hdfs源码分析第二弹

以写文件为例，串联整个流程的源码：

 FSDataOutputStream out = fs.create(outFile);

1. DistributedFileSystem

继承并实现了FileSystem，该对象是终端用户和hadoop分布式文件系统交互的接口。

原文说明：

/****************************************************************
 * Implementation of the abstract FileSystem for the DFS system.
 * This object is the way end-user code interacts with a Hadoop
 * DistributedFileSystem.
 *
 *****************************************************************/

调用create方法：

 @Override
  public FSDataOutputStream create(final Path f, final FsPermission permission,
    final EnumSet<CreateFlag> cflags, final int bufferSize,
    final short replication, final long blockSize, final Progressable progress,
    final ChecksumOpt checksumOpt) throws IOException {
    statistics.incrementWriteOps(1);
    Path absF = fixRelativePart(f);
    return new FileSystemLinkResolver<FSDataOutputStream>() {
      @Override
      public FSDataOutputStream doCall(final Path p)
          throws IOException, UnresolvedLinkException {
        final DFSOutputStream dfsos = dfs.create(getPathName(p), permission,
                cflags, replication, blockSize, progress, bufferSize,
                checksumOpt);
        return dfs.createWrappedOutputStream(dfsos, statistics);
      }
      @Override
      public FSDataOutputStream next(final FileSystem fs, final Path p)
          throws IOException {
        return fs.create(p, permission, cflags, bufferSize,
            replication, blockSize, progress, checksumOpt);
      }
    }.resolve(this, absF);
  }

2. DFSClient

调用Create方法：

 /**
   * Same as {@link #create(String, FsPermission, EnumSet, boolean, short, long,
   * Progressable, int, ChecksumOpt)} with the addition of favoredNodes that is
   * a hint to where the namenode should place the file blocks.
   * The favored nodes hint is not persisted in HDFS. Hence it may be honored
   * at the creation time only. HDFS could move the blocks during balancing or
   * replication, to move the blocks from favored nodes. A value of null means
   * no favored nodes for this create
   */
  public DFSOutputStream create(String src,
                             FsPermission permission,
                             EnumSet<CreateFlag> flag,
                             boolean createParent,
                             short replication,
                             long blockSize,
                             Progressable progress,
                             int buffersize,
                             ChecksumOpt checksumOpt,
                             InetSocketAddress[] favoredNodes) throws IOException {
    checkOpen();
    if (permission == null) {
      permission = FsPermission.getFileDefault();
    }
    FsPermission masked = permission.applyUMask(dfsClientConf.uMask);
    if(LOG.isDebugEnabled()) {
      LOG.debug(src + ": masked=" + masked);
    }
    String[] favoredNodeStrs = null;
    if (favoredNodes != null) {
      favoredNodeStrs = new String[favoredNodes.length];
      for (int i = 0; i < favoredNodes.length; i++) {
        favoredNodeStrs[i] =
            favoredNodes[i].getHostName() + ":"
                         + favoredNodes[i].getPort();
      }
    }
    final DFSOutputStream result = DFSOutputStream.newStreamForCreate(this,
        src, masked, flag, createParent, replication, blockSize, progress,
        buffersize, dfsClientConf.createChecksum(checksumOpt),
        favoredNodeStrs);
    beginFileLease(result.getFileId(), result);
    return result;
  }

3. DFSOutputStream

　　DFSOutputStream根据字节流创建文件。客户端应用先将数据写入流的缓存中，然后数据分解成包的形式，每个报文包（packet）通常为64k，一个报文包由多个块(chuck)组成，每个块通常为512比特，且存在一个关联的checksum（类似于文件的md5值）。

　　当客户端应用向当前包报文写入数据时，数据排队进入数据队列（dataQueue），DataStreamer线程从数据队列中接收这些数据，然后发送到管道的第一个数据节点(datanode),并将它从数据队列中移动到响应队列(ackQueue)。响应处理器(ResponseProcessor)接收数据节点的响应。 当从所有的数据节点接收到一个成功的响应包报文时，ResponseProcessor将相应的包报文从响应队列中移除。

