实现半同步复制
1.1 实验环境
- 三台主机实现半同步复制
- 在192.168.66.8 master
- 在192.168.66.18 slave
- 在192.168.66.28 slave
- 主机系统为CentOS8
- 需要干净的系统，关闭防火墙，selinux=disabled
- dnf仓库需自己创建
注意：此实验采用dnf 安装的mariadb-server
1.2 简单搭建主从复制
1.2.1 master
dnf install mariadb-server -y
systemctl start mariadb
vim /etc/my.cnf.d/mariadb-server.cnf
[mysqld]
server-id=8 # 8为ip最后一位 个人设置喜好问题
log-bin
#保存退出
systemctl start mariadb
mysql
MariaDB [(none)]> show master logs;
+--------------------+-----------+
| Log_name | File_size |
+--------------------+-----------+
| mariadb-bin.000001 | 330 |
+--------------------+-----------+
MariaDB [(none)]> grant replication slave on *.* to repluser@'192.168.66.%' identified by 'ming';
Query OK, 0 rows affected (0.002 sec)
MariaDB [(none)]> select user,host,password from mysql.user;
+----------+--------------+-------------------------------------------+
| user | host | password |
+----------+--------------+-------------------------------------------+
| root | localhost | |
| root | centos8 | |
| root | 127.0.0.1 | |
| root | ::1 | |
| repluser | 192.168.66.% | *C6B2B8EC628390A22B99AB0609E5E70A0F962E9F |
+----------+--------------+-------------------------------------------+
1.2.2 slave
- 分别在192.168.66.18 slave 和192.168.66.28 slave安装MariaDB-Server
[root@centos8 ~]dnf install mariadb-server -y
[root@centos8 ~]systemctl start mariadb
[root@centos8 ~]vim /etc/my.cnf.d/mariadb-server.cnf
[mysqld]
server-id=# #个人喜欢将ip最后一位作为 server-id
[root@centos8 ~]# systemctl restart mariadb.service
[root@centos8 ~]# mysql
- 分别在192.168.66.18 slave 和192.168.66.28 slave
MariaDB [(none)]> CHANGE MASTER TO
-> MASTER_HOST='192.168.66.8',
-> MASTER_USER='repluser',
-> MASTER_PASSWORD='ming',
-> MASTER_PORT=3306,
-> MASTER_LOG_FILE='mariadb-bin.000001', MASTER_LOG_POS=330;
MariaDB [(none)]> start slave; #开启线程
Query OK, 0 rows affected (0.001 sec)
MariaDB [(none)]> show slave status\G;
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 192.168.66.8
Master_User: repluser
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mariadb-bin.000001
Read_Master_Log_Pos: 529
Relay_Log_File: mariadb-relay-bin.000002
Relay_Log_Pos: 756
Relay_Master_Log_File: mariadb-bin.000001
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB:
Replicate_Ignore_DB:
Replicate_Do_Table:
Replicate_Ignore_Table:
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 0
Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 529
Relay_Log_Space: 1067
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed: No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
Last_IO_Errno: 0
Last_IO_Error:
Last_SQL_Errno: 0
Last_SQL_Error:
Replicate_Ignore_Server_Ids:
Master_Server_Id: 8
Master_SSL_Crl:
Master_SSL_Crlpath:
Using_Gtid: No
Gtid_IO_Pos:
Replicate_Do_Domain_Ids:
Replicate_Ignore_Domain_Ids:
Parallel_Mode: conservative
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
Slave_DDL_Groups: 1
Slave_Non_Transactional_Groups: 0
Slave_Transactional_Groups: 0
1 row in set (0.000 sec)
ERROR: No query specified
1.2.3 测试主从
+ 主：
MariaDB [(none)]> create database db1;
Query OK, 1 row affected (0.002 sec)
+ 从:
MariaDB [(none)]> show databases;
+--------------------+
| Database |
+--------------------+
| db1 |
| information_schema |
| mysql |
| performance_schema |
+--------------------+
