一、文件的打开
1.1、客户端
HDFS打开一个文件，需要在客户端调用DistributedFileSystem.open(Path f, int bufferSize)，其实现为：
public FSDataInputStream open(Path f, int bufferSize) throws IOException {
return new DFSClient.DFSDataInputStream(
dfs.open(getPathName(f), bufferSize, verifyChecksum, statistics));
}
其中dfs为DistributedFileSystem的成员变量DFSClient，其open函数被调用，其中创建一个DFSInputStream(src, buffersize, verifyChecksum)并返回。
在DFSInputStream的构造函数中，openInfo函数被调用，其主要从namenode中得到要打开的文件所对应的blocks的信息，实现如下：
synchronized void openInfo() throws IOException {
LocatedBlocks newInfo = callGetBlockLocations(namenode, src, 0, prefetchSize);
this.locatedBlocks = newInfo;
this.currentNode = null;
}
private static LocatedBlocks callGetBlockLocations(ClientProtocol namenode,
String src, long start, long length) throws IOException {
return namenode.getBlockLocations(src, start, length);
}
LocatedBlocks主要包含一个链表的List<LocatedBlock> blocks，其中每个LocatedBlock包含如下信息：
Block b：此block的信息
long offset：此block在文件中的偏移量
DatanodeInfo[] locs：此block位于哪些DataNode上
上面namenode.getBlockLocations是一个RPC调用，最终调用NameNode类的getBlockLocations函数。
1.2、NameNode
NameNode.getBlockLocations实现如下：
public LocatedBlocks getBlockLocations(String src,
long offset,
long length) throws IOException {
return namesystem.getBlockLocations(getClientMachine(),
src, offset, length);
}
namesystem是NameNode一个成员变量，其类型为FSNamesystem，保存的是NameNode的name space树，其中一个重要的成员变量为FSDirectory dir。
FSDirectory和Lucene中的FSDirectory没有任何关系，其主要包括FSImage fsImage，用于读写硬盘上的fsimage文件，FSImage类有成员变量FSEditLog editLog，用于读写硬盘上的edit文件，这两个文件的关系在上一篇文章中已经解释过。
FSDirectory还有一个重要的成员变量INodeDirectoryWithQuota rootDir，INodeDirectoryWithQuota的父类为INodeDirectory，实现如下：
public class INodeDirectory extends INode {
……
private List<INode> children;
……
｝
由此可见INodeDirectory本身是一个INode，其中包含一个链表的INode，此链表中，如果仍为文件夹，则是类型INodeDirectory，如果是文件，则是类型INodeFile，INodeFile中有成员变量BlockInfo blocks[]，是此文件包含的block的信息。显然这是一棵树形的结构。
FSNamesystem.getBlockLocations函数如下：
public LocatedBlocks getBlockLocations(String src, long offset, long length,
boolean doAccessTime) throws IOException {
final LocatedBlocks ret = getBlockLocationsInternal(src, dir.getFileINode(src),
offset, length, Integer.MAX_VALUE, doAccessTime);
return ret;
}
dir.getFileINode(src)通过路径名从文件系统树中找到INodeFile，其中保存的是要打开的文件的INode的信息。
getBlockLocationsInternal的实现如下：
private synchronized LocatedBlocks getBlockLocationsInternal(String src,
INodeFile inode,
long offset,
long length,
int nrBlocksToReturn,
boolean doAccessTime)
throws IOException {
//得到此文件的block信息
Block[] blocks = inode.getBlocks();
List<LocatedBlock> results = new ArrayList<LocatedBlock>(blocks.length);
//计算从offset开始，长度为length所涉及的blocks
int curBlk = 0;
long curPos = 0, blkSize = 0;
int nrBlocks = (blocks[0].getNumBytes() == 0) ? 0 : blocks.length;
for (curBlk = 0; curBlk < nrBlocks; curBlk++) {
blkSize = blocks[curBlk].getNumBytes();
if (curPos + blkSize > offset) {
//当offset在curPos和curPos + blkSize之间的时候，curBlk指向offset所在的block
break;
}
curPos += blkSize;
}
long endOff = offset + length;
//循环，依次遍历从curBlk开始的每个block，直到当前位置curPos越过endOff
do {
int numNodes = blocksMap.numNodes(blocks[curBlk]);
int numCorruptNodes = countNodes(blocks[curBlk]).corruptReplicas();
int numCorruptReplicas = corruptReplicas.numCorruptReplicas(blocks[curBlk]);
boolean blockCorrupt = (numCorruptNodes == numNodes);
int numMachineSet = blockCorrupt ? numNodes :
(numNodes - numCorruptNodes);
//依次找到此block所对应的datanode，将其中没有损坏的放入machineSet中
DatanodeDescriptor[] machineSet = new DatanodeDescriptor[numMachineSet];
