MySQL 5.7增强半同步测试

   

    we've know the machenism of semi-synchronous replication in my previous article,let's do some test to learn more details about it.now i have three virtual machines in my test environment,one master and the other two slaves are replicated from master which based on GTID mode.the replication method is asynchronous at the moment.okay here we go.

1.Install the plugin of semi-synchronous replication on master.
root@localhost:mysql3306.sock [(none)]02:17:05>INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
Query OK, 0 rows affected (0.11 sec)

root@localhost:mysql3306.sock [(none)]02:29:53>SELECT PLUGIN_NAME, PLUGIN_STATUS
    ->        FROM INFORMATION_SCHEMA.PLUGINS
    ->        WHERE PLUGIN_NAME LIKE '%semi%';
+----------------------+---------------+
| PLUGIN_NAME          | PLUGIN_STATUS |
+----------------------+---------------+
| rpl_semi_sync_master | ACTIVE        |
+----------------------+---------------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:29:57>show plugins;
+----------------------------+----------+--------------------+--------------------+---------+
| Name                       | Status   | Type               | Library            | License |
+----------------------------+----------+--------------------+--------------------+---------+
| binlog                     | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| mysql_native_password      | ACTIVE   | AUTHENTICATION     | NULL               | GPL     |
| sha256_password            | ACTIVE   | AUTHENTICATION     | NULL               | GPL     |
| PERFORMANCE_SCHEMA         | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| MRG_MYISAM                 | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| MEMORY                     | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| InnoDB                     | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| INNODB_TRX                 | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_LOCKS               | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_LOCK_WAITS          | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_CMP                 | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_CMP_RESET           | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_CMPMEM              | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_CMPMEM_RESET        | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_CMP_PER_INDEX       | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_CMP_PER_INDEX_RESET | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_BUFFER_PAGE         | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_BUFFER_PAGE_LRU     | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_BUFFER_POOL_STATS   | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_TEMP_TABLE_INFO     | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_METRICS             | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_FT_DEFAULT_STOPWORD | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_FT_DELETED          | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_FT_BEING_DELETED    | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_FT_CONFIG           | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_FT_INDEX_CACHE      | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_FT_INDEX_TABLE      | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_TABLES          | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_TABLESTATS      | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_INDEXES         | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_COLUMNS         | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_FIELDS          | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_FOREIGN         | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_FOREIGN_COLS    | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_TABLESPACES     | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_DATAFILES       | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| INNODB_SYS_VIRTUAL         | ACTIVE   | INFORMATION SCHEMA | NULL               | GPL     |
| CSV                        | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| MyISAM                     | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| ARCHIVE                    | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| partition                  | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| BLACKHOLE                  | ACTIVE   | STORAGE ENGINE     | NULL               | GPL     |
| FEDERATED                  | DISABLED | STORAGE ENGINE     | NULL               | GPL     |
| ngram                      | ACTIVE   | FTPARSER           | NULL               | GPL     |
| rpl_semi_sync_master       | ACTIVE   | REPLICATION        | semisync_master.so | GPL     |  -- Here you'll see plugin "rpl_semi_sync_master" has been installed
+----------------------------+----------+--------------------+--------------------+---------+
45 rows in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:39:08>

2.Install the plugin of semi-synchronous replication on slave 1.
root@localhost:mysql.sock [(none)]02:17:39>INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
Query OK, 0 rows affected (0.06 sec)

root@localhost:mysql.sock [(none)]02:30:32>SELECT PLUGIN_NAME, PLUGIN_STATUS
    ->        FROM INFORMATION_SCHEMA.PLUGINS
    ->        WHERE PLUGIN_NAME LIKE '%semi%';
+---------------------+---------------+
| PLUGIN_NAME         | PLUGIN_STATUS |
+---------------------+---------------+
| rpl_semi_sync_slave | ACTIVE        |
+---------------------+---------------+
1 row in set (0.00 sec)

root@localhost:mysql.sock [(none)]02:31:05>
    
3.Install the plugin of semi-synchronous replication on slave 2.
root@localhost:mysql3306.sock [(none)]02:31:59>INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
Query OK, 0 rows affected (0.09 sec)

root@localhost:mysql3306.sock [(none)]02:32:09>SELECT PLUGIN_NAME, PLUGIN_STATUS
    ->        FROM INFORMATION_SCHEMA.PLUGINS
    ->        WHERE PLUGIN_NAME LIKE '%semi%';
+---------------------+---------------+
| PLUGIN_NAME         | PLUGIN_STATUS |
+---------------------+---------------+
| rpl_semi_sync_slave | ACTIVE        |
+---------------------+---------------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:32:17>

