synchronized void startAll() throws IOException {
    try {
      UserGroupInformation.getLoginUser().doAs(
          new PrivilegedExceptionAction<Object>() {
            @Override
            public Object run() throws Exception {
              for (BPOfferService bpos : offerServices) {
                bpos.start();
              }
              return null;
            }
          });
    } catch (InterruptedException ex) {
      IOException ioe = new IOException();
      ioe.initCause(ex.getCause());
      throw ioe;
    }
  }
  

    //This must be called only by blockPoolManager
  void start() {
    for (BPServiceActor actor : bpServices) {
      actor.start();
    }
  }
  

 
   /**
   * 无论发生任何异常，都不会停止offerService方法，除非shouldRun 或者shouldServiceRun返回false
   * No matter what kind of exception we get, keep retrying to offerService().
   * That's the loop that connects to the NameNode and provides basic DataNode
   * functionality.
   *
   * Only stop when "shouldRun" or "shouldServiceRun" is turned off, which can
   * happen either at shutdown or due to refreshNamenodes.
   */
  @Override
  public void run() {
    LOG.info(this + " starting to offer service");

    try {
      while (true) {
        // init stuff
        try {
          // setup storage
          //连接到namenode，注册datanode
          connectToNNAndHandshake();
          break;
        } catch (IOException ioe) {
                .......
        }
      }

      runningState = RunningState.RUNNING;

      while (shouldRun()) {
        try {
          //心跳报告
          offerService();
        } catch (Exception ex) {
           .....
        }
      }
      runningState = RunningState.EXITED;
    } catch (Throwable ex) {
      ...
    } finally {
     ....
    }
  }
 

 
   @Override
  public RegisterDatanodeResponseProto registerDatanode(
      RpcController controller, RegisterDatanodeRequestProto request)
      throws ServiceException {
    DatanodeRegistration registration = PBHelper.convert(request
        .getRegistration());
    DatanodeRegistration registrationResp;
    try {
      registrationResp = impl.registerDatanode(registration);
    } catch (IOException e) {
      throw new ServiceException(e);
    }
    return RegisterDatanodeResponseProto.newBuilder()
        .setRegistration(PBHelper.convert(registrationResp)).build();
  }
 
 

 
   @Override
  public HeartbeatResponseProto sendHeartbeat(RpcController controller,
      HeartbeatRequestProto request) throws ServiceException {
    HeartbeatResponse response;
    try {
        ......
      //发送心跳
      response = impl.sendHeartbeat(PBHelper.convert(request.getRegistration()),
          report, request.getCacheCapacity(), request.getCacheUsed(),
          request.getXmitsInProgress(),
          request.getXceiverCount(), request.getFailedVolumes(),
          volumeFailureSummary);
    } catch (IOException e) {
      throw new ServiceException(e);
    }
    
    //返回命令
    HeartbeatResponseProto.Builder builder = HeartbeatResponseProto
        .newBuilder();
    DatanodeCommand[] cmds = response.getCommands();
    .............
  }
 
 

 
   /**
   *
   * sendHeartbeat方法告诉namenode这个datanode还活着，当然也包含一些状态的信息. namenode也会通过心跳信息给datanode发送一些命令，通过DatanodeCommand对象来封装命令。通过这些DatanodeCommand命令，DataNode做一些删除本地无效的块、或者将本地的块复制到其他的datanode的操作。
   *
   * sendHeartbeat() tells the NameNode that the DataNode is still
   * alive and well.  Includes some status info, too. 
   * It also gives the NameNode a chance to return 
   * an array of "DatanodeCommand" objects in HeartbeatResponse.
   * A DatanodeCommand tells the DataNode to invalidate local block(s), 
   * or to copy them to other DataNodes, etc.
   * @param registration datanode registration information datanode的注册信息
   * @param reports utilization report per storage datanode上每个存储的利用率报告（datanode可以配置多个存储目录，这些存储目录可以是异构的，如内存、disk、ssd等）
   * @param xmitsInProgress number of transfers from this datanode to others
   * @param xceiverCount number of active transceiver threads
   * @param failedVolumes number of failed volumes
   * @param volumeFailureSummary info about volume failures
   * @throws IOException on error
   */
  @Idempotent
  public HeartbeatResponse sendHeartbeat(DatanodeRegistration registration,
                                       StorageReport[] reports,
                                       long dnCacheCapacity,
                                       long dnCacheUsed,
                                       int xmitsInProgress,
                                       int xceiverCount,
                                       int failedVolumes,
                                       VolumeFailureSummary volumeFailureSummary)
      throws IOException;
 

 
 
