单副本模式运行

如果DN集群中只有单个节点存活，将无法完成paxos的选举，便无法对外提供服务，此时需要以单副本模式运行。登录存活DN所在的容器，执行ps -ef | grep mysql获取mysqld启动命令及进程号，

[root@polardbx-demo /]# ps -ef | grep mysql
root         172       1  0 Apr15 ?        00:00:00 /bin/sh /opt/galaxy_engine/bin/mysqld_safe --defaults-file=/data/mysql/conf/my.cnf --loose-pod-name=pxd_test-dn-0-Cand-0-17777
mysql       5087     172  1 11:43 ?        00:03:11 /opt/galaxy_engine/bin/mysqld --defaults-file=/data/mysql/conf/my.cnf --basedir=/opt/galaxy_engine --datadir=/data/mysql/data --plugin-dir=/opt/galaxy_engine/lib/plugin --user=mysql --loose-pod-name=pxd_test-dn-0-Cand-0-17777 --log-error=/data/mysql/log/alert.log --open-files-limit=65535 --pid-file=/data/mysql/run/mysql.pid --socket=/data/mysql/run/mysql.sock --port=17777
root        5329    5309  0 16:17 pts/1    00:00:00 grep --color=auto mysql

根据defaults-file指向的路径，修改配置文件，在[mysqld]这个section中添加如下配置：

[mysqld]
cluster-force-single-mode = 1

之后将原mysqld进程终止，等待mysqld_safe将其重新拉起：

[root@polardbx-demo /]# kill 5087

重新拉起后可以执行myc命令，登录DN的数据库，查看DN集群状态是否正常：

[root@polardbx-demo /]# myc
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 130
Server version: 8.0.30 Source distribution

Copyright (c) 2000, 2019, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select * from information_schema.alisql_cluster_global;
+-----------+---------------------+-------------+------------+--------+-----------+------------+-----------------+----------------+---------------+------------+--------------+
| SERVER_ID | IP_PORT             | MATCH_INDEX | NEXT_INDEX | ROLE   | HAS_VOTED | FORCE_SYNC | ELECTION_WEIGHT | LEARNER_SOURCE | APPLIED_INDEX | PIPELINING | SEND_APPLIED |
+-----------+---------------------+-------------+------------+--------+-----------+------------+-----------------+----------------+---------------+------------+--------------+
|         1 | 172.16.201.11:25777 |         124 |          0 | Leader | Yes       | No         |               5 |              0 |           123 | No         | No           |
+-----------+---------------------+-------------+------------+--------+-----------+------------+-----------------+----------------+---------------+------------+--------------+
1 row in set (0.00 sec)

此时集群中只有该节点存活了，单节点拉起成功，可以进一步验证数据库访问是否正常。