Hadoop之DataNode启动源码解析

添加依赖

为了能够编译和运行 Hadoop 的 DataNode 组件，我们需要在项目的 pom.xml 文件中添加以下依赖：

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<dependencies>
  <dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-client</artifactId>
    <version>3.1.3</version>
  </dependency>

  <dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-hdfs</artifactId>
    <version>3.1.3</version>
  </dependency>

  <dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-hdfs-client</artifactId>
    <version>3.1.3</version>
    <scope>provided</scope>
  </dependency>
</dependencies>

DataNode 类介绍

DataNode 类是 Hadoop 分布式文件系统 (HDFS) 中的一个核心组件，它负责存储文件系统的数据块。每个部署可以包含一个或多个 DataNode 实例。DataNode 与 NameNode 通信以报告其存储状态，并响应来自 NameNode 的指令，如删除或复制块等操作。此外，DataNode 还需要与客户端代码和其他 DataNode 进行交互。

DataNode 主程序入口

DataNode 的主程序入口点位于 main 方法中，该方法首先检查命令行参数是否存在帮助请求，然后调用 secureMain 方法初始化 DataNode 实例。

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public static void main(String args[]) {
  if (DFSUtil.parseHelpArgument(args, DataNode.USAGE, System.out, true)) {
    System.exit(0);
  }

  secureMain(args, null);
}

public static void secureMain(String args[], SecureResources resources) {
  int errorCode = 0;
  try {
    StringUtils.startupShutdownMessage(DataNode.class, args, LOG);

    DataNode datanode = createDataNode(args, null, resources);

    ... ...
  } catch (Throwable e) {
    LOG.error("Exception in secureMain", e);
    terminate(1, e);
  } finally {
    LOG.warn("Exiting Datanode");
    terminate(errorCode);
  }
}

DataNode 实例化

createDataNode 方法用于实例化 DataNode 对象，并启动其守护进程。

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public static DataNode createDataNode(String args[], Configuration conf,
    SecureResources resources) throws IOException {
  // 初始化DN
  DataNode dn = instantiateDataNode(args, conf, resources);

  if (dn != null) {
    // 启动DN进程
    dn.runDatanodeDaemon();
  }
  return dn;
}

public static DataNode instantiateDataNode(String args[], Configuration conf,
    SecureResources resources) throws IOException {
  ... ...
  
  return makeInstance(dataLocations, conf, resources);
}

static DataNode makeInstance(Collection<StorageLocation> dataDirs,
    Configuration conf, SecureResources resources) throws IOException {
  ... ...
  return new DataNode(conf, locations, storageLocationChecker, resources);
}

DataNode 构造函数

构造函数初始化了 DataNode 的主要属性，并启动了必要的组件。

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DataNode(final Configuration conf,
         final List<StorageLocation> dataDirs,
         final StorageLocationChecker storageLocationChecker,
         final SecureResources resources) throws IOException {
  super(conf);
  ... ...

  try {
    hostName = getHostName(conf);
    LOG.info("Configured hostname is {}", hostName);
    // 启动DN
    startDataNode(dataDirs, resources);
  } catch (IOException ie) {
    shutdown();
    throw ie;
  }
  ... ...
}

DataNode 启动过程

startDataNode 方法初始化了 DataNode 的关键组件，包括数据存储 (DataStorage)、MXBean 注册、DataXceiver 服务器、HTTP 服务器等。

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void startDataNode(List<StorageLocation> dataDirectories,
                   SecureResources resources
                   ) throws IOException {
  ... ...
  // 创建数据存储对象
  storage = new DataStorage();
  
  // global DN settings
  registerMXBean();
  // 初始化DataXceiver
  initDataXceiver();
  
  // 启动HttpServer
  startInfoServer();

  pauseMonitor = new JvmPauseMonitor();
  pauseMonitor.init(getConf());
  pauseMonitor.start();

  // BlockPoolTokenSecretManager is required to create ipc server.
  this.blockPoolTokenSecretManager = new BlockPoolTokenSecretManager();

  // Login is done by now. Set the DN user name.
  dnUserName = UserGroupInformation.getCurrentUser().getUserName();
  LOG.info("dnUserName = {}", dnUserName);
  LOG.info("supergroup = {}", supergroup);
  
  // 初始化RPC服务
  initIpcServer();

  metrics = DataNodeMetrics.create(getConf(), getDisplayName());
  peerMetrics = dnConf.peerStatsEnabled ?
      DataNodePeerMetrics.create(getDisplayName(), getConf()) : null;
  metrics.getJvmMetrics().setPauseMonitor(pauseMonitor);

  ecWorker = new ErasureCodingWorker(getConf(), this);
  blockRecoveryWorker = new BlockRecoveryWorker(this);
  
  // 创建BlockPoolManager
  blockPoolManager = new BlockPoolManager(this);
  // 心跳管理
  blockPoolManager.refreshNamenodes(getConf());

