转载自：https://www.jianshu.com/p/ca1b78b5d615
可以在mysql_servers表和mysql_replication_hostgroups表(可选)中配置后端的MySQL Servers。
注意：在阅读下面内容之前，请确定你已经理解了ProxySQL中的多层次配置系统，不理解的返回去看手册第二节。
特别是：
修改mysql_servers表和mysql_replication_hostgroups表后，在没有运行LOAD MYSQL SERVERS TO RUNTIME时是不会生效的。
修改mysql_servers表和mysql_replication_hostgroups表后，在没有运行SAVE MYSQL SERVERS TO DISK时，ProxySQL的重启或崩溃都会丢失这些修改。
也就是说，修改mysql_servers和mysql_replication_hostgroups表后，需要运行LOAD MYSQL SERVERS TO RUNTIME才能让这些修改生效，需要运行SAVE MYSQL SERVERS TO DISK才能持久化这些修改。
重要：下面的所有例子中，都需要执行LOAD MYSQL SERVERS TO RUNTIME才生效，都需要执行SAVE MYSQL SERVERS TO DISK才永久保存。
将MySQL Servers从内存数据库拷贝到runtime
例如，要将MySQL Servers从内存数据库拷贝到runtime数据结构：
Admin> LOAD MYSQL SERVERS TO RUNTIME;
等价写法：
LOAD MYSQL SERVERS TO RUN
LOAD MYSQL SERVERS FROM MEM
LOAD MYSQL SERVERS FROM MEMORY
将MySQL Servers从内存数据库持久化到磁盘数据库
例如，要将MySQL Servers从内存数据库持久化到磁盘数据库：
Admin> SAVE MYSQL SERVERS TO DISK;
等价写法：
SAVE MYSQL SERVERS FROM MEM
SAVE MYSQL SERVERS FROM MEMORY
将MySQL Servers从runtime数据结构拷贝到内存数据库
例如，将MySQL Servers从runtime数据结构拷贝到内存数据库：
Admin> SAVE MYSQL SERVERS TO MEMORY;
等价写法：
SAVE MYSQL SERVERS TO MEM
SAVE MYSQL SERVERS FROM RUN
SAVE MYSQL SERVERS FROM RUNTIME
将MySQL Servers从磁盘数据结构拷贝到内存数据库
例如，要将MySQL Servers从磁盘数据结构拷贝到内存数据库：
Admin> LOAD MYSQL SERVERS TO MEMORY;
等价写法：
LOAD MYSQL SERVERS TO MEM
LOAD MYSQL SERVERS FROM DISK
添加一个新的后端MySQL节点
向mysql_servers表中插入一个新行，就表示添加一个新的后端MySQL节点。
需要注意，这个表中有几个字段带有默认值属性。
例如，下面完全使用默认配置来添加一个后端节点：
Admin> SELECT * FROM mysql_servers;
Empty set (0.00 sec)
Admin> INSERT INTO mysql_servers (hostname) VALUES ('172.16.0.1');
Query OK, 1 row affected (0.00 sec)
Admin> SELECT * FROM mysql_servers\G
*************************** 1. row ***************************
hostgroup_id: 0
hostname: 172.16.0.1
port: 3306
status: ONLINE
weight: 1
compression: 0
max_connections: 1000
max_replication_lag: 0
use_ssl: 0
max_latency_ms: 0
comment:
1 row in set (0.00 sec)
(译注：可见，如果不需要将后端节点分组，添加时只需指定一个hostname字段即可。同时也发现，不指定主机组的后端，默认都属于id=0的主机组)
添加新的后端到主机组中
如下面的例子，将新增的主机添加到1号组中：
Admin> SELECT hostgroup_id,hostname FROM mysql_servers;
+--------------+------------+
| hostgroup_id | hostname |
+--------------+------------+
| 0 | 172.16.0.1 |
+--------------+------------+
1 row in set (0.00 sec)
Admin> INSERT INTO mysql_servers (hostgroup_id, hostname) VALUES (1, '172.16.0.2'), (1,'172.16.0.3');
Query OK, 2 rows affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname FROM mysql_servers;
+--------------+------------+
| hostgroup_id | hostname |
+--------------+------------+
| 0 | 172.16.0.1 |
| 1 | 172.16.0.2 |
| 1 | 172.16.0.3 |
+--------------+------------+
3 rows in set (0.00 sec)
限制某后端节点的连接数量
Admin> SELECT hostgroup_id,hostname,max_connections FROM mysql_servers;
+--------------+------------+-----------------+
| hostgroup_id | hostname | max_connections |
+--------------+------------+-----------------+
| 0 | 172.16.0.1 | 1000 |
| 1 | 172.16.0.2 | 1000 |
| 1 | 172.16.0.3 | 1000 |
+--------------+------------+-----------------+
3 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET max_connections=10 WHERE hostname='172.16.0.2';
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,max_connections FROM mysql_servers;
