INSTALL PLUGIN 'rpl_semi_sync_master' SONAME 'semisync_master.so' not working
I think the problem with your attempt is that you put quotes around the plugin name, along with the fact that you used a .so suffix to the library name.

Step-by-step:

Check that your MySQL install is dynamic_loading capable:

mysql&gt; show variables like '%dynamic%';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| have_dynamic_loading | YES   |
+----------------------+-------+
1 row in set (0.00 sec)

mysql&gt;


Check that your plugin directory is set correctly:

mysql&gt; show variables like '%plugin%';
+---------------+-----------------------------------------------------+
| Variable_name | Value                                               |
+---------------+-----------------------------------------------------+
| plugin_dir    | C:\Program Files\MySQL\MySQL Server 5.6\lib\plugin\ |
+---------------+-----------------------------------------------------+
1 row in set (0.00 sec)

mysql&gt;


Then verify the .dll exists in that location:



The plugin install should then work OK:

mysql&gt; INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.dll';
Query OK, 0 rows affected (0.01 sec)

mysql&gt;