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

[root@GreatSQL ~]# cat /usr/lib/systemd/system/greatsql.service
...
Restart=on-failure
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[root@GreatSQL ~]# mysqlsh --uri greatsql@yejr-mgr3:3306
...
-- 不管干啥，都先看 help，这是玩转Linux的必备素养，啥事不清楚都先找男人（man）
-- 注意到有这样的一个方法 rebootClusterFromCompleteOutage()，看起来没跑了



MySQL  yejr-mgr3:3306 ssl  JS > \help dba
      rebootClusterFromCompleteOutage([clusterName][, options])
            Brings a cluster back ONLINE when all members are OFFLINE.

-- 跑一个试试看
MySQL  yejr-mgr3:3306 ssl  JS > dba.rebootClusterFromCompleteOutage()
Restoring the default cluster from complete outage...

The instance 'yejr-mgr4:3306' was part of the cluster configuration.
Would you like to rejoin it to the cluster? [y/N]: y

The instance 'yejr-mgr2:3306' was part of the cluster configuration.
Would you like to rejoin it to the cluster? [y/N]: y

Dba.rebootClusterFromCompleteOutage: The active session instance (yejr-mgr3:3306) isn't the most updated in comparison with the ONLINE instances of the Cluster's metadata. Please use the most up to date instance: 'yejr-mgr4:3306'. (RuntimeError)
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[root@GreatSQL ~]# mysqlsh --uri greatsql@yejr-mgr3:3306
...
MySQL  yejr-mgr4:3306 ssl  JS > dba.rebootClusterFromCompleteOutage()
Restoring the default cluster from complete outage...

The instance 'yejr-mgr3:3306' was part of the cluster configuration.
Would you like to rejoin it to the cluster? [y/N]: y

The instance 'yejr-mgr2:3306' was part of the cluster configuration.
Would you like to rejoin it to the cluster? [y/N]: y

yejr-mgr4:3306 was restored.
Rejoining 'yejr-mgr3:3306' to the cluster.
Rejoining instance 'yejr-mgr3:3306' to cluster 'GreatSQLMGR'...
The instance 'yejr-mgr3:3306' was successfully rejoined to the cluster.

Rejoining 'yejr-mgr2:3306' to the cluster.
Rejoining instance 'yejr-mgr2:3306' to cluster 'GreatSQLMGR'...
The instance 'yejr-mgr2:3306' was successfully rejoined to the cluster.

The cluster was successfully rebooted.
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-- yejr-mgr2节点
root@GreatSQL [none]> select RECEIVED_TRANSACTION_SET from performance_schema.replication_connection_status where 
channel_name = 'group_replication_applier' union all 
select variable_value from performance_schema.global_variables where 
variable_name = 'gtid_executed'\G
*************************** 1. row ***************************
RECEIVED_TRANSACTION_SET:
*************************** 2. row ***************************
RECEIVED_TRANSACTION_SET: 1c293e90-3bdc-11ec-bca1-525400e2078a:1-4537605,
4b7b3b88-3b13-11ec-86e9-525400e2078a:1

-- yejr-mgr3节点
...
*************************** 1. row ***************************
RECEIVED_TRANSACTION_SET:
*************************** 2. row ***************************
RECEIVED_TRANSACTION_SET: 1c293e90-3bdc-11ec-bca1-525400e2078a:1-4542304,
4b7b3b88-3b13-11ec-86e9-525400e2078a:1

-- yejr-mgr4节点
...
*************************** 1. row ***************************
RECEIVED_TRANSACTION_SET:
*************************** 2. row ***************************
RECEIVED_TRANSACTION_SET: 1c293e90-3bdc-11ec-bca1-525400e2078a:1-4652391,
4b7b3b88-3b13-11ec-86e9-525400e2078a:1
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[root@GreatSQL ~]# mysql -hyejr-mgr4 -P3306 -ugreatsql -p
...
greatsql@mgr4:3306 [(none)]>set global group_replication_bootstrap_group=ON;

greatsql@mgr4:3306 [(none)]>start group_replication;

-- 启动完MGR后，记得立即将其设置为OFF
greatsql@mgr4:3306 [(none)]>set global group_replication_bootstrap_group=OFF;
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