5. MGR管理维护 | 深入浅出MGR
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今天介绍MGR集群的日常管理维护操作，包括主节点切换，单主&多主模式切换等。手工操作以及利用MySQL Shell两种方式都会分别介绍。
现在有个三节点的MGR集群：
mysql> select * from performance_schema.replication_group_members;+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| CHANNEL_NAME | MEMBER_ID | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE | MEMBER_ROLE | MEMBER_VERSION |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| group_replication_applier | af39db70-6850-11ec-94c9-00155d064000 | 172.16.16.10 | 3306 | ONLINE | PRIMARY | 8.0.25 || group_replication_applier | b05c0838-6850-11ec-a06b-00155d064000 | 172.16.16.11 | 3306 | ONLINE | SECONDARY | 8.0.25 || group_replication_applier | b0f86046-6850-11ec-92fe-00155d064000 | 172.16.16.12 | 3306 | ONLINE | SECONDARY | 8.0.25 |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+
1. 切换主节点
当主节点需要进行维护时，或者执行滚动升级时，就可以对其进行切换，将主节点切换到其他节点。
在命令行模式下，可以使用 group_replication_set_as_primary() 这个udf实现切换，例如：
-- 将Primary角色切换到第二个节点mysql> select group_replication_set_as_primary('b05c0838-6850-11ec-a06b-00155d064000');+--------------------------------------------------------------------------+| group_replication_set_as_primary('b05c0838-6850-11ec-a06b-00155d064000') |+--------------------------------------------------------------------------+| Primary server switched to: b05c0838-6850-11ec-a06b-00155d064000 |+--------------------------------------------------------------------------+1 row in set (1.00 sec)[root@yejr.run:mysql.sock] [(none)]>select * from performance_schema.replication_group_members;+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| CHANNEL_NAME | MEMBER_ID | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE | MEMBER_ROLE | MEMBER_VERSION |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| group_replication_applier | af39db70-6850-11ec-94c9-00155d064000 | 172.16.16.10 | 3306 | ONLINE | SECONDARY | 8.0.25 || group_replication_applier | b05c0838-6850-11ec-a06b-00155d064000 | 172.16.16.11 | 3306 | ONLINE | PRIMARY | 8.0.25 || group_replication_applier | b0f86046-6850-11ec-92fe-00155d064000 | 172.16.16.12 | 3306 | ONLINE | SECONDARY | 8.0.25 |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+
顺便提一下，在MySQL 5.7版本中，只能通过重启以实现主节点的自动切换，不能手动切换。从这个角度来说，如果想要使用MGR，最好是选择MySQL 8.0版本，而不要使用5.7版本。
如果是用MySQL Shell，则可以调用 setPrimaryInstance() 函数进行切换：
#首先获取cluster对象MySQL 172.16.16.10:3306 ssl JS > var c=dba.getCluster()#查看当前各节点列表MySQL 172.16.16.10:3306 ssl JS > c.status()"status": "OK","statusText": "Cluster is ONLINE and can tolerate up to ONE failure.","topology": {"172.16.16.10:3306": {"address": "172.16.16.10:3306","memberRole": "PRIMARY","mode": "R/W","readReplicas": {},"replicationLag": null,"role": "HA","status": "ONLINE","version": "8.0.25"},"172.16.16.11:3306": {"address": "172.16.16.11:3306","memberRole": "SECONDARY","mode": "R/O","readReplicas": {},"replicationLag": null,"role": "HA","status": "ONLINE","version": "8.0.25"},"172.16.16.12:3306": {"address": "172.16.16.12:3306","memberRole": "SECONDARY","mode": "R/O","readReplicas": {},"replicationLag": null,"role": "HA","status": "ONLINE","version": "8.0.25"}},"topologyMode": "Single-Primary"},"groupInformationSourceMember": "172.16.16.10:3306"}#执行切换MySQL 172.16.16.10:3306 ssl JS > c.setPrimaryInstance('172.16.16.11:3306')Setting instance '172.16.16.11:3306' as the primary instance of cluster 'MGR1'...#罗列了三个节点各自发生的变化Instance '172.16.16.10:3306' was switched from PRIMARY to SECONDARY.Instance '172.16.16.11:3306' was switched from SECONDARY to PRIMARY.Instance '172.16.16.12:3306' remains SECONDARY.WARNING: The cluster internal session is not the primary member anymore. For cluster management operations please obtain a fresh cluster handle using dba.getCluster().#完成切换The instance '172.16.16.11:3306' was successfully elected as primary.
