hadoop3 写数据流程（四）：写数据1_storeallocatedblock

参考源码hadoop-3.3.0，这部分代码写的质量真的不太行，好些方法几百行。

1 概述

HDFS使用数据流管道方式（数据管道的建立可以参见写数据系列第三篇）来写数据。 DFSClient通过使用写数据流程（一）创建的数据输出流out调用write方法完成数据的传输，传输中将数据切分成多个packet，而后往dataQueue中存储切分之后的packet，在写入完成后调用notifyAll方法重新激活在wait等待的DataStreamer线程，而后在此线程中的run()方法中调用DataStreaamer#nextBlockOutputStream方法，这里会调用Sender.writeBlock()方法（此方法在DataStreamer#createBlockOutputStream）触发一个写数据块请求， 这个请求会传送到数据流管道中的每一个数据节点， 这个写数据块请求通过流式接口到达Datanode之后， Datanode上监听流式接口请求的DataXceiverServer会接收这个请求， 并构造一个DataXceiver对象， 然后在DataXceiver对象上调用DataXceiver.writeBlock()方法响应这个请求。 当前Datanode的DataXceiver.writeBlock()方法会级联向数据流管道中的下一个Datanode发送写数据块请求， 这个流式请求会一直在数据流管道中传递下去， 直到写数据块请求到达数据流管道中的最后一个Datanode。数据流管道中的最后一个数据节点会回复请求确认， 这个确认消息逆向地通过数据流管道送回DFSClient。

当一个数据块中的所有数据包都成功发送完毕， 并且收到确认消息后， DFSClient会发送一个空数据包标识当前数据块发送完毕。 至此， 整个数据块发送流程结束。

2 源码解析

2.1 往dataQueue队列中写数据

DFSClient收到请求确认后， 将要写入的数据块切分成若干个数据包（packet） ， 然后依次向数据流管道中发送这些数据包。 数据包会首先从DFSClient发送到数据流管道中的第一个数据节点（这里是dn1）， dn1成功接收数据包后， 会将数据包写入磁盘， 然后将数据包发送到数据流管道中的第二个节点（dn2） 。 依此类推， 当数据包到达数据流管道中的最后一个节点（dn3）时， dn3会对收到的数据包进行校验， 如果校验成功， dn3会发送数据包确认消息， 这个确认消息会逆向地通过数据流管道送回DFSClient。

先从DFSOutputStream#write方法开始吧：

public class DataWriteTest {
    public static void main(String[] args) throws IOException {
        Configuration conf = new Configuration();
        FileSystem fileSystem = FileSystem.newInstance(conf);
        // 上传文件
        FSDataOutputStream fsDataOutputStream = fileSystem.create(new Path("/user/clare/test.txt"));
        fsDataOutputStream.write("Hello World!".getBytes(StandardCharsets.UTF_8));
    }
}

这个write方法经过多层封装调用，最后调用的是DFSOutputStream的父类FSOutputSummer中的write方法：

/** Write one byte */
@Override
public synchronized void write(int b) throws IOException {
    buf[count++] = (byte)b;
    if(count == buf.length) {
        // 强制对任何缓冲的输出字节进行校验 和 写出到基础输出流
        flushBuffer();
    }
}
 
/* Forces buffered output bytes to be checksummed and written out to
   * the underlying output stream. If there is a trailing partial chunk in the
   * buffer,
   * 1) flushPartial tells us whether to flush that chunk
   * 2) if flushPartial is true, keep tells us whether to keep that chunk in the
   * buffer (if flushPartial is false, it is always kept in the buffer)
   *
   * Returns the number of bytes that were flushed but are still left in the
   * buffer (can only be non-zero if keep is true).
   * 如果设置为在buf中保留chunk(keep 为true)，则返回值为非0
   */
protected synchronized int flushBuffer(boolean keep,
                                       boolean flushPartial) throws IOException {
    int bufLen = count;
    int partialLen = bufLen % sum.getBytesPerChecksum();
    int lenToFlush = flushPartial ? bufLen : bufLen - partialLen;
    if (lenToFlush != 0) {
        // 为给定的数据块生成校验和，并将数据块和校验和输出到底层输出流。
        writeChecksumChunks(buf, 0, lenToFlush);
        if (!flushPartial || keep) {
            count = partialLen;
            System.arraycopy(buf, bufLen - count, buf, 0, count);
        } else {
            count = 0;
        }
    }
 
    // total bytes left minus unflushed bytes left
    return count - (bufLen - lenToFlush);
}
 
/** Generate checksums for the given data chunks and output chunks & checksums
   * to the underlying output stream.
   */
private void writeChecksumChunks(byte b[], int off, int len)
    throws IOException {
    // 计算校验和
    sum.calculateChunkedSums(b, off, len, checksum, 0);
    TraceScope scope = createWriteTraceScope();
    try {
        // 虽然这个命名是getBytesPerChecksum，但我看了下貌似这个是指chunk，命名不规范
        // 因此这里通过for循环一个个chunk写
        for (int i = 0; i < len; i += sum.getBytesPerChecksum()) {
            int chunkLen = Math.min(sum.getBytesPerChecksum(), len - i);
            int ckOffset = i / sum.getBytesPerChecksum() * getChecksumSize();
            // 写出chunk
            writeChunk(b, off + i, chunkLen, checksum, ckOffset,
                       getChecksumSize());
        }
    } finally {
        if (scope != null) {
            scope.close();
        }
    }
}

