1. 背景
MySQL Group Replication（MGR）是MySQL官方在5.7.17版本引进的一个数据库高可用与高扩展的解决方案，以插件形式提供，实现了分布式下数据的最终一致性，总结MGR特点如下：
- 高一致性：基于分布式paxos协议实现组复制，保证数据一致性；
- 高容错性：自动检测机制，只要不是大多数节点都宕机就可以继续工作，内置防脑裂保护机制；
- 高扩展性：节点的增加与移除会自动更新组成员信息，新节点加入后，自动从其他节点同步增量数据，直到与其他节点数据一致；
- 高灵活性：提供单主模式和多主模式，单主模式在主库宕机后能够自动选主，所有写入都在主节点进行，多主模式支持多节点写入。
2. 环境：
3台服务器搭建3节点MGR集群，MySQL版本8.0.11，操作系统版本CentOS 7.3。
- 192.168.56.101
- 192.168.56.102
- 192.168.56.103
3. 安装步骤
3.1 安装MySQL8.0
MySQL官方版8.0.11，采用Linux Generic安装方式，直接下载压缩包，解压到目录/usr/local/mysql8.0。
3.2 修改配置文件
编辑配置文件 /home/mysql/etc/my_mgr_8.0.cnf，3个节点除了server_id、loose-group_replication_local_address、report_host 三个参数不一样外，其他保持一致。
# /home/mysql/etc/my_mgr_8.0.cnf
[mysqld]
port=3306
basedir=/usr/local/mysql8.0
datadir=/mysql/data_mgr_8.0/
socket=/mysql/data_mgr_8.0/mysql.sock
pid_file=/mysql/data_mgr_8.0/mysql.pid
server_id=1
gtid_mode=ON
enforce_gtid_consistency=ON
binlog_checksum=NONE
log_bin=binlog
log_slave_updates=ON
binlog_format=ROW
master_info_repository=TABLE
relay_log_info_repository=TABLE
transaction_write_set_extraction=XXHASH64
loose-group_replication_group_name="aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"
loose-group_replication_start_on_boot=OFF
loose-group_replication_local_address= "192.168.56.101:33061"
loose-group_replication_group_seeds= "192.168.56.101:33061,192.168.56.102:33061,192.168.56.103:33061"
loose-group_replication_bootstrap_group=OFF
report_host=192.168.56.101
report_port=3306
3.3 初始化数据库（所有节点执行）
useradd mysql
mkdir -p /mysql/data_mgr_8.0
sudo chown -R mysql.mysql /mysql/data_mgr_8.0/
sudo chown -R mysql.mysql /home/mysql/etc/
/usr/local/mysql8.0/bin/mysqld --initialize-insecure --basedir=/usr/local/mysql8.0 --datadir=/mysql/data_mgr_8.0 --user=mysql
3.4 启动数据库，安装MGR插件，设置复制账号（所有节点执行）
# 启动数据库
su - mysql -c "/usr/local/mysql8.0/bin/mysqld_safe --defaults-file=/home/mysql/etc/my_mgr_8.0.cnf &"
# 登录数据库
/usr/local/mysql8.0/bin/mysql -S /mysql/data_mgr_8.0/mysql.sock
# 安装MGR插件
mysql>INSTALL PLUGIN group_replication SONAME 'group_replication.so';
#设置复制账号
mysql> SET SQL_LOG_BIN=0;
mysql> CREATE USER repl@'%' IDENTIFIED BY 'repl';
mysql> GRANT REPLICATION SLAVE ON *.* TO repl@'%';
mysql> FLUSH PRIVILEGES;
mysql> SET SQL_LOG_BIN=1;
mysql> CHANGE MASTER TO MASTER_USER='repl', MASTER_PASSWORD='repl' FOR CHANNEL 'group_replication_recovery';
3.5 启动MGR单主模式
# 启动MGR，在主库(192.168.56.101)上执行
mysql> SET GLOBAL group_replication_bootstrap_group=ON;
mysql> START GROUP_REPLICATION;
mysql> SET GLOBAL group_replication_bootstrap_group=OFF;
# 查看MGR组信息
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 | 8cb3f19b-8414-11e8-9d34-fa163eda7360 | 192.168.56.101 | 3306 | ONLINE | PRIMARY | 8.0.11 |
+---------------------------+--------------------------------------+------------------------+-------------+--------------+-------------+----------------+