2. 切换单主/多主模式
在命令行模式下，可以调用 group_replication_switch_to_single_primary_mode() 和 group_replication_switch_to_multi_primary_mode() 来切换单主/多主模式。
#直接调用函数即可mysql> select group_replication_switch_to_multi_primary_mode();+--------------------------------------------------+| group_replication_switch_to_multi_primary_mode() |+--------------------------------------------------+| Mode switched to multi-primary successfully. |+--------------------------------------------------+#查看各节点状态mysql> select * from performance_schema.replication_group_members;+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| CHANNEL_NAME | MEMBER_ID | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE | MEMBER_ROLE | MEMBER_VERSION |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+| group_replication_applier | af39db70-6850-11ec-94c9-00155d064000 | 172.16.16.10 | 3306 | ONLINE | PRIMARY | 8.0.25 || group_replication_applier | b05c0838-6850-11ec-a06b-00155d064000 | 172.16.16.11 | 3306 | ONLINE | PRIMARY | 8.0.25 || group_replication_applier | b0f86046-6850-11ec-92fe-00155d064000 | 172.16.16.12 | 3306 | ONLINE | PRIMARY | 8.0.25 |+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+#切换成单主模式时可以指定某个节点的 server_uuid，如果不指定则会根据规则自动选择一个新的主节点#在这里，我选择了指定mgr3节点作为新主mysql> select group_replication_switch_to_single_primary_mode('b0f86046-6850-11ec-92fe-00155d064000');+-----------------------------------------------------------------------------------------+| group_replication_switch_to_single_primary_mode('b0f86046-6850-11ec-92fe-00155d064000') |+-----------------------------------------------------------------------------------------+| Mode switched to single-primary successfully. |+-----------------------------------------------------------------------------------------+
在MySQL Shell中，可以调用 switchToSinglePrimaryMode() 以及 switchToMultiPrimaryMode() 函数进行切换。同样地，函数 switchToSinglePrimaryMode() 里也可以指定某个节点作为新的主节点。
MySQL 172.16.16.10:3306 ssl JS > var c=dba.getCluster()#切换到多主模式MySQL 172.16.16.10:3306 ssl JS > c.switchToMultiPrimaryMode()#切换到单主模式，这里我指定mgr2节点作为新主MySQL 172.16.16.10:3306 ssl JS > c.switchToSinglePrimaryMode("172.16.16.11:3306")
注意，在已经是单主模式时，无论是 group_replication_switch_to_single_primary_mode() 还是 switchToSinglePrimaryMode() 函数中指定另一个节点时是不会发生切换的，但也不会报错，只有提示。
3. 添加新节点
接下来我们演示如何向MGR集群中添加一个新节点。
首先，要先完成MySQL Server初始化，创建好MGR专用账户、设置好MGR服务通道等前置工作，这部分的操作可以参考前文 3. 安装部署MGR集群。
接下来，直接执行命令 start group_replication 启动MGR服务即可，新增的节点会进入分布式恢复这个步骤，它会从已有节点中自动选择一个作为捐献者（donor），并自行决定是直接读取binlog进行恢复，还是利用Clone进行全量恢复。
如果是已经在线运行一段时间的MGR集群，有一定存量数据，这时候新节点加入可能会比较慢，建议手动利用Clone进行一次全量复制。还记得前面创建MGR专用账户时，给加上了 BACKUP_ADMIN 授权吗，这时候就排上用场了，Clone需要用到这个权限。
下面演示如何利用Clone进行一次全量数据恢复，假定要新增的节点是 172.16.16.13 （给它命名为 mgr4）。
#在mgr4上设置捐献者#为了降低对Primary节点的影响，建议选择其他Secondary节点mysql> set global clone_valid_donor_list='172.16.16.11:3306';#停掉mgr服务（如果有的话），关闭super_read_only模式，然后开始复制数据#注意这里要填写的端口是3306（MySQL正常服务端口），而不是33061这个MGR服务专用端口mysql> stop group_replication; set global super_read_only=0; clone INSTANCE FROM GreatSQL@172.16.16.11:3306 IDENTIFIED BY 'GreatSQL';
全量复制完数据后，该节点会进行一次自动重启。重启完毕后，再次确认 group_replication_group_name、group_replication_local_address、group_replication_group_seeds 这些选项值是否正确，如果没问题，执行 start group_replication 后，该节点应该就可以正常加入集群了。
如果是用MySQL Shell添加新节点则更简单。先执行MySQL Server初始化，并执行 dba.dba.configureInstance() 创建MGR专用账号后。而后，连接到Primary节点，直接调用 addInstance() 函数即可：