而后调用DFSOutputStream#writeChunk

@Override
protected synchronized void writeChunk(byte[] b, int offset, int len,
                                       byte[] checksum, int ckoff, int cklen) throws IOException {
    // 做一些校验，如
    writeChunkPrepare(len, ckoff, cklen);
 
    currentPacket.writeChecksum(checksum, ckoff, cklen);
    currentPacket.writeData(b, offset, len);
    currentPacket.incNumChunks();
    getStreamer().incBytesCurBlock(len);
 
    // If packet is full, enqueue it for transmission
    if (currentPacket.getNumChunks() == currentPacket.getMaxChunks() ||
        getStreamer().getBytesCurBlock() == blockSize) {
        // 如果packet满了，则将其写入dataQueue便于后续传输
        enqueueCurrentPacketFull();
    }
}
 
synchronized void enqueueCurrentPacketFull() throws IOException {
    LOG.debug("enqueue full {}, src={}, bytesCurBlock={}, blockSize={},"
              + " appendChunk={}, {}", currentPacket, src, getStreamer()
              .getBytesCurBlock(), blockSize, getStreamer().getAppendChunk(),
              getStreamer());
    // 此方法调用DataStreamer#waitAndQueuePacket方法等待dataQueue的空间写入packet
    enqueueCurrentPacket();
    adjustChunkBoundary();
    endBlock();
}
 
void enqueueCurrentPacket() throws IOException {
    getStreamer().waitAndQueuePacket(currentPacket);
    currentPacket = null;
}

调用waitAndQueuePacket方法将packet添加到队列中，其中当dataQueue满了的时候，会进行等待，但在finally中最终还是会将packet添加到队列中：

/**
   * wait for space of dataQueue and queue the packet
   *
   * @param packet  the DFSPacket to be queued
   * @throws IOException
   */
void waitAndQueuePacket(DFSPacket packet) throws IOException {
    synchronized (dataQueue) {
        try {
            // If queue is full, then wait till we have enough space
            boolean firstWait = true;
            try {
                while (!streamerClosed && dataQueue.size() + ackQueue.size() >
                       dfsClient.getConf().getWriteMaxPackets()) {
                    if (firstWait) {
                        Span span = Tracer.getCurrentSpan();
                        if (span != null) {
                            span.addTimelineAnnotation("dataQueue.wait");
                        }
                        firstWait = false;
                    }
                    try {
                        dataQueue.wait();
                    } catch (InterruptedException e) {
                        // If we get interrupted while waiting to queue data, we still need to get rid
                        // of the current packet. This is because we have an invariant that if
                        // currentPacket gets full, it will get queued before the next writeChunk.
                        //
                        // Rather than wait around for space in the queue, we should instead try to
                        // return to the caller as soon as possible, even though we slightly overrun
                        // the MAX_PACKETS length.
                        Thread.currentThread().interrupt();
                        break;
                    }
                }
            } finally {
                Span span = Tracer.getCurrentSpan();
                if ((span != null) && (!firstWait)) {
                    span.addTimelineAnnotation("end.wait");
                }
            }
            checkClosed();
            queuePacket(packet);
        } catch (ClosedChannelException cce) {
            LOG.debug("Closed channel exception", cce);
        }
    }
}

2.2 writeBlock：数据块请求

数据块的写请求主要在DataStreamer#run()中触发

if (stage == BlockConstructionStage.PIPELINE_SETUP_CREATE) {
    // 此逻辑用于创建新文件
    LOG.debug("Allocating new block: {}", this);
    //nextBlockOutputStream()方法用来向Namenode 申请块信息，返回LocatedBlock 对象，
    // 其包含了 数据流pipeline 数据流节点信息 DatanodeInfo
    setPipeline(nextBlockOutputStream());
    // 初始化数据流，在其中会启动一个ResponseProcessor线程，此线程用来处理来自dn的响应
    // 所谓响应即ack，每当我们发出一个数据Packet，DataNode都需要发送ACK回复我们表示他收到了
    // 因此这样可以看出是每一个block对应一个响应线程，当此block写完关闭时，则会关闭此线程
    initDataStreaming();
} else if (stage == BlockConstructionStage.PIPELINE_SETUP_APPEND) {
    // 此逻辑用于往文件添加数据
    LOG.debug("Append to block {}", block);
    // 这里也是创建一个dataStreamer
    setupPipelineForAppendOrRecovery();
    if (streamerClosed) {
        continue;
    }
    // 初始化dataStream，在其中会启动一个ResponseProcessor线程，此线程用来处理来自dn的响应
    // 所谓响应即ack，每当我们发出一个数据Packet，DataNode都需要发送ACK回复我们表示他收到了
    initDataStreaming();
}

在DataStreamer#nextBlockOutputStream方法中调用DataStreamer#locateFollowingBlock方法向nn请求创建一个block，而后触发写数据块请求：

// 在DataStreamer#nextBlockOutputStream方法中
lb = locateFollowingBlock(
    excluded.length > 0 ? excluded : null, oldBlock);
 
private LocatedBlock locateFollowingBlock(DatanodeInfo[] excluded,
                                          ExtendedBlock oldBlock) throws IOException {
    // 这个方法就是在配置各种参数，而后dfsClient通过rpc调用namenodeRpcServer#addBlock
    return DFSOutputStream.addBlock(excluded, dfsClient, src, oldBlock,
                                    stat.getFileId(), favoredNodes, addBlockFlags);
}
 
