A replication group uses a Group Replication communication protocol version that can differ from the MySQL Server version of the members. To check the group's communication protocol version, issue the following statement on any member:

SELECT group_replication_get_communication_protocol();

The return value shows the oldest MySQL Server version that can join this group and use the group's communication protocol. Versions from MySQL 5.7.14 allow compression of messages, and versions from MySQL 8.0.16 also allow fragmentation of messages. Note that the group_replication_get_communication_protocol() UDF returns the minimum MySQL version that the group supports, which might differ from the version number that was passed to the group_replication_set_communication_protocol() UDF, and from the MySQL Server version that is installed on the member where you use the UDF.

When you upgrade all the members of a replication group to a new MySQL Server release, the Group Replication communication protocol version is not automatically upgraded, in case there is still a requirement to allow members at earlier releases to join. If you do not need to support older members and want to allow the upgraded members to use any added communication capabilities, after the upgrade use the group_replication_set_communication_protocol() UDF to upgrade the communication protocol, specifying the new MySQL Server version to which you have upgraded the members. For more information, see Section 18.4.1.4, “Setting a Group's Communication Protocol Version”.