§ 构建读写分离集群
本文介绍如何在MGR集群前端部署MySQL Router以实现读写分离、读负载均衡，以及故障自动转移。
MySQL Router是一个轻量级的中间件，它采用多端口的方案实现读写分离以及读负载均衡，而且同时支持mysql和mysql x协议。
建议把MySQL Router部署在应用服务器上，每个应用服务器都部署一套，这样应用程序可以直接连接本机IP，连接的效率更高，而且后端数据库发生变化时，程序端也无需修改IP配置。
§ 部署MySQL Router
MySQL Router第一次启动时要先初始化
参数解释参数 ：
--bootstrap 表示开始初始化参数
GreatSQL@172.16.16.10:3306 是MGR服务专用账号
--user=mysqlrouter 是运行mysqlrouter进程的系统用户名
$ mysqlrouter --bootstrap GreatSQL@172.16.16.10:3306 --user=mysqlrouter
Please enter MySQL password for GreatSQL: <-- 输入密码
# 然后mysqlrouter开始自动进行初始化
# 它会自动读取MGR的元数据信息，自动生成配置文件
Please enter MySQL password for GreatSQL:
# Bootstrapping system MySQL Router instance...
- Creating account(s) (only those that are needed, if any)
- Using existing certificates from the '/var/lib/mysqlrouter' directory
- Verifying account (using it to run SQL queries that would be run by Router)
- Storing account in keyring
- Adjusting permissions of generated files
- Creating configuration /etc/mysqlrouter/mysqlrouter.conf
# MySQL Router configured for the InnoDB Cluster 'MGR1'
After this MySQL Router has been started with the generated configuration
$ /etc/init.d/mysqlrouter restart
or
$ systemctl start mysqlrouter
or
$ mysqlrouter -c /etc/mysqlrouter/mysqlrouter.conf
the cluster 'MGR1' can be reached by connecting to:
## MySQL Classic protocol <-- MySQL协议的两个端口
- Read/Write Connections: localhost:6446
- Read/Only Connections: localhost:6447
## MySQL X protocol <-- MySQL X协议的两个端口
- Read/Write Connections: localhost:6448
- Read/Only Connections: localhost:6449
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如果想自定义名字和目录，还可以在初始化时自行指定 --name 和 --directory 选项，这样可以实现在同一个服务器上部署多个Router实例，参考这篇文章：MySQL Router可以在同一个系统环境下跑多实例吗 (opens new window)
§ 启动mysqlrouter服务
这就初始化完毕了，按照上面的提示，直接启动 mysqlrouter 服务即可：
$ systemctl start mysqlrouter
$ ps -ef | grep -v grep | grep mysqlrouter
mysqlro+ 6026 1 5 09:28 ? 00:00:00 /usr/bin/mysqlrouter
$ netstat -lntp | grep mysqlrouter
tcp 0 0 0.0.0.0:6446 0.0.0.0:* LISTEN 6026/mysqlrouter
tcp 0 0 0.0.0.0:6447 0.0.0.0:* LISTEN 6026/mysqlrouter
tcp 0 0 0.0.0.0:6448 0.0.0.0:* LISTEN 6026/mysqlrouter
tcp 0 0 0.0.0.0:6449 0.0.0.0:* LISTEN 6026/mysqlrouter
tcp 0 0 0.0.0.0:8443 0.0.0.0:* LISTEN 6026/mysqlrouter
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可以看到 mysqlrouter 服务正常启动了。
mysqlrouter 初始化时自动生成的配置文件是 /etc/mysqlrouter/mysqlrouter.conf，主要是关于R/W、RO不同端口的配置，例如：
[routing:greatsqlMGR_rw]
bind_address=0.0.0.0
bind_port=6446
destinations=metadata-cache://greatsqlMGR/?role=PRIMARY
routing_strategy=first-available
protocol=classic
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可以根据需要自行修改绑定的IP地址和端口，也可以在初始化时指定 --conf-base-port 选项自定义初始端口号。
§ 确认读写分离效果
现在，用客户端连接到6446（读写）端口，确认连接的是PRIMARY节点：
-- 连接登入GreatSQL
-- mysql -h172.16.16.10 -u GreatSQL -p -P6446
-- 记住下面几个 MEMBER_ID
greatsql> SELECT MEMBER_ID,MEMBER_ROLE FROM performance_schema.replication_group_members;
+--------------------------------------+-------------+
| MEMBER_ID | MEMBER_ROLE |
