code: protected void reportRegister(DatanodeID node) {
      if (node != null
          && shouldUpdateNodes()) {
        if (!liveDatanodes.contains(node)) {
          // A new node has checked in, we want to send a ClearPrimary command to
          // it as well.
          outStandingHeartbeats.add(node);
          liveDatanodes.add(node);
        }
      }
    }
docs: Processes a register from the datanode. First, we will
  await a heartbeat, and later for a incremental block
  report.
  
  @param node
  the datanode that has reported