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，半同步复制提高了数据的安全性，同时它也造成了一定程度的延迟，这个延迟最少是一个TCP/IP往返的时间。所以，半同步复制最好在低延时的网络中使用。
潜在的问题
客户端事务在存储引擎层提交后，在得到从库确认的过程中，主库宕机了，此时，可能的情况有两种：
1.事务还没发送到从库上：此时，客户端会收到事务提交失败的信息，客户端会重新提交该事务到新的主上，当宕机的主库重新启动后，以从库的身份重新加入到该主从结构中，会发现，该事务在从库中被提交了两次，一次是之前作为主的时候，一次是被新主同步过来的。
2.事务已经发送到从库上：此时，从库已经收到并应用了该事务，但是客户端仍然会收到事务提交失败的信息，重新提交该事务到新的主上。
半同步复制支持多种插件：/usr/lib64/mysql/plugin/*
需要安装方可使用：
MariaDB [mydb]> INSTALL PLUGIN plugin_name SONAME 'shared_library_name';
半同步复制插件：
semisync_master.so
semisync_slave.so
主从复制演示示例：
分别在配置文件server.cnf的[mysqld]段添加如下配置：
主节点：
[root@node1 ~]# vim /etc/my.cnf.d/server.cnf [mysqld] skip_name_resolve=ON innodb_file_per_table=ON server_id=1 log_bin=log-bin
从节点：
[root@node2 ~]# vim /etc/my.cnf.d/server.cnf [mysqld] skip_name_resolve=ON innodb_file_per_table=ON server_id=2 relay_log=relay-log
查看插件是否安装：
MariaDB [mydb]> SHOW PLUGINS; +--------------------------------+----------+--------------------+---------+---------+ | Name | Status | Type | Library | License | +--------------------------------+----------+--------------------+---------+---------+ | binlog | ACTIVE | STORAGE ENGINE | NULL | GPL | | mysql_native_password | ACTIVE | AUTHENTICATION | NULL | GPL | | mysql_old_password | ACTIVE | AUTHENTICATION | NULL | GPL | | MRG_MYISAM | ACTIVE | STORAGE ENGINE | NULL | GPL | | MEMORY | ACTIVE | STORAGE ENGINE | NULL | GPL | | MyISAM | ACTIVE | STORAGE ENGINE | NULL | GPL | | CSV | ACTIVE | STORAGE ENGINE | NULL | GPL | | PERFORMANCE_SCHEMA | ACTIVE | STORAGE ENGINE | NULL | GPL | | ARCHIVE | ACTIVE | STORAGE ENGINE | NULL | GPL | | InnoDB | ACTIVE | STORAGE ENGINE | NULL | GPL | | INNODB_RSEG | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_UNDO_LOGS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_TRX | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_LOCKS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_LOCK_WAITS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_CMP | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_CMP_RESET | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_CMPMEM | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_CMPMEM_RESET | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_TABLES | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_TABLESTATS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_INDEXES | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_COLUMNS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_FIELDS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_FOREIGN | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_FOREIGN_COLS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_SYS_STATS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_TABLE_STATS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_INDEX_STATS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_BUFFER_POOL_PAGES | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_BUFFER_POOL_PAGES_INDEX | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_BUFFER_POOL_PAGES_BLOB | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | XTRADB_ADMIN_COMMAND | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_CHANGED_PAGES | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_BUFFER_PAGE | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_BUFFER_PAGE_LRU | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | INNODB_BUFFER_POOL_STATS | ACTIVE | INFORMATION SCHEMA | NULL | GPL | | FEDERATED | ACTIVE | STORAGE ENGINE | NULL | GPL | | BLACKHOLE | ACTIVE | STORAGE ENGINE | NULL | GPL | | Aria | ACTIVE | STORAGE ENGINE | NULL | GPL | | FEEDBACK | DISABLED | INFORMATION SCHEMA | NULL | GPL | | partition | ACTIVE | STORAGE ENGINE | NULL | GPL | +--------------------------------+----------+--------------------+---------+---------+
经过查看半同步插件并没有安装，我们需要安装上去：
MariaDB [mydb]> HELP INSTALL; Name: 'INSTALL PLUGIN' Description: Syntax: INSTALL PLUGIN plugin_name SONAME 'shared_library_name'
在主节点安装插件：
MariaDB [(none)]> INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so'; MariaDB [(none)]> SHOW PLUGINS; +--------------------------------+----------+--------------------+--------------------+---------+ | Name | Status | Type | Library | License | +--------------------------------+----------+--------------------+--------------------+---------+ | rpl_semi_sync_master | ACTIVE | REPLICATION | semisync_master.so | GPL | +--------------------------------+----------+--------------------+--------------------+---------+ MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE '%semi%'; +------------------------------------+-------+ | 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 | +------------------------------------+-------+ MariaDB [(none)]> SET @@global.rpl_semi_sync_master_enabled=ON; MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE '%semi%'; +------------------------------------+-------+ | Variable_name | Value | +------------------------------------+-------+ | rpl_semi_sync_master_enabled | ON | | rpl_semi_sync_master_timeout | 10000 | | rpl_semi_sync_master_trace_level | 32 | | rpl_semi_sync_master_wait_no_slave | ON | +------------------------------------+-------+
