动清除和定期清除旧的中继日志

(2)binlog-do-db和replicate-ignore-db设置必须相同。MHA在启动时候会检测过滤规则，如果过滤规则不同，MHA不启动监控和故障转移

(3)master.info和relay.info必须是file，不能是table

(2)彻底关闭master连接的阶段，避免master未关闭导致的脑裂

[info] * Phase 2: Dead Master Shutdown Phase..

具体关闭命令是：

/etc/masterha/master_ip_failover --orig_master_host=192.168.118.3 --orig_master_ip=192.168.118.3 --orig_master_port= --command=stopssh --ssh_user=root  

关闭完成后给出报告

[info] * Phase : Dead Master Shutdown Phase completed.

(3)master恢复阶段：

[info] * Phase : Master Recovery Phase..

›1   确认relay log最新的slave实例

›2   确定新的master

如果在配置文件中设置了候选master,会直接确定预设机器实例为master；如果没有预设，会选择含有最新的relay log的那个slave

›3   确认新的master后，会先设置sql_log_bin=0以阻塞master日志写入使其他slave赶上复制

会选取含有最新relay log 的slave，在该slave上设置sql_log_bin=0并在其余slaves上应用该最新relay log，最终获得这个层次的数据一致性，之后再set sql_log_bin=1使恢复日志写入。可以通过半同步复制来解决无法ssh到master所在机器所造成的事务丢失问题

待全部数据一致后，通过show master status确定新master的日志位置并在其他slave上执行change master语句创建新的主从连接：

Fri Jul 1 13:35:33 2016 - [info] Getting new master's binlog name and position..
Fri Jul 1 13:35:33 2016 - [info] mysql-bin.000004:191
Fri Jul 1 13:35:33 2016 - [info] All other slaves should start replication from here. Statement should be: CHANGE MASTER TO MASTER_HOST='192.168.118.2', MASTER_PORT=3306, MAST
ER_AUTO_POSITION=1, MASTER_USER='repl', MASTER_PASSWORD='xxx';

执行该切换的具体语句是

/etc/masterha/master_ip_failover --command=start --ssh_user=root --orig_master_host=192.168.118.3 --orig_master_ip=192.168.118.3 --orig_master_port= --new_master_host=192.168.118.2 --new_master_ip=192.168.118.2 --new_master_port= --new_master_user='user' --new_master_password='password'  

该阶段成果后给出报告：

Fri Jul   ::  - [info] ** Finished master recovery successfully.
Fri Jul   ::  - [info] * Phase : Master Recovery Phase completed.

(4)slaves恢复阶段：

[info] * Phase : Slaves Recovery Phase..

先停止IO线程，等待SQL线程执行完成后，stop slave，清除原slave信息，重新change master指向新的master，start slave ，over

 Query     SHOW SLAVE STATUS
                    Query     STOP SLAVE IO_THREAD
                    Query     SHOW SLAVE STATUS
                    Query     SHOW SLAVE STATUS
                    Query     STOP SLAVE
                    Query     SHOW SLAVE STATUS
                    Query     RESET SLAVE
                    Query     CHANGE MASTER TO MASTER_HOST = '192.168.118.62' MASTER_USER = 'repl' MASTER_PASSWORD = <secret> MASTER_PORT =
                    Query     START SLAVE
                    Connect Out       repl@192.168.118.62:
                    Query     SHOW SLAVE STATUS

(5)清除新选出的master上的slave信息

[info] * Phase : New master cleanup phase..

reset slave all;

  Query     STOP SLAVE
                     Query     SHOW SLAVE STATUS
                     Query     RESET SLAVE /*!50516 ALL */
                     Query     SHOW SLAVE STATUS

至此，整个切换过程完成，最后给出切换报告，over

----- Failover Report -----

app1: MySQL Master failover 192.168.118.(192.168.118.:) to 192.168.118.(192.168.118.:) succeeded

Master 192.168.118.(192.168.118.:) is down!

Check MHA Manager logs at localhost.localdomain:/var/log/masterha/app1/app1.log for details.

Started automated(non-interactive) failover.
Invalidated master IP address on 192.168.118.(192.168.118.:)
Selected 192.168.118.(192.168.118.:) as a new master.
192.168.118.(192.168.118.:): OK: Applying all logs succeeded.
192.168.118.(192.168.118.:): OK: Activated master IP address.
192.168.118.(192.168.118.:): OK: Slave started, replicating from 192.168.118.(192.168.118.:)
192.168.118.(192.168.118.:): Resetting slave info succeeded.
Master failover to 192.168.118.(192.168.118.:) completed successfully.