主从搭建完成
1.3 实现半同步
1.3.1 在master实现，启用半同步功能
- 官方文档：https://mariadb.com/kb/en/library/semisynchronous-replication/
[root@centos8 ~]#systemctl restart mariadb
[root@centos8 ~]#vim /etc/my.cnf.d/mariadb-server.cnf
[mysqld] **#号注释的为 所需插件**
server-id=8
log-bin
plugin_load_add = semisync_master #
rpl_semi_sync_master_enabled=ON #
rpl_semi_sync_master_timeout=3000 # 设置3s内无法同步，也将返回成功信息给客户端
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
log-error=/var/log/mariadb/mariadb.log
pid-file=/run/mariadb/mariadb.pid
[root@centos8 ~]#systemctl restart mariadb
[root@centos8 ~]#mysql
MariaDB [(none)]> show global variables like '%semi%';
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 10
Current database: *** NONE ***
+---------------------------------------+--------------+
| Variable_name | Value |
+---------------------------------------+--------------+
| rpl_semi_sync_master_enabled | ON |
| rpl_semi_sync_master_timeout | 3000 |
| rpl_semi_sync_master_trace_level | 32 |
| rpl_semi_sync_master_wait_no_slave | ON |
| rpl_semi_sync_master_wait_point | AFTER_COMMIT |
| rpl_semi_sync_slave_delay_master | OFF |
| rpl_semi_sync_slave_enabled | OFF |
| rpl_semi_sync_slave_kill_conn_timeout | 5 |
| rpl_semi_sync_slave_trace_level | 32 |
+---------------------------------------+--------------+
9 rows in set (0.002 sec)
MariaDB [(none)]> show global status like '%semi%';
+--------------------------------------------+-------+
| Variable_name | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients | 0 |
| Rpl_semi_sync_master_get_ack | 0 |
| Rpl_semi_sync_master_net_avg_wait_time | 0 |
| Rpl_semi_sync_master_net_wait_time | 0 |
| Rpl_semi_sync_master_net_waits | 0 |
| Rpl_semi_sync_master_no_times | 0 |
| Rpl_semi_sync_master_no_tx | 0 |
| Rpl_semi_sync_master_request_ack | 0 |
| Rpl_semi_sync_master_status | ON |
| Rpl_semi_sync_master_timefunc_failures | 0 |
| Rpl_semi_sync_master_tx_avg_wait_time | 0 |
| Rpl_semi_sync_master_tx_wait_time | 0 |
| Rpl_semi_sync_master_tx_waits | 0 |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0 |
| Rpl_semi_sync_master_wait_sessions | 0 |
| Rpl_semi_sync_master_yes_tx | 0 |
| Rpl_semi_sync_slave_send_ack | 0 |
| Rpl_semi_sync_slave_status | OFF |
+--------------------------------------------+-------+
18 rows in set (0.001 sec)
1.3.2 在其它所有slave节点上都实现，启用半同步功能
[root@slave ~]#vim /etc/my.cnf.d/mariadb-server.cnf
[mysqld]
server-id=18
plugin_load_add = semisync_slave
rpl_semi_sync_slave_enabled=ON
[root@slave ~]#systemctl restart mariadb
[root@slave ~]#mysql
MariaDB [(none)]> show global variables like '%semi%';
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 11
Current database: *** NONE ***
+---------------------------------------+--------------+
| Variable_name | Value |
+---------------------------------------+--------------+
| rpl_semi_sync_master_enabled | OFF |
| rpl_semi_sync_master_timeout | 10000 |
| rpl_semi_sync_master_trace_level | 32 |
| rpl_semi_sync_master_wait_no_slave | ON |
| rpl_semi_sync_master_wait_point | AFTER_COMMIT |
| rpl_semi_sync_slave_delay_master | OFF |
| rpl_semi_sync_slave_enabled | ON |
| rpl_semi_sync_slave_kill_conn_timeout | 5 |
| rpl_semi_sync_slave_trace_level | 32 |
+---------------------------------------+--------------+
9 rows in set (0.003 sec)
