MHA在线切换的步骤及原理

在日常工作中，会碰到如下的场景，如mysql数据库升级，主服务器硬件升级等，这个时候就需要将写操作切换到另外一台服务器上，那么如何进行在线切换呢？同时，要求切换过程短，对业务的影响比较小。

MHA就提供了这样一种优雅的方式，只会堵塞业务0.5~2s的时间，在这段时间内，业务无法读取和写入。

集群信息

角色 IP地址 ServerID 类型

Master 192.168.244.10 1 写入

Candicate master 192.168.244.20 2 读

Slave 192.168.244.30 3 读

Monitor host 192.168.244.40 监控集群组

MHA具体的搭建步骤和原理，可参考另外一篇博客：

MySQL高可用方案MHA的部署和原理

在线切换的步骤

1. 关闭MHA监控

# masterha_stop --conf=/etc/masterha/app1.cnf

2. 在线切换

# /usr/local/bin/masterha_master_switch --conf=/etc/masterha/app1.cnf --master_state=alive --new_master_host=192.168.244.20 --new_master_port=3306 --orig_master_is_new_slave --running_updates_limit=10000

其中，

--orig_master_is_new_slave是将原master切换为新主的slave，默认情况下，是不添加的。

--running_updates_limit默认为1s，即如果主从延迟时间（Seconds_Behind_Master），或master show processlist中dml操作大于1s，则不会执行切换。

在线切换的输出

Tue Apr  ::  - [info] MHA::MasterRotate version 0.56.

Tue Apr  ::  - [info] Starting online master switch..

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] * Phase : Configuration Check Phase..

Tue Apr  ::  - [info]

Tue Apr  ::  - [warning] Global configuration file /etc/masterha_default.cnf not found. Skipping.

Tue Apr  ::  - [info] Reading application default configuration from /etc/masterha/app1.cnf..

Tue Apr  ::  - [info] Reading server configuration from /etc/masterha/app1.cnf..

Tue Apr  ::  - [info] GTID failover mode =

Tue Apr  ::  - [info] Current Alive Master: 192.168.244.10(192.168.244.10:)

Tue Apr  ::  - [info] Alive Slaves:

Tue Apr  ::  - [info]   192.168.244.20(192.168.244.20:)  Version=5.6.-log (oldest major version between slaves) log

-bin:enabledTue Apr  ::  - [info]     Replicating from 192.168.244.10(192.168.244.10:)

Tue Apr  ::  - [info]     Primary candidate for the new Master (candidate_master is set)

Tue Apr  ::  - [info]   192.168.244.30(192.168.244.30:)  Version=5.6.-log (oldest major version between slaves) log

-bin:enabledTue Apr  ::  - [info]     Replicating from 192.168.244.10(192.168.244.10:)

It is better to execute FLUSH NO_WRITE_TO_BINLOG TABLES on the master before switching. Is it ok to execute on 192.168.244.10(192.168

.244.10:)? (YES/no): yes
Tue Apr  ::  - [info] Executing FLUSH NO_WRITE_TO_BINLOG TABLES. This may take long time..

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] Checking MHA is not monitoring or doing failover..

Tue Apr  ::  - [info] Checking replication health on 192.168.244.20..

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] Checking replication health on 192.168.244.30..

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] 192.168.244.20 can be new master.

Tue Apr  ::  - [info]

From:

192.168.244.10(192.168.244.10:) (current master)

 +--192.168.244.20(192.168.244.20:)

 +--192.168.244.30(192.168.244.30:)

To:

192.168.244.20(192.168.244.20:) (new master)

 +--192.168.244.30(192.168.244.30:)

 +--192.168.244.10(192.168.244.10:)

Starting master switch from 192.168.244.10(192.168.244.10:) to 192.168.244.20(192.168.244.20:)? (yes/NO): yes

Tue Apr  ::  - [info] Checking whether 192.168.244.20(192.168.244.20:) is ok for the new master..

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] 192.168.244.10(192.168.244.10:): SHOW SLAVE STATUS returned empty result. To check replication

filtering rules, temporarily executing CHANGE MASTER to a dummy host.Tue Apr  ::  - [info] 192.168.244.10(192.168.244.10:): Resetting slave pointing to the dummy host.

Tue Apr  ::  - [info] ** Phase : Configuration Check Phase completed.

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] * Phase : Rejecting updates Phase..

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] Executing master ip online change script to disable write on the current master:

Tue Apr  ::  - [info]   /usr/local/bin/master_ip_online_change --command=stop --orig_master_host=192.168.244.10 --orig_ma

ster_ip=192.168.244.10 --orig_master_port= --orig_master_user='monitor' --orig_master_password='monitor123' --new_master_host=192.168.244.20 --new_master_ip=192.168.244.20 --new_master_port= --new_master_user='monitor' --new_master_password='monitor123' --orig_master_ssh_user=root --new_master_ssh_user=root   --orig_master_is_new_slaveTue Apr  ::   Set read_only on the new master.. ok.

