code: private void checkReplicationPolicyCompatibility(Configuration conf) {
      if (!(BlockPlacementPolicy.getInstance(conf, null, null, null, null, null)
          instanceof BlockPlacementPolicyDefault)) {
        throw new IllegalArgumentException("Configuration lacks BlockPlacementPolicyDefault");
      }
    }
docs: /*
  Check that this Balancer is compatible with the Block Placement Policy
  used by the Namenode.
  
  In case it is not compatible, throw IllegalArgumentException