if (numMachineSet > 0) {
numNodes = 0;
for(Iterator<DatanodeDescriptor> it =
blocksMap.nodeIterator(blocks[curBlk]); it.hasNext();) {
DatanodeDescriptor dn = it.next();
boolean replicaCorrupt = corruptReplicas.isReplicaCorrupt(blocks[curBlk], dn);
if (blockCorrupt || (!blockCorrupt && !replicaCorrupt))
machineSet[numNodes++] = dn;
}
}
//使用此machineSet和当前的block构造一个LocatedBlock
results.add(new LocatedBlock(blocks[curBlk], machineSet, curPos,
blockCorrupt));
curPos += blocks[curBlk].getNumBytes();
curBlk++;
} while (curPos < endOff
&& curBlk < blocks.length
&& results.size() < nrBlocksToReturn);
//使用此LocatedBlock链表构造一个LocatedBlocks对象返回
return inode.createLocatedBlocks(results);
}
1.3、客户端
通过RPC调用，在NameNode得到的LocatedBlocks对象，作为成员变量构造DFSInputStream对象，最后包装为FSDataInputStream返回给用户。
二、文件的读取
2.1、客户端
文件读取的时候，客户端利用文件打开的时候得到的FSDataInputStream.read(long position, byte[] buffer, int offset, int length)函数进行文件读操作。
FSDataInputStream会调用其封装的DFSInputStream的read(long position, byte[] buffer, int offset, int length)函数，实现如下：
public int read(long position, byte[] buffer, int offset, int length)
throws IOException {
long filelen = getFileLength();
int realLen = length;
if ((position + length) > filelen) {
realLen = (int)(filelen - position);
}
//首先得到包含从offset到offset + length内容的block列表
//比如对于64M一个block的文件系统来说，欲读取从100M开始，长度为128M的数据，则block列表包括第2，3，4块block
List<LocatedBlock> blockRange = getBlockRange(position, realLen);
int remaining = realLen;
//对每一个block，从中读取内容
//对于上面的例子，对于第2块block，读取从36M开始，读取长度28M，对于第3块，读取整一块64M，对于第4块，读取从0开始，长度为36M，共128M数据
for (LocatedBlock blk : blockRange) {
long targetStart = position - blk.getStartOffset();
long bytesToRead = Math.min(remaining, blk.getBlockSize() - targetStart);
fetchBlockByteRange(blk, targetStart,
targetStart + bytesToRead - 1, buffer, offset);
remaining -= bytesToRead;
position += bytesToRead;
offset += bytesToRead;
}
assert remaining == 0 : "Wrong number of bytes read.";
if (stats != null) {
stats.incrementBytesRead(realLen);
}
return realLen;
}
其中getBlockRange函数如下：
private synchronized List<LocatedBlock> getBlockRange(long offset,
long length)
throws IOException {
List<LocatedBlock> blockRange = new ArrayList<LocatedBlock>();
//首先从缓存的locatedBlocks中查找offset所在的block在缓存链表中的位置
int blockIdx = locatedBlocks.findBlock(offset);
if (blockIdx < 0) { // block is not cached
blockIdx = LocatedBlocks.getInsertIndex(blockIdx);
}
long remaining = length;
long curOff = offset;
while(remaining > 0) {
LocatedBlock blk = null;
//按照blockIdx的位置找到block
if(blockIdx < locatedBlocks.locatedBlockCount())
blk = locatedBlocks.get(blockIdx);
//如果block为空，则缓存中没有此block，则直接从NameNode中查找这些block，并加入缓存
if (blk == null || curOff < blk.getStartOffset()) {
LocatedBlocks newBlocks;
newBlocks = callGetBlockLocations(namenode, src, curOff, remaining);
locatedBlocks.insertRange(blockIdx, newBlocks.getLocatedBlocks());
continue;
}
//如果block找到，则放入结果集
blockRange.add(blk);
long bytesRead = blk.getStartOffset() + blk.getBlockSize() - curOff;
remaining -= bytesRead;
curOff += bytesRead;
//取下一个block
blockIdx++;
}
return blockRange;
}
其中fetchBlockByteRange实现如下：
private void fetchBlockByteRange(LocatedBlock block, long start,
long end, byte[] buf, int offset) throws IOException {
Socket dn = null;
int numAttempts = block.getLocations().length;
//此while循环为读取失败后的重试次数
while (dn == null && numAttempts-- > 0 ) {
//选择一个DataNode来读取数据
DNAddrPair retval = chooseDataNode(block);
DatanodeInfo chosenNode = retval.info;
InetSocketAddress targetAddr = retval.addr;
BlockReader reader = null;
