The automatic primary member election process involves each member looking at the new view of the group, ordering the potential new primary members, and choosing the member that qualifies as the most suitable. Each member makes its own decision locally, following the primary election algorithm in its MySQL Server release. Because all members must reach the same decision, members adapt their primary election algorithm if other group members are running lower MySQL Server versions, so that they have the same behavior as the member with the lowest MySQL Server version in the group.
The factors considered by members when electing a primary, in order, are as follows:
The first factor considered is which member or members are running the lowest MySQL Server version. If all group members are running MySQL 8.0.17 or higher, members are first ordered by the patch version of their release. If any members are running MySQL Server 5.7 or MySQL 8.0.16 or lower, members are first ordered by the major version of their release, and the patch version is ignored.
If more than one member is running the lowest MySQL Server version, the second factor considered is the member weight of each of those members, as specified by the group_replication_member_weight system variable on the member. If any member of the group is running MySQL Server 5.7, where this system variable was not available, this factor is ignored.
The group_replication_member_weight system variable specifies a number in the range 0-100. All members default to a weight of 50, so set a weight below this to lower their ordering, and a weight above it to increase their ordering. You can use this weighting function to prioritize the use of better hardware or to ensure failover to a specific member during scheduled maintenance of the primary.
If more than one member is running the lowest MySQL Server version, and more than one of those members has the highest member weight (or member weighting is being ignored), the third factor considered is the lexographical order of the generated server UUIDs of each member, as specified by the server_uuid system variable. The member with the lowest server UUID is chosen as the primary. This factor acts as a guaranteed and predictable tie-breaker so that all group members reach the same decision if it cannot be determined by any important factors.