Tue Apr  ::   Set read_only= on the orig master.. ok.

Tue Apr  ::   Killing all application threads..

Tue Apr  ::   done.

Disabling the VIP an old master: 192.168.244.10

SIOCSIFFLAGS: Cannot assign requested address

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] Locking all tables on the orig master to reject updates from everybody (including root):

Tue Apr  ::  - [info] Executing FLUSH TABLES WITH READ LOCK..

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] Orig master binlog:pos is mysql-bin.:.

Tue Apr  ::  - [info]  Waiting to execute all relay logs on 192.168.244.20(192.168.244.20:)..

Tue Apr  ::  - [info]  master_pos_wait(mysql-bin.:) completed on 192.168.244.20(192.168.244.20:). Executed

 events.Tue Apr  ::  - [info]   done.

Tue Apr  ::  - [info] Getting new master's binlog name and position..

Tue Apr  ::  - [info]  mysql-bin.:

Tue Apr  ::  - [info]  All other slaves should start replication from here. Statement should be: CHANGE MASTER TO MASTER_

HOST='192.168.244.20', MASTER_PORT=, MASTER_LOG_FILE='mysql-bin.000009', MASTER_LOG_POS=, MASTER_USER='repl', MASTER_PASSWORD='xxx';Tue Apr  ::  - [info] Executing master ip online change script to allow write on the new master:

Tue Apr  ::  - [info]   /usr/local/bin/master_ip_online_change --command=start --orig_master_host=192.168.244.10 --orig_m

aster_ip=192.168.244.10 --orig_master_port= --orig_master_user='monitor' --orig_master_password='monitor123' --new_master_host=192.168.244.20 --new_master_ip=192.168.244.20 --new_master_port= --new_master_user='monitor' --new_master_password='monitor123' --orig_master_ssh_user=root --new_master_ssh_user=root   --orig_master_is_new_slaveTue Apr  ::   Set read_only= on the new master.

Enabling the VIP 192.168.244.188 on the new master: 192.168.244.20

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] * Switching slaves in parallel..

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] -- Slave switch on host 192.168.244.30(192.168.244.30:) started, pid:

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] Log messages from 192.168.244.30 ...

Tue Apr  ::  - [info]

Tue Apr  ::  - [info]  Waiting to execute all relay logs on 192.168.244.30(192.168.244.30:)..

Tue Apr  ::  - [info]  master_pos_wait(mysql-bin.:) completed on 192.168.244.30(192.168.244.30:). Executed

 events.Tue Apr  ::  - [info]   done.

Tue Apr  ::  - [info]  Resetting slave 192.168.244.30(192.168.244.30:) and starting replication from the new master

92.168.244.20(192.168.244.20:)..Tue Apr  ::  - [info]  Executed CHANGE MASTER.

Tue Apr  ::  - [info]  Slave started.

Tue Apr  ::  - [info] End of log messages from 192.168.244.30 ...

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] -- Slave switch on host 192.168.244.30(192.168.244.30:) succeeded.

Tue Apr  ::  - [info] Unlocking all tables on the orig master:

Tue Apr  ::  - [info] Executing UNLOCK TABLES..

Tue Apr  ::  - [info]  ok.

Tue Apr  ::  - [info] Starting orig master as a new slave..

Tue Apr  ::  - [info]  Resetting slave 192.168.244.10(192.168.244.10:) and starting replication from the new master

92.168.244.20(192.168.244.20:)..Tue Apr  ::  - [info]  Executed CHANGE MASTER.

Tue Apr  ::  - [info]  Slave started.

Tue Apr  ::  - [info] All new slave servers switched successfully.

Tue Apr  ::  - [info]

Tue Apr  ::  - [info] * Phase : New master cleanup phase..

Tue Apr  ::  - [info]

Tue Apr  ::  - [info]  192.168.244.20: Resetting slave info succeeded.

Tue Apr  ::  - [info] Switching master to 192.168.244.20(192.168.244.20:) completed successfully.

MHA在线切换的原理

1. 检查当前的配置信息及主从服务器的信息

包括读取MHA的配置文件/etc/masterha/app1.cnf及检查当前slave的健康状态

2. 阻止对当前master的更新

主要通过如下步骤：

1> 等待1.5s（$time_until_kill_threads*100ms），等待当前连接断开。

2> 执行 read_only=1，阻止新的DML操作

3> 等待0.5s，等待当前DML操作完成。

4> kill掉所有连接。

5> FLUSH NO_WRITE_TO_BINLOG TABLES

6> FLUSH TABLES WITH READ LOCK

3. 等待新master执行完所有的relay log

Waiting to execute all relay logs on 192.168.244.20(192.168.244.20:)..

