hadoop2.5.2学习及实践笔记（四）—— namenode启动过程源码概览

对namenode启动时的相关操作及相关类有一个大体了解，后续深入研究时，再对本文进行补充

>实现类

HDFS启动脚本为$HADOOP_HOME/sbin/start-dfs.sh，查看start-dfs.sh可以看出，namenode是通过bin/hdfs命令来启动

$ vi  start-dfs.sh

# namenodes
NAMENODES=$($HADOOP_PREFIX/bin/hdfs getconf -namenodes)
echo "Starting namenodes on [$NAMENODES]"
"$HADOOP_PREFIX/sbin/hadoop-daemons.sh" \
  --config "$HADOOP_CONF_DIR" \
  --hostnames "$NAMENODES" \
  --script "$bin/hdfs" start namenode $nameStartOpt
#---------------------------------------------------------

查看$HADOOP_HOME/bin/hdfs，可以找到namenode启动所调用的java类。

$ vi bin/hdfs:

if [ "$COMMAND" = "namenode" ] ; then
  CLASS='org.apache.hadoop.hdfs.server.namenode.NameNode'
  HADOOP_OPTS="$HADOOP_OPTS $HADOOP_NAMENODE_OPTS"

>源码查看

按照前文hadoop2.5.2学习及实践笔记（二）——编译源代码及导入源码至eclipse步骤，源码已经导入到eclipse中，快捷键ctrl+shift+R搜索并打开NameNode.java查看源码

NameNode类中有一个静态代码块，表示在加载器加载NameNode类过程中的准备阶段，就会执行代码块中的代码。HdfsConfiguration的init()方法的方法体是空的，这里的作用是触发对HdfsConfiguration的主动调用，从而保证在执行NameNode类相关调用时，如果HdfsConfiguration类没有被加载和初始化，先触发HdfsConfiguration的初始化过程。

//org.apache.hadoop.hdfs.server.namenode.NameNode.java

static{
     //HdfsConfiguration类init()方法：public static void init() {}
    HdfsConfiguration.init();
}

查看其main方法，可以看出namenode相关操作的主要入口方法是createNameNode(String argv[], Configuration conf)方法。

//org.apache.hadoop.hdfs.server.namenode.NameNode.java

public static void main(String argv[]) throws Exception {
    if (DFSUtil.parseHelpArgument(argv, NameNode.USAGE, System.out, true)) {
      System.exit(0);
    }

    try {
      //打印namenode启动或关闭日志信息
      StringUtils.startupShutdownMessage(NameNode.class, argv, LOG);
     //namenode相关主要操作
      NameNode namenode = createNameNode(argv, null);
      if (namenode != null) {
          //向客户端和datanode提供RPC服务，直到RPC服务器结束运行
        namenode.join();
      }
    } catch (Throwable e) {
      LOG.fatal("Exception in namenode join", e);
      terminate(1, e);
    }
  }

createNameNode方法中通过一个switch语句对不同的命令执行不同的操作。比如搭建环境时格式化文件系统时的操作，可以查看FORMAT分支。

//org.apache.hadoop.hdfs.server.namenode.NameNode.java

public static NameNode createNameNode(String argv[], Configuration conf)
      throws IOException {
    LOG.info("createNameNode " + Arrays.asList(argv));
    if (conf == null)
      conf = new HdfsConfiguration();
    //参数为空时默认： -regular
    StartupOption startOpt = parseArguments(argv);
    if (startOpt == null) {
      printUsage(System.err);
      return null;
    }
    setStartupOption(conf, startOpt);

