MySQL半同步安装以及参数
基于MySQL5.5
官档地址：
Semisynchronous Replication Administrative Interface
https://dev.mysql.com/doc/refman/5.5/en/replication-semisync-interface.html
控制半同步复制的系统参数
rpl_semi_sync_master_enabled
Controls whether semisynchronous replication is enabled on the master. To enable or disable the plugin, set this variable to 1 or 0, respectively. The default is 0 (off).
控制是否启用半同步复制
rpl_semi_sync_master_timeout
A value in milliseconds that controls how long the master waits on a commit for acknowledgment from a slave before timing out and reverting to asynchronous replication. The default value is 10000 (10 seconds).
等待从库提交并反馈的超时时间，超时则转向异步同步
rpl_semi_sync_slave_enabled
Similar to rpl_semi_sync_master_enabled, but controls the slave plugin.
类似于 rpl_semi_sync_master_enabled，但是用于控制 slave 的插件。
启用半同步复制监视的状态变量
Rpl_semi_sync_master_clients
The number of semisynchronous slaves.
半同步复制的 slave 的数量
Rpl_semi_sync_master_status
Whether semisynchronous replication currently is operational on the master. The value is 1 if the plugin has been enabled and a commit acknowledgment has occurred. It is 0 if the plugin is not enabled or the master has fallen back to asynchronous replication due to commit acknowledgment timeout.
半同步复制目前是否运行在主机上。如果已启用该插件，提交确认发生，值为 1。如果未启用该插件或 master 已经回落到异步复制导致提交确认超时，为 0。
Rpl_semi_sync_master_no_tx
The number of commits that were not acknowledged successfully by a slave.
提交但是没有反馈成功的slave的数量。
Rpl_semi_sync_master_yes_tx
The number of commits that were acknowledged successfully by a slave.
提交后反馈成功的slave的数量。
Rpl_semi_sync_slave_status
Whether semisynchronous replication currently is operational on the slave. This is 1 if the plugin has been enabled and the slave I/O thread is running, 0 otherwise.
是否同步复制目前是业务上的slave。如果已启用该插件，slave的 I/O 线程正在运行，是1 ，否则 0。
安装和配置
前提条件
1. MySQL 5.5 or higher must be installed.
2. The capability of installing plugins requires a MySQL server that supports dynamic loading. To verify this, check that the value of the have_dynamic_loading system variable is YES. Binary distributions should support dynamic loading.
3. Replication must already be working.
安装
1.设置主从库参数文件
[mysqld]
plugin_dir=/path/to/plugin/directory
主库：
INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
从库:
INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
查询安装情况
mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS FROM INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME LIKE '%semi%';
+----------------------+---------------+
| PLUGIN_NAME | PLUGIN_STATUS |
+----------------------+---------------+
| rpl_semi_sync_master | ACTIVE |
+----------------------+---------------+
安装完成后，半同步默认是关闭的，需要在主从库上面都开启，才能使用，只开启主或者从，都无法使用
mysql运行时，可以使用以下参数
主库：
SET GLOBAL rpl_semi_sync_master_enabled = {0|1}; #默认是0，1是启用，0是关闭
SET GLOBAL rpl_semi_sync_master_timeout = N; #单位milliseconds（毫秒），10000毫秒=10秒
从库：
SET GLOBAL rpl_semi_sync_slave_enabled = {0|1};
如果要在mysql运行时启用半同步，必须重启IO_THREAD，slave会重新连上master并且被注册为 semisynchronous slave
STOP SLAVE IO_THREAD;
START SLAVE IO_THREAD;
如果不重启的话，slave将会继续使用 异步复制
可以重启mysql时
配置my.cof参数
主库：
[mysqld]
rpl_semi_sync_master_enabled=1
rpl_semi_sync_master_timeout=1000 # 1 second
从库：
[mysqld]
rpl_semi_sync_slave_enabled=1
监控
mysql> SHOW VARIABLES LIKE 'rpl_semi_sync%';
mysql> SHOW STATUS LIKE 'Rpl_semi_sync%';
卸载
主库：
UNINSTALL PLUGIN rpl_semi_sync_master;
从库:
UNINSTALL PLUGIN rpl_semi_sync_slave;