NameNode源码分析-NameNode结构和功能

在hadoop体系中，HDFS是最基础也是最核心的部分，本次hadoop源码阅读，首先会从HDFS开始，逐一从各个组件开始分析，然后细化到各个单独的features。

NameNode部分，主要从以下几个方面开始：

NameNode 结构和功能

NameNode启动和关闭流程

NameNode如何管理Slave

NameNode 重要组件

NameNode HA机制

……待续

1.NameNode

NameNode作为hdfs的master节点，负责管理整个集群元数据和集群管理功能。NameNode代码在org.apache.hadoop.hdfs.server.namenode包下。由于NN是通过master/slave方式管理整个集群，它的主要功能如下：

1.文件系统元数据操作
通过FSDirectory管理整个文件系统元数据的目录树
2.数据块的管理
保存NN中所有数据块的元数据，用于block管理，维护这BlocksMap和接收来自dn的块汇报等
3.数据节点管理
namenode用于管理datanode，包含了decommission和activities的节点，用于接收dn注册，心跳等
4.租约管理
用于管理文件访问的租约
5.缓存管理
缓存管理器处理DataNode上的缓存。这个类由FSNamesystem实例化。它通过datanode缓存report来维护cache block到datanode的映射。根据这些report,添加和删除缓存指令
6.提供客户端和slave远程方法调用
创建了ServerRPCServer和ClientRPCServer用于处理客户端和slave请求

重点说明一下NameNode中比较重要的数据结构：
FSDirectory

  INodeDirectory rootDir;//用于记录目录树的根路径
  private final FSNamesystem namesystem; //FSNameSystem对象
  private volatile boolean skipQuotaCheck = false; //skip while consuming edits 是否跳过quota检测
  private final int maxComponentLength; //目录中每个Component的最大长度
  private final int maxDirItems; //目录里最大的文件数量
  private final INodeMap inodeMap; // Synchronized by dirLock 存储了inodeid和Inode的对应关系
  private int quotaInitThreads; //用于在启动NN后，初始化quota的线程数量
  private final FSEditLog editLog; //FSEditlog对象，用于写editlog

主要方法有：

我们详细看一下在我们调用FileSystem.create之后，在FSDirectory是如何添加文件的：

  在指定的Inode
  @VisibleForTesting
  public INodesInPath addLastINode(INodesInPath existing, INode inode,
      boolean checkQuota) throws QuotaExceededException {//在一个存在的目录下边创建文件
    assert existing.getLastINode() != null &&
        existing.getLastINode().isDirectory();

    final int pos = existing.length();//查看路径level
    // Disallow creation of /.reserved. This may be created when loading
    // editlog/fsimage during upgrade since /.reserved was a valid name in older
    // release. This may also be called when a user tries to create a file
    // or directory /.reserved.
    if (pos == 1 && existing.getINode(0) == rootDir && isReservedName(inode)) {
      throw new HadoopIllegalArgumentException(
          "File name \"" + inode.getLocalName() + "\" is reserved and cannot "
              + "be created. If this is during upgrade change the name of the "
              + "existing file or directory to another name before upgrading "
              + "to the new release.");
    }
    //获取创建目录的上一级InodeD
    final INodeDirectory parent = existing.getINode(pos - 1).asDirectory();
    // The filesystem limits are not really quotas, so this check may appear
    // odd. It's because a rename operation deletes the src, tries to add
    // to the dest, if that fails, re-adds the src from whence it came.
    // The rename code disables the quota when it's restoring to the
    // original location because a quota violation would cause the the item
    // to go "poof".  The fs limits must be bypassed for the same reason.
    if (checkQuota) {//是否检测quota
      final String parentPath = existing.getPath();
      verifyMaxComponentLength(inode.getLocalNameBytes(), parentPath);
      verifyMaxDirItems(parent, parentPath);
    }
    // always verify inode name 验证inode name
    verifyINodeName(inode.getLocalNameBytes());
    //通过块存储策略来计算quota使用量
    final QuotaCounts counts = inode.computeQuotaUsage(getBlockStoragePolicySuite());
    //更新父目录的quota
    updateCount(existing, pos, counts, checkQuota);

    boolean isRename = (inode.getParent() != null);
    boolean added;
    try {
      added = parent.addChild(inode, true, existing.getLatestSnapshotId());
    } catch (QuotaExceededException e) {
      updateCountNoQuotaCheck(existing, pos, counts.negation());
      throw e;
    }
    if (!added) {
      updateCountNoQuotaCheck(existing, pos, counts.negation());
      return null;
    } else {
      if (!isRename) {
        AclStorage.copyINodeDefaultAcl(inode);
      }
      addToInodeMap(inode);
    }
    //quota更新成功后，在对应父目录的InodePath中添加此inode对象
    return INodesInPath.append(existing, inode, inode.getLocalNameBytes());
  }
  

FSNamesystem
FSNamesystem 是NN中最重要的类，实际记录了dn上所有的工作状态。

重要的参数有

  FSDirectory dir; FSDirectory
  private final BlockManager blockManager; //保存NN中所有数据块的元数据，用于block管理，维护这BlocksMap和接收来自dn的块汇报等
  private final SnapshotManager snapshotManager; //用于Snapshot的管理
  private final CacheManager cacheManager; //用于管理dn上的Cache
  private final DatanodeStatistics datanodeStatistics; //用于记录dn的统计信息，例如心跳汇报等

此类的方法很多，主要用于管理dn上报的比如心跳汇报，块汇报等，用于管理集群slave和数据块。

我们重点看一下FSNameSystem是如何处理dn发送过来的心跳汇报的

  //用于记录dn发送过来的心跳，确保dn不心跳超时，同时发送command给dn
  HeartbeatResponse handleHeartbeat(DatanodeRegistration nodeReg,
      StorageReport[] reports, long cacheCapacity, long cacheUsed,
      int xceiverCount, int xmitsInProgress, int failedVolumes,
      VolumeFailureSummary volumeFailureSummary,
      boolean requestFullBlockReportLease) throws IOException {
    readLock();
    try {
      //get datanode commands
      final int maxTransfer = blockManager.getMaxReplicationStreams()
          - xmitsInProgress;
      //nn通过调用DataNodeManager的handleHeartbeat方法进行心跳汇报的处理，生成一组需要被dn执行的cmds命令
      DatanodeCommand[] cmds = blockManager.getDatanodeManager().handleHeartbeat(
          nodeReg, reports, blockPoolId, cacheCapacity, cacheUsed,
          xceiverCount, maxTransfer, failedVolumes, volumeFailureSummary);
      long blockReportLeaseId = 0;
      if (requestFullBlockReportLease) {
        blockReportLeaseId =  blockManager.requestBlockReportLeaseId(nodeReg);
      }
      //create ha status 获取当前NS的HA状态
      final NNHAStatusHeartbeat haState = new NNHAStatusHeartbeat(
          haContext.getState().getServiceState(),
          getFSImage().getCorrectLastAppliedOrWrittenTxId());
      //生成HeartbeatResponse返回给dn
      return new HeartbeatResponse(cmds, haState, rollingUpgradeInfo,
          blockReportLeaseId);
    } finally {
      readUnlock("handleHeartbeat");
    }
  }

I am reading codes

目录

1.NameNode