  /**
   * 
   * 
   * datanodeMap这个map主要是存储了StorageID到DatanodeDescriptor的映射关系，如注释所说，datanode向namenode注册的时候分为三种情况。
   * 
   *  1.如果是以一个新的storage id注册，直接放到map里。
   *  2.现有节点重复注册，这个时候用新的替换旧的就行
   *  3.一个已经存在的datanode以一个不同的storage id来注册
   * 
   * Stores the datanode -> block map.  
   * <p>
   * Done by storing a set of {@link DatanodeDescriptor} objects, sorted by 
   * storage id. In order to keep the storage map consistent it tracks 
   * all storages ever registered with the namenode.
   * A descriptor corresponding to a specific storage id can be
   * <ul> 
   * <li>added to the map if it is a new storage id;</li>
   * <li>updated with a new datanode started as a replacement for the old one 
   * with the same storage id; and </li>
   * <li>removed if and only if an existing datanode is restarted to serve a
   * different storage id.</li>
   * </ul> <br> 
   * <p>
   * Mapping: StorageID -> DatanodeDescriptor
   */
  private final NavigableMap<String, DatanodeDescriptor> datanodeMap
      = new TreeMap<String, DatanodeDescriptor>();

  /**
   * 集群的网络结构
   * Cluster network topology
   */
  private final NetworkTopology networktopology;

  /**
   *  host和DatanodeDescriptor和映射，因为一个节点上可能会有多个datanode。
   *  所以在Host2NodesMap内部其实是String到DatanodeDescriptor[]的映射，
   *  这样的话对这个节点上的某一个datanode进行增删改查操作一个普通的map就无能为力了，
   *  Host2NodesMap主要就是对这些增删改查操作做了一下封装
   *  Host names to datanode descriptors mapping.
   */
  private final Host2NodesMap host2DatanodeMap = new Host2NodesMap();

 
//这里用 nodeS表示从datanodeMap中获取的datanode信息
DatanodeDescriptor nodeS = getDatanode(nodeReg.getDatanodeUuid());
//用nodeN表示从host2DatanodeMap获取的信息
DatanodeDescriptor nodeN = host2DatanodeMap.getDatanodeByXferAddr(nodeReg.getIpAddr(), nodeReg.getXferPort());
          

 
      //此情况为数据节点存在,但是使用了新的存储ID
      if (nodeN != null && nodeN != nodeS) {
        NameNode.LOG.info("BLOCK* registerDatanode: " + nodeN);
        // nodeN previously served a different data storage, 
        // which is not served by anybody anymore.
        //移除
        removeDatanode(nodeN);
        // physically remove node from datanodeMap
        //物理层面的移除，包含移除这个datanode下面的数据块等
        wipeDatanode(nodeN);
        nodeN = null;
      }

      //重复注册的情况，主要就是更新信息
      if (nodeS != null) {
            ................
          nodeS.updateRegInfo(nodeReg);

          nodeS.setSoftwareVersion(nodeReg.getSoftwareVersion());
          nodeS.setDisallowed(false); // Node is in the include list

          //重新解析网络的位置信息
          // resolve network location
          if(this.rejectUnresolvedTopologyDN) {
            nodeS.setNetworkLocation(resolveNetworkLocation(nodeS));
            nodeS.setDependentHostNames(getNetworkDependencies(nodeS));
          } else {
            nodeS.setNetworkLocation(
                resolveNetworkLocationWithFallBackToDefaultLocation(nodeS));
            nodeS.setDependentHostNames(
                getNetworkDependenciesWithDefault(nodeS));
          }
          getNetworkTopology().add(nodeS);
            
          // also treat the registration message as a heartbeat
          heartbeatManager.register(nodeS);
          incrementVersionCount(nodeS.getSoftwareVersion());
          startDecommissioningIfExcluded(nodeS);
          success = true;
         ...........................
      }



    //接下来处理从未注册的过的新节点注册的情况
    
    
            // resolve network location
            //解析网络信息，将其加入集群的网络拓扑中
        if(this.rejectUnresolvedTopologyDN) {
          nodeDescr.setNetworkLocation(resolveNetworkLocation(nodeDescr));
          nodeDescr.setDependentHostNames(getNetworkDependencies(nodeDescr));
        } else {
          nodeDescr.setNetworkLocation(
              resolveNetworkLocationWithFallBackToDefaultLocation(nodeDescr));
          nodeDescr.setDependentHostNames(
              getNetworkDependenciesWithDefault(nodeDescr));
        }
        networktopology.add(nodeDescr);
        nodeDescr.setSoftwareVersion(nodeReg.getSoftwareVersion());
  
        // register new datanode
        addDatanode(nodeDescr);
        // also treat the registration message as a heartbeat
        // no need to update its timestamp
        // because its is done when the descriptor is created
        heartbeatManager.addDatanode(nodeDescr);
        incrementVersionCount(nodeReg.getSoftwareVersion());
        startDecommissioningIfExcluded(nodeDescr);
        success = true;

  /** Add a datanode. */
  void addDatanode(final DatanodeDescriptor node) {
    // To keep host2DatanodeMap consistent with datanodeMap,
    // remove  from host2DatanodeMap the datanodeDescriptor removed
    // from datanodeMap before adding node to host2DatanodeMap.
    