    switch (startOpt) {
      case FORMAT: {//格式化文件系统，伪分布式环境搭建时调用过namenode -format命令
        boolean aborted = format(conf, startOpt.getForceFormat(),
            startOpt.getInteractiveFormat());
        terminate(aborted ? 1 : 0);
        return null; // avoid javac warning
      }
      case GENCLUSTERID: {
        System.err.println("Generating new cluster id:");
        System.out.println(NNStorage.newClusterID());
        terminate(0);
        return null;
      }
      case FINALIZE: {
        System.err.println("Use of the argument '" + StartupOption.FINALIZE +
            "' is no longer supported. To finalize an upgrade, start the NN " +
            " and then run `hdfs dfsadmin -finalizeUpgrade'");
        terminate(1);
        return null; // avoid javac warning
      }
      case ROLLBACK: {
        boolean aborted = doRollback(conf, true);
        terminate(aborted ? 1 : 0);
        return null; // avoid warning
      }
      case BOOTSTRAPSTANDBY: {
        String toolArgs[] = Arrays.copyOfRange(argv, 1, argv.length);
        int rc = BootstrapStandby.run(toolArgs, conf);
        terminate(rc);
        return null; // avoid warning
      }
      case INITIALIZESHAREDEDITS: {
        boolean aborted = initializeSharedEdits(conf,
            startOpt.getForceFormat(),
            startOpt.getInteractiveFormat());
        terminate(aborted ? 1 : 0);
        return null; // avoid warning
      }
      case BACKUP:
      case CHECKPOINT: {//backupnode和checkpointnode启动
        NamenodeRole role = startOpt.toNodeRole();
        DefaultMetricsSystem.initialize(role.toString().replace(" ", ""));
          //backupnode继承NameNode类，代码最终执行的还是NameNode的构造方法
        return new BackupNode(conf, role);
      }
      case RECOVER: {
        NameNode.doRecovery(startOpt, conf);
        return null;
      }
      case METADATAVERSION: {
        printMetadataVersion(conf);
        terminate(0);
        return null; // avoid javac warning
      }
      default: {
        DefaultMetricsSystem.initialize("NameNode");
          //启动时startOpt=“-regular”，代码执行default分支，通过构造函数返回一个namenode实例
        return new NameNode(conf);
      }
    }
  }

namenode的构造方法

//org.apache.hadoop.hdfs.server.namenode.NameNode.java

 public NameNode(Configuration conf) throws IOException {
    this(conf, NamenodeRole.NAMENODE);
  }
 protected NameNode(Configuration conf, NamenodeRole role)
      throws IOException {
    this.conf = conf;
    this.role = role;
    //获取fs.defaultFS，设置namenode地址
    setClientNamenodeAddress(conf);
    String nsId = getNameServiceId(conf);
    String namenodeId = HAUtil.getNameNodeId(conf, nsId);
    //是否启用HA
    this.haEnabled = HAUtil.isHAEnabled(conf, nsId);
    //HA状态：启用/备用
    state = createHAState(getStartupOption(conf));
    //读取dfs.ha.allow.stale.reads，设置namenode在备用状态时是否允许读操作,默认为false
    this.allowStaleStandbyReads = HAUtil.shouldAllowStandbyReads(conf);
    this.haContext = createHAContext();
    try {
     //联邦环境下，使用该方法配置一系列使用一个逻辑上的nsId组合在一起的namenode
      initializeGenericKeys(conf, nsId, namenodeId);
    //namenode初始化
    initialize(conf);
      try {
        haContext.writeLock();
        state.prepareToEnterState(haContext);
        //namenode进入相应状态：active state/backup state/standby state
       state.enterState(haContext);
      } finally {
        haContext.writeUnlock();
      }
    } catch (IOException e) {
      this.stop();
      throw e;
    } catch (HadoopIllegalArgumentException e) {
      this.stop();
      throw e;
    }
  }

namenode初始化方法代码

//org.apache.hadoop.hdfs.server.namenode.NameNode.java

protected void initialize(Configuration conf) throws IOException {
    if (conf.get(HADOOP_USER_GROUP_METRICS_PERCENTILES_INTERVALS) == null) {
      String intervals = conf.get(DFS_METRICS_PERCENTILES_INTERVALS_KEY);
      if (intervals != null) {
        conf.set(HADOOP_USER_GROUP_METRICS_PERCENTILES_INTERVALS,
          intervals);
      }
    }
    //设置权限，根据hadoop.security.authentication获取认证方式及规则
    UserGroupInformation.setConfiguration(conf);
    //登录：如果认证方式为simple则退出该方法
    //否则调用UserGroupInformation.loginUserFromKeytab进行登陆，登陆使用dfs.namenode.kerberos.principal作为用户名
    loginAsNameNodeUser(conf);

    //初始化度量系统，用于度量namenode服务状态
    NameNode.initMetrics(conf, this.getRole());
    StartupProgressMetrics.register(startupProgress);

    if (NamenodeRole.NAMENODE == role) {
      //启动http服务器
      startHttpServer(conf);
    }
    //根据命令对命名空间进行操作，如：前文所述启动时加载本地命名空间镜像和应用编辑日志，在内存中建立命名空间的映像
    loadNamesystem(conf);

