在异步复制（async replication）中，Master不用关心Slave是否接收到二进制日志，所以Master与Slave没有任何的依赖关系。你可以认为Master和Slave是分别独自工作的两台服务器，数据最终会通过二进制日志达到一致。
异步复制的性能最好，因为它对数据库本身几乎没有任何开销，除非主从延迟非常大，Dump Thread需要读取大量二进制日志文件。
如果业务对于数据一致性要求不高，当发生故障时，能容忍数据的丢失，甚至大量的丢失，推荐用异步复制，这样性能最好（比如像微博这样的业务，虽然它对性能的要求极高，但对于数据丢失，通常可以容忍）。但往往核心业务系统最关心的就是数据安全，比如监控业务、告警系统。
异步复制环境的规划：
master（docker），端口3310
slave（docker），端口3311
master的配置配置文件my.cnf
$ sudo cat /home/mysql/docker-data/3311/conf/my.cnf# For advice on how to change settings please see# http://dev.mysql.com/doc/refman/5.7/en/server-configuration-defaults.html[mysqld]## Remove leading # and set to the amount of RAM for the most important data# cache in MySQL. Start at 70% of total RAM for dedicated server, else 10%.# innodb_buffer_pool_size = 128M## Remove leading # to turn on a very important data integrity option: logging# changes to the binary log between backups.# log_bin## Remove leading # to set options mainly useful for reporting servers.# The server defaults are faster for transactions and fast SELECTs.# Adjust sizes as needed, experiment to find the optimal values.# join_buffer_size = 128M# sort_buffer_size = 2M# read_rnd_buffer_size = 2M#datadir=/home/mysql/docker-data/3307/data#socket=/home/mysql/docker-data/3307/mysql.sockcharacter_set_server=utf8init_connect='SET NAMES utf8'# Disabling symbolic-links is recommended to prevent assorted security riskssymbolic-links=0#log-error=/home/mysql/docker-data/3307/logs/mysqld.log#pid-file=/home/mysql/docker-data/3307/mysqld.pidlower_case_table_names=1 # 表名是否小写server-id=1403311log-bin=mysql-bin # 开启binlogbinlog-format=ROW # binlog的格式auto_increment_increment=1 # 自增的步长，适用于主主复制，为了避免id冲突，步长设置为master的个数auto_increment_offset=1 # 自增的偏移，主主复制每个master的偏移需要不一致# binlog-do-db=mstest # 要同步的数据库# binlog-ignore-db=mysql # 要忽略的数据库#rpl_semi_sync_master_enabled=1#rpl_semi_sync_master_timeout=10000
启动mysql：
$ docker run --name mysql3310 -p 3310:3306 --privileged=true -ti -e MYSQL_ROOT_PASSWORD=root -e MYSQL_DATABASE=order -e MYSQL_USER=user -e MYSQL_PASSWORD=pass -v /home/mysql/docker-data/3310/conf:/etc/mysql/conf.d -v /home/mysql/docker-data/3310/data/:/var/lib/mysql -v /home/mysql/docker-data/3310/logs/:/var/log/mysql -d mysql:5.7
创建用于同步的用户：
mysql> GRANT REPLICATION SLAVE,FILE,REPLICATION CLIENT ON *.* TO 'repluser'@'%' IDENTIFIED BY '123456';Query OK, 0 rows affected, 1 warning (0.01 sec)mysql> FLUSH PRIVILEGES;Query OK, 0 rows affected (0.01 sec)
在master上查看master的二进制日志：
mysql> show master status;+------------------+----------+--------------+------------------+-------------------+| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |+------------------+----------+--------------+------------------+-------------------+| mysql-bin.000003 | 1147 | | | |+------------------+----------+--------------+------------------+-------------------+1 row in set (0.00 sec)
查看master上的进程列表：
mysql> show processlist;+----+----------+------------------+-------+-------------+------+---------------------------------------------------------------+------------------+| Id | User | Host | db | Command | Time | State | Info |+----+----------+------------------+-------+-------------+------+---------------------------------------------------------------+------------------+| 2 | root | localhost | order | Query | 0 | starting | show processlist || 6 | repluser | 172.17.0.1:48450 | NULL | Binlog Dump | 448 | Master has sent all binlog to slave; waiting for more updates | NULL |+----+----------+------------------+-------+-------------+------+---------------------------------------------------------------+------------------+2 rows in set (0.00 sec)slave的配置
配置文件my.cnf与master的配置一致，除了server-id字段需唯一。
启动mysql：