　　当发送错误时，所有未完成的报文从响应队列中移除。从最初的管道线中关闭旧的坏的数据节点，然后新建一个管道线。此时DataStreamer开始从数据节点中发送数据包了。

原文如下：

/****************************************************************
 * DFSOutputStream creates files from a stream of bytes.
 *
 * The client application writes data that is cached internally by
 * this stream. Data is broken up into packets, each packet is
 * typically 64K in size. A packet comprises of chunks. Each chunk
 * is typically 512 bytes and has an associated checksum with it.
 *
 * When a client application fills up the currentPacket, it is
 * enqueued into dataQueue.  The DataStreamer thread picks up
 * packets from the dataQueue, sends it to the first datanode in
 * the pipeline and moves it from the dataQueue to the ackQueue.
 * The ResponseProcessor receives acks from the datanodes. When an
 * successful ack for a packet is received from all datanodes, the
 * ResponseProcessor removes the corresponding packet from the
 * ackQueue.
 *
 * In case of error, all outstanding packets and moved from
 * ackQueue. A new pipeline is setup by eliminating the bad
 * datanode from the original pipeline. The DataStreamer now
 * starts sending packets from the dataQueue.
****************************************************************/

 static DFSOutputStream newStreamForCreate(DFSClient dfsClient, String src,
      FsPermission masked, EnumSet<CreateFlag> flag, boolean createParent,
      short replication, long blockSize, Progressable progress, int buffersize,
      DataChecksum checksum, String[] favoredNodes) throws IOException {
    HdfsFileStatus stat = null;

    // Retry the create if we get a RetryStartFileException up to a maximum
    // number of times
    boolean shouldRetry = true;
    int retryCount = CREATE_RETRY_COUNT;
    while (shouldRetry) {
      shouldRetry = false;
      try {
        stat = dfsClient.namenode.create(src, masked, dfsClient.clientName,
            new EnumSetWritable<CreateFlag>(flag), createParent, replication,
            blockSize, SUPPORTED_CRYPTO_VERSIONS);
        break;
      } catch (RemoteException re) {
        IOException e = re.unwrapRemoteException(
            AccessControlException.class,
            DSQuotaExceededException.class,
            FileAlreadyExistsException.class,
            FileNotFoundException.class,
            ParentNotDirectoryException.class,
            NSQuotaExceededException.class,
            RetryStartFileException.class,
            SafeModeException.class,
            UnresolvedPathException.class,
            SnapshotAccessControlException.class,
            UnknownCryptoProtocolVersionException.class);
        if (e instanceof RetryStartFileException) {
          if (retryCount > 0) {
            shouldRetry = true;
            retryCount--;
          } else {
            throw new IOException("Too many retries because of encryption" +
                " zone operations", e);
          }
        } else {
          throw e;
        }
      }
    }
    Preconditions.checkNotNull(stat, "HdfsFileStatus should not be null!");
    final DFSOutputStream out = new DFSOutputStream(dfsClient, src, stat,
        flag, progress, checksum, favoredNodes);
    out.start();
    return out;
  }

Packet

 private static class Packet {
    private static final long HEART_BEAT_SEQNO = -1L;
    long seqno; // sequencenumber of buffer in block
    final long offsetInBlock; // offset in block
    boolean syncBlock; // this packet forces the current block to disk
    int numChunks; // number of chunks currently in packet
    final int maxChunks; // max chunks in packet
    private byte[] buf;
    private boolean lastPacketInBlock; // is this the last packet in block?

    /**
     * buf is pointed into like follows:
     *  (C is checksum data, D is payload data)
     *
     * [_________CCCCCCCCC________________DDDDDDDDDDDDDDDD___]
     *           ^        ^               ^               ^
     *           |        checksumPos     dataStart       dataPos
     *           checksumStart
     *
     * Right before sending, we move the checksum data to immediately precede
     * the actual data, and then insert the header into the buffer immediately
     * preceding the checksum data, so we make sure to keep enough space in
     * front of the checksum data to support the largest conceivable header.
     */
    int checksumStart;
    int checksumPos;
    final int dataStart;
    int dataPos;

