MySQL 9.5 Reference Manual Including MySQL NDB Cluster 9.5
      The state of a Group Replication group member shows its current
      role in the group. The Performance Schema table
      replication_group_members shows the
      state for each member in a group. If the group is fully functional
      and all members are communicating properly, all members report the
      same state for all other members. However, a member that has left
      the group or is part of a network partition cannot report accurate
      information on the other servers. In this situation, the member
      does not attempt to guess the status of the other servers, and
      instead reports them as unreachable.
    
A group member can be in the following states:
ONLINE
        
            The server is an active member of a group and in a fully
            functioning state. Other group members can connect to it, as
            can clients if applicable. A member is only fully
            synchronized with the group, and participating in it, when
            it is in the ONLINE state.
          
RECOVERING
        The server has joined a group and is in the process of becoming an active member. Distributed recovery is currently taking place, where the member is receiving state transfer from a donor using a remote cloning operation or the donor's binary log. This state is
For more information, see Section 20.5.4, “Distributed Recovery”.
OFFLINE
        The Group Replication plugin is loaded but the member does not belong to any group. This status may briefly occur while a member is joining or rejoining a group.
ERROR
        The member is in an error state and is not functioning correctly as a group member. A member can enter error state either while applying transactions or during the recovery phase. A member in this state does not participate in the group's transactions. For more information on possible reasons for error states, see Section 20.7.7, “Responses to Failure Detection and Network Partitioning”.
            Depending on the exit action set by
            group_replication_exit_state_action,
            the member is in read-only mode
            (super_read_only=ON) and
            could also be in offline mode
            (offline_mode=ON). Note
            that a server in offline mode following the
            OFFLINE_MODE exit action is displayed
            with ERROR status, not
            OFFLINE. A server with the exit action
            ABORT_SERVER shuts down and is removed
            from the view of the group. For more information, see
            Section 20.7.7.4, “Exit Action”.
          
            While a member is joining or rejoining a replication group,
            its status can be displayed as ERROR
            before the group completes the compatibility checks and
            accepts it as a member.
          
UNREACHABLE
        The local failure detector suspects that the member cannot be contacted, because the group's messages are timing out. This can happen if a member is disconnected involuntarily, for example. If you see this status for other servers, it can also mean that the member where you query this table is part of a partition, where a subset of the group's servers can contact each other but cannot contact the other servers in the group. For more information, see Section 20.7.8, “Handling a Network Partition and Loss of Quorum”.
See Section 20.4.3, “The replication_group_members Table” for an example of the Performance Schema table contents.