+--------------+------------+-----------------+
| hostgroup_id | hostname | max_connections |
+--------------+------------+-----------------+
| 0 | 172.16.0.1 | 1000 |
| 1 | 172.16.0.2 | 10 |
| 1 | 172.16.0.3 | 1000 |
+--------------+------------+-----------------+
3 rows in set (0.00 sec)
通过修改权重来确定流量的优先级
权重属性只在主机组内部才有效。
Admin> SELECT hostgroup_id,hostname,weight FROM mysql_servers;
+--------------+------------+--------+
| hostgroup_id | hostname | weight |
+--------------+------------+--------+
| 0 | 172.16.0.1 | 1 |
| 1 | 172.16.0.2 | 1 |
| 1 | 172.16.0.3 | 1 |
+--------------+------------+--------+
3 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET weight=1000 WHERE hostname NOT IN ('172.16.0.2', '172.16.0.1') AND hostgroup_id=1;
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,weight FROM mysql_servers;
+--------------+------------+--------+
| hostgroup_id | hostname | weight |
+--------------+------------+--------+
| 0 | 172.16.0.1 | 1 |
| 1 | 172.16.0.2 | 1 |
| 1 | 172.16.0.3 | 1000 |
+--------------+------------+--------+
3 rows in set (0.00 sec)
和某个后端指定使用SSL连接
下面的示例演示了如何配置和后端建立SSL连接。此处没有说明如何配置全局的SSL，全局SSL配置可参见https://github.com/malongshuai/proxysql/wiki/SSL-Support。
Admin> SELECT hostgroup_id,hostname,use_ssl FROM mysql_servers;
+--------------+------------+---------+
| hostgroup_id | hostname | use_ssl |
+--------------+------------+---------+
| 0 | 172.16.0.1 | 1 |
| 1 | 172.16.0.2 | 0 |
| 1 | 172.16.0.3 | 0 |
+--------------+------------+---------+
3 rows in set (0.00 sec)
自动避开拖后腿的slave节点
如果设置了某个节点的max_replication_lag为非0值，则Monitor模块会对该节点是否拖后腿(replication lag)做常规检查。如果该字段设置为0，则Monitor模块不会做replication lag的检查。
Admin> SELECT hostgroup_id,hostname,max_replication_lag FROM mysql_servers;
+--------------+------------+---------------------+
| hostgroup_id | hostname | max_replication_lag |
+--------------+------------+---------------------+
| 0 | 172.16.0.1 | 0 |
| 1 | 172.16.0.2 | 0 |
| 1 | 172.16.0.3 | 0 |
+--------------+------------+---------------------+
3 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET max_replication_lag=30 WHERE hostname='172.16.0.3';
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,max_replication_lag FROM mysql_servers;
+--------------+------------+---------------------+
| hostgroup_id | hostname | max_replication_lag |
+--------------+------------+---------------------+
| 0 | 172.16.0.1 | 0 |
| 1 | 172.16.0.2 | 0 |
| 1 | 172.16.0.3 | 30 |
+--------------+------------+---------------------+
3 rows in set (0.00 sec)
上面的配置中，如果 172.16.0.3 相比master的数据延迟了30秒，则ProxySQL会暂时自动避开(忽略)该slave节点。
注意，如果将该字段设置为0，那么后端slave节点永远不会被自动避开，即使这个后端节点的复制线程工作不正常(译注：例如，SQL线程未开启，IO线程未开启，未连接上master等)，详见：mysql-monitor_slave_lag_when_null。
将一个后端节点添加到不同主机组中
mysql_servers表的主键字段定义方式为PRIMARY KEY (hostgroup_id,hostname,port)。这意味着同一个后端节点可以加入不同主机组中。
至于为什么要让一个节点加入多个组，有多种原因。例如，在一个复制拓扑结构中，有1个master节点，2个slave节点，你希望在slave故障时(例如它们宕机、或者拖后腿而被ProxySQL避开等)可以向master节点发送读请求。
(译注：为了让读尽量路由到slave上，可以将那个跨多组的节点(可能是master)在组中的权重设置的足够小)
一个示例：
Admin> SELECT hostgroup_id,hostname,weight,max_replication_lag FROM mysql_servers;
+--------------+------------+--------+---------------------+