在从节点安装插件：
MariaDB [(none)]> INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so'; MariaDB [(none)]> SHOW PLUGINS; +--------------------------------+----------+--------------------+-------------------+---------+ | Name | Status | Type | Library | License | +--------------------------------+----------+--------------------+-------------------+---------+ | rpl_semi_sync_slave | ACTIVE | REPLICATION | semisync_slave.so | GPL | +--------------------------------+----------+--------------------+-------------------+---------+ MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE '%semi%'; +---------------------------------+-------+ | Variable_name | Value | +---------------------------------+-------+ | rpl_semi_sync_slave_enabled | OFF | | rpl_semi_sync_slave_trace_level | 32 | +---------------------------------+-------+ MariaDB [(none)]> SET @@global.rpl_semi_sync_slave_enabled=ON; MariaDB [(none)]> SHOW GLOBAL VARIABLES LIKE '%semi%'; +---------------------------------+-------+ | Variable_name | Value | +---------------------------------+-------+ | rpl_semi_sync_slave_enabled | ON | | rpl_semi_sync_slave_trace_level | 32 | +---------------------------------+-------+
主节点：
MariaDB [(none)]> SHOW GLOBAL STATUS LIKE '%semi%'; +--------------------------------------------+-------+ | Variable_name | Value | +--------------------------------------------+-------+ | Rpl_semi_sync_master_clients | 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_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 | +--------------------------------------------+-------+
从节点启动IO线程：
MariaDB [(none)]> STOP SLAVE IO_THREAD; MariaDB [(none)]> START SLAVE IO_THREAD;
主节点：
MariaDB [(none)]> SHOW GLOBAL STATUS LIKE '%semi%'; +--------------------------------------------+-------+ | Variable_name | Value | +--------------------------------------------+-------+ | Rpl_semi_sync_master_clients | 1 | | 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_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 | +--------------------------------------------+-------+
在主节点为事先创建的用户进行主从复制授权：
MariaDB [(none)]> GRANT REPLICATION SLAVE,REPLICATION CLIENT ON *.* TO 'repluser'@'172.18.67.%' IDENTIFIED BY 'replpass'; MariaDB [(none)]> FLUSH PRIVILEGES; MariaDB [(none)]> SHOW MASTER STATUS; +----------------+----------+--------------+------------------+ | File | Position | Binlog_Do_DB | Binlog_Ignore_DB | +----------------+----------+--------------+------------------+ | log-bin.000003 | 497 | | | +----------------+----------+--------------+------------------+
从节点：
MariaDB [(none)]> CHANGE MASTER TO MASTER_HOST='172.18.67.11',MASTER_USER='repluser',MASTER_PASSWORD='replpass',MASTER_LOG_FILE='log-bin.000003',MASTER_LOG_POS=497; MariaDB [(none)]> START SLAVE;
在主节点创建数据库mydb：
MariaDB [(none)]> CREATE DATABASE mydb CHARACTER SET 'utf8';
在从节点查看数据库是否复制成功：
MariaDB [(none)]> SHOW DATABASES; +--------------------+ | Database | +--------------------+ | information_schema | | mydb | | mysql | | performance_schema | | test | +--------------------+
发现数据库mydb成功复制
使用数据库mydb在主节点进行数据操作：
MariaDB [(none)]> use mydb; MariaDB [mydb]> CREATE TABLE tbl1 (id INT,name VARCHAR(100));
在从节点上观察：
MariaDB [mydb]> SHOW GLOBAL STATUS LIKE '%semi%'; +--------------------------------------------+-------+ | Variable_name | Value | +--------------------------------------------+-------+ | Rpl_semi_sync_master_clients | 1 | | Rpl_semi_sync_master_net_avg_wait_time | 487 | | Rpl_semi_sync_master_net_wait_time | 487 | | Rpl_semi_sync_master_net_waits | 1 | | Rpl_semi_sync_master_no_times | 0 | | Rpl_semi_sync_master_no_tx | 0 | | Rpl_semi_sync_master_status | ON | | Rpl_semi_sync_master_timefunc_failures | 0 | | Rpl_semi_sync_master_tx_avg_wait_time | 646 | | Rpl_semi_sync_master_tx_wait_time | 646 | | Rpl_semi_sync_master_tx_waits | 1 | | Rpl_semi_sync_master_wait_pos_backtraverse | 0 | | Rpl_semi_sync_master_wait_sessions | 0 | | Rpl_semi_sync_master_yes_tx | 1 | +--------------------------------------------+-------+
主节点继续操作：
MariaDB [mydb]> INSERT INTO tbl1 VALUES (1,'tom');
我们发现从节点的数据都在变化：
MariaDB [mydb]> SHOW GLOBAL STATUS LIKE '%semi%'; +--------------------------------------------+-------+ | Variable_name | Value | +--------------------------------------------+-------+ | Rpl_semi_sync_master_clients | 1 | | Rpl_semi_sync_master_net_avg_wait_time | 910 | | Rpl_semi_sync_master_net_wait_time | 1821 | | Rpl_semi_sync_master_net_waits | 2 | | Rpl_semi_sync_master_no_times | 0 | | Rpl_semi_sync_master_no_tx | 0 | | Rpl_semi_sync_master_status | ON | | Rpl_semi_sync_master_timefunc_failures | 0 | | Rpl_semi_sync_master_tx_avg_wait_time | 734 | | Rpl_semi_sync_master_tx_wait_time | 1469 | | Rpl_semi_sync_master_tx_waits | 2 | | Rpl_semi_sync_master_wait_pos_backtraverse | 0 | | Rpl_semi_sync_master_wait_sessions | 0 | | Rpl_semi_sync_master_yes_tx | 2 | +--------------------------------------------+-------+