4.Enbale the semi-synchronous replication on master.
root@localhost:mysql3306.sock [(none)]02:39:08>SET GLOBAL rpl_semi_sync_master_enabled=1;  -- The parameter can be changed at runtime.
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:41:47>show global status like 'rpl%';  -- We can see all the status about semi-synchronous replication here.
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 0     |
| Rpl_semi_sync_master_net_avg_wait_time     | 0     |
| Rpl_semi_sync_master_net_wait_time         | 0     |
| Rpl_semi_sync_master_net_waits             | 0     |
| 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      | 0     |
| Rpl_semi_sync_master_tx_wait_time          | 0     |
| Rpl_semi_sync_master_tx_waits              | 0     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 0     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

5.Enbale the semi-synchronous replication on salve 1.
root@localhost:mysql.sock [(none)]02:43:29>SET GLOBAL rpl_semi_sync_slave_enabled=1;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql.sock [(none)]02:43:44>show global status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | OFF   |  -- It's still "OFF" now,'cause the sql IO_Thread should be restart.
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql.sock [(none)]02:50:45>stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql.sock [(none)]02:51:10>start slave io_thread;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql.sock [(none)]02:51:19>show global status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |  -- Now,the value has become "ON"
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql.sock [(none)]02:51:25>

6.Enbale the semi-synchronous replication on salve 2.
root@localhost:mysql3306.sock [(none)]02:33:13>SET GLOBAL rpl_semi_sync_slave_enabled=1;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:44:54>show global status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | OFF   |
+----------------------------+-------+
1 row in set (0.01 sec)

root@localhost:mysql3306.sock [(none)]02:45:09>stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:51:33>start slave io_thread;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:51:40>show global status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:51:49>

7.check the status of master again.
root@localhost:mysql3306.sock [(none)]02:42:17>show global status like 'rpl%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 2     |  -- This value of status will show how many slave in semi_synchronous mode you really have.
| Rpl_semi_sync_master_net_avg_wait_time     | 0     |
| Rpl_semi_sync_master_net_wait_time         | 0     |
| Rpl_semi_sync_master_net_waits             | 0     |
| 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      | 0     |
| Rpl_semi_sync_master_tx_wait_time          | 0     |
| Rpl_semi_sync_master_tx_waits              | 0     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 0     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]02:55:36>

8.Let's do some tests with semi-synchronous replication,create a table and insert several records on master.
root@localhost:mysql3306.sock [(none)]02:55:36>use zlm;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
root@localhost:mysql3306.sock [zlm]02:59:39>create table semi_sync_test(
    -> id int auto_increment,
    -> name char(15) not null default '',
    -> primary key(id)
    -> ) engine=innodb charset=utf8;
Query OK, 0 rows affected (0.04 sec)

root@localhost:mysql3306.sock [zlm]03:03:08>insert into semi_sync_test(name) values('aaa');
Query OK, 1 row affected (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:03:28>show global status like 'rpl%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 2     |
| Rpl_semi_sync_master_net_avg_wait_time     | 0     |
| Rpl_semi_sync_master_net_wait_time         | 0     |
| Rpl_semi_sync_master_net_waits             | 4     |  -- The times of master waiting for slave replies(two slaves plus two transcations got 4).
| 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      | 3257  |  -- Total avertage time master has waited for transaction.
| Rpl_semi_sync_master_tx_wait_time          | 6514  |  -- Total time master has waited for transaction.(two transactions plus 3257 got 6514).
| Rpl_semi_sync_master_tx_waits              | 2     |  -- I generated two transactions just now,the figure here is "2"
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 2     |  -- The two transactions were all acknowleged by the two slaves.
+--------------------------------------------+-------+
14 rows in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:03:40>

here's some messages from error log:

2018-06-10T12:41:47.265030Z 5 [Note] Semi-sync replication initialized for transactions.
2018-06-10T12:41:47.265060Z 5 [Note] Semi-sync replication enabled on the master.
2018-06-10T12:41:47.265349Z 0 [Note] Starting ack receiver thread  -- There's a individual thread called "ack receiver"
2018-06-10T12:47:17.754330Z 5 [Note] Aborted connection 5 to db: 'unconnected' user: 'root' host: 'localhost' (Got timeout reading communication packets)
2018-06-10T12:51:19.658621Z 6 [Note] While initializing dump thread for slave with UUID <27af30ca-6800-11e8-ad7e-080027de0e0e>, found a zombie dump thread with the same UUID. Master is killing the zombie dump thread(4).
2018-06-10T12:51:19.658773Z 6 [Note] Start binlog_dump to master_thread_id(6) slave_server(1013306), pos(, 4)
2018-06-10T12:51:19.658797Z 6 [Note] Start semi-sync binlog_dump to slave (server_id: 1013306), pos(, 4)
2018-06-10T12:51:19.658989Z 4 [Note] Stop asynchronous binlog_dump to slave (server_id: 1013306)
2018-06-10T12:51:40.294491Z 7 [Note] While initializing dump thread for slave with UUID <e00ef9f5-6c4b-11e8-8389-080027de0e0e>, found a zombie dump thread with the same UUID. Master is killing the zombie dump thread(3).
2018-06-10T12:51:40.294611Z 7 [Note] Start binlog_dump to master_thread_id(7) slave_server(1023306), pos(, 4)
2018-06-10T12:51:40.294663Z 3 [Note] Stop asynchronous binlog_dump to slave (server_id: 1023306)
2018-06-10T12:51:40.694773Z 7 [Note] Start semi-sync binlog_dump to slave (server_id: 1023306), pos(, 4)
2018-06-10T13:08:41.071298Z 8 [Note] Aborted connection 8 to db: 'zlm' user: 'root' host: 'localhost' (Got timeout reading communication packets)

my parameter about semi-synchronous replication listed below:

root@localhost:mysql3306.sock [zlm]03:22:51>show variables like 'rpl%';
+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 10000      |  -- 10s is the default value,it's the threshold when the master will turn into async mode.
| rpl_semi_sync_master_trace_level          | 32         |  -- There're 4 different value of it such as 1,16,32(default),64 of debug level.
| rpl_semi_sync_master_wait_for_slave_count | 1          |
| rpl_semi_sync_master_wait_no_slave        | ON         |  -- "ON" is default values,the semi-sync will continue even if the slave remain is less than the value of                                                        value of "rpl_semi_sync_master_wait_for_slave_count".  
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |  -- This values is only permitted in 5.7 or above which can guarantee consistency in replication.
| rpl_stop_slave_timeout                    | 31536000   |  -- This is the maxium and default value which means 1 year(it seems hardly to timeout^^).
+-------------------------------------------+------------+
7 rows in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:22:58>

Now i'm gonna to test when the semi-sync mode will turned into async mode:

1.set the parameter "rpl_semi_sync_master_wait_for_slave_count"=2 and "rpl_semi_sync_master_wait_no_slave"=OFF
root@localhost:mysql3306.sock [zlm]03:44:01>set global rpl_semi_sync_master_wait_for_slave_count=2;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:44:40>set global rpl_semi_sync_master_wait_no_slave=OFF;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:44:58>show variables like 'rpl%';
+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 10000      |
| rpl_semi_sync_master_trace_level          | 32         |
| rpl_semi_sync_master_wait_for_slave_count | 2          |
| rpl_semi_sync_master_wait_no_slave        | OFF        |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_stop_slave_timeout                    | 31536000   |
+-------------------------------------------+------------+
7 rows in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:45:06>

2.turn off the io_thread in one of the slave such as slave 2 and check the status of semi-sync mode both slave and master
slave 2:
root@localhost:mysql3306.sock [(none)]03:46:49>stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]03:47:15>show status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | OFF   |  -- As soo as the IO_Thread had been stopped,the semi-sync mode immediately turned into async mode.
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]03:47:37>

master:
root@localhost:mysql3306.sock [zlm]03:47:50>show status like 'rpl_semi_sync_master_status';
+-----------------------------+-------+
| Variable_name               | Value |
+-----------------------------+-------+
| Rpl_semi_sync_master_status | OFF   |  -- The value of status changed follow the value on slave.
+-----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:48:49>

whicn also can be seen in the error log:
2018-06-10T13:47:15.870173Z 0 [ERROR] mysqld: Got an error reading communication packets  -- Here occured an error.
2018-06-10T13:47:58.552523Z 7 [Note] Stop semi-sync binlog_dump to slave (server_id: 1023306)
2018-06-10T13:47:58.942023Z 7 [Note] Semi-sync replication switched OFF.  -- The semi-sync mode was OFF.
2018-06-10T13:47:58.942052Z 7 [Note] Aborted connection 7 to db: 'unconnected' user: 'repl' host: 'zlm3' (failed on flush_net())  -- lost connection with slave 2.
2018-06-10T13:53:49.983185Z 10 [Note] Aborted connection 10 to db: 'zlm' user: 'root' host: 'localhost' (Got timeout reading communication packets)