| hostgroup_id | hostname | weight | max_replication_lag |
+--------------+------------+--------+---------------------+
| 0 | 172.16.0.1 | 1 | 0 |
| 1 | 172.16.0.2 | 1 | 0 |
| 1 | 172.16.0.3 | 1000 | 30 |
+--------------+------------+--------+---------------------+
3 rows in set (0.00 sec)
Admin> INSERT INTO mysql_servers(hostgroup_id,hostname) VALUES (1,'172.16.0.1');
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,weight,max_replication_lag FROM mysql_servers;
+--------------+------------+--------+---------------------+
| hostgroup_id | hostname | weight | max_replication_lag |
+--------------+------------+--------+---------------------+
| 0 | 172.16.0.1 | 1 | 0 |
| 1 | 172.16.0.2 | 1 | 0 |
| 1 | 172.16.0.3 | 1000 | 30 |
| 1 | 172.16.0.1 | 1 | 0 |
+--------------+------------+--------+---------------------+
4 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET max_replication_lag=30, weight=1000;
Query OK, 4 rows affected (0.00 sec)
Admin> UPDATE mysql_servers SET weight=1 WHERE hostname='172.16.0.1' AND hostgroup_id=1;
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,weight,max_replication_lag FROM mysql_servers;
+--------------+------------+--------+---------------------+
| hostgroup_id | hostname | weight | max_replication_lag |
+--------------+------------+--------+---------------------+
| 0 | 172.16.0.1 | 1000 | 30 |
| 1 | 172.16.0.2 | 1000 | 30 |
| 1 | 172.16.0.3 | 1000 | 30 |
| 1 | 172.16.0.1 | 1 | 30 |
+--------------+------------+--------+---------------------+
4 rows in set (0.00 sec)
上面的例子中，如果我们让hostgroup_id=1的组负责read请求，那么99.95%的读请求都会均衡到 172.16.0.2 和 172.16.0.3 上，只有0.05%的读操作才会路由到 172.16.0.1 上。如果 172.16.0.2 和 172.16.0.3 都不可用了， 172.16.0.1 将负责所有的read请求。
注意：max_replication_lag只对slave节点有效。如果某后端MySQL未启用复制功能，则Monitor模块不会做任何和复制相关的动作。
压缩传输到后端节点的数据
只需设置mysql_servers.compression字段为一个非0值即可。注意，启用压缩功能(已load到runtime)后，只对从此之后建立的新连接才有效，已建立的连接(包括线程池中的空闲连接)不会对数据进行压缩传输。
Admin> SELECT hostgroup_id,hostname,compression FROM mysql_servers;
+--------------+------------+-------------+
| hostgroup_id | hostname | compression |
+--------------+------------+-------------+
| 0 | 172.16.0.1 | 0 |
| 1 | 172.16.0.2 | 0 |
| 1 | 172.16.0.3 | 0 |
| 1 | 172.16.0.1 | 0 |
+--------------+------------+-------------+
4 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET compression=1 WHERE hostname='172.16.0.2' AND hostgroup_id=1;
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,compression FROM mysql_servers;
+--------------+------------+-------------+
| hostgroup_id | hostname | compression |
+--------------+------------+-------------+
| 0 | 172.16.0.1 | 0 |
| 1 | 172.16.0.2 | 1 |
| 1 | 172.16.0.3 | 0 |
| 1 | 172.16.0.1 | 0 |
+--------------+------------+-------------+
4 rows in set (0.00 sec)
平滑禁用一个后端节点
要平滑禁用一个后端节点，需要修改该后端节点的status字段值为OFFLINE_SOFT，这表示该节点正在处理的事务会继续执行，但ProxySQL不会向该节点发送新的请求。
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------+
| hostgroup_id | hostname | status |
+--------------+------------+--------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | ONLINE |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | ONLINE |
+--------------+------------+--------+