    /**
     * Create a new packet.
     *
     * @param pktSize maximum size of the packet,
     *                including checksum data and actual data.
     * @param chunksPerPkt maximum number of chunks per packet.
     * @param offsetInBlock offset in bytes into the HDFS block.
     */
    private Packet(byte[] buf, int chunksPerPkt, long offsetInBlock, long seqno,
        int checksumSize) {
      this.lastPacketInBlock = false;
      this.numChunks = 0;
      this.offsetInBlock = offsetInBlock;
      this.seqno = seqno;

      this.buf = buf;

      checksumStart = PacketHeader.PKT_MAX_HEADER_LEN;
      checksumPos = checksumStart;
      dataStart = checksumStart + (chunksPerPkt * checksumSize);
      dataPos = dataStart;
      maxChunks = chunksPerPkt;
    }
}

DataStreamer

DataStreamer负责发送数据报文包到管道中的数据节点。它从名称节点获取到新的blockid和block位置后，开始发送流报文到它的管道中。每个报文包有一个唯一的序列号。当块中所有报文发送完成并接受到响应报文时，DataStreamer将会关闭当前的block。

  private synchronized void start() {
    streamer.start();
  }

原文如下：

  //
  // The DataStreamer class is responsible for sending data packets to the
  // datanodes in the pipeline. It retrieves a new blockid and block locations
  // from the namenode, and starts streaming packets to the pipeline of
  // Datanodes. Every packet has a sequence number associated with
  // it. When all the packets for a block are sent out and acks for each
  // if them are received, the DataStreamer closes the current block.
  //

继承了Daemon(后台线程)，间接继承了Thread类，因此其核心方法为run():

 /*
     * streamer thread is the only thread that opens streams to datanode,
     * and closes them. Any error recovery is also done by this thread.
     */
    @Override
    public void run() {
      long lastPacket = Time.now();
      TraceScope traceScope = null;
      if (traceSpan != null) {
        traceScope = Trace.continueSpan(traceSpan);
      }
      while (!streamerClosed && dfsClient.clientRunning) {

        // if the Responder encountered an error, shutdown Responder
        if (hasError && response != null) {
          try {
            response.close();
            response.join();
            response = null;
          } catch (InterruptedException  e) {
            DFSClient.LOG.warn("Caught exception ", e);
          }
        }

        Packet one;
        try {
          // process datanode IO errors if any
          boolean doSleep = false;
          if (hasError && (errorIndex >= 0 || restartingNodeIndex >= 0)) {
            doSleep = processDatanodeError();
          }

          synchronized (dataQueue) {
            // wait for a packet to be sent.
            long now = Time.now();
            while ((!streamerClosed && !hasError && dfsClient.clientRunning
                && dataQueue.size() == 0 &&
                (stage != BlockConstructionStage.DATA_STREAMING ||
                 stage == BlockConstructionStage.DATA_STREAMING &&
                 now - lastPacket < dfsClient.getConf().socketTimeout/2)) || doSleep ) {
              long timeout = dfsClient.getConf().socketTimeout/2 - (now-lastPacket);
              timeout = timeout <= 0 ? 1000 : timeout;
              timeout = (stage == BlockConstructionStage.DATA_STREAMING)?
                 timeout : 1000;
              try {
                dataQueue.wait(timeout);
              } catch (InterruptedException  e) {
                DFSClient.LOG.warn("Caught exception ", e);
              }
              doSleep = false;
              now = Time.now();
            }
            if (streamerClosed || hasError || !dfsClient.clientRunning) {
              continue;
            }
            // get packet to be sent.
            if (dataQueue.isEmpty()) {
              one = createHeartbeatPacket();
            } else {
              one = dataQueue.getFirst(); // regular data packet
            }
          }
          assert one != null;