3.set the parameter "rpl_semi_sync_master_wait_no_slave"=ON on master again.
root@localhost:mysql3306.sock [zlm]03:48:49>set global rpl_semi_sync_master_wait_no_slave=ON;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:58:35>show status like 'rpl_semi_sync_master_status';
+-----------------------------+-------+
| Variable_name               | Value |
+-----------------------------+-------+
| Rpl_semi_sync_master_status | ON    |  -- After set "rpl_semi_sync_master_wait_no_slave"=ON,it turned back again with semi-sync mode.
+-----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:58:54>

slave 1:
root@localhost:mysql.sock [(none)]03:59:58>show status like 'rpl_semi_sync_slave_status';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |  -- slave 1 still can be replicated from master using semi-sync mode.
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql.sock [(none)]04:00:11>

slave 2:
root@localhost:mysql3306.sock [(none)]03:47:37>show status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | OFF   |  -- only if IO_Thread is stated,the value cannnot be changed.
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]04:00:53>

4.continue to insert into test table "semi_sync_test" wait until up to timeout happen.
master:
root@localhost:mysql3306.sock [zlm]03:58:35>show status like 'rpl_semi_sync_master_status';
+-----------------------------+-------+
| Variable_name               | Value |
+-----------------------------+-------+
| Rpl_semi_sync_master_status | ON    |
+-----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]03:58:54>insert into semi_sync_test(name) values('bbb');
Query OK, 1 row affected (10.01 sec)  -- Notice,the time used is "10.01" secsonds which is matched with the parameter "rpl_semi_sync_master_timeout"=10000.

root@localhost:mysql3306.sock [zlm]04:07:00>show status like 'rpl_semi_sync_master_status';
+-----------------------------+-------+
| Variable_name               | Value |
+-----------------------------+-------+
| Rpl_semi_sync_master_status | OFF   |
+-----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]04:07:12>

The master become using async mode again on account of the limit time of parameter "rpl_semi_sync_master_timeout".

error log of master:
2018-06-10T13:53:49.983185Z 10 [Note] Aborted connection 10 to db: 'zlm' user: 'root' host: 'localhost' (Got timeout reading communication packets)
2018-06-10T14:03:54.321177Z 11 [Note] Aborted connection 11 to db: 'zlm' user: 'root' host: 'localhost' (Got timeout reading communication packets)
2018-06-10T14:07:00.367988Z 12 [Warning] Timeout waiting for reply of binlog (file: mysql-bin.000041, pos: 1002), semi-sync up to file mysql-bin.000041, position 732.
2018-06-10T14:07:00.368027Z 12 [Note] Semi-sync replication switched OFF.  -- the error log also has told you about the change.

slave 1:
root@localhost:mysql.sock [(none)]04:00:11>show status like 'rpl_semi_sync_slave_status';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |  -- Even though the value is "ON" in slave 1,but it still cannot use semi-sync mode duo to master has changed.
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql.sock [(none)]04:08:08>

slave 2:
root@localhost:mysql3306.sock [(none)]04:08:38>show status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | OFF   |  -- Slave won't have any change about the value.
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]04:08:46>

5.In the end,let's start the IO_Thread on slave 2 and see what will happen.
slave1:
root@localhost:mysql3306.sock [(none)]04:08:46>start slave;
Query OK, 0 rows affected (0.00 sec)

root@localhost:mysql3306.sock [(none)]04:17:12>show status like 'rpl%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [(none)]04:17:16>

slave2:
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |
+----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql.sock [(none)]04:17:27>

master:
root@localhost:mysql3306.sock [zlm]04:07:12>show status like 'rpl_semi_sync_master_status';
+-----------------------------+-------+
| Variable_name               | Value |
+-----------------------------+-------+
| Rpl_semi_sync_master_status | ON    |
+-----------------------------+-------+
1 row in set (0.00 sec)

root@localhost:mysql3306.sock [zlm]04:17:34>

master error log:
2018-06-10T14:12:12.905142Z 12 [Note] Aborted connection 12 to db: 'zlm' user: 'root' host: 'localhost' (Got timeout reading communication packets)
2018-06-10T14:17:12.140168Z 13 [Note] Start binlog_dump to master_thread_id(13) slave_server(1023306), pos(, 4)
2018-06-10T14:17:12.438034Z 13 [Note] Start semi-sync binlog_dump to slave (server_id: 1023306), pos(, 4)
2018-06-10T14:17:12.447797Z 0 [Note] Semi-sync replication switched ON at (mysql-bin.000041, 1002)  -- became semi-sync mode eventually.

Summary:

• there're sevaral restrictions when using semi-synchronous replication:
• based on MySQL 5.5 or above version.
• parameter "have_dynamic_loading" must be set as "yes".
• replication must has been normally worked such as asynchronous mode.
• merely compatible with default replication channel.
• take care of the converting condition between semi-sync and async mode due to timeout.
• in order to keep consistency of replication,increase the value of "rpl_semi_sync_master_timeout" close to an infinite value maybe makeshift.