    //创建RPC服务器
    rpcServer = createRpcServer(conf);
    if (clientNamenodeAddress == null) {
      // This is expected for MiniDFSCluster. Set it now using
      // the RPC server's bind address.
      clientNamenodeAddress =
          NetUtils.getHostPortString(rpcServer.getRpcAddress());
      LOG.info("Clients are to use " + clientNamenodeAddress + " to access"
          + " this namenode/service.");
    }
    if (NamenodeRole.NAMENODE == role) {
      httpServer.setNameNodeAddress(getNameNodeAddress());
      httpServer.setFSImage(getFSImage());
    }

    pauseMonitor = new JvmPauseMonitor(conf);
    pauseMonitor.start();
    metrics.getJvmMetrics().setPauseMonitor(pauseMonitor);

    //启动活动状态和备用状态的公共服务：RPC服务和namenode的插件程序启动
    startCommonServices(conf);
  }

loadNamesystem(Configuration conf)方法调用FSNamesystem类的loadFromDisk(Configuration conf)。前文提到的，namenode启动时从本地文件系统加载镜像并重做编辑日志，都在此方法中实现。

//org.apache.hadoop.hdfs.server.namenode.FSNamesystem.java

static FSNamesystem loadFromDisk(Configuration conf) throws IOException {
    //必须的编辑日志目录检查
    checkConfiguration(conf);
    //设在NNStorage，并初始化编辑日志目录。NNStorage主要功能是管理namenode使用的存储目录
    FSImage fsImage = new FSImage(conf,
        FSNamesystem.getNamespaceDirs(conf),
        FSNamesystem.getNamespaceEditsDirs(conf));
    //根据指定的镜像创建FSNamesystem对象
    FSNamesystem namesystem = new FSNamesystem(conf, fsImage, false);
    StartupOption startOpt = NameNode.getStartupOption(conf);
    if (startOpt == StartupOption.RECOVER) {
      namesystem.setSafeMode(SafeModeAction.SAFEMODE_ENTER);
    }

    long loadStart = now();
    try {
      //加载镜像、重做编辑日志，并打开一个新编辑文件都在此方法中
      namesystem.loadFSImage(startOpt);
    } catch (IOException ioe) {
      LOG.warn("Encountered exception loading fsimage", ioe);
      fsImage.close();
      throw ioe;
    }
    long timeTakenToLoadFSImage = now() - loadStart;
    LOG.info("Finished loading FSImage in " + timeTakenToLoadFSImage + " msecs");
    NameNodeMetrics nnMetrics = NameNode.getNameNodeMetrics();
    if (nnMetrics != null) {
      nnMetrics.setFsImageLoadTime((int) timeTakenToLoadFSImage);
    }
    return namesystem;
  }

private void loadFSImage(StartupOption startOpt) throws IOException {
    final FSImage fsImage = getFSImage();

    // format before starting up if requested
    if (startOpt == StartupOption.FORMAT) {

      fsImage.format(this, fsImage.getStorage().determineClusterId());// reuse current id

      startOpt = StartupOption.REGULAR;
    }
    boolean success = false;
    writeLock();
    try {
      // We shouldn't be calling saveNamespace if we've come up in standby state.
      MetaRecoveryContext recovery = startOpt.createRecoveryContext();
      final boolean staleImage
          = fsImage.recoverTransitionRead(startOpt, this, recovery);
      if (RollingUpgradeStartupOption.ROLLBACK.matches(startOpt)) {
        rollingUpgradeInfo = null;
      }
      final boolean needToSave = staleImage && !haEnabled && !isRollingUpgrade();
      LOG.info("Need to save fs image? " + needToSave
          + " (staleImage=" + staleImage + ", haEnabled=" + haEnabled
          + ", isRollingUpgrade=" + isRollingUpgrade() + ")");
      if (needToSave) {
        fsImage.saveNamespace(this);
      } else {
        // No need to save, so mark the phase done.
        StartupProgress prog = NameNode.getStartupProgress();
        prog.beginPhase(Phase.SAVING_CHECKPOINT);
        prog.endPhase(Phase.SAVING_CHECKPOINT);
      }
      // This will start a new log segment and write to the seen_txid file, so
      // we shouldn't do it when coming up in standby state
      if (!haEnabled || (haEnabled && startOpt == StartupOption.UPGRADE)) {
        fsImage.openEditLogForWrite();
      }
      success = true;
    } finally {
      if (!success) {
        fsImage.close();
      }
      writeUnlock();
    }
    imageLoadComplete();
  }