MySQL数据库的主从同步和读写分离

文章目录

MySQL数据库的主从同步和读写分离1.案例2.主从同步1.主从同步的类型2.主从复制的工作过程3.案例实施3.读写分离1.读写分离的原理2.案例实施

1.案例

企业拓扑图

1.在上述拓扑图中，后端MySQL数据库只有一台时，会有以下问题：

单点故障，导致服务不可用

无法处理大量的并发数据请求

数据丢失是大灾难

改造过后企业的拓扑图

2.改造的办法

增加MySQL数据库的服务器，对数据进行备份，形成主备

确保主备MySQL数据库服务器数据是一样的

主服务器宕机了，备份服务器继续工作，数据有保障

更高级的解决方案的拓扑图

重点：

1.我们需要主从同步账号

2.节点被调度的权限或者账号

3.代理账号

2.主从同步

1.主从同步的类型

1.基于语句的同步（默认）

在主服务器上执行的语句，从服务器执行同样的语句

2.基于行的同步

把改变的内容复制到从服务器

3.混合类型的复制

一旦发现基于语句无法精确复制时，就会采用基于行的复制

2.主从复制的工作过程

3.案例实施

1.配置MySQL master服务器

[root@localhost ~]# hostnamectl set-hostname master[root@localhost ~]# su//安装ntp，同步时间[root@master ~]# yum -y install ntp//配置ntp的主配置文件[root@master ~]# vim /etc/ntp.conf//本地时钟源server 127.127.195.0 //设置时间层级为8fudge 127.127.195.0 stratum 8 [root@master ~]# systemctl start ntpd//关闭防火墙[root@master ~]# systemctl stop firewalld[root@master ~]# setenforce 0//安装MySQL5.7//安装MySQL的源码编译包[root@master ~]# yum -y install ncurses ncurses-devel bison cmake gcc gcc-c++//创建MySQL程序用户[root@master ~]# useradd -s /sbin/nologin mysql//将压缩包解压到/opt目录下面[root@master ~]# tar -zxvf mysql-boost-5.7.20.tar.gz -C /opt[root@master ~]# cd /opt/mysql-5.7.20///cmake 编译安装MySQL5.7[root@master mysql-5.7.20]# cmake \-DCMAKE_INSTALL_PREFIX=/usr/local/mysql \-DMYSQL_UNIX_ADDR=/usr/local/mysql/mysql.sock \-DSYSCONFDIR=/etc \-DSYSTEMD_PID_DIR=/usr/local/mysql \-DDEFAULT_CHARSET=utf8 \-DDEFAULT_COLLATION=utf8_general_ci \-DWITH_INNOBASE_STORAGE_ENGINE=1 \-DWITH_ARCHIVE_STORAGE_ENGINE=1 \-DWITH_BLACKHOLE_STORAGE_ENGINE=1 \-DWITH_PERFSCHEMA_STORAGE_ENGINE=1 \-DMYSQL_DATADIR=/usr/local/mysql/data \-DWITH_BOOST=boost \-DWITH_SYSTEMD=1[root@master mysql-5.7.20]# make [root@master mysql-5.7.20]# make install//将/usr/local/mysql目录下面的所有文件的属主和属组都给mysql[root@master mysql-5.7.20]# chown -R mysql:mysql /usr/local/mysql///配置MySQL的f文件[root@master mysql-5.7.20]# vi /etc/f[client]port = 3306default-character-set=utf8socket = /usr/local/mysql/mysql.sock[mysql]port = 3306default-character-set=utf8socket = /usr/local/mysql/mysql.sock[mysqld]user = mysqlbasedir = /usr/local/mysqldatadir = /usr/local/mysql/dataport = 3306character_set_server=utf8pid-file = /usr/local/mysql/mysqld.pidsocket = /usr/local/mysql/mysql.sockserver-id = 1sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_AUTO_VALUE_ON_ZERO,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,PIPES_AS_CONCAT,ANSI_QUOTES//修改f的属主和属组[root@master mysql-5.7.20]# chown mysql:mysql /etc/f//配置mysql的环境变量[root@master mysql-5.7.20]# echo 'PATH=/usr/local/mysql/bin:/usr/local/mysql/lib:$PATH' >> /etc/profile[root@master mysql-5.7.20]# echo 'export PATH' >> /etc/profile//加载环境变量[root@master mysql-5.7.20]# source /etc/profile//初始化mysql数据库[root@master mysql-5.7.20]# cd /usr/local/mysql/[root@master mysql]# bin/mysqld \--initialize-insecure \--user=mysql \--basedir=/usr/local/mysql \--datadir=/usr/local/mysql/data[root@master mysql]# cp usr/lib/systemd/system/mysqld.service /usr/lib/systemd/system/[root@master mysql]# systemctl enable mysqldCreated symlink from /etc/systemd/system/multi-user.target.wants/mysqld.service to /usr/lib/systemd/system/mysqld.service.[root@master mysql]# netstat -ntap | grep 3306tcp6 00 :::3306 :::*LISTEN22547/mysqld [root@master mysql]# vim /etc/fserver-id = 11log-bin=master-binlog-slave-updates=true[root@master mysql]# systemctl restart mysqld[root@master ~]# mysql -uroot -pabc123mysql: [Warning] Using a password on the command line interface can be insecure.Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 3Server version: 5.7.20-log Source distributionCopyright (c) 2000, , Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql> grant replication slave on *.* to 'myslave'@'192.168.73.%' identified by '123456';Query OK, 0 rows affected, 1 warning (0.01 sec)mysql> flush privileges;Query OK, 0 rows affected (0.00 sec)mysql> show master status;+-------------------+----------+--------------+------------------+-------------------+| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |+-------------------+----------+--------------+------------------+-------------------+| master-bin.000001 |604 | | | |+-------------------+----------+--------------+------------------+-------------------+1 row in set (0.00 sec)mysql> grant all on *.* to test@'192.168.73.%' identified by '';Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> flush privileges;Query OK, 0 rows affected (0.00 sec)