4. 将新master的read_only设置为off，并添加VIP

5. slave切换到新master上。

1> 等待slave（192.168.244.30）应用完原主从复制产生的relay log，然后执行change master操作切换到新master上。

2> 释放原master上加的锁。

3> 因masterha_master_switch命令行中带有--orig_master_is_new_slave参数，故原master也切换为新master的从。

6. 清理新master的相关信息。

主要是执行了reset slave all操作，清除之前的复制信息。

MHA在线切换需满足的条件

MHA在执行在线切换之前，会判断当前的主从复制信息，只有满足了以下条件，才能执行切换动作：

1. 所有SLAVE的IO线程和SQL线程都在运行。

2. 所有slave的Seconds_Behind_Master小于或等于running_updates_limit的值，该参数如果没有显示指定的话，则默认为1s

3. 在master上，通过show processlist输出，没有一个DML操作的时间大于running_updates_limit的值。

在线切换时，打开general log，各个服务器的操作信息

注：在执行masterha_master_switch命令时，会有两次确认操作

1. It is better to execute FLUSH NO_WRITE_TO_BINLOG TABLES on the master before switching. Is it ok to execute on 192.168.244.10(192.168
.244.10:3306)? (YES/no):

2. Starting master switch from 192.168.244.10(192.168.244.10:3306) to 192.168.244.20(192.168.244.20:3306)? (yes/NO):

以下输出中间都有两次空白，其中第一次空白之前的输出对应第一次确认之前，第二次之前的输出对应第二次确认之前。

原master 192.168.244.10

 ::     Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

 ::     Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SET wait_timeout=

                    Query    SELECT @@global.server_id As Value

                    Query    SELECT VERSION() AS Value

                    Query    SELECT @@global.gtid_mode As Value

                    Query    SHOW GLOBAL VARIABLES LIKE 'log_bin'

                    Query    SHOW MASTER STATUS

                    Query    SELECT @@global.datadir AS Value

                    Query    SELECT @@global.slave_parallel_workers AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SELECT @@global.read_only As Value

                    Query    SELECT @@global.relay_log_purge As Value

 ::     Query    FLUSH NO_WRITE_TO_BINLOG TABLES

                    Query    SELECT GET_LOCK('MHA_Master_High_Availability_Monitor', '') AS Value

                    Query    SHOW PROCESSLIST

 ::     Query    SHOW SLAVE STATUS

                    Query    CHANGE MASTER TO MASTER_HOST='dummy_host'

 ::     Query    SHOW SLAVE STATUS

                    Query    RESET SLAVE /*!50516 ALL */

                    Query    SELECT RELEASE_LOCK('MHA_Master_High_Availability_Monitor') As Value

                    Quit

                    Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SET sql_log_bin=

                    Query    SHOW PROCESSLIST

                    Query    SELECT @@global.read_only As Value

                    Query    SET GLOBAL read_only=

                    Query    SELECT @@global.read_only As Value

                    Query    SHOW PROCESSLIST

                    Query    SET sql_log_bin=

                    Quit

                    Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SET wait_timeout=

                    Query    FLUSH TABLES WITH READ LOCK

                    Query    SHOW MASTER STATUS

 ::     Query    UNLOCK TABLES

                    Query    CHANGE MASTER TO MASTER_HOST = '192.168.244.20' MASTER_USER = 'repl' MASTER_PASSWORD = <secret> MASTE

R_PORT =  MASTER_LOG_FILE = 'mysql-bin.000010' MASTER_LOG_POS =             Query    SET GLOBAL relay_log_purge=

                    Query    START SLAVE

                    Connect Out    repl@192.168.244.20:

                    Query    SHOW SLAVE STATUS

                    Query    SELECT RELEASE_LOCK('MHA_Master_High_Availability_Failover') As Value

                    Quit

新master 192.168.244.20

 ::     Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

 ::     Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SET wait_timeout=

                    Query    SELECT @@global.server_id As Value

                    Query    SELECT VERSION() AS Value

                    Query    SELECT @@global.gtid_mode As Value

                    Query    SHOW GLOBAL VARIABLES LIKE 'log_bin'

                    Query    SHOW MASTER STATUS

                    Query    SELECT @@global.datadir AS Value

                    Query    SELECT @@global.slave_parallel_workers AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SELECT @@global.read_only As Value

                    Query    SELECT @@global.relay_log_purge As Value

                    Query    SELECT @@global.relay_log_info_repository AS Value

                    Query    SELECT @@global.datadir AS Value

                    Query    SELECT @@global.relay_log_info_file AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SELECT Repl_slave_priv AS Value FROM mysql.user WHERE user = 'repl'