// NanenodeRpcServer#addBlock
// 而这个方法就是调用FSNamesystem中的方法去后续的添加block
@Override
public LocatedBlock addBlock(String src, String clientName,
                             ExtendedBlock previous, DatanodeInfo[] excludedNodes, long fileId,
                             String[] favoredNodes, EnumSet<AddBlockFlag> addBlockFlags)
    throws IOException {
    checkNNStartup();
    LocatedBlock locatedBlock = namesystem.getAdditionalBlock(src, fileId,
                                                              clientName, previous, excludedNodes, favoredNodes, addBlockFlags);
    if (locatedBlock != null) {
        metrics.incrAddBlockOps();
    }
    return locatedBlock;
}

下面看下FSNamesystem#getAdditionalBlock方法，此方法为指定的文件名（正在写入）获取附加块。返回一个由块和一组机器组成的数组。此列表中的第一个应该是客户端写入数据的位置。列表中的后续项目必须在与第一个数据节点的连接中提供。确保之前的块已被 datanodes 报告并被复制。其实就是一个pipeline中的机器和block位置。

在这个类中，分为两部分：

1. 首先调用validateAddBlock()方法检查Namenode是否有写权限， 是否处于安全模式中， 以及当前文件系统中的对象是否过多。 然后对当前Namenode中记录的该文件的最后一个数据块与Client上报的最后一个数据块进行比较， 最后判断是否发生了RPC请求的重传， 或者请求异常等情况
2. 调用FSDirWriteFileOp.storeAllocatedBlock创建一个新的block，过程中会再次调用analyzeFileState方法对文件状态进行分析，如果过程中状态未变化，则进行后续的block的申请，申请之前会先将当前file的最后一个block commit，最后才是申请新的block并添加到文件中

/**
   * The client would like to obtain an additional block for the indicated
   * filename (which is being written-to).  Return an array that consists
   * of the block, plus a set of machines.  The first on this list should
   * be where the client writes data.  Subsequent items in the list must
   * be provided in the connection to the first datanode.
   *
   * Make sure the previous blocks have been reported by datanodes and
   * are replicated.  Will return an empty 2-elt array if we want the
   * client to "try again later".
   */
LocatedBlock getAdditionalBlock(
    String src, long fileId, String clientName, ExtendedBlock previous,
    DatanodeInfo[] excludedNodes, String[] favoredNodes,
    EnumSet<AddBlockFlag> flags) throws IOException {
    final String operationName = "getAdditionalBlock";
    NameNode.stateChangeLog.debug("BLOCK* getAdditionalBlock: {}  inodeId {}" +
                                  " for {}", src, fileId, clientName);
 
    LocatedBlock[] onRetryBlock = new LocatedBlock[1];
    FSDirWriteFileOp.ValidateAddBlockResult r;
    checkOperation(OperationCategory.READ);
    final FSPermissionChecker pc = getPermissionChecker();
    FSPermissionChecker.setOperationType(operationName);
    readLock();
    try {
        checkOperation(OperationCategory.READ);
        // 分析处于读取锁定状态的文件的状态，以确定客户端是否可以添加新块、检测潜在的重试、
        // 租用不匹配以及倒数第二个块的最小复制。为新块生成目标 DataNode 位置，但不要创建新块。
        r = FSDirWriteFileOp.validateAddBlock(this, pc, src, fileId, clientName,
                                              previous, onRetryBlock);
    } finally {
        readUnlock(operationName);
    }
 
    if (r == null) {
        assert onRetryBlock[0] != null : "Retry block is null";
        // This is a retry. Just return the last block.
        return onRetryBlock[0];
    }
 
    // 选择存储这个新申请的block的存储
    DatanodeStorageInfo[] targets = FSDirWriteFileOp.chooseTargetForNewBlock(
        blockManager, src, excludedNodes, favoredNodes, flags, r);
 
    checkOperation(OperationCategory.WRITE);
    writeLock();
    LocatedBlock lb;
    try {
        checkOperation(OperationCategory.WRITE);
        // 申请新block，过程还会将block添加到对应的文件中
        lb = FSDirWriteFileOp.storeAllocatedBlock(
            this, src, fileId, clientName, previous, targets);
    } finally {
        writeUnlock(operationName);
    }
    getEditLog().logSync();
    return lb;
}

因此下面根据上文中提及的两个步骤分别看：

2.2.1 第一部分：权限检查

在这个方法中主要通过FSDirWriteFileOp.validateAddBlock去进行校验，分析处于读取锁定状态的文件的状态，以确定客户端是否可以添加新块、检测潜在的重试、租用不匹配以及倒数第二个块的最小复制。为新块生成目标 DataNode 位置，但不要创建新块。

/**
   * Part I of getAdditionalBlock().
   * Analyze the state of the file under read lock to determine if the client
   * can add a new block, detect potential retries, lease mismatches,
   * and minimal replication of the penultimate block.
   *
   * Generate target DataNode locations for the new block,
   * but do not create the new block yet.
   */
static ValidateAddBlockResult validateAddBlock(
    FSNamesystem fsn, FSPermissionChecker pc,
    String src, long fileId, String clientName,
    ExtendedBlock previous, LocatedBlock[] onRetryBlock) throws IOException {
    final long blockSize;
    final short numTargets;
    final byte storagePolicyID;
    String clientMachine;
    final BlockType blockType;
 
    // 解析文件路径
    INodesInPath iip = fsn.dir.resolvePath(pc, src, fileId);
    // 分析状态，对当前Namenode中记录的该文件的最后一个数据块与Client上报的最后一个数据块进行比较， 
    // 最后判断是否发生了RPC请求的重传， 或者请求异常等情况
    FileState fileState = analyzeFileState(fsn, iip, fileId, clientName,
                                           previous, onRetryBlock);
    if (onRetryBlock[0] != null && onRetryBlock[0].getLocations().length > 0) {
        // This is a retry. No need to generate new locations.
        // Use the last block if it has locations.
        return null;
    }
 