#连接到Primary节点$ mysqlsh --uri GreatSQL@172.16.16.10:3306MySQL 172.16.16.10:3306 ssl JS > var c=dba.getCluster()MySQL 172.16.16.10:3306 ssl JS > c.addInstance('GreatSQL@172.16.16.13:3306')WARNING: A GTID set check of the MySQL instance at '172.16.16.13:3306' determined that it contains transactions that do not originate from the cluster, which must be discarded before it can join the cluster....NOTE: 172.16.16.13:3306 is being cloned from 172.16.16.10:3306 #<--自动选择一个donor节点** Stage DROP DATA: Completed** Clone TransferFILE COPY ############################################################ 100% CompletedPAGE COPY ############################################################ 100% CompletedREDO COPY ############################################################ 100% CompletedNOTE: 172.16.16.13:3306 is shutting down...* Waiting for server restart... ready* 172.16.16.13:3306 has restarted, waiting for clone to finish...** Stage RESTART: Completed* Clone process has finished: 72.43 MB transferred in about 1 second (~72.43 MB/s)Incremental state recovery is now in progress.* Waiting for distributed recovery to finish...NOTE: '172.16.16.13:3306' is being recovered from '172.16.16.12:3306'* Distributed recovery has finished#新节点成功加入完毕The instance '172.16.16.13:3306' was successfully added to the cluster.#确认添加成功，已在MGR集群列表中MySQL 172.16.16.10:3306 ssl JS > c.describe(){"clusterName": "MGR1","defaultReplicaSet": {"name": "default","topology": [{"address": "172.16.16.10:3306","label": "172.16.16.10:3306","role": "HA"},{"address": "172.16.16.11:3306","label": "172.16.16.11:3306","role": "HA"},{"address": "172.16.16.12:3306","label": "172.16.16.12:3306","role": "HA"},{"address": "172.16.16.13:3306", <--新加入的节点"label": "172.16.16.13:3306","role": "HA"}],"topologyMode": "Single-Primary"}}
确认新节点添加成功。
4. 删除节点
在命令行模式下，一个节点想退出MGR集群，直接执行 stop group_replication 即可，如果这个节点只是临时退出集群，后面还想加回集群，则执行 start group_replication 即可自动再加入。而如果是想彻底退出集群，则停止MGR服务后，执行 reset master; reset slave all; 重置所有复制（包含MGR）相关的信息就可以了。
在MySQL Shell里，只需调用 removeInstance() 函数即可删除某个节点，例如：
MySQL 172.16.16.10:3306 ssl JS > c.removeInstance('172.16.16.13:3306');The instance will be removed from the InnoDB cluster. Depending on the instancebeing the Seed or not, the Metadata session might become invalid. If so, pleasestart a new session to the Metadata Storage R/W instance.Instance '172.16.16.13:3306' is attempting to leave the cluster...The instance '172.16.16.13:3306' was successfully removed from the cluster.
这就将该节点踢出集群了，并且会重置 group_replication_group_seeds 和 group_replication_local_address 两个选项值。之后该节点如果想再加入集群，需要调用 addInstance() 重新加回。
5. 异常退出的节点重新加回
当节点因为网络断开、实例crash等异常情况与MGR集群断开连接后，这个节点的状态会变成 UNREACHABLE，待到超过 group_replication_member_expel_timeout + 5 秒后，集群会踢掉该节点。等到这个节点再次启动并执行 start group_replication，正常情况下，该节点应能自动重新加回集群。
在MySQL Shell里，可以调用 rejoinInstance() 函数将异常的节点重新加回集群：
MySQL 172.16.16.10:3306 ssl JS > c.rejoinInstance('172.16.16.13:3306');Rejoining instance '172.16.16.13:3306' to cluster 'MGR1'...The instance '172.16.16.13:3306' was successfully rejoined to the cluster.
6. 重启MGR集群
正常情况下，MGR集群中的Primary节点退出时，剩下的节点会自动选出新的Primary节点。当最后一个节点也退出时，相当于整个MGR集群都关闭了。这时候任何一个节点启动MGR服务后，都不会自动成为Primary节点，需要在启动MGR服务前，先设置 group_replication_bootstrap_group=ON，使其成为引导节点，再启动MGR服务，它才会成为Primary节点，后续启动的其他节点也才能正常加入集群。可自行测试，这里不再做演示。
P.S，第一个节点启动完毕后，记得重置选项 group_replication_bootstrap_group=OFF，避免在后续的操作中导致MGR集群分裂。
如果是用MySQL Shell重启MGR集群，调用 rebootClusterFromCompleteOutage() 函数即可，它会自动判断各节点的状态，选择其中一个作为Primary节点，然后拉起各节点上的MGR服务，完成MGR集群重启。可以参考这篇文章：万答#12，MGR整个集群挂掉后，如何才能自动选主，不用手动干预
7. 小结
本文介绍了MGR集群几种常见管理维护操作方法，包括切换主节点，切换单主/多主模式，添加节点，删除节点，异常节点重加入，重启整个MGR集群等。总的来看，利用MySQL Shell管理MGR集群会更简单方便些，也有利于管理平台的封装，不过手工操作的方式也不能忘记，有些时候可能没有配套的MySQL Shell工具，就得靠手工了。
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