2.配置MySQL从服务器slave1

//MySQL5.7的安装参照master的安装[root@localhost mysql]# hostnamectl set-hostname slave1[root@localhost mysql]# su[root@slave1 mysql]# yum -y install ntp ntpdate[root@slave1 mysql]# systemctl stop firewalld[root@slave1 mysql]# setenforce 0[root@slave1 mysql]# /usr/sbin/ntpdate 192.168.73.1419 Jan 11:16:04 ntpdate[5477]: the NTP socket is in use, exiting[root@slave1 mysql]# vim /etc/fserver-id = 22relay-log=relay-log-binrelay-log-index=slave-relay-bin.index[root@slave1 mysql]# systemctl restart mysqld[root@slave1 mysql]# mysql -u root -pabc123mysql: [Warning] Using a password on the command line interface can be insecure.Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 3Server version: 5.7.20 Source distributionCopyright (c) 2000, , Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql> change master to master_host='192.168.73.141',master_user='myslave',master_password='123456'',master_log_file='master-bin.000001',master_log_pos=604;Query OK, 0 rows affected, 2 warnings (0.04 sec)mysql> start slave;Query OK, 0 rows affected (0.00 sec)mysql> grant all on *.* to test@'192.168.73.%' identified by '';Query OK, 0 rows affected, 1 warning (0.01 sec)mysql> show slave status\G;*************************** 1. row ***************************Slave_IO_State: Connecting to masterMaster_Host: 192.168.73.141Master_User: myslaveMaster_Port: 3306Connect_Retry: 60Master_Log_File: master-bin.000001Read_Master_Log_Pos: 604Relay_Log_File: relay-log-bin.000001Relay_Log_Pos: 4Relay_Master_Log_File: master-bin.000001Slave_IO_Running: ConnectingSlave_SQL_Running: YesReplicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0Last_Error: Skip_Counter: 0Exec_Master_Log_Pos: 604Relay_Log_Space: 154Until_Condition: NoneUntil_Log_File: Until_Log_Pos: 0Master_SSL_Allowed: NoMaster_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: NULLMaster_SSL_Verify_Server_Cert: NoLast_IO_Errno: 1130Last_IO_Error: error connecting to master 'myslave@192.168.73.141:3306' - retry-time: 60 retries: 9Last_SQL_Errno: 0Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 0Master_UUID: Master_Info_File: /usr/local/mysql/data/master.infoSQL_Delay: 0SQL_Remaining_Delay: NULLSlave_SQL_Running_State: Slave has read all relay log; waiting for more updatesMaster_Retry_Count: 86400Master_Bind: Last_IO_Error_Timestamp: 200109 11:29:56Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: Auto_Position: 0Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version: 1 row in set (0.00 sec)mysql> grant all on *.* to test@'192.168.73.%' identified by '';Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> flush privileges;Query OK, 0 rows affected (0.00 sec)