1 row in set (0.01 sec)
# 其他节点加入MGR，在从库（192.168.56.102，192.168.56.103）上执行
mysql> START GROUP_REPLICATION;
# 查看MGR组信息
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 | 5254cf46-8415-11e8-af09-fa163eab3dcf | 192.168.56.102 | 3306 | ONLINE | SECONDARY | 8.0.11 |
| group_replication_applier | 601a4025-8415-11e8-b2b6-fa163e767b9a | 192.168.56.103 | 3306 | ONLINE | SECONDARY | 8.0.11 |
| group_replication_applier | 8cb3f19b-8414-11e8-9d34-fa163eda7360 | 192.168.56.101 | 3306 | ONLINE | PRIMARY | 8.0.11 |
+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+
3 rows in set (0.00 sec)
可以看到，3个节点状态为online，并且主节点为192.168.56.101，只有主节点可以写入，其他节点只读，MGR单主模式搭建成功。
3.6 切换到多主模式
MGR切换模式需要重新启动组复制，因些需要在所有节点上先关闭组复制，设置 group_replication_single_primary_mode=OFF 等参数，再启动组复制。
# 停止组复制(所有节点执行)：
mysql> stop group_replication;
mysql> set global group_replication_single_primary_mode=OFF;
mysql> set global group_replication_enforce_update_everywhere_checks=ON;
# 随便选择某个节点执行
mysql> SET GLOBAL group_replication_bootstrap_group=ON;
mysql> START GROUP_REPLICATION;
mysql> SET GLOBAL group_replication_bootstrap_group=OFF;
# 其他节点执行
mysql> START GROUP_REPLICATION;
# 查看组信息，所有节点的 MEMBER_ROLE 都为 PRIMARY
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 | 5254cf46-8415-11e8-af09-fa163eab3dcf | 192.168.56.102 | 3306 | ONLINE | PRIMARY | 8.0.11 |
| group_replication_applier | 601a4025-8415-11e8-b2b6-fa163e767b9a | 192.168.56.103 | 3306 | ONLINE | PRIMARY | 8.0.11 |
| group_replication_applier | 8cb3f19b-8414-11e8-9d34-fa163eda7360 | 192.168.56.101 | 3306 | ONLINE | PRIMARY | 8.0.11 |
+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+
3 rows in set (0.00 sec)
可以看到所有节点状态都是online，角色都是PRIMARY，MGR多主模式搭建成功。
3.7 切回单主模式
# 所有节点执行
mysql> stop group_replication;
mysql> set global group_replication_enforce_update_everywhere_checks=OFF;
mysql> set global group_replication_single_primary_mode=ON;
# 主节点（192.168.56.101）执行
SET GLOBAL group_replication_bootstrap_group=ON;
START GROUP_REPLICATION;
SET GLOBAL group_replication_bootstrap_group=OFF;
# 从节点（192.168.56.102、192.168.56.103）执行
START GROUP_REPLICATION;
# 查看MGR组信息
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 | 5254cf46-8415-11e8-af09-fa163eab3dcf | 192.168.56.102 | 3306 | ONLINE | SECONDARY | 8.0.11 |
| group_replication_applier | 601a4025-8415-11e8-b2b6-fa163e767b9a | 192.168.56.103 | 3306 | ONLINE | SECONDARY | 8.0.11 |
| group_replication_applier | 8cb3f19b-8414-11e8-9d34-fa163eda7360 | 192.168.56.101 | 3306 | ONLINE | PRIMARY | 8.0.11 |
+---------------------------+--------------------------------------+--------------+-------------+--------------+-------------+----------------+
3 rows in set (0.00 sec)
本文简单介绍了MySQL8.0 MGR 单主模式、多主模式的搭建方法，关于MySQL MGR更多内容，后面会持续更新，如有问题，欢迎留言讨论。
