MySQL Group Replication: who is the primary master ??

As you know, MySQL Group Replication runs by default in single primary mode.

mysql2 mysql> show global variables like 'group_replication_single_primary_mode';
+---------------------------------------+-------+
| Variable_name                         | Value |
+---------------------------------------+-------+
| group_replication_single_primary_mode | ON    |
+---------------------------------------+-------+

But how can we easily find which member of the group is the Primary-Master ?

Of course you could check which one is not in read_only:

mysql2 mysql> select @@read_only;
+-------------+
| @@read_only |
+-------------+
|           1 |
+-------------+

But then you need to perform this on all the nodes one by one until you find the right one.

The primary master is exposed through a status variable: group_replication_primary_member:

mysql2 mysql> show global status like 'group_replication_primary_member';
+----------------------------------+--------------------------------------+
| Variable_name                    | Value                                |
+----------------------------------+--------------------------------------+
| group_replication_primary_member | f7aa830d-0f02-11e7-83ba-08002718d305 |
+----------------------------------+--------------------------------------+

But the value is not that obvious to know which MySQL server it refers to.

Once again, as we are able to know who are the members in the group via Performance_Schema, we can verify this:

mysql2 mysql> select * from performance_schema.replication_group_members\G                                            
*************************** 1. row ***************************
CHANNEL_NAME: group_replication_applier
   MEMBER_ID: 73f48dcc-0f02-11e7-99b8-08002718d305
 MEMBER_HOST: mysql3
 MEMBER_PORT: 3306
MEMBER_STATE: ONLINE
*************************** 2. row ***************************
CHANNEL_NAME: group_replication_applier
   MEMBER_ID: f7aa830d-0f02-11e7-83ba-08002718d305
 MEMBER_HOST: mysql1
 MEMBER_PORT: 3306
MEMBER_STATE: ONLINE
*************************** 3. row ***************************
CHANNEL_NAME: group_replication_applier
   MEMBER_ID: fbe0b0a1-0f02-11e7-a1e5-08002718d305
 MEMBER_HOST: mysql2
 MEMBER_PORT: 3306
MEMBER_STATE: ONLINE

So we can here see that f7aa830d-0f02-11e7-83ba-08002718d305 is in fact mysql1 !

Can we merge all this in one single query ? Of course !

Here is the query you need to use:

mysql2 mysql> SELECT member_host as "primary master"
              FROM performance_schema.global_status         
              JOIN performance_schema.replication_group_members         
              WHERE variable_name = 'group_replication_primary_member'         
                AND member_id=variable_value;
+----------------+
| primary master |
+----------------+
| mysql1         |
+----------------+

Simple and useful tip 😉