  // Create the ReadaheadPool from the DataNode context so we can
  // exit without having to explicitly shutdown its thread pool.
  readaheadPool = ReadaheadPool.getInstance();
  saslClient = new SaslDataTransferClient(dnConf.getConf(),
      dnConf.saslPropsResolver, dnConf.trustedChannelResolver);
  saslServer = new SaslDataTransferServer(dnConf, blockPoolTokenSecretManager);
  startMetricsLogger();

  if (dnConf.diskStatsEnabled) {
    diskMetrics = new DataNodeDiskMetrics(this,
        dnConf.outliersReportIntervalMs);
  }
}

初始化DataXceiverServer

initDataXceiver 方法创建并启动了 DataXceiverServer，它是 DataNode 用来接收客户端和其他 DataNode 发送过来的数据的服务。

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private void initDataXceiver() throws IOException {
  // dataXceiverServer是一个服务，DN用来接收客户端和其他DN发送过来的数据服务
  this.dataXceiverServer = new Daemon(threadGroup, xserver);
  this.threadGroup.setDaemon(true); // auto destroy when empty

  ... ...
}

初始化HTTP服务

startInfoServer 方法初始化并启动了 HTTP 服务器，用于提供有关 DataNode 的信息和服务。

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private void startInfoServer()
  throws IOException {
  // SecureDataNodeStarter will bind the privileged port to the channel if
  // the DN is started by JSVC, pass it along.
  ServerSocketChannel httpServerChannel = secureResources != null ?
      secureResources.getHttpServerChannel() : null;

  httpServer = new DatanodeHttpServer(getConf(), this, httpServerChannel);
  httpServer.start();
  if (httpServer.getHttpAddress() != null) {
    infoPort = httpServer.getHttpAddress().getPort();
  }
  if (httpServer.getHttpsAddress() != null) {
    infoSecurePort = httpServer.getHttpsAddress().getPort();
  }
}

DatanodeHttpServer 构造函数

DatanodeHttpServer 构造函数用于创建 HTTP 服务器实例。

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public DatanodeHttpServer(final Configuration conf,
    final DataNode datanode,
    final ServerSocketChannel externalHttpChannel)
  throws IOException {

  ... ...
  HttpServer2.Builder builder = new HttpServer2.Builder()
      .setName("datanode")
      .setConf(confForInfoServer)
      .setACL(new AccessControlList(conf.get(DFS_ADMIN, " ")))
      .hostName(getHostnameForSpnegoPrincipal(confForInfoServer))
      .addEndpoint(URI.create("http://localhost:" + proxyPort))
      .setFindPort(true);
  ... ...
}

初始化DataNode的RPC服务端

初始化 DataNode 的 RPC 服务端涉及到配置和启动相关的服务，以供客户端连接。

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private void initIpcServer() throws IOException {
  InetSocketAddress ipcAddr = NetUtils.createSocketAddr(
      getConf().getTrimmed(DFS_DATANODE_IPC_ADDRESS_KEY));

  ... ...
  ipcServer = new RPC.Builder(getConf())
      .setProtocol(ClientDatanodeProtocolPB.class)
      .setInstance(service)
      .setBindAddress(ipcAddr.getHostName())
      .setPort(ipcAddr.getPort())
      .setNumHandlers(
          getConf().getInt(DFS_DATANODE_HANDLER_COUNT_KEY,
              DFS_DATANODE_HANDLER_COUNT_DEFAULT)).setVerbose(false)
      .setSecretManager(blockPoolTokenSecretManager).build();
  ... ...
}

DataNode 向 NameNode 注册

DataNode 需要向 NameNode 注册自身，以便 NameNode 可以跟踪和管理集群中的所有 DataNode。

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void refreshNamenodes(Configuration conf)
    throws IOException {
  ... ...

  synchronized (refreshNamenodesLock) {
    doRefreshNamenodes(newAddressMap, newLifelineAddressMap);
  }
}

private void doRefreshNamenodes(
    Map<String, Map<String, InetSocketAddress>> addrMap,
    Map<String, Map<String, InetSocketAddress>> lifelineAddrMap)
    throws IOException {
  ... ...
  
  synchronized (this) {
    ... ...

    // Step 3. Start new nameservices
    if (!toAdd.isEmpty()) {

      for (String nsToAdd : toAdd) {
        ... ...
        BPOfferService bpos = createBPOS(nsToAdd, addrs, lifelineAddrs);
        bpByNameserviceId.put(nsToAdd, bpos);
        offerServices.add(bpos);
      }
    }
    startAll();
  }

  ... ...
}

protected BPOfferService createBPOS(
    final String nameserviceId,
    List<InetSocketAddress> nnAddrs,
    List<InetSocketAddress> lifelineNnAddrs) {
  // 根据NameNode个数创建对应的服务
  return new BPOfferService(nameserviceId, nnAddrs, lifelineNnAddrs, dn);
}

