Group replication enables you to reset the group membership list by forcing a specific configuration. For instance in the case above, where s1 and s2 are the only servers online, you could chose to force a membership configuration consisting of only s1 and s2. This requires checking some information about s1 and s2 and then using the group_replication_force_members variable.

Figure 17.8 Forcing a New Membership

Suppose that you are back in the situation where s1 and s2 are the only servers left in the group. Servers s3, s4 and s5 have left the group unexpectedly. To make servers s1 and s2 continue, you want to force a membership configuration that contains only s1 and s2.

Recall that the system is blocked and the current configuration is the following (as perceived by the local failure detector on s1):

mysql> SELECT * FROM performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| CHANNEL_NAME              | MEMBER_ID                            | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| group_replication_applier | 1999b9fb-4aaf-11e6-bb54-28b2bd168d07 | 127.0.0.1   |       13002 | UNREACHABLE  |
| group_replication_applier | 199b2df7-4aaf-11e6-bb16-28b2bd168d07 | 127.0.0.1   |       13001 | ONLINE       |
| group_replication_applier | 199bb88e-4aaf-11e6-babe-28b2bd168d07 | 127.0.0.1   |       13000 | ONLINE       |
| group_replication_applier | 19ab72fc-4aaf-11e6-bb51-28b2bd168d07 | 127.0.0.1   |       13003 | UNREACHABLE  |
| group_replication_applier | 19b33846-4aaf-11e6-ba81-28b2bd168d07 | 127.0.0.1   |       13004 | UNREACHABLE  |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
5 rows in set (0,00 sec)

The first thing to do is to check what is the peer address (group communication identifier) for s1 and s2. Log in to s1 and s2 and get that information as follows.

mysql> SELECT @@group_replication_local_address;
+-----------------------------------+
| @@group_replication_local_address |
+-----------------------------------+
| 127.0.0.1:10000                   |
+-----------------------------------+
1 row in set (0,00 sec)

Then log in to s2 and do the same thing:

mysql> SELECT @@group_replication_local_address;
+-----------------------------------+
| @@group_replication_local_address |
+-----------------------------------+
| 127.0.0.1:10001                   |
+-----------------------------------+
1 row in set (0,00 sec)

Once you know the group communication addresses of s1 (127.0.0.1:10000) and s2 (127.0.0.1:10001), you can use that on one of the two servers to inject a new membership configuration, thus overriding the existing one that has lost quorum. To do that on s1:

mysql> SET GLOBAL group_replication_force_members="127.0.0.1:10000,127.0.0.1:10001";
Query OK, 0 rows affected (7,13 sec)

This unblocks the group by forcing a different configuration. Check replication_group_members on both s1 and s2 to verify the group membership after this change. First on s1.

mysql> select * from performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| CHANNEL_NAME              | MEMBER_ID                            | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| group_replication_applier | b5ffe505-4ab6-11e6-b04b-28b2bd168d07 | 127.0.0.1   |       13000 | ONLINE       |
| group_replication_applier | b60907e7-4ab6-11e6-afb7-28b2bd168d07 | 127.0.0.1   |       13001 | ONLINE       |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
2 rows in set (0,00 sec)

And then on s2.

mysql> select * from performance_schema.replication_group_members;
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| CHANNEL_NAME              | MEMBER_ID                            | MEMBER_HOST | MEMBER_PORT | MEMBER_STATE |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
| group_replication_applier | b5ffe505-4ab6-11e6-b04b-28b2bd168d07 | 127.0.0.1   |       13000 | ONLINE       |
| group_replication_applier | b60907e7-4ab6-11e6-afb7-28b2bd168d07 | 127.0.0.1   |       13001 | ONLINE       |
+---------------------------+--------------------------------------+-------------+-------------+--------------+
2 rows in set (0,00 sec)

When forcing a new membership configuration, make sure that whatever servers are going to be forced out of the group are indeed stopped. In the scenario depicted above, if s3, s4 and s5 are not really unreachable but instead are online, they may have formed their own functional partition (they are 3 out of 5, hence they have the majority). In that case, forcing a group membership list with s1 and s2 could create an artificial split-brain situation. Therefore it is important before forcing a new membership configuration to ensure that the servers to be excluded are indeed shutdown and if they are not, shut them down before proceding.