    // 根据文件状态对象获取内存中的文件对象
    final INodeFile pendingFile = fileState.inode;
    if (!fsn.checkFileProgress(src, pendingFile, false)) {
        throw new NotReplicatedYetException("Not replicated yet: " + src);
    }
    if (pendingFile.getBlocks().length >= fsn.maxBlocksPerFile) {
        throw new IOException("File has reached the limit on maximum number of"
                              + " blocks (" + DFSConfigKeys.DFS_NAMENODE_MAX_BLOCKS_PER_FILE_KEY
                              + "): " + pendingFile.getBlocks().length + " >= "
                              + fsn.maxBlocksPerFile);
    }
    blockSize = pendingFile.getPreferredBlockSize();
    clientMachine = pendingFile.getFileUnderConstructionFeature()
        .getClientMachine();
    blockType = pendingFile.getBlockType();
    ErasureCodingPolicy ecPolicy = null;
    if (blockType == BlockType.STRIPED) {
        ecPolicy =
            FSDirErasureCodingOp.unprotectedGetErasureCodingPolicy(fsn, iip);
        numTargets = (short) (ecPolicy.getSchema().getNumDataUnits()
                              + ecPolicy.getSchema().getNumParityUnits());
    } else {
        numTargets = pendingFile.getFileReplication();
    }
    storagePolicyID = pendingFile.getStoragePolicyID();
    // 从INodeFile对象中获取的变量填充成一个ValidateAddBlockResult对象
    return new ValidateAddBlockResult(blockSize, numTargets, storagePolicyID,
                                      clientMachine, blockType, ecPolicy);
}

2.2.2 第二部分：block的申请

调用FSDirWriteFileOp.storeAllocatedBlock创建一个新的block，过程中会再次调用analyzeFileState方法对文件状态进行分析，如果过程中状态未变化，则进行后续的block的申请，申请之前会先将当前file的最后一个block commit，最后才是申请新的block并添加到文件中

/**
   * Part II of getAdditionalBlock().
   * Should repeat the same analysis of the file state as in Part 1,
   * but under the write lock.
   * If the conditions still hold, then allocate a new block with
   * the new targets, add it to the INode and to the BlocksMap.
   */
static LocatedBlock storeAllocatedBlock(FSNamesystem fsn, String src,
                                        long fileId, String clientName, ExtendedBlock previous,
                                        DatanodeStorageInfo[] targets) throws IOException {
    long offset;
    // Run the full analysis again, since things could have changed
    // while chooseTarget() was executing.
    LocatedBlock[] onRetryBlock = new LocatedBlock[1];
    INodesInPath iip = fsn.dir.resolvePath(null, src, fileId);
    // 再次调用analyzeFileState()方法以防止chooseTarget4NewBlock()
    // 方法执行期间Namenode状态发生改变， 例如在调用FSDirWriteFileOp.chooseTarget4NewBlock()
    // 方法时， 上一次触发的分配请求执行完成， 这时无须再次分配新的数据块， 返回上一次分配的数据块即可。
    FileState fileState = analyzeFileState(fsn, iip, fileId, clientName,
                                           previous, onRetryBlock);
    final INodeFile pendingFile = fileState.inode;
    src = fileState.path;
 
    if (onRetryBlock[0] != null) {
        if (onRetryBlock[0].getLocations().length > 0) {
            // This is a retry. Just return the last block if having locations.
            return onRetryBlock[0];
        } else {
            // add new chosen targets to already allocated block and return
            BlockInfo lastBlockInFile = pendingFile.getLastBlock();
            lastBlockInFile.getUnderConstructionFeature().setExpectedLocations(
                lastBlockInFile, targets, pendingFile.getBlockType());
            offset = pendingFile.computeFileSize();
            return makeLocatedBlock(fsn, lastBlockInFile, targets, offset);
        }
    }
 
    // commit the last block and complete it if it has minimum replicas
    // 提交上一个block，提交时判断是否副本数等要求已满足
    fsn.commitOrCompleteLastBlock(pendingFile, fileState.iip,
                                  ExtendedBlock.getLocalBlock(previous));
 
    // allocate new block, record block locations in INode.
    final BlockType blockType = pendingFile.getBlockType();
    // allocate new block, record block locations in INode.
    // 申请block
    Block newBlock = fsn.createNewBlock(blockType);
    INodesInPath inodesInPath = INodesInPath.fromINode(pendingFile);
    // 将新建的block添加到对应的文件上
    saveAllocatedBlock(fsn, src, inodesInPath, newBlock, targets, blockType);
 
    // 往editLog中添加对应的元数据（add block）
    persistNewBlock(fsn, src, pendingFile);
    offset = pendingFile.computeFileSize();
 
    // Return located block
    // 构建一个LocatedBlock对象
    return makeLocatedBlock(fsn, fsn.getStoredBlock(newBlock), targets, offset);
}

2.3 建立数据管道

在上文申请block完成之后，代码依据createBlockOutputStream方法去创建管道，方法中主要完成三件事：

1. 与流水线第一个dn建立socket连接；
2. 发送写数据的请求；

1. 接收响应信息

// connects to the first datanode in the pipeline
// Returns true if success, otherwise return failure.
//
boolean createBlockOutputStream(DatanodeInfo[] nodes,
                                StorageType[] nodeStorageTypes, String[] nodeStorageIDs,
                                long newGS, boolean recoveryFlag) {
    if (nodes.length == 0) {
        LOG.info("nodes are empty for write pipeline of " + block);
        return false;
    }
    String firstBadLink = "";
    boolean checkRestart = false;
    if (LOG.isDebugEnabled()) {
        LOG.debug("pipeline = " + Arrays.toString(nodes) + ", " + this);
    }
 