3.配置MySQL从服务器slave2

[root@localhost mysql]# hostnamectl set-hostname slave2[root@localhost mysql]# su[root@slave2 mysql]# yum -y install ntp ntpdate[root@slave2 mysql]# systemctl start ntpd[root@slave2 mysql]# systemctl stop firewalld[root@slave2 mysql]# setenforce 0[root@slave2 mysql]# /usr/sbin/ntpdate 192.168.73.1419 Jan 11:27:05 ntpdate[23734]: the NTP socket is in use, exiting[root@slave2 mysql]# vim /etc/fserver-id = 23relay-log=relay-log-binrelay-log-index=slave-relay-bin.index[root@slave2 mysql]# systemctl restart mysqld[root@slave2 mysql]# mysql -uroot -pabc123mysql: [Warning] Using a password on the command line interface can be insecure.Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 3Server version: 5.7.20 Source distributionCopyright (c) 2000, , Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql> change master to master_host='192.168.73.141',master_user='myslave',master_password='123456',master_log_file='master-bin.000001',master_log_pos=604;Query OK, 0 rows affected, 2 warnings (0.12 sec)mysql> start slave;Query OK, 0 rows affected (0.01 sec)mysql> show slave status\G;*************************** 1. row ***************************Slave_IO_State: Connecting to masterMaster_Host: 192.168.73.141Master_User: myslaveMaster_Port: 3306Connect_Retry: 60Master_Log_File: master-bin.000001Read_Master_Log_Pos: 604Relay_Log_File: relay-log-bin.000001Relay_Log_Pos: 4Relay_Master_Log_File: master-bin.000001Slave_IO_Running: ConnectingSlave_SQL_Running: YesReplicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0Last_Error: Skip_Counter: 0Exec_Master_Log_Pos: 604Relay_Log_Space: 154Until_Condition: NoneUntil_Log_File: Until_Log_Pos: 0Master_SSL_Allowed: NoMaster_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: NULLMaster_SSL_Verify_Server_Cert: NoLast_IO_Errno: 1130Last_IO_Error: error connecting to master 'myslave@192.168.73.141:3306' - retry-time: 60 retries: 1Last_SQL_Errno: 0Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 0Master_UUID: Master_Info_File: /usr/local/mysql/data/master.infoSQL_Delay: 0SQL_Remaining_Delay: NULLSlave_SQL_Running_State: Slave has read all relay log; waiting for more updatesMaster_Retry_Count: 86400Master_Bind: Last_IO_Error_Timestamp: 200109 11:33:22Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: Auto_Position: 0Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version: 1 row in set (0.00 sec)mysql> grant all on *.* to test@'192.168.73.%' identified by '';Query OK, 0 rows affected, 1 warning (0.00 sec)mysql> flush privileges;Query OK, 0 rows affected (0.00 sec)

4.测试主从同步

在master主机上

mysql> show databases;+--------------------+| Database |+--------------------+| information_schema || mysql || performance_schema || sys|+--------------------+4 rows in set (0.11 sec)mysql> create database school;Query OK, 1 row affected (0.10 sec)mysql> use school;Database changedmysql> create table info (id int,name char(20));Query OK, 0 rows affected (0.01 sec)mysql> insert into info (id,name) values(1,'1hao');Query OK, 1 row affected (0.01 sec)mysql> insert into info (id,name) values(2,'2hao');Query OK, 1 row affected (0.01 sec)mysql> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao |+------+------+2 rows in set (0.00 sec)

在slave1主机上

mysql> show databases;+--------------------+| Database |+--------------------+| information_schema || mysql || performance_schema || school || sys|+--------------------+5 rows in set (0.00 sec)mysql> use school;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao |+------+------+2 rows in set (0.00 sec)