          // get new block from namenode.
          if (stage == BlockConstructionStage.PIPELINE_SETUP_CREATE) {
            if(DFSClient.LOG.isDebugEnabled()) {
              DFSClient.LOG.debug("Allocating new block");
            }
            setPipeline(nextBlockOutputStream());
            initDataStreaming();
          } else if (stage == BlockConstructionStage.PIPELINE_SETUP_APPEND) {
            if(DFSClient.LOG.isDebugEnabled()) {
              DFSClient.LOG.debug("Append to block " + block);
            }
            setupPipelineForAppendOrRecovery();
            initDataStreaming();
          }

          long lastByteOffsetInBlock = one.getLastByteOffsetBlock();
          if (lastByteOffsetInBlock > blockSize) {
            throw new IOException("BlockSize " + blockSize +
                " is smaller than data size. " +
                " Offset of packet in block " +
                lastByteOffsetInBlock +
                " Aborting file " + src);
          }

          if (one.lastPacketInBlock) {
            // wait for all data packets have been successfully acked
            synchronized (dataQueue) {
              while (!streamerClosed && !hasError &&
                  ackQueue.size() != 0 && dfsClient.clientRunning) {
                try {
                  // wait for acks to arrive from datanodes
                  dataQueue.wait(1000);
                } catch (InterruptedException  e) {
                  DFSClient.LOG.warn("Caught exception ", e);
                }
              }
            }
            if (streamerClosed || hasError || !dfsClient.clientRunning) {
              continue;
            }
            stage = BlockConstructionStage.PIPELINE_CLOSE;
          }

          // send the packet
          synchronized (dataQueue) {
            // move packet from dataQueue to ackQueue
            if (!one.isHeartbeatPacket()) {
              dataQueue.removeFirst();
              ackQueue.addLast(one);
              dataQueue.notifyAll();
            }
          }

          if (DFSClient.LOG.isDebugEnabled()) {
            DFSClient.LOG.debug("DataStreamer block " + block +
                " sending packet " + one);
          }

          // write out data to remote datanode
          try {
            one.writeTo(blockStream);
            blockStream.flush();
          } catch (IOException e) {
            // HDFS-3398 treat primary DN is down since client is unable to
            // write to primary DN. If a failed or restarting node has already
            // been recorded by the responder, the following call will have no
            // effect. Pipeline recovery can handle only one node error at a
            // time. If the primary node fails again during the recovery, it
            // will be taken out then.
            tryMarkPrimaryDatanodeFailed();
            throw e;
          }
          lastPacket = Time.now();

          // update bytesSent
          long tmpBytesSent = one.getLastByteOffsetBlock();
          if (bytesSent < tmpBytesSent) {
            bytesSent = tmpBytesSent;
          }

          if (streamerClosed || hasError || !dfsClient.clientRunning) {
            continue;
          }

          // Is this block full?
          if (one.lastPacketInBlock) {
            // wait for the close packet has been acked
            synchronized (dataQueue) {
              while (!streamerClosed && !hasError &&
                  ackQueue.size() != 0 && dfsClient.clientRunning) {
                dataQueue.wait(1000);// wait for acks to arrive from datanodes
              }
            }
            if (streamerClosed || hasError || !dfsClient.clientRunning) {
              continue;
            }

            endBlock();
          }
          if (progress != null) { progress.progress(); }

          // This is used by unit test to trigger race conditions.
          if (artificialSlowdown != 0 && dfsClient.clientRunning) {
            Thread.sleep(artificialSlowdown);
          }
        } catch (Throwable e) {
          // Log warning if there was a real error.
          if (restartingNodeIndex == -1) {
            DFSClient.LOG.warn("DataStreamer Exception", e);
          }
          if (e instanceof IOException) {
            setLastException((IOException)e);
          } else {
            setLastException(new IOException("DataStreamer Exception: ",e));
          }
          hasError = true;
          if (errorIndex == -1 && restartingNodeIndex == -1) {
            // Not a datanode issue
            streamerClosed = true;
          }
        }
      }
      if (traceScope != null) {
        traceScope.close();
      }
      closeInternal();
    }

ResponseProcessor

处理数据节点的响应。当接收到响应时，将一个包报文从响应队列中删除。

DataStreamer的run方法启动了ResponseProcessor线程：

    /**
     * Initialize for data streaming
     */
    private void initDataStreaming() {
      this.setName("DataStreamer for file " + src +
          " block " + block);
      response = new ResponseProcessor(nodes);
      response.start();
      stage = BlockConstructionStage.DATA_STREAMING;
    }