启动所有的服务

启动所有的服务涉及到创建服务实例并开始它们的生命周期。

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

void start() {
  for (BPServiceActor actor : bpServices) {
    actor.start();
  }
}

void start() {
  ... ...
  bpThread = new Thread(this);
  bpThread.setDaemon(true); // needed for JUnit testing
  // 表示开启一个线程，所有查找该线程的 run 方法
  bpThread.start();

  if (lifelineSender != null) {
    lifelineSender.start();
  }
}

DataNode 服务线程运行

DataNode 服务线程运行涉及初始化和心跳发送。

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public void run() {
  LOG.info(this + " starting to offer service");

  try {
    while (true) {
      // init stuff
      try {
        // setup storage
		// 向NN 注册
        connectToNNAndHandshake();
        break;
      } catch (IOException ioe) {
        // Initial handshake, storage recovery or registration failed
        runningState = RunningState.INIT_FAILED;
        if (shouldRetryInit()) {
          // Retry until all namenode's of BPOS failed initialization
          LOG.error("Initialization failed for " + this + " "
              + ioe.getLocalizedMessage());
		  // 注册失败，5s后重试
          sleepAndLogInterrupts(5000, "initializing");
        } else {
          runningState = RunningState.FAILED;
          LOG.error("Initialization failed for " + this + ". Exiting. ", ioe);
          return;
        }
      }
    }
    ... ...
    while (shouldRun()) {
      try {
        // 发送心跳
        offerService();
      } catch (Exception ex) {
        ... ...
      }
    }
}

private void connectToNNAndHandshake() throws IOException {
  // get NN proxy 获取NN的RPC客户端对象
  bpNamenode = dn.connectToNN(nnAddr);

  // First phase of the handshake with NN - get the namespace
  // info.
  NamespaceInfo nsInfo = retrieveNamespaceInfo();

  // Verify that this matches the other NN in this HA pair.
  // This also initializes our block pool in the DN if we are
  // the first NN connection for this BP.
  bpos.verifyAndSetNamespaceInfo(this, nsInfo);

  /* set thread name again to include NamespaceInfo when it's available. */
  this.bpThread.setName(formatThreadName("heartbeating", nnAddr));

  // 注册
  register(nsInfo);
}

DataNode 注册至 NameNode

DataNode 注册至 NameNode 包括创建注册信息以及实际的注册过程。

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void register(NamespaceInfo nsInfo) throws IOException {
  // 创建注册信息
  DatanodeRegistration newBpRegistration = bpos.createRegistration();

  LOG.info(this + " beginning handshake with NN");

  while (shouldRun()) {
    try {
      // Use returned registration from namenode with updated fields
	  // 把注册信息发送给NN（DN调用接口方法，执行在NN）
      newBpRegistration = bpNamenode.registerDatanode(newBpRegistration);
      newBpRegistration.setNamespaceInfo(nsInfo);
      bpRegistration = newBpRegistration;
      break;
    } catch(EOFException e) {  // namenode might have just restarted
      LOG.info("Problem connecting to server: " + nnAddr + " :"
          + e.getLocalizedMessage());
      sleepAndLogInterrupts(1000, "connecting to server");
    } catch(SocketTimeoutException e) {  // namenode is busy
      LOG.info("Problem connecting to server: " + nnAddr);
      sleepAndLogInterrupts(1000, "connecting to server");
    }
  }
  ... ...
}

NameNode 处理注册

NameNode 接收到 DataNode 的注册请求后，会进行处理。

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public DatanodeRegistration registerDatanode(DatanodeRegistration nodeReg)
    throws IOException {
  checkNNStartup();
  verifySoftwareVersion(nodeReg);
  // 注册DN
  namesystem.registerDatanode(nodeReg);

  return nodeReg;
}

void registerDatanode(DatanodeRegistration nodeReg) throws IOException {
  writeLock();
  try {
    blockManager.registerDatanode(nodeReg);
  } finally {
    writeUnlock("registerDatanode");
  }
}

public void registerDatanode(DatanodeRegistration nodeReg)
    throws DisallowedDatanodeException, UnresolvedTopologyException {
	... ...
	// register new datanode 注册DN
    addDatanode(nodeDescr);
    blockManager.getBlockReportLeaseManager().register(nodeDescr);
    // also treat the registration message as a heartbeat
    // no need to update its timestamp
    // because its is done when the descriptor is created
	// 将DN添加到心跳管理
    heartbeatManager.addDatanode(nodeDescr);
    heartbeatManager.updateDnStat(nodeDescr);
    incrementVersionCount(nodeReg.getSoftwareVersion());
    startAdminOperationIfNecessary(nodeDescr);
    success = true;
	... ...
}

void addDatanode(final DatanodeDescriptor node) {
  // To keep host2DatanodeMap consistent with datanodeMap,
  // remove  from host2DatanodeMap the datanodeDescriptor removed
  // from datanodeMap before adding node to host2DatanodeMap.
  synchronized(this) {
    host2DatanodeMap.remove(datanodeMap.put(node.getDatanodeUuid(), node));
  }

  networktopology.add(node); // may throw InvalidTopologyException
  host2DatanodeMap.add(node);
  checkIfClusterIsNowMultiRack(node);
  resolveUpgradeDomain(node);

  ... ...
}