在slave2主机上

mysql> show databases;+--------------------+| Database |+--------------------+| information_schema || mysql || performance_schema || school || sys|+--------------------+5 rows in set (0.00 sec)mysql> use school;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao |+------+------+2 rows in set (0.00 sec)

3.读写分离

1.读写分离的原理

1.读写分离就是只在主服务器上写，只在从服务器上读

2.主数据库处理事务性查询，而从数据库处理select查询

3.数据库复制被用来把事务性查询导致的变更同步到集群中的从数据库

2.案例实施

1.搭建amoeba代理服务器

[root@localhost ~]# hostnamectl set-hostname amoeba[root@localhost ~]# su[root@amoeba ~]# lsamoeba-mysql-binary-2.2.0.tar.gz jdk-6u14-linux-x64.bin模板 文档 桌面anaconda-ks.cfg mysql-boost-5.7.20.tar.gz 视频 下载initial-setup-ks.cfg 公共 图片 音乐[root@amoeba ~]# systemctl stop firewalld[root@amoeba ~]# setenforce 0[root@amoeba ~]# cp jdk-6u14-linux-x64.bin /usr/local[root@amoeba ~]# cd /usr/local[root@amoeba local]# lsbin etc games include jdk-6u14-linux-x64.bin lib lib64 libexec mysql sbin share src[root@amoeba local]# chmod +x jdk-6u14-linux-x64.bin [root@amoeba local]# ./jdk-6u14-linux-x64.bin Please enter "yes" or "no".Do you agree to the above license terms? [yes or no]yes[root@amoeba local]# lsbin games jdk1.6.0_14 lib libexec sbin srcetc include jdk-6u14-linux-x64.bin lib64 mysql share[root@amoeba local]# mv jdk1.6.0_14/ jdk1.6[root@amoeba local]# lsbin games jdk1.6 lib libexec sbin srcetc include jdk-6u14-linux-x64.bin lib64 mysql share[root@amoeba local]# echo '> export JAVA_HOME=/usr/local/jdk1.6> export CLASSPATH=$CLASSPATH:$JAVA_HOME/lib:$JAVA_HOME/jre/lib> export PATH=$JAVA_HOME/lib:$JAVA_HOME/jre/bin:$PATH:$HOME/bin> export AMOEBA_HOME=/usr/local/amoeba> export PATH=$PATH:$AMOEBA_HOME/bin' >> /etc/profile[root@amoeba local]# source /etc/profile[root@amoeba local]# java -versionjava version "1.6.0_14"Java(TM) SE Runtime Environment (build 1.6.0_14-b08)Java HotSpot(TM) 64-Bit Server VM (build 14.0-b16, mixed mode)[root@amoeba local]# mkdir /usr/local/amoeba[root@amoeba local]# cd[root@amoeba ~]# tar -zxvf amoeba-mysql-binary-2.2.0.tar.gz -C /usr/local/amoeba[root@amoeba ~]# chmod -R 755 /usr/local/amoeba[root@amoeba ~]# /usr/local/amoeba/bin/amoebaamoeba start|stop[root@amoeba ~]# cd /usr/local/amoeba[root@amoeba amoeba]# vim conf/amoeba.xml30 <property name="user">amoeba</property>31 32 <property name="password">123456</property>117 <property name="defaultPool">master</property>118 <property name="writePool">master</property>119 <property name="readPool">slaves</property>[root@amoeba amoeba]# vim conf/amoeba.xml26<property name="user">test</property>272829<property name="password"></property>45 <dbServer name="user" parent="abstractServer">46 <factoryConfig>47<!-- mysql ip -->48<property name="ipAddress">192.168.73.141</property>49 </factoryConfig>50 </dbServer>45 <dbServer name="master" parent="abstractServer">46 <factoryConfig>47<!-- mysql ip -->48<property name="ipAddress">192.168.73.141</property>49 </factoryConfig>50 </dbServer>51 52 <dbServer name="slave1" parent="abstractServer">53 <factoryConfig>54<!-- mysql ip -->55<property name="ipAddress">192.168.73.138</property>56 </factoryConfig>57 </dbServer>58 59 <dbServer name="slave2" parent="abstractServer">60 <factoryConfig>61<!-- mysql ip -->62<property name="ipAddress">192.168.73.142</property>63 </factoryConfig>64 </dbServer>6566 <dbServer name="slaves" virtual="true">67 <poolConfig class="com.meidusa.amoeba.server.MultipleServerPool">68<!-- Load balancing strategy: 1=ROUNDROBIN , 2=WEIGHTBASED , 3=HA-->69<property name="loadbalance">1</property>70 71<!-- Separated by commas,such as: server1,server2,server1 -->72<property name="poolNames">slave1,slave2</property>73 </poolConfig>74 </dbServer>[root@amoeba ~]# netstat -ntap | grep javatcp6 00 :::8066 :::*LISTEN6097/java tcp6 00 127.0.0.1:24038 :::*LISTEN6097/java tcp6370 192.168.73.140:45008 192.168.73.141:3306CLOSE_WAIT 5838/java tcp6 00 192.168.73.140:42792 192.168.73.141:3306ESTABLISHED 5975/java tcp6 00 192.168.73.140:54100 192.168.73.142:3306ESTABLISHED 6097/java tcp6 00 192.168.73.140:35558 192.168.73.138:3306ESTABLISHED 5975/java tcp6 00 192.168.73.140:44924 192.168.73.141:3306ESTABLISHED 6097/java tcp6 00 192.168.73.140:51968 192.168.73.142:3306ESTABLISHED 5975/java tcp6 00 192.168.73.140:37690 192.168.73.138:3306ESTABLISHED 6097/java

