MySQL Replication--半同步复制(Semi-Sync Replication)

AFTER_COMMIT：
1>将事务写入binlog和备库
2>将binlog刷新到磁盘
3>提交事务,等待备库返回确认信息
4>接收到备库确认信息
5>返回结果给客户端

1>将事务写入binlog和备库
2>将binlog刷新到磁盘
3>等待备库返回确认信息
4>接收到备库确认信息，提交事务
5>返回结果给客户端

两者区别在于：
AFTER_COMMIT：在接收到备库确认信息前将事务提交
AFTER_SYNC：在接收到备库确认信息后将事务提交


造成影响：
对于AFTER_SYNC，所有客户端可以在同一时间看到事务被提交；
对于AFTER_COMMIT，由于事务提交后还需要等待备库的确认信息，在提交事务的客户端等待的这段时间内，其他客户端已经可以看到事务提交，并能访问到被提交的数据。


主库发生宕机时：
对于AFTER_SYNC，所有在主库上提交的事务都已复制到备库，因此备库数据是实时的，主备切换不丢失数据。
对于AFTER_COMMIT，不能保证所有主库上提交的事务都已复制到备库，假设回话S1提交事务T1将D1值从1修改为2，事务T1在主库上已提交但等待备库返回确认信息，此时回话S2访问到D1的值为2，此时主库宕机并主备切换，备库可能没有正常应用该事务(也可能还未收到主库发送过来的日志)，新主库的D1值还为1，因此数据丢失。

## 查看MYSQL服务器是否支持动态插件
SELECT @@have_dynamic_loading;
+------------------------+
| @@have_dynamic_loading |
+------------------------+
| YES                    |
+------------------------+


##在主库上安装PLUGIN，执行：
INSTALL PLUGIN rpl_semi_sync_master SONAME ‘semisync_master.so‘;


##在从库上安装PLUGIN，执行：
INSTALL PLUGIN rpl_semi_sync_slave SONAME ‘semisync_slave.so‘;


##在主库上启用半同步复制，并设置等待时长为1000ms，执行：
SET GLOBAL rpl_semi_sync_master_enabled = 1;
SET GLOBAL rpl_semi_sync_master_timeout = 1000;


##在从库上启用版同步复制,执行：
SET GLOBAL rpl_semi_sync_slave_enabled = 1;


##在从库上重启IO_THREAD,执行：
STOP SLAVE IO_THREAD; 
START SLAVE IO_THREAD;

##============================================================
##查看半同步复制的状态和参数：
SHOW VARIABLES LIKE ‘rpl_semi_sync%‘;
+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 1000       |
| 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 |
+-------------------------------------------+------------+


##============================================================
##查看半同步复制的状态
SHOW STATUS LIKE ‘rpl_semi_sync%‘;
+--------------------------------------------+-------+
| 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             | 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      | 1336  |
| Rpl_semi_sync_master_tx_wait_time          | 1336  |
| 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     |
+--------------------------------------------+-------+


Rpl_semi_sync_master_clients：当前有多少个从库被设置为半同步模式。

Rpl_semi_sync_master_net_avg_wait_time：等待从库返回去确认消息的平均时间(单位：ms)

Rpl_semi_sync_master_net_wait_time：等到从库返回确认消息的总时间(单位：ms)

Rpl_semi_sync_master_net_waits：等待从库返回确认消息的总次数

Rpl_semi_sync_master_no_tx：事务没有得到从库返回确认消息就提交的次数，The number of commits that were not acknowledged successfully by a slave.

Rpl_semi_sync_master_timefunc_failures：调用函数gettimeofday()失败的次数，The number of times the master failed when calling time functions such as gettimeofday(). 

Rpl_semi_sync_master_tx_avg_wait_time：主库上平均每个事务等待时长，The average time in microseconds the master waited for each transaction. 

Rpl_semi_sync_master_tx_wait_time：主库上事务等待的总时长，The total time in microseconds the master waited for transactions. 

Rpl_semi_sync_master_tx_waits：主库上事务等待的次数，The total number of times the master waited for transactions. 

Rpl_semi_sync_master_wait_pos_backtraverse：改变当前等待最小二进制日志的次数，The total number of times the master waited for an event with binary coordinates lower than events waited for previously. This can occur when the order in which transactions start waiting for a reply is different from the order in which their binary log events are written.  

Rpl_semi_sync_master_wait_sessions：当前等待从库返回确认消息的回话数，The number of sessions currently waiting for slave replies. 

Rpl_semi_sync_master_yes_tx：得到从库返回消息的事务数，The number of commits that were acknowledged successfully by a slave. 

当半同步复制开启后，Rpl_semi_sync_master_status 变量用动态地显示半同步复制的状态。
Rpl_semi_sync_master_status设置为OFF,表明当前处于异步复制下，当等待从库返回确认消息时间超过rpl_semi_sync_master_timeout值后修改为OFF.；Rpl_semi_sync_master_status设置为ON,表明当前处于半同步复制下。当从库追赶上主库后修改为ON，
If a timeout occurs without any slave having acknowledged the transaction, the master reverts to asynchronous replication. When at least one semisynchronous slave catches up, the master returns to semisynchronous replication.