$ docker run --name mysql3311 -p 3311:3306 --privileged=true -ti -e MYSQL_ROOT_PASSWORD=root -e MYSQL_DATABASE=order -e MYSQL_USER=user -e MYSQL_PASSWORD=pass -v /home/mysql/docker-data/3311/conf:/etc/mysql/conf.d -v /home/mysql/docker-data/3311/data/:/var/lib/mysql -v /home/mysql/docker-data/3311/logs/:/var/log/mysql -d mysql:5.7
在slave中设置master的信息：
# master_log_file和master_log_pos取上面show master status显示的值mysql> change master to master_host='172.23.252.98',master_port=3310,master_user='repluser',master_password='123456',master_log_file='mysql-bin.000003',master_log_pos=1147;
开启slave，启动SQL和IO线程：
mysql> start slave;Query OK, 0 rows affected (0.00 sec)
查看slave的状态：
mysql> show slave status\G;*************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 192.168.54.214 Master_User: repluser Master_Port: 3310 Connect_Retry: 60 Master_Log_File: mysql-bin.000003 Read_Master_Log_Pos: 1147 Relay_Log_File: 2da789531bf3-relay-bin.000002 Relay_Log_Pos: 320 Relay_Master_Log_File: mysql-bin.000003 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: 1147 Relay_Log_Space: 534 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: 0Master_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: 1403311 Master_UUID: cd2eaa0a-7a59-11ec-b3b4-0242ac110002 Master_Info_File: /var/lib/mysql/master.info SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates Master_Retry_Count: 86400 Master_Bind: Last_IO_Error_Timestamp: Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: Auto_Position: 0 Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version:1 row in set (0.00 sec)
只有新增的数据才会进行复制，存量的数据需要手动使用工具（如mysqldump）进行同步。
查看slave上的进程列表：
mysql> show processlist;+----+-------------+-----------+-------+---------+------+--------------------------------------------------------+------------------+| Id | User | Host | db | Command | Time | State | Info |+----+-------------+-----------+-------+---------+------+--------------------------------------------------------+------------------+| 4 | root | localhost | order | Query | 0 | starting | show processlist || 7 | system user | | NULL | Connect | 533 | Waiting for master to send event | NULL || 8 | system user | | NULL | Connect | 127 | Slave has read all relay log; waiting for more updates | NULL |+----+-------------+-----------+-------+---------+------+--------------------------------------------------------+------------------+3 rows in set (0.00 sec)半同步复制
半同步复制是MySQL 5.7版本前的半同步复制机制。
半同步复制要求Master事务提交过程中，至少有N个Slave接收到二进制日志，这样就能保证当Master发生宕机，至少有N 台Slave服务器中的数据是完整的。
半同步复制并不是MySQL内置的功能，而是要安装半同步插件，并启用半同步复制功能，设置N个Slave接受二进制日志成功。
缺点：假设user1在主库插入了一条数据，正在等待数据返回，user2这时就能查询到这条数据了，如果此时master挂了，user2又查不到这条数据了，产生了类似幻读的现象。
增强半同步复制增强半同步复制解决了半同步复制的缺点，WAIT和ACK发生在事务提交之前，这样即便Slave没有收到二进制日志，但是 Master宕机了，由于最后一个事务还没有提交，所以本身这个数据对外也不可见，不存在丢失的问题。
所以，对于任何有数据一致性要求的业务，如电商的核心订单业务、银行、保险、证券等与资金密切相关的业务，务必使用增强半同步复制。这样数据才是安全的、有保障的、即使发生宕机，从机也有一份完整的数据。
增强半同步复制环境是基于异步复制的基础上进行的。
安装插件，建议master和slave都安装，因为会有主从切换的情景：
# mastermysql> install plugin rpl_semi_sync_master soname 'semisync_master.so';# slavemysql> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';
确保插件安装成功：
mysql> show plugins;... ...| rpl_semi_sync_master | ACTIVE | REPLICATION | semisync_master.so | GPL || rpl_semi_sync_slave | ACTIVE | REPLICATION | semisync_slave.so | GPL |+----------------------------+----------+--------------------+--------------------+---------+46 rows in set (0.00 sec)
先启动slave上的半同步：
mysql> set global rpl_semi_sync_slave_enabled = {0|1}; # 1：启用，0：禁止再启动master上的半同步：
mysql> set global rpl_semi_sync_master_enabled = {0|1}; # 1：启用，0：禁止# mysql> set global rpl_semi_sync_master_timeout = 10000; # 单位为ms，默认为10s从库重启io_thread：
mysql> stop slave io_thread;mysql> start slave io_thread;半同步的参数为什么不建议写入配置文件
