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。Master和Slave可以被视为两个独立地工作的服务器，在二进制日志的作用下，它们最终能够保持数据的一致性。
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
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.sock character_set_server=utf8 init_connect='SET NAMES utf8' # Disabling symbolic-links is recommended to prevent assorted security risks symbolic-links=0 #log-error=/home/mysql/docker-data/3307/logs/mysqld.log #pid-file=/home/mysql/docker-data/3307/mysqld.pid lower_case_table_names=1 # 表名是否小写 server-id=1403311 log-bin=mysql-bin # 开启binlog binlog-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)
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。即使出现宕机，从机也会有完整的数据备份，因此数据将是安全且可靠的。
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：
# master mysql> install plugin rpl_semi_sync_master soname 'semisync_master.so'; # slave mysql> 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;
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。
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。
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 thread 2022-01-25T02:33:59.644444Z 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 packets 2022-01-25T02:38:27.228952Z 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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