 ::     Query    SELECT GET_LOCK('MHA_Master_High_Availability_Failover', '') AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SHOW SLAVE STATUS

 ::     Query    SHOW PROCESSLIST

                    Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SELECT @@global.read_only As Value

                    Query    SELECT @@global.read_only As Value

                    Quit

                    Query    SHOW SLAVE STATUS

                    Query    SELECT MASTER_POS_WAIT('mysql-bin.000017','',) AS Result

                    Query    STOP SLAVE SQL_THREAD

                    Query    SHOW SLAVE STATUS

                    Query    SHOW MASTER STATUS

                    Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SET sql_log_bin=

                    Query    SELECT @@global.read_only As Value

                    Query    SET GLOBAL read_only=

                    Query    SET sql_log_bin=

                    Quit

                    Query    SELECT @@global.read_only As Value

                    Connect    repl@node3 on

                    Query    SELECT UNIX_TIMESTAMP()

                    Query    SHOW VARIABLES LIKE 'SERVER_ID'

                    Query    SET @master_heartbeat_period=

                    Query    SET @master_binlog_checksum= @@global.binlog_checksum

                    Query    SELECT @master_binlog_checksum

                    Query    SELECT @@GLOBAL.GTID_MODE

                    Query    SHOW VARIABLES LIKE 'SERVER_UUID'

                    Query    SET @slave_uuid= '8a1093c8-1d00-11e7-954f-000c299a5715'

                    Binlog Dump    Log: 'mysql-bin.000010'  Pos:

 ::     Connect    repl@node1 on

                    Query    SELECT UNIX_TIMESTAMP()

                    Query    SHOW VARIABLES LIKE 'SERVER_ID'

                    Query    SET @master_heartbeat_period=

                    Query    SET @master_binlog_checksum= @@global.binlog_checksum

                    Query    SELECT @master_binlog_checksum

                    Query    SELECT @@GLOBAL.GTID_MODE

                    Query    SHOW VARIABLES LIKE 'SERVER_UUID'

                    Query    STOP SLAVE

                    Query    SET @slave_uuid= '2a6365e0-1d05-11e7-956d-000c29c64704'

                    Binlog Dump    Log: 'mysql-bin.000010'  Pos:

                    Query    SHOW SLAVE STATUS

                    Query    RESET SLAVE /*!50516 ALL */

                    Query    SHOW SLAVE STATUS

                    Query    SELECT RELEASE_LOCK('MHA_Master_High_Availability_Failover') As Value

                    Quit

slave 192.168.244.30

 ::     Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

 ::     Connect    monitor@node4 on

                    Query    set autocommit=

                    Query    SELECT CONNECTION_ID() AS Value

                    Query    SET wait_timeout=

                    Query    SELECT @@global.server_id As Value

                    Query    SELECT VERSION() AS Value

                    Query    SELECT @@global.gtid_mode As Value

                    Query    SHOW GLOBAL VARIABLES LIKE 'log_bin'

                    Query    SHOW MASTER STATUS

                    Query    SELECT @@global.datadir AS Value

                    Query    SELECT @@global.slave_parallel_workers AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SELECT @@global.read_only As Value

                    Query    SELECT @@global.relay_log_purge As Value

                    Query    SELECT @@global.relay_log_info_repository AS Value

                    Query    SELECT @@global.datadir AS Value

                    Query    SELECT @@global.relay_log_info_file AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SELECT Repl_slave_priv AS Value FROM mysql.user WHERE user = 'repl'

 ::     Query    SELECT GET_LOCK('MHA_Master_High_Availability_Failover', '') AS Value

                    Query    SHOW SLAVE STATUS

                    Query    SHOW SLAVE STATUS

 ::     Query    SHOW SLAVE STATUS

 ::     Query    SHOW SLAVE STATUS

                    Query    SELECT MASTER_POS_WAIT('mysql-bin.000017','',) AS Result

                    Query    STOP SLAVE SQL_THREAD

                    Query    SHOW SLAVE STATUS

                    Query    STOP SLAVE

                    Query    STOP SLAVE

                    Query    SHOW SLAVE STATUS

                    Query    RESET SLAVE

                    Query    CHANGE MASTER TO MASTER_HOST = '192.168.244.20' MASTER_USER = 'repl' MASTER_PASSWORD = <secret> MASTE

R_PORT =  MASTER_LOG_FILE = 'mysql-bin.000010' MASTER_LOG_POS =             Query    SET GLOBAL relay_log_purge=

                    Query    START SLAVE

                    Connect Out    repl@192.168.244.20:

                    Query    SHOW SLAVE STATUS

 ::     Query    SELECT RELEASE_LOCK('MHA_Master_High_Availability_Failover') As Value

                    Quit

参考

《深入浅出MySQL》

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