2.在客户机上面

[root@localhost ~]# hostnamectl set-hostname client[root@localhost ~]# su[root@client ~]# yum -y install mysql[root@client ~]# systemctl stop firewalld[root@client ~]# setenforce 0[root@client ~]# mysql -u amoeba -p 123456 -h 192.168.73.140 -P8066Enter password: \\密码123456Welcome to the MariaDB monitor. Commands end with ; or \g.Your MySQL connection id is 1218932998Server version: 5.1.45-mysql-amoeba-proxy-2.2.0Copyright (c) 2000, , Oracle, MariaDB Corporation Ab and others.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.MySQL [(none)]> show databases;ERROR (HY000): MySQL server has gone awayNo connection. Trying to reconnect...Connection id: 1218932998Current database: *** NONE ***+--------------------+| Database |+--------------------+| information_schema || mysql || performance_schema || school || sys|+--------------------+5 rows in set (0.01 sec)MySQL [(none)]> use school;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedMySQL [school]> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao |+------+------+2 rows in set (0.01 sec)

3.关闭slave的slave

//slave2[root@slave1 ~]# mysql -uroot -pabc123mysql: [Warning] Using a password on the command line interface can be insecure.Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 1534Server version: 5.7.20 Source distributionCopyright (c) 2000, , Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql> stop slave;Query OK, 0 rows affected (0.01 sec)mysql> use school;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> insert into info (id,name) values(4,'4hao');Query OK, 1 row affected (0.01 sec)//slave1[root@slave1 ~]# mysql -uroot -pabc123mysql: [Warning] Using a password on the command line interface can be insecure.Welcome to the MySQL monitor. Commands end with ; or \g.Your MySQL connection id is 1534Server version: 5.7.20 Source distributionCopyright (c) 2000, , Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.mysql> stop slave;Query OK, 0 rows affected (0.01 sec)mysql> show databases;+--------------------+| Database |+--------------------+| information_schema || mysql || performance_schema || school || sys|+--------------------+5 rows in set (0.00 sec)mysql> use school;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> insert into info (id,name) values (3,'3hao');Query OK, 1 row affected (0.00 sec)

4.在client里面测试

MySQL [school]> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao || 4 | 4hao |+------+------+3 rows in set (0.02 sec)MySQL [school]> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao || 3 | 3hao |+------+------+3 rows in set (0.01 sec)MySQL [school]> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao || 4 | 4hao |+------+------+3 rows in set (0.00 sec)MySQL [school]> select * from info;+------+------+| id | name |+------+------+| 1 | 1hao || 2 | 2hao || 3 | 3hao |+------+------+3 rows in set (0.00 sec)

从这个测试中，我们可以看出： MySQL对读请求做了负载均衡，对多次请求采用了轮询算法，所有的用于处理读请求的MySQL会轮流(或者按权重)来响应 。