    // persist blocks on namenode on next flush
    persistBlocks.set(true);
 
    int refetchEncryptionKey = 1;
    while (true) {
        boolean result = false;
        DataOutputStream out = null;
        try {
            assert null == s : "Previous socket unclosed";
            assert null == blockReplyStream : "Previous blockReplyStream unclosed";
            // 步骤1，建立socket连接，返回第一个dn建立连接的socket
            s = createSocketForPipeline(nodes[0], nodes.length, dfsClient);
            long writeTimeout = dfsClient.getDatanodeWriteTimeout(nodes.length);
            long readTimeout = dfsClient.getDatanodeReadTimeout(nodes.length);
 
            OutputStream unbufOut = NetUtils.getOutputStream(s, writeTimeout);
            InputStream unbufIn = NetUtils.getInputStream(s, readTimeout);
            IOStreamPair saslStreams = dfsClient.saslClient.socketSend(s,
                                                                       unbufOut, unbufIn, dfsClient, accessToken, nodes[0]);
            unbufOut = saslStreams.out;
            unbufIn = saslStreams.in;
            // 创建输出流对象，使用装饰者模式，真实使用BufferedOutputStream
            out = new DataOutputStream(new BufferedOutputStream(unbufOut,
                                                                DFSUtilClient.getSmallBufferSize(dfsClient.getConfiguration())));
            blockReplyStream = new DataInputStream(unbufIn);
 
            //
            // Xmit header info to datanode
            //
 
            BlockConstructionStage bcs = recoveryFlag ?
                stage.getRecoveryStage() : stage;
 
            // We cannot change the block length in 'block' as it counts the number
            // of bytes ack'ed.
            ExtendedBlock blockCopy = block.getCurrentBlock();
            blockCopy.setNumBytes(stat.getBlockSize());
 
            boolean[] targetPinnings = getPinnings(nodes);
            // send the request
            // 步骤二：发送写数据请求，这个方法将buffer中的数据刷写到dn中
            new Sender(out).writeBlock(blockCopy, nodeStorageTypes[0], accessToken,
                                       dfsClient.clientName, nodes, nodeStorageTypes, null, bcs,
                                       nodes.length, block.getNumBytes(), bytesSent, newGS,
                                       checksum4WriteBlock, cachingStrategy.get(), isLazyPersistFile,
                                       (targetPinnings != null && targetPinnings[0]), targetPinnings,
                                       nodeStorageIDs[0], nodeStorageIDs);
 
            // receive ack for connect
            // 步骤3，接收响应
            BlockOpResponseProto resp = BlockOpResponseProto.parseFrom(
                PBHelperClient.vintPrefixed(blockReplyStream));
            Status pipelineStatus = resp.getStatus();
            firstBadLink = resp.getFirstBadLink();
 
            // Got an restart OOB ack.
            // If a node is already restarting, this status is not likely from
            // the same node. If it is from a different node, it is not
            // from the local datanode. Thus it is safe to treat this as a
            // regular node error.
            // 检查通道中dn是否处于重启状态，如果客户端对应这个dn不是重启状态才进入此判断
            if (PipelineAck.isRestartOOBStatus(pipelineStatus) &&
                !errorState.isRestartingNode()) {
                checkRestart = true;
                throw new IOException("A datanode is restarting.");
            }
 
            String logInfo = "ack with firstBadLink as " + firstBadLink;
            DataTransferProtoUtil.checkBlockOpStatus(resp, logInfo);
 
            assert null == blockStream : "Previous blockStream unclosed";
            blockStream = out;
            result =  true; // success
            errorState.resetInternalError();
            lastException.clear();
            // remove all restarting nodes from failed nodes list
            failed.removeAll(restartingNodes);
            restartingNodes.clear();
        } catch (IOException ie) {
            if (!errorState.isRestartingNode()) {
                LOG.warn("Exception in createBlockOutputStream " + this, ie);
            }
            if (ie instanceof InvalidEncryptionKeyException &&
                refetchEncryptionKey > 0) {
                LOG.info("Will fetch a new encryption key and retry, "
                         + "encryption key was invalid when connecting to "
                         + nodes[0] + " : " + ie);
                // The encryption key used is invalid.
                refetchEncryptionKey--;
                dfsClient.clearDataEncryptionKey();
                // Don't close the socket/exclude this node just yet. Try again with
                // a new encryption key.
                continue;
            }
 
            // find the datanode that matches
            // 设置BadNode，把第一个不能正常连接的DataNode标记为BadNode，
            // 如果返回的消息不能认定谁是BadNode，那么BadNode就是你啦，第一个节点
            if (firstBadLink.length() != 0) {
                for (int i = 0; i < nodes.length; i++) {
                    // NB: Unconditionally using the xfer addr w/o hostname
                    if (firstBadLink.equals(nodes[i].getXferAddr())) {
                        errorState.setBadNodeIndex(i);
                        break;
                    }
                }
            } else {
                assert !checkRestart;
                errorState.setBadNodeIndex(0);
            }
 
            final int i = errorState.getBadNodeIndex();
            // Check whether there is a restart worth waiting for.
            if (checkRestart) {
                errorState.initRestartingNode(i,
                                              "Datanode " + i + " is restarting: " + nodes[i],
                                              shouldWaitForRestart(i));
            }
            errorState.setInternalError();
            lastException.set(ie);
            result =  false;  // error
        } finally {
            if (!result) {
                IOUtils.closeSocket(s);
                s = null;
                IOUtils.closeStream(out);
                IOUtils.closeStream(blockReplyStream);
                blockReplyStream = null;
            }
        }
        return result;
    }
}

2.4 dn进行数据接受

写数据块请求通过流式接口到达Datanode之后， Datanode上监听流式接口请求的DataXceiverServer会接收这个请求， 并构造一个DataXceiver对象， 然后在DataXceiver对象上调用DataXceiver.writeBlock()方法响应这个请求。 当前Datanode的DataXceiver.writeBlock()方法会级联向数据流管道中的下一个Datanode发送写数据块请求， 这个流式请求会一直在数据流管道中传递下去， 直到写数据块请求到达数据流管道中的最后一个Datanode。

上文中Sender(out).writeBlock方法触发写数据请求后，dn的DataXceiverServer接收，接受到后交由DataXceiver进行处理，这两个都是线程对象，所以这里可以先看DataXceiverServer#run()方法，在run方法中创建了DataXceiver线程供后续操作，这里过程也是在DataXceiver#run()方法中实现，经过调用，后面会到DataXceiver#writeBlock方法（这个方法能看下来挺需要耐心。。。。太长了）：

@Override
public void writeBlock(final ExtendedBlock block,
                       final StorageType storageType, 
                       final Token<BlockTokenIdentifier> blockToken,
                       final String clientname,
                       final DatanodeInfo[] targets,
                       final StorageType[] targetStorageTypes,
                       final DatanodeInfo srcDataNode,
                       final BlockConstructionStage stage,
                       final int pipelineSize,
                       final long minBytesRcvd,
                       final long maxBytesRcvd,
                       final long latestGenerationStamp,
                       DataChecksum requestedChecksum,
                       CachingStrategy cachingStrategy,
                       boolean allowLazyPersist,
                       final boolean pinning,
                       final boolean[] targetPinnings,
                       final String storageId,
                       final String[] targetStorageIds) throws IOException {
    previousOpClientName = clientname;
    updateCurrentThreadName("Receiving block " + block);
    // isDatanode变量指示当前写操作是否是DFSClient发起的 : false
    final boolean isDatanode = clientname.length() == 0;
    // isClient变量与isDatanode相反， 表示是Datanode触发的写操作  true
    final boolean isClient = !isDatanode;
    // isTransfer变量指示当前的写操作是否为数据块复制操作， 利用数据流管道状态来判断 false
    final boolean isTransfer = stage == BlockConstructionStage.TRANSFER_RBW
        || stage == BlockConstructionStage.TRANSFER_FINALIZED;
 
    // 是否开启lazyPersist false
    allowLazyPersist = allowLazyPersist &&
        (dnConf.getAllowNonLocalLazyPersist() || peer.isLocal());
    long size = 0;
    // reply to upstream datanode or client 
    final DataOutputStream replyOut = getBufferedOutputStream();
 
    int nst = targetStorageTypes.length;
    StorageType[] storageTypes = new StorageType[nst + 1];
    storageTypes[0] = storageType;
    if (targetStorageTypes.length > 0) {
        System.arraycopy(targetStorageTypes, 0, storageTypes, 1, nst);
    }
 
    // To support older clients, we don't pass in empty storageIds
    final int nsi = targetStorageIds.length;
    final String[] storageIds;
    if (nsi > 0) {
        storageIds = new String[nsi + 1];
        storageIds[0] = storageId;
        if (targetStorageTypes.length > 0) {
            System.arraycopy(targetStorageIds, 0, storageIds, 1, nsi);
        }
    } else {
        storageIds = new String[0];
    }
    checkAccess(replyOut, isClient, block, blockToken, Op.WRITE_BLOCK,
                BlockTokenIdentifier.AccessMode.WRITE,
                storageTypes, storageIds);
 
    // check single target for transfer-RBW/Finalized
    if (isTransfer && targets.length > 0) {
        throw new IOException(stage + " does not support multiple targets "
                              + Arrays.asList(targets));
    }
 
    if (LOG.isDebugEnabled()) {
        LOG.debug("opWriteBlock: stage={}, clientname={}\n  " +
                  "block  ={}, newGs={}, bytesRcvd=[{}, {}]\n  " +
                  "targets={}; pipelineSize={}, srcDataNode={}, pinning={}",
                  stage, clientname, block, latestGenerationStamp, minBytesRcvd,
                  maxBytesRcvd, Arrays.asList(targets), pipelineSize, srcDataNode,
                  pinning);
        LOG.debug("isDatanode={}, isClient={}, isTransfer={}",
                  isDatanode, isClient, isTransfer);
        LOG.debug("writeBlock receive buf size {} tcp no delay {}",
                  peer.getReceiveBufferSize(), peer.getTcpNoDelay());
    }
 
    // We later mutate block's generation stamp and length, but we need to
    // forward the original version of the block to downstream mirrors, so
    // make a copy here.
    final ExtendedBlock originalBlock = new ExtendedBlock(block);
    if (block.getNumBytes() == 0) {
        block.setNumBytes(dataXceiverServer.estimateBlockSize);
    }
    LOG.info("Receiving {} src: {} dest: {}",
             block, remoteAddress, localAddress);
 
    // 到下游数据节点的输出流
    DataOutputStream mirrorOut = null;  // stream to next target
    //下游数据节点的输入流
    DataInputStream mirrorIn = null;    // reply from next target
    //到下游节点的Socket
    Socket mirrorSock = null;           // socket to next target
    //下游节点的地址，即名称:端口
    String mirrorNode = null;           // the name:port of next target
    String firstBadLink = "";           // first datanode that failed in connection setup
    Status mirrorInStatus = SUCCESS;
    final String storageUuid;
    final boolean isOnTransientStorage;
    try {
        final Replica replica;
        if (isDatanode || 
            stage != BlockConstructionStage.PIPELINE_CLOSE_RECOVERY) {
            // open a block receiver
            //打开一个BlockReceiver， 用于从上游节点接收数据块
            setCurrentBlockReceiver(getBlockReceiver(block, storageType, in,
                                                     peer.getRemoteAddressString(),
                                                     peer.getLocalAddressString(),
                                                     stage, latestGenerationStamp, minBytesRcvd, maxBytesRcvd,
                                                     clientname, srcDataNode, datanode, requestedChecksum,
                                                     cachingStrategy, allowLazyPersist, pinning, storageId));
            replica = blockReceiver.getReplica();
        } else {
            replica = datanode.data.recoverClose(
                block, latestGenerationStamp, minBytesRcvd);
        }
        storageUuid = replica.getStorageUuid();
        isOnTransientStorage = replica.isOnTransientStorage();
 
        //
        // Connect to downstream machine, if appropriate
        // 连接到下游节点
        //
        if (targets.length > 0) {
            InetSocketAddress mirrorTarget = null;
            // Connect to backup machine
            mirrorNode = targets[0].getXferAddr(connectToDnViaHostname);
            LOG.debug("Connecting to datanode {}", mirrorNode);
            mirrorTarget = NetUtils.createSocketAddr(mirrorNode);
            mirrorSock = datanode.newSocket();
            try {
 
                DataNodeFaultInjector.get().failMirrorConnection();
 
                int timeoutValue = dnConf.socketTimeout +
                    (HdfsConstants.READ_TIMEOUT_EXTENSION * targets.length);
                int writeTimeout = dnConf.socketWriteTimeout +
                    (HdfsConstants.WRITE_TIMEOUT_EXTENSION * targets.length);
                // 建立到下游节点的Socket连接
                NetUtils.connect(mirrorSock, mirrorTarget, timeoutValue);
                mirrorSock.setTcpNoDelay(dnConf.getDataTransferServerTcpNoDelay());
                mirrorSock.setSoTimeout(timeoutValue);
                mirrorSock.setKeepAlive(true);
                if (dnConf.getTransferSocketSendBufferSize() > 0) {
                    mirrorSock.setSendBufferSize(
                        dnConf.getTransferSocketSendBufferSize());
                }
 
                OutputStream unbufMirrorOut = NetUtils.getOutputStream(mirrorSock,
                                                                       writeTimeout);
                InputStream unbufMirrorIn = NetUtils.getInputStream(mirrorSock);
                DataEncryptionKeyFactory keyFactory =
                    datanode.getDataEncryptionKeyFactoryForBlock(block);
                SecretKey secretKey = null;
                if (dnConf.overwriteDownstreamDerivedQOP) {
                    String bpid = block.getBlockPoolId();
                    BlockKey blockKey = datanode.blockPoolTokenSecretManager
                        .get(bpid).getCurrentKey();
                    secretKey = blockKey.getKey();
                }
                IOStreamPair saslStreams = datanode.saslClient.socketSend(
                    mirrorSock, unbufMirrorOut, unbufMirrorIn, keyFactory,
                    blockToken, targets[0], secretKey);
                unbufMirrorOut = saslStreams.out;
                unbufMirrorIn = saslStreams.in;
                // 创建下游节点的输入输出流对象
                mirrorOut = new DataOutputStream(new BufferedOutputStream(unbufMirrorOut,
                                                                          smallBufferSize));
                mirrorIn = new DataInputStream(unbufMirrorIn);
 
                String targetStorageId = null;
                if (targetStorageIds.length > 0) {
                    // Older clients may not have provided any targetStorageIds
                    targetStorageId = targetStorageIds[0];
                }
                if (targetPinnings != null && targetPinnings.length > 0) {
                    // 向下游节点发送数据块写入请求，这里发送请求后，经过BlockReceiver接受
                    new Sender(mirrorOut).writeBlock(originalBlock, targetStorageTypes[0],
                                                     blockToken, clientname, targets, targetStorageTypes,
                                                     srcDataNode, stage, pipelineSize, minBytesRcvd, maxBytesRcvd,
                                                     latestGenerationStamp, requestedChecksum, cachingStrategy,
                                                     allowLazyPersist, targetPinnings[0], targetPinnings,
                                                     targetStorageId, targetStorageIds);
                } else {
                    //接收来自下游节点的请求确认， 并记录请求确认状态
                    new Sender(mirrorOut).writeBlock(originalBlock, targetStorageTypes[0],
                                                     blockToken, clientname, targets, targetStorageTypes,
                                                     srcDataNode, stage, pipelineSize, minBytesRcvd, maxBytesRcvd,
                                                     latestGenerationStamp, requestedChecksum, cachingStrategy,
                                                     allowLazyPersist, false, targetPinnings,
                                                     targetStorageId, targetStorageIds);
                }
 
                mirrorOut.flush();
 
                DataNodeFaultInjector.get().writeBlockAfterFlush();
 
                // read connect ack (only for clients, not for replication req)
                if (isClient) {
                    BlockOpResponseProto connectAck =
                        BlockOpResponseProto.parseFrom(PBHelperClient.vintPrefixed(mirrorIn));
                    mirrorInStatus = connectAck.getStatus();
                    firstBadLink = connectAck.getFirstBadLink();
                    if (mirrorInStatus != SUCCESS) {
                        LOG.debug("Datanode {} got response for connect" +
                                  "ack  from downstream datanode with firstbadlink as {}",
                                  targets.length, firstBadLink);
                    }
                }
 
            } catch (IOException e) {
                // 出现异常， 向上游节点发送异常响应
                if (isClient) {
                    BlockOpResponseProto.newBuilder()
                        .setStatus(ERROR)
                        // NB: Unconditionally using the xfer addr w/o hostname
                        .setFirstBadLink(targets[0].getXferAddr())
                        .build()
                        .writeDelimitedTo(replyOut);
                    replyOut.flush();
                }
 
                // 关闭到下游节点的Socket、 输入流以及输出流
                IOUtils.closeStream(mirrorOut);
                mirrorOut = null;
                IOUtils.closeStream(mirrorIn);
                mirrorIn = null;
                IOUtils.closeSocket(mirrorSock);
                mirrorSock = null;
                if (isClient) {
                    LOG.error("{}:Exception transfering block {} to mirror {}",
                              datanode, block, mirrorNode, e);
                    throw e;
                } else {
                    LOG.info("{}:Exception transfering {} to mirror {}- continuing " +
                             "without the mirror", datanode, block, mirrorNode, e);
                    incrDatanodeNetworkErrors();
                }
            }
        }
 
        // send connect-ack to source for clients and not transfer-RBW/Finalized
        if (isClient && !isTransfer) {
            if (mirrorInStatus != SUCCESS) {
                LOG.debug("Datanode {} forwarding connect ack to upstream " +
                          "firstbadlink is {}", targets.length, firstBadLink);
            }
            // 向上游节点返回请求确认
            BlockOpResponseProto.newBuilder()
                .setStatus(mirrorInStatus)
                .setFirstBadLink(firstBadLink)
                .build()
                .writeDelimitedTo(replyOut);
            replyOut.flush();
        }
 
        // receive the block and mirror to the next target
        if (blockReceiver != null) {
            String mirrorAddr = (mirrorSock == null) ? null : mirrorNode;
            // 调用BlockReceiver.receiveBlock()从上游节点接收数据块， 然后将数据块发送到下游节点
            blockReceiver.receiveBlock(mirrorOut, mirrorIn, replyOut, mirrorAddr,
                                       dataXceiverServer.getWriteThrottler(), targets, false);
 
            // send close-ack for transfer-RBW/Finalized 
            if (isTransfer) {
                LOG.trace("TRANSFER: send close-ack");
                writeResponse(SUCCESS, null, replyOut);
            }
        }
 
        // 成功执行了BlockReceiver.receiveBlock()之后， writeBlock()方法就会更新
        // 当前数据节点上新写入数据块副本的时间戳、 副本文件长度等信息。 
        // 如果是数据流管道关闭的恢复操作或者是数据块的复制操作， 则调用
        // Datanode.closeBlock()方法向Namenode汇报Datanode接收了新的数据块， 
        // Datanode.closeBlock()调用BPOfferService.notifyNamenodeReceivedBlock()
        // 通知Namenode。 对于客户端发起的写数据请求， 在PacketResponder线程中已经
        // 通过调用Datanode.closeBlock()方法关闭了数据块
        // update its generation stamp
        if (isClient && 
            stage == BlockConstructionStage.PIPELINE_CLOSE_RECOVERY) {
            block.setGenerationStamp(latestGenerationStamp);
            block.setNumBytes(minBytesRcvd);
        }
 
        // if this write is for a replication request or recovering
        // a failed close for client, then confirm block. For other client-writes,
        // the block is finalized in the PacketResponder.
        if (isDatanode ||
            stage == BlockConstructionStage.PIPELINE_CLOSE_RECOVERY) {
            datanode.closeBlock(block, null, storageUuid, isOnTransientStorage);
            LOG.info("Received {} src: {} dest: {} of size {}",
                     block, remoteAddress, localAddress, block.getNumBytes());
        }
 
        // 对于复制操作， 不需要向下游节点转发数据块， 也不需要接收下游节点的确认
        // 所以成功接收完数据块之后， 在当前节点直接返回确认消息
        if(isClient) {
            size = block.getNumBytes();
        }
    } catch (IOException ioe) {
        LOG.info("opWriteBlock {} received exception {}",
                 block, ioe.toString());
        incrDatanodeNetworkErrors();
        throw ioe;
    } finally {
        // close all opened streams
        // 关闭上下游节点的输入／ 输 出流， 同时关闭blockReceiver对象
        IOUtils.closeStream(mirrorOut);
        IOUtils.closeStream(mirrorIn);
        IOUtils.closeStream(replyOut);
        IOUtils.closeSocket(mirrorSock);
        IOUtils.closeStream(blockReceiver);
        setCurrentBlockReceiver(null);
    }
 
    //update metrics
    datanode.getMetrics().addWriteBlockOp(elapsed());
    datanode.getMetrics().incrWritesFromClient(peer.isLocal(), size);
}

上述过程成功建立了与下游节点的输入／ 输出流后， writeBlock()方法就会调用blockReceiver.receiveBlock()方法从上游节点接收数据块， 然后将数据块发送到下游节点。同时blockReceiver对象还会从下游节点接收数据块中数据包的确认消息， 并且将这个确认消息转发到上游节点。

BlockReceiver下一篇继续肝。。。。