code: @Nullable
      public static EndpointSelector getNodeSelector(String groupName) {
          groupName = normalizeGroupName(groupName);
          return serverGroups.get(groupName);
      }
docs: Returns the {@link EndpointSelector} for the specified case-insensitive {@code groupName}.
  
  @return the {@link EndpointSelector}, or {@code null} if {@code groupName} has not been registered yet.