From MySQL 8.0.16, Group Replication has the concept of a communication protocol for the group. The Group Replication communication protocol version can be managed explicitly, and set to accommodate the oldest MySQL Server version that you want the group to support. This enables groups to be formed from members at different MySQL Server versions while ensuring backward compatibility. Versions from MySQL 5.7.14 allow compression of messages, and versions from MySQL 8.0.16 also allow fragmentation of messages. All members of the group must use the same communication protocol version, so that group members can be at different MySQL Server releases but only send messages that can be understood by all group members.
A MySQL server at version X can only join and reach
ONLINE status in a replication group if the
group's communication protocol version is less than or equal to
X. When a new member joins a replication group, it checks the
communication protocol version that is announced by the existing
members of the group. If the joining member supports that
version, it joins the group and uses the communication protocol
that the group has announced, even if the member supports
additional communication capabilities. If the joining member
does not support the communication protocol version, it is
expelled from the group.
If two members attempt to join in the same membership change event, they can only join if the communication protocol version for both members is already compatible with the group's communication protocol version. Members with different communication protocol versions from the group must join in isolation. For example:
One MySQL Server 8.0.16 instance can successfully join a group that uses the communication protocol version 5.7.24.
One MySQL Server 5.7.24 instance cannot successfully join a group that uses the communication protocol version 8.0.16.
Two MySQL Server 8.0.16 instances cannot simultaneously join a group that uses the communication protocol version 5.7.24.
Two MySQL Server 8.0.16 instances can simultaneously join a group that uses the communication protocol version 8.0.16.
You can inspect the communication protocol in use by a group by
UDF, which returns the oldest MySQL Server version that the
group supports. All existing members of the group return the
same communication protocol version. For example:
SELECT group_replication_get_communication_protocol(); +------------------------------------------------+ | group_replication_get_communication_protocol() | +------------------------------------------------+ | 8.0.16 | +------------------------------------------------+
Note that the
UDF returns the minimum MySQL version that the group supports,
which might differ from the version number that was passed to
UDF, and from the MySQL Server version that is installed on the
member where you use the UDF.
If you need to change the communication protocol version of a
group so that members at earlier releases can join, use the
UDF to specify the MySQL Server version of the oldest member
that you want to allow. This makes the group fall back to a
compatible communication protocol version if possible. The
is required to use this UDF, and all existing group members must
be online when you issue the statement, with no loss of
majority. For example:
If you upgrade all the members of a replication group to a new
MySQL Server release, the group's communication protocol version
is not automatically upgraded to match. If you no longer need to
support members at earlier releases, you can use the
UDF to set the communication protocol version to the new MySQL
Server version to which you have upgraded the members. For
UDF is implemented as a group action, so it is executed at the
same time on all members of the group. The group action starts
buffering messages and waits for delivery of any outgoing
messages that were already in progress to complete, then changes
the communication protocol version and sends the buffered
messages. If a member attempts to join the group at any time
after you change the communication protocol version, the group
members announce the new protocol version.
MySQL InnoDB cluster automatically and transparently manages the communication protocol versions of its members, whenever the cluster topology is changed using AdminAPI operations. An InnoDB cluster always uses the most recent communication protocol version that is supported by all the instances that are currently part of the cluster or joining it.
When an instance is added to, removed from, or rejoins the cluster, or a rescan or reboot operation is carried out on the cluster, the communication protocol version is automatically set to a version supported by the instance that is now at the earliest MySQL Server version.
When you carry out a rolling upgrade by removing instances from the cluster, upgrading them, and adding them back into the cluster, the communication protocol version is automatically upgraded when the last remaining instance at the old MySQL Server version is removed from the cluster prior to its upgrade.
To see the communication protocol version in use in an InnoDB
cluster, use the
function with the
'extended' option enabled.
The communication protocol version is returned in the
'GRProtocolVersion' field, provided that the
cluster has quorum and no cluster members are unreachable.