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)