+--------------------------------------+-------------+
| 4ebd3504-11d9-11ec-8f92-70b5e873a570 | PRIMARY |
| 549b92bf-11d9-11ec-88e1-70b5e873a570 | SECONDARY |
| 5596116c-11d9-11ec-8624-70b5e873a570 | SECONDARY |
+--------------------------------------+-------------+
greatsql> SELECT @@server_uuid;
+--------------------------------------+
| @@server_uuid |
+--------------------------------------+
| 4ebd3504-11d9-11ec-8f92-70b5e873a570 |
+--------------------------------------+
# 确实是连接的PRIMARY节点
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同样地，连接6447（只读）端口，确认连接的是SECONDARY节点：
-- 连接登入GreatSQL
-- mysql -h172.16.16.10 -u GreatSQL -p -P6447
greatsql> SELECT @@server_uuid;
+--------------------------------------+
| @@server_uuid |
+--------------------------------------+
| 549b92bf-11d9-11ec-88e1-70b5e873a570 |
+--------------------------------------+
# 确实是连接的SECONDARY节点
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§ 确认只读负载均衡效果
MySQL Router连接读写节点（Primary节点）默认的策略是 first-available，即只连接第一个可用的节点。Router连接只读节点（Secondary节点）默认的策略是 round-robin-with-fallback，会在各个只读节点间轮询。
保持6447端口原有的连接不退出，继续新建到6447端口的连接，查看 server_uuid，这时应该会发现读取到的是其他只读节点的值，因为 mysqlrouter 的读负载均衡机制是在几个只读节点间自动轮询。在默认的 round-robin-with-fallback 策略下，只有当所有只读节点都不可用时，只读请求才会打到PRIMARY节点上。
关于Router的连接策略，可以参考 FAQ文档中的：24. MySQL Router可以配置在MGR主从节点间轮询吗 (opens new window)，或者MySQL Router官方文档：routing_strategy参数/选项 (opens new window)
§ 确认故障自动转移功能
接下来模拟PRIMARY节点宕机或切换时，mysqlrouter 也能实现自动故障转移。
登入MGR集群任意节点：
$ mysqlsh --uri GreatSQL@172.16.16.10:3306
...
MySQL 172.16.16.10:3306 ssl JS > var c=dba.getCluster();
MySQL 172.16.16.10:3306 ssl JS > c.setPrimaryInstance('172.16.16.11:3306'); <-- 切换PRIMARY节点
Setting instance '172.16.16.11:3306' as the primary instance of cluster 'MGR1'...
Instance '172.16.16.10:3306' was switched from PRIMARY to SECONDARY. <-- 切换了，从PRIMARY到SECONDARY
Instance '172.16.16.11:3306' was switched from SECONDARY to PRIMARY. <-- 切换了，从SECONDARY到PRIMARY
Instance '172.16.16.12:3306' remains SECONDARY. <-- 保持不变
WARNING: The cluster internal session is not the primary member anymore. For cluster management operations please obtain a fresh cluster handle using dba.getCluster().
The instance '172.16.16.11:3306' was successfully elected as primary.
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回到前面连接6446端口的那个会话，再次查询 server_uuid，此时会发现连接自动断开了：
greatsql> SELECT @@server_uuid;
ERROR 2013 (HY000): Lost connection to MySQL server during query
greatsql> SELECT @@server_uuid;
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 157990
Current database: *** NONE ***
+--------------------------------------+
| @@server_uuid |
+--------------------------------------+
| 549b92bf-11d9-11ec-88e1-70b5e873a570 | <-- 确认server_uuid变成新的
+--------------------------------------+
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这就实现了自动故障转移。
至此，利用MySQL Router配合GreatSQL构建一套支持读写分离、读负载均衡以及故障自动转移的MGR集群就部署完毕了。
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