4 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET status='OFFLINE_SOFT' WHERE hostname='172.16.0.2';
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------------+
| hostgroup_id | hostname | status |
+--------------+------------+--------------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | OFFLINE_SOFT |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | ONLINE |
+--------------+------------+--------------+
4 rows in set (0.00 sec)
直接禁用后端节点
将status字段的值设置为OFFLINE_HARD，即表示强制禁用，可认为直接杀掉了该节点。该节点上正在执行的事务会停止，也不会向其发送任何请求。
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------------+
| hostgroup_id | hostname | status |
+--------------+------------+--------------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | OFFLINE_SOFT |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | ONLINE |
+--------------+------------+--------------+
4 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET status='OFFLINE_HARD' WHERE hostname='172.16.0.1' AND hostgroup_id=1;
Query OK, 1 row affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------------+
| hostgroup_id | hostname | status |
+--------------+------------+--------------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | OFFLINE_SOFT |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | OFFLINE_HARD |
+--------------+------------+--------------+
4 rows in set (0.00 sec)
重新启用被禁用的后端节点
直接将status字段的值改回ONLINE即可：
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------------+
| hostgroup_id | hostname | status |
+--------------+------------+--------------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | OFFLINE_SOFT |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | OFFLINE_HARD |
+--------------+------------+--------------+
4 rows in set (0.00 sec)
Admin> UPDATE mysql_servers SET status='ONLINE' WHERE status NOT IN ('ONLINE');
Query OK, 2 rows affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------+
| hostgroup_id | hostname | status |
+--------------+------------+--------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | ONLINE |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | ONLINE |
+--------------+------------+--------+
4 rows in set (0.00 sec)
移除一个后端节点
删除mysql_servers表中的行可以完全移除对应的后端节点：
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------+
| hostgroup_id | hostname | status |
+--------------+------------+--------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.2 | ONLINE |
| 1 | 172.16.0.3 | ONLINE |
| 1 | 172.16.0.1 | ONLINE |
+--------------+------------+--------+
4 rows in set (0.00 sec)
Admin> DELETE FROM mysql_servers WHERE hostgroup_id=1 AND hostname IN ('172.16.0.1','172.16.0.2');
Query OK, 2 rows affected (0.00 sec)
Admin> SELECT hostgroup_id,hostname,status FROM mysql_servers;
+--------------+------------+--------+
| hostgroup_id | hostname | status |
+--------------+------------+--------+
| 0 | 172.16.0.1 | ONLINE |
| 1 | 172.16.0.3 | ONLINE |
+--------------+------------+--------+
2 rows in set (0.00 sec)
在ProxySQL的内部，删除一个后端节点或将它设置为OFFLINE_HARD的处理方式是一样的。当执行LOAD MYSQL SERVERS TO RUNTIME后，主机组管理器(Hostgroup Manager)将探测到该节点已被移除，并在内部将器标记为OFFLINE_HARD。