try {
//创建Socket连接到DataNode
dn = socketFactory.createSocket();
dn.connect(targetAddr, socketTimeout);
dn.setSoTimeout(socketTimeout);
int len = (int) (end - start + 1);
//利用建立的Socket链接，生成一个reader负责从DataNode读取数据
reader = BlockReader.newBlockReader(dn, src,
block.getBlock().getBlockId(),
block.getBlock().getGenerationStamp(),
start, len, buffersize,
verifyChecksum, clientName);
//读取数据
int nread = reader.readAll(buf, offset, len);
return;
} finally {
IOUtils.closeStream(reader);
IOUtils.closeSocket(dn);
dn = null;
}
//如果读取失败，则将此DataNode标记为失败节点
addToDeadNodes(chosenNode);
}
}
BlockReader.newBlockReader函数实现如下：
public static BlockReader newBlockReader( Socket sock, String file,
long blockId,
long genStamp,
long startOffset, long len,
int bufferSize, boolean verifyChecksum,
String clientName)
throws IOException {
//使用Socket建立写入流，向DataNode发送读指令
DataOutputStream out = new DataOutputStream(
new BufferedOutputStream(NetUtils.getOutputStream(sock,HdfsConstants.WRITE_TIMEOUT)));
out.writeShort( DataTransferProtocol.DATA_TRANSFER_VERSION );
out.write( DataTransferProtocol.OP_READ_BLOCK );
out.writeLong( blockId );
out.writeLong( genStamp );
out.writeLong( startOffset );
out.writeLong( len );
Text.writeString(out, clientName);
out.flush();
//使用Socket建立读入流，用于从DataNode读取数据
DataInputStream in = new DataInputStream(
new BufferedInputStream(NetUtils.getInputStream(sock),
bufferSize));
DataChecksum checksum = DataChecksum.newDataChecksum( in );
long firstChunkOffset = in.readLong();
//生成一个reader，主要包含读入流，用于读取数据
return new BlockReader( file, blockId, in, checksum, verifyChecksum,
startOffset, firstChunkOffset, sock );
}
BlockReader的readAll函数就是用上面生成的DataInputStream读取数据。
2.2、DataNode
在DataNode启动的时候，会调用函数startDataNode，其中与数据读取有关的逻辑如下：
void startDataNode(Configuration conf,
AbstractList<File> dataDirs
) throws IOException {
……
// 建立一个ServerSocket，并生成一个DataXceiverServer来监控客户端的链接
ServerSocket ss = (socketWriteTimeout > 0) ?
ServerSocketChannel.open().socket() : new ServerSocket();
Server.bind(ss, socAddr, 0);
ss.setReceiveBufferSize(DEFAULT_DATA_SOCKET_SIZE);
// adjust machine name with the actual port
tmpPort = ss.getLocalPort();
selfAddr = new InetSocketAddress(ss.getInetAddress().getHostAddress(),
tmpPort);
this.dnRegistration.setName(machineName + ":" + tmpPort);
this.threadGroup = new ThreadGroup("dataXceiverServer");
this.dataXceiverServer = new Daemon(threadGroup,
new DataXceiverServer(ss, conf, this));
this.threadGroup.setDaemon(true); // auto destroy when empty
……
}
DataXceiverServer.run()函数如下：
public void run() {
while (datanode.shouldRun) {
//接受客户端的链接
Socket s = ss.accept();
s.setTcpNoDelay(true);
//生成一个线程DataXceiver来对建立的链接提供服务
new Daemon(datanode.threadGroup,
new DataXceiver(s, datanode, this)).start();
}
try {
ss.close();
} catch (IOException ie) {
LOG.warn(datanode.dnRegistration + ":DataXceiveServer: "
+ StringUtils.stringifyException(ie));
}
}
DataXceiver.run()函数如下：
public void run() {
DataInputStream in=null;
try {
//建立一个输入流，读取客户端发送的指令
in = new DataInputStream(
new BufferedInputStream(NetUtils.getInputStream(s),
SMALL_BUFFER_SIZE));
short version = in.readShort();
boolean local = s.getInetAddress().equals(s.getLocalAddress());
byte op = in.readByte();
// Make sure the xciver count is not exceeded
int curXceiverCount = datanode.getXceiverCount();
long startTime = DataNode.now();
switch ( op ) {
//读取
case DataTransferProtocol.OP_READ_BLOCK:
//真正的读取数据
readBlock( in );
datanode.myMetrics.readBlockOp.inc(DataNode.now() - startTime);
if (local)
datanode.myMetrics.readsFromLocalClient.inc();
else
datanode.myMetrics.readsFromRemoteClient.inc();
break;
//写入
case DataTransferProtocol.OP_WRITE_BLOCK:
//真正的写入数据
writeBlock( in );
datanode.myMetrics.writeBlockOp.inc(DataNode.now() - startTime);
if (local)
datanode.myMetrics.writesFromLocalClient.inc();
else
datanode.myMetrics.writesFromRemoteClient.inc();
break;
//其他的指令
……
}
} catch (Throwable t) {
LOG.error(datanode.dnRegistration + ":DataXceiver",t);
} finally {
IOUtils.closeStream(in);
IOUtils.closeSocket(s);
dataXceiverServer.childSockets.remove(s);
}
}
private void readBlock(DataInputStream in) throws IOException {
//读取指令
long blockId = in.readLong();
Block block = new Block( blockId, 0 , in.readLong());
long startOffset = in.readLong();
long length = in.readLong();
String clientName = Text.readString(in);
//创建一个写入流，用于向客户端写数据
OutputStream baseStream = NetUtils.getOutputStream(s,
datanode.socketWriteTimeout);
DataOutputStream out = new DataOutputStream(
new BufferedOutputStream(baseStream, SMALL_BUFFER_SIZE));
//生成BlockSender用于读取本地的block的数据，并发送给客户端
//BlockSender有一个成员变量InputStream blockIn用于读取本地block的数据
BlockSender blockSender = new BlockSender(block, startOffset, length,
true, true, false, datanode, clientTraceFmt);
out.writeShort(DataTransferProtocol.OP_STATUS_SUCCESS); // send op status
//向客户端写入数据
long read = blockSender.sendBlock(out, baseStream, null);
……
} finally {
IOUtils.closeStream(out);
IOUtils.closeStream(blockSender);
}
}
三、文件的写入
下面解析向hdfs上传一个文件的过程。
3.1、客户端
上传一个文件到hdfs，一般会调用DistributedFileSystem.create，其实现如下：
public FSDataOutputStream create(Path f, FsPermission permission,
boolean overwrite,
int bufferSize, short replication, long blockSize,
Progressable progress) throws IOException {
return new FSDataOutputStream
(dfs.create(getPathName(f), permission,
overwrite, replication, blockSize, progress, bufferSize),
statistics);
}
其最终生成一个FSDataOutputStream用于向新生成的文件中写入数据。其成员变量dfs的类型为DFSClient，DFSClient的create函数如下：
public OutputStream create(String src,
FsPermission permission,
boolean overwrite,
short replication,
long blockSize,
Progressable progress,
int buffersize
) throws IOException {
checkOpen();
if (permission == null) {
permission = FsPermission.getDefault();
}
FsPermission masked = permission.applyUMask(FsPermission.getUMask(conf));
OutputStream result = new DFSOutputStream(src, masked,
overwrite, replication, blockSize, progress, buffersize,
conf.getInt("io.bytes.per.checksum", 512));
leasechecker.put(src, result);
return result;
}
其中构造了一个DFSOutputStream，在其构造函数中，同过RPC调用NameNode的create来创建一个文件。
当然，构造函数中还做了一件重要的事情，就是streamer.start()，也即启动了一个pipeline，用于写数据，在写入数据的过程中，我们会仔细分析。
DFSOutputStream(String src, FsPermission masked, boolean overwrite,
short replication, long blockSize, Progressable progress,
int buffersize, int bytesPerChecksum) throws IOException {
this(src, blockSize, progress, bytesPerChecksum);
computePacketChunkSize(writePacketSize, bytesPerChecksum);
try {
namenode.create(
src, masked, clientName, overwrite, replication, blockSize);
} catch(RemoteException re) {
throw re.unwrapRemoteException(AccessControlException.class,
QuotaExceededException.class);
}
streamer.start();
}
3.2、NameNode
NameNode的create函数调用namesystem.startFile函数，其又调用startFileInternal函数，实现如下：
private synchronized void startFileInternal(String src,
PermissionStatus permissions,
String holder,
String clientMachine,
boolean overwrite,
boolean append,
short replication,
long blockSize
) throws IOException {
......
//创建一个新的文件，状态为under construction，没有任何data block与之对应
long genstamp = nextGenerationStamp();
INodeFileUnderConstruction newNode = dir.addFile(src, permissions,
replication, blockSize, holder, clientMachine, clientNode, genstamp);
......
}
3.3、客户端
下面轮到客户端向新创建的文件中写入数据了，一般会使用FSDataOutputStream的write函数，最终会调用DFSOutputStream的writeChunk函数：
按照hdfs的设计，对block的数据写入使用的是pipeline的方式，也即将数据分成一个个的package，如果需要复制三分，分别写入DataNode 1, 2, 3，则会进行如下的过程：
首先将package 1写入DataNode 1
然后由DataNode 1负责将package 1写入DataNode 2，同时客户端可以将pacage 2写入DataNode 1
然后DataNode 2负责将package 1写入DataNode 3, 同时客户端可以讲package 3写入DataNode 1，DataNode 1将package 2写入DataNode 2
就这样将一个个package排着队的传递下去，直到所有的数据全部写入并复制完毕
protected synchronized void writeChunk(byte[] b, int offset, int len, byte[] checksum)
throws IOException {
//创建一个package，并写入数据
currentPacket = new Packet(packetSize, chunksPerPacket,
bytesCurBlock);
currentPacket.writeChecksum(checksum, 0, cklen);
currentPacket.writeData(b, offset, len);
currentPacket.numChunks++;
bytesCurBlock += len;
//如果此package已满，则放入队列中准备发送
if (currentPacket.numChunks == currentPacket.maxChunks ||
bytesCurBlock == blockSize) {
......
dataQueue.addLast(currentPacket);
//唤醒等待dataqueue的传输线程，也即DataStreamer
dataQueue.notifyAll();
currentPacket = null;
......
}
}
DataStreamer的run函数如下：
public void run() {
while (!closed && clientRunning) {
Packet one = null;
synchronized (dataQueue) {
//如果队列中没有package，则等待
while ((!closed && !hasError && clientRunning
&& dataQueue.size() == 0) || doSleep) {
try {
dataQueue.wait(1000);
} catch (InterruptedException e) {
}
doSleep = false;
}
try {
//得到队列中的第一个package
one = dataQueue.getFirst();
long offsetInBlock = one.offsetInBlock;
//由NameNode分配block，并生成一个写入流指向此block
if (blockStream == null) {
nodes = nextBlockOutputStream(src);
response = new ResponseProcessor(nodes);
response.start();
}
ByteBuffer buf = one.getBuffer();
//将package从dataQueue移至ackQueue,等待确认
dataQueue.removeFirst();
dataQueue.notifyAll();
synchronized (ackQueue) {
ackQueue.addLast(one);
ackQueue.notifyAll();
}
//利用生成的写入流将数据写入DataNode中的block
blockStream.write(buf.array(), buf.position(), buf.remaining());
if (one.lastPacketInBlock) {
blockStream.writeInt(0); //表示此block写入完毕
}
blockStream.flush();
} catch (Throwable e) {
}
}
......
}
其中重要的一个函数是nextBlockOutputStream，实现如下：
private DatanodeInfo[] nextBlockOutputStream(String client) throws IOException {
LocatedBlock lb = null;
boolean retry = false;
DatanodeInfo[] nodes;
int count = conf.getInt("dfs.client.block.write.retries", 3);
boolean success;
do {
......
//由NameNode为文件分配DataNode和block
lb = locateFollowingBlock(startTime);
block = lb.getBlock();
nodes = lb.getLocations();
//创建向DataNode的写入流
success = createBlockOutputStream(nodes, clientName, false);
......
} while (retry && --count >= 0);
return nodes;
}
locateFollowingBlock中通过RPC调用namenode.addBlock(src, clientName)函数
3.4、NameNode
NameNode的addBlock函数实现如下：
public LocatedBlock addBlock(String src,
String clientName) throws IOException {
LocatedBlock locatedBlock = namesystem.getAdditionalBlock(src, clientName);
return locatedBlock;
}
FSNamesystem的getAdditionalBlock实现如下：
public LocatedBlock getAdditionalBlock(String src,
String clientName
) