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MySQL复制原理
mysql从3.23开始提供复制功能，复制指将主库的ddl和dml操作通过binlog文件传送到从库上执行，从而保持主库和从库数据同步。mysql支持一台主库同时向多台从库复制，从库同时也可以作为其他从库的主库，从而实现级联复制功能。mysql复制功能相当于oracle数据库的逻辑dg功能。
mysql复制原理大致如下：
1）mysql主库事务提交时会把数据变更作为event记录在binlog文件中，mysql主库的sync_binlog参数控制binlog日志刷新到磁盘。
2）主库推送binlog中的event到从库的中继日志relay log中，之后从库根据中继日志重做数据变更操作，通过逻辑复制来达到与主库同步的目的。
mysql通过3个线程完成复制功能：其中binlog dump线程跑在主库上，io线程和sql线程跑在从库上。当从库启动复制（start slave）时，首先创建io线程连接主库，主库随后创建binlog dump线程读取event发送给io线程，io线程获取到event后更新到从库中继日志relay log中，之后从库sql线程根据relay log内容在从库从做操作。从而完成mysql复制功能。mysql复制结构如图：
主库通过show processlist命令可以看到binlog dump进程，如下：
mysql> show processlist \G;
*************************** 1. row ***************************
Id: 125
User: root
Host: localhost
db: test
Command: Query
Time: 0
State: init
Info: show processlist
*************************** 2. row ***************************
Id: 127
User: rep1
Host: 192.168.80.136:44889
db: NULL
Command: Binlog Dump
Time: 16991
State: Master has sent all binlog to slave; waiting for binlog to be updated
Info: NULL
2 rows in set (0.00 sec)
ERROR:
No query specified
mysql>
从库通过show processlist命令可以看到io线程和sql线程，如下：
mysql> show processlist \G;
*************************** 1. row ***************************
Id: 4
User: root
Host: localhost
db: test
Command: Query
Time: 0
State: init
Info: show processlist
*************************** 2. row ***************************
Id: 7
User: system user
Host:
db: NULL
Command: Connect
Time: 17079
State: Waiting for master to send event
Info: NULL
*************************** 3. row ***************************
Id: 8
User: system user
Host:
db: NULL
Command: Connect
Time: 17107
State: Slave has read all relay log; waiting for the slave I/O thread to update it
Info: NULL
3 rows in set (0.00 sec)
ERROR:
No query specified
mysql复制涉及到两种文件：binlog和relay log文件。其中根据binlog的不同设置，mysql复制分为3中模式。
复制的3中常见架构：一主多从、级联复制、双主架构
1）mysql一主多从架构：解决主库读请求压力解决方案。
2）mysql级联架构：解决一主多从架构中主库io和网络压力，缺点时额外增加了主库到从库的应用延迟，其中master2可以设置为blackhold（黑洞）模式来缓解复制延迟。
3）双主复制/dual master架构：
4）双主级联复制架构：
通过show master status查看主库状态：
mysql> show master status \G;
*************************** 1. row ***************************
File: dbking-bin.000004
Position: 1729
Binlog_Do_DB:
Binlog_Ignore_DB:
Executed_Gtid_Set:
1 row in set (0.00 sec)
ERROR:
No query specified
通过show slave status查看从库状态：
mysql> show slave status \G;
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 192.168.80.133
Master_User: rep1
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: dbking-bin.000004
Read_Master_Log_Pos: 1729
Relay_Log_File: chavinking-relay-bin.000010
Relay_Log_Pos: 714
Relay_Master_Log_File: dbking-bin.000004
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: 1729
Relay_Log_Space: 1056
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: 1
Master_UUID: 9b92b2a8-b7e0-11e6-81e4-000c29fa5a95
Master_Info_File: /usr/local/software/mysql-5.6.24-linux-glibc2.5-x86_64/data/master.info
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
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
1 row in set (0.00 sec)
MySQL复制原理