default void preBalance(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    BalanceRequest request) throws IOException {
  }
Called prior to requesting rebalancing of the cluster regions, though after the initial checks
for regions in transition and the balance switch flag.
@param ctx     the environment to interact with the framework and master
@param request the request used to trigger the balancer
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java