原文描述：

    //
    // Processes responses from the datanodes.  A packet is removed
    // from the ackQueue when its response arrives.
    //

继承了Daemon(后台线程)，间接继承了Thread类，因此其核心方法为run():

 public void run() {

        setName("ResponseProcessor for block " + block);
        PipelineAck ack = new PipelineAck();

        while (!responderClosed && dfsClient.clientRunning && !isLastPacketInBlock) {
          // process responses from datanodes.
          try {
            // read an ack from the pipeline
            long begin = Time.monotonicNow();
            ack.readFields(blockReplyStream);
            long duration = Time.monotonicNow() - begin;
            if (duration > dfsclientSlowLogThresholdMs
                && ack.getSeqno() != Packet.HEART_BEAT_SEQNO) {
              DFSClient.LOG
                  .warn("Slow ReadProcessor read fields took " + duration
                      + "ms (threshold=" + dfsclientSlowLogThresholdMs + "ms); ack: "
                      + ack + ", targets: " + Arrays.asList(targets));
            } else if (DFSClient.LOG.isDebugEnabled()) {
              DFSClient.LOG.debug("DFSClient " + ack);
            }

            long seqno = ack.getSeqno();
            // processes response status from datanodes.
            for (int i = ack.getNumOfReplies()-1; i >=0  && dfsClient.clientRunning; i--) {
              final Status reply = ack.getReply(i);
              // Restart will not be treated differently unless it is
              // the local node or the only one in the pipeline.
              if (PipelineAck.isRestartOOBStatus(reply) &&
                  shouldWaitForRestart(i)) {
                restartDeadline = dfsClient.getConf().datanodeRestartTimeout +
                    Time.now();
                setRestartingNodeIndex(i);
                String message = "A datanode is restarting: " + targets[i];
                DFSClient.LOG.info(message);
               throw new IOException(message);
              }
              // node error
              if (reply != SUCCESS) {
                setErrorIndex(i); // first bad datanode
                throw new IOException("Bad response " + reply +
                    " for block " + block +
                    " from datanode " +
                    targets[i]);
              }
            }

            assert seqno != PipelineAck.UNKOWN_SEQNO :
              "Ack for unknown seqno should be a failed ack: " + ack;
            if (seqno == Packet.HEART_BEAT_SEQNO) {  // a heartbeat ack
              continue;
            }

            // a success ack for a data packet
            Packet one;
            synchronized (dataQueue) {
              one = ackQueue.getFirst();
            }
            if (one.seqno != seqno) {
              throw new IOException("ResponseProcessor: Expecting seqno " +
                                    " for block " + block +
                                    one.seqno + " but received " + seqno);
            }
            isLastPacketInBlock = one.lastPacketInBlock;

            // Fail the packet write for testing in order to force a
            // pipeline recovery.
            if (DFSClientFaultInjector.get().failPacket() &&
                isLastPacketInBlock) {
              failPacket = true;
              throw new IOException(
                    "Failing the last packet for testing.");
            }

            // update bytesAcked
            block.setNumBytes(one.getLastByteOffsetBlock());

            synchronized (dataQueue) {
              lastAckedSeqno = seqno;
              ackQueue.removeFirst();
              dataQueue.notifyAll();

              one.releaseBuffer(byteArrayManager);
            }
          } catch (Exception e) {
            if (!responderClosed) {
              if (e instanceof IOException) {
                setLastException((IOException)e);
              }
              hasError = true;
              // If no explicit error report was received, mark the primary
              // node as failed.
              tryMarkPrimaryDatanodeFailed();
              synchronized (dataQueue) {
                dataQueue.notifyAll();
              }
              if (restartingNodeIndex == -1) {
                DFSClient.LOG.warn("DFSOutputStream ResponseProcessor exception "
                     + " for block " + block, e);
              }
              responderClosed = true;
            }
          }
        }
      }

小结：

　　从上面的源码分析我们可以知道：

　　DFSOutputStream是hdfs写文件的主类，它通过DataStreamer来写文件，并通过ResponseProcessor来处理数据节点的返回信息。