    //加到datanodeMap和 host2DatanodeMap中
    synchronized(datanodeMap) {
      host2DatanodeMap.remove(datanodeMap.put(node.getDatanodeUuid(), node));
    }

    //加到网络中
    networktopology.add(node); // may throw InvalidTopologyException
    host2DatanodeMap.add(node);
    checkIfClusterIsNowMultiRack(node);

    if (LOG.isDebugEnabled()) {
      LOG.debug(getClass().getSimpleName() + ".addDatanode: "
          + "node " + node + " is added to datanodeMap.");
    }
  }

  /** Handle heartbeat from datanodes. */
  public DatanodeCommand[] handleHeartbeat(DatanodeRegistration nodeReg,
      StorageReport[] reports, final String blockPoolId,
      long cacheCapacity, long cacheUsed, int xceiverCount, 
      int maxTransfers, int failedVolumes,
      VolumeFailureSummary volumeFailureSummary) throws IOException {
    synchronized (heartbeatManager) {
      synchronized (datanodeMap) {
        DatanodeDescriptor nodeinfo = null;
        try {
          //获取datanode的信息
          nodeinfo = getDatanode(nodeReg);
        } catch(UnregisteredNodeException e) {
          return new DatanodeCommand[]{RegisterCommand.REGISTER};
        }
        //是否允许连接
        // Check if this datanode should actually be shutdown instead. 
        if (nodeinfo != null && nodeinfo.isDisallowed()) {
          setDatanodeDead(nodeinfo);
          throw new DisallowedDatanodeException(nodeinfo);
        }

        //检查是否注册过
        if (nodeinfo == null || !nodeinfo.isRegistered()) {
          return new DatanodeCommand[]{RegisterCommand.REGISTER};
        }

        //更新datanode的信息，如使用空间，剩余空间等
        heartbeatManager.updateHeartbeat(nodeinfo, reports,
                                         cacheCapacity, cacheUsed,
                                         xceiverCount, failedVolumes,
                                         volumeFailureSummary);

        //是否处于安全模式
        // If we are in safemode, do not send back any recovery / replication
        // requests. Don't even drain the existing queue of work.
        if(namesystem.isInSafeMode()) {
          return new DatanodeCommand[0];
        }

         //检查租约情况
        //check lease recovery
        BlockInfoContiguousUnderConstruction[] blocks = nodeinfo
            .getLeaseRecoveryCommand(Integer.MAX_VALUE);
        if (blocks != null) {
          BlockRecoveryCommand brCommand = new BlockRecoveryCommand(
              blocks.length);
           .................................
          return new DatanodeCommand[] { brCommand };
        }

       
        final List<DatanodeCommand> cmds = new ArrayList<DatanodeCommand>();
        //生成复制命令
        //check pending replication
        List<BlockTargetPair> pendingList = nodeinfo.getReplicationCommand(
              maxTransfers);
        if (pendingList != null) {
          cmds.add(new BlockCommand(DatanodeProtocol.DNA_TRANSFER, blockPoolId,
              pendingList));
        }
        //检查无效的数据块，生成删除命令
        //check block invalidation
        Block[] blks = nodeinfo.getInvalidateBlocks(blockInvalidateLimit);
        if (blks != null) {
          cmds.add(new BlockCommand(DatanodeProtocol.DNA_INVALIDATE,
              blockPoolId, blks));
        }
        //生成缓存相关的命令
        boolean sendingCachingCommands = false;
        long nowMs = monotonicNow();
        if (shouldSendCachingCommands && 
            ((nowMs - nodeinfo.getLastCachingDirectiveSentTimeMs()) >=
                timeBetweenResendingCachingDirectivesMs)) {
          DatanodeCommand pendingCacheCommand =
              getCacheCommand(nodeinfo.getPendingCached(), nodeinfo,
                DatanodeProtocol.DNA_CACHE, blockPoolId);
          if (pendingCacheCommand != null) {
            cmds.add(pendingCacheCommand);
            sendingCachingCommands = true;
          }
          DatanodeCommand pendingUncacheCommand =
              getCacheCommand(nodeinfo.getPendingUncached(), nodeinfo,
                DatanodeProtocol.DNA_UNCACHE, blockPoolId);
          if (pendingUncacheCommand != null) {
            cmds.add(pendingUncacheCommand);
            sendingCachingCommands = true;
          }
          if (sendingCachingCommands) {
            nodeinfo.setLastCachingDirectiveSentTimeMs(nowMs);
          }
        }

        blockManager.addKeyUpdateCommand(cmds, nodeinfo);
         //生成带宽相关的命令
        // check for balancer bandwidth update
        if (nodeinfo.getBalancerBandwidth() > 0) {
          cmds.add(new BalancerBandwidthCommand(nodeinfo.getBalancerBandwidth()));
          // set back to 0 to indicate that datanode has been sent the new value
          nodeinfo.setBalancerBandwidth(0);
        }

        //返回所有的命令
        if (!cmds.isEmpty()) {
          return cmds.toArray(new DatanodeCommand[cmds.size()]);
        }
      }
    }

    return new DatanodeCommand[0];
  }