MariaDB [(none)]> show global status like '%semi%';
+--------------------------------------------+-------+
| Variable_name | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients | 0 |
| Rpl_semi_sync_master_get_ack | 0 |
| Rpl_semi_sync_master_net_avg_wait_time | 0 |
| Rpl_semi_sync_master_net_wait_time | 0 |
| Rpl_semi_sync_master_net_waits | 0 |
| Rpl_semi_sync_master_no_times | 0 |
| Rpl_semi_sync_master_no_tx | 0 |
| Rpl_semi_sync_master_request_ack | 0 |
| Rpl_semi_sync_master_status | OFF |
| Rpl_semi_sync_master_timefunc_failures | 0 |
| Rpl_semi_sync_master_tx_avg_wait_time | 0 |
| Rpl_semi_sync_master_tx_wait_time | 0 |
| Rpl_semi_sync_master_tx_waits | 0 |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0 |
| Rpl_semi_sync_master_wait_sessions | 0 |
| Rpl_semi_sync_master_yes_tx | 0 |
| Rpl_semi_sync_slave_send_ack | 0 |
| Rpl_semi_sync_slave_status | ON |
+--------------------------------------------+-------+
18 rows in set (0.001 sec)
1.3.3 再次在master上查看
MariaDB [(none)]> show global status like '%semi%';
+--------------------------------------------+-------+
| Variable_name | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients | 2 | #两个从节点
| Rpl_semi_sync_master_get_ack | 0 |
| Rpl_semi_sync_master_net_avg_wait_time | 0 |
| Rpl_semi_sync_master_net_wait_time | 0 |
| Rpl_semi_sync_master_net_waits | 0 |
| Rpl_semi_sync_master_no_times | 0 |
| Rpl_semi_sync_master_no_tx | 0 |
| Rpl_semi_sync_master_request_ack | 0 |
| Rpl_semi_sync_master_status | ON |
| Rpl_semi_sync_master_timefunc_failures | 0 |
| Rpl_semi_sync_master_tx_avg_wait_time | 0 |
| Rpl_semi_sync_master_tx_wait_time | 0 |
| Rpl_semi_sync_master_tx_waits | 0 |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0 |
| Rpl_semi_sync_master_wait_sessions | 0 |
| Rpl_semi_sync_master_yes_tx | 0 |
| Rpl_semi_sync_slave_send_ack | 0 |
| Rpl_semi_sync_slave_status | OFF |
+--------------------------------------------+-------+
18 rows in set (0.002 sec)
1.3.4 测试
# 在master实现，创建数据库，立即成功
MariaDB [db1]> create database db2;
Query OK, 1 row affected (0.004 sec)
# 在所有slave节点实现，停止复制线程
MariaDB [(none)]> stop slave;
Query OK, 0 rows affected (0.011 sec)
# 在master实现，创建数据库，等待3s才能成功
MariaDB [db1]> create database db3;
Query OK, 1 row affected (3.003 sec)
# 在任意一个slave节点实现，恢复复制线程
MariaDB [(none)]> start slave;
Query OK, 0 rows affected (0.006 sec)
# 在master实现，创建数据库，立即成功
MariaDB [db1]> create database db4;
Query OK, 1 row affected (0.002 sec)
2 半同步复制总结及其原理
2.1 异步、同步和半同步复制概念
-
异步复制（Asynchronous replication），MySQL默认的复制是异步的，主库在执行完客户端提交的事务后会立即将结果返给给客户端，并不关心从库是否已经接收并处理。原理最简单，性能最好，但是主从之间数据不一致的概率很大。
-
全同步复制（Fully synchronous replication），指当主库执行完一个事务，所有的从库都执行了该事务才返回给客户端。因为需要等待所有从库执行完该事务才能返回，所以全同步复制的性能必然会收到严重的影响。
-
半同步复制（Semisynchronous replication），介于异步复制和全同步复制之间，主库在执行完客户端提交的事务后不是立刻返回给客户端，而是等待至少一个从库接收到并写到relay log中才返回给客户端。相对于异步复制，半同步复制牺牲了一定的性能，提高了数据的安全性。
2.2 半同步复制原理
- 默认情况下，MySQL的复制功能是异步的，异步复制可以提供最佳的性能，主库把binlog日志发送给从库即结束，并不验证从库是否接收完毕。这意味着当主服务器或从服务器端发生故障时，有可能从服务器没有接收到主服务器发送过来的binlog日志，这就会造成主服务器和从服务器的数据不一致，甚至在
恢复时造成数据的丢失 - 该模式可以确保从库接收完主库发送的binlog日志文件并写入到自己的中继日志relay log里，然后会给主库一个反馈，告诉主库已经接收完毕，这时主库才返回结果给客户端告知操作完成。当出现从库响应超时情况时，主库会暂时切换到异步复制模式，直到下一次同步没有超时转为半同步复制为止。（master的dump线程除了发送binlog数据到slave，还承担了接收slave的ack工作。如果出现异常，没有收到ack，那么将自动降为普通的异步复制，直到异常修复）