参数写入配置文件的话，会使实例启动后立即进入半同步模式，如果发生长时间断连的实例重新运行启动，有可能导致主库被拖垮。
长时间断开的从库，重新连接后，要等待追完全部事务后，手动开启半同步模式，而不是启动后直接切换，防止冲击主库。
查询主库状态信息mysql> 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 || Rpl_semi_sync_slave_status | ON |+--------------------------------------------+-------+15 rows in set (0.00 sec)
重要参数说明：
Rpl_semi_sync_master_clients：支持和注册半同步复制的已连Slave数
Rpl_semi_sync_master_no_times：master关闭半同步复制的次数
Rpl_semi_sync_master_no_tx：master没有收到slave的回复而提交的次数，可以理解为master等待超时的次数，即半同步模式不成功提交数量
Rpl_semi_sync_master_status：ON是活动状态（半同步），OFF是非活动状态（异步），用于表示主服务器使用的是异步复制模式，还是半同步复制模式
Rpl_semi_sync_master_tx_avg_wait_time：master花在每个事务上的平均等待时间
Rpl_semi_sync_master_tx_waits：master等待成功的次数，即master没有等待超时的次数，也就是成功提交的次数
Rpl_semi_sync_master_yes_tx：master成功接收到slave的回复的次数，即半同步模式成功提交数量。
查询主库参数信息mysql> show global variables like '%sync%';+-------------------------------------------+------------+| Variable_name | Value |+-------------------------------------------+------------+| binlog_group_commit_sync_delay | 0 || binlog_group_commit_sync_no_delay_count | 0 || innodb_flush_sync | ON || innodb_sync_array_size | 1 || innodb_sync_spin_loops | 30 || rpl_semi_sync_master_enabled | ON || rpl_semi_sync_master_timeout | 10000 || rpl_semi_sync_master_trace_level | 32 || rpl_semi_sync_master_wait_for_slave_count | 1 || rpl_semi_sync_master_wait_no_slave | ON || rpl_semi_sync_master_wait_point | AFTER_SYNC || rpl_semi_sync_slave_enabled | ON || rpl_semi_sync_slave_trace_level | 32 || sync_binlog | 1 || sync_frm | ON || sync_master_info | 10000 || sync_relay_log | 10000 || sync_relay_log_info | 10000 |+-------------------------------------------+------------+18 rows in set (0.01 sec)
重要参数说明：
rpl_semi_sync_master_enabled：（主库）是否启动半同步
rpl_semi_sync_master_timeout：等待多时毫秒后变成异步复制，默认是10000ms
rpl_semi_sync_master_wait_point：5.7默认AFTER_SYNC（增强版半同步复制，无损复制模式），在得到slave的应答后再commit，可选值AFTER_COMMIT，在master提交后同步数据给slave，然后master等待slave应答，应答成功返回客户端。
可以在slave端执行stop slave，测试master端会发生什么情况？
在master上执行下面的sql：
mysql> insert into t_order values(3,"C");Query OK, 1 row affected (10.05 sec)
会发现这条语句会阻塞10s（对应上面的参数rpl_semi_sync_master_timeout），然后执行成功，最后看看看master的日志描述：
2022-01-25T02:31:57.016430Z 4 [Note] Start binlog_dump to master_thread_id(4) slave_server(1403312), pos(mysql-bin.000005, 154)2022-01-25T02:31:57.016515Z 4 [Note] Start asynchronous binlog_dump to slave (server_id: 1403312), pos(mysql-bin.000005, 154)2022-01-25T02:33:32.183819Z 2 [Note] Semi-sync replication initialized for transactions.2022-01-25T02:33:32.183865Z 2 [Note] Semi-sync replication enabled on the master.2022-01-25T02:33:32.184004Z 0 [Note] Starting ack receiver thread2022-01-25T02:33:59.644444Z 5 [Note] While initializing dump thread for slave with UUID <aba2eb12-7cbc-11ec-9c1d-0242ac110003>, found a zombie dump thread with the same UUID. Master is killing the zombie dump thread(4).2022-01-25T02:33:59.644612Z 5 [Note] Start binlog_dump to master_thread_id(5) slave_server(1403312), pos(mysql-bin.000005, 154)2022-01-25T02:33:59.644632Z 4 [Note] Stop asynchronous binlog_dump to slave (server_id: 1403312)2022-01-25T02:33:59.644727Z 5 [Note] Start semi-sync binlog_dump to slave (server_id: 1403312), pos(mysql-bin.000005, 154)2022-01-25T02:38:02.847879Z 0 [ERROR] mysqld: Got an error reading communication packets2022-01-25T02:38:27.228952Z 2 [Warning] Timeout waiting for reply of binlog (file: mysql-bin.000005, pos: 684), semi-sync up to file mysql-bin.000005, position 419.2022-01-25T02:38:27.229063Z 2 [Note] Semi-sync replication switched OFF.2022-01-25T02:39:47.230189Z 5 [Note] Stop semi-sync binlog_dump to slave (server_id: 1403312)
可以发现半同步关闭了，变成异步模式。
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