MySQL 8.3 Release Notes
During an online upgrade procedure, if the group is in
single-primary mode, all the servers that are not currently
offline for upgrading function as they did before. The group
elects a new primary whenever necessary, following the election
policies described in
Section 20.1.3.1, “Single-Primary Mode”. Note
that if you require the primary to remain the same throughout
(except when it is being upgraded itself), you must first
upgrade all of the secondaries to a version higher than or equal
to the target primary member version, then upgrade the primary
last. The primary cannot remain as the primary unless it is
running the lowest MySQL Server version in the group. After the
primary has been upgraded, you can use the
group_replication_set_as_primary()
function to reappoint it as the primary.
If the group is in multi-primary mode, fewer online members are available to perform writes during the upgrade procedure, because upgraded members join in read-only mode after their upgrade. When all members have been upgraded to the same release, they all change back to read-write mode automatically.
To deal with a problem situation, for example if you have to
roll back an upgrade or add extra capacity to a group in an
emergency, it is possible to allow a member to join an online
group although it is running a lower MySQL Server version than
the lowest version in use by other group members. The Group
Replication system variable
group_replication_allow_local_lower_version_join
can be used in such situations to override the normal
compatibility policies.
Important
Setting
group_replication_allow_local_lower_version_join
to ON does not make
the new member compatible with the group; doing this allows it
to join the group without any safeguards against incompatible
behaviors by the existing members. This must therefore only be
used carefully in specific situations, and you must take
additional precautions to avoid the new member failing due to
normal group activity. See the description of this variable
for more information.