MySQL 9.1 Release Notes
Group Replication in MySQL 9.1 uses the concept of a group communication protocol whose 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 using different MySQL Server versions while ensuring backward compatibility.
MySQL 5.7.14 and later allows compression of messages (see Section 20.7.4, “Message Compression”).
MySQL 8.0.16 and later also allows fragmentation of messages (see Section 20.7.5, “Message Fragmentation”).
MySQL 8.0.27 and later also allow the group communication engine to operate with a single consensus leader when the group is in single-primary mode and
group_replication_paxos_single_leaderis set to true (see Section 20.7.3, “Single Consensus Leader”).
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
using the
group_replication_get_communication_protocol()
function, 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
group_replication_get_communication_protocol()
function 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()
function, and from the MySQL Server version that is installed on
the member where you use the function.
If you need to change the communication protocol version of a
group so that members at earlier releases can join, use the
group_replication_set_communication_protocol()
function 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
GROUP_REPLICATION_ADMIN privilege
is required to use this function, and all existing group members
must be online when you issue the statement, with no loss of
majority. For example:
SELECT group_replication_set_communication_protocol("5.7.25");
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
group_replication_set_communication_protocol()
function to set the communication protocol version to the new
MySQL Server version to which you have upgraded the members. For
example:
SELECT group_replication_set_communication_protocol("8.0.16");
The
group_replication_set_communication_protocol()
function 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. For details, see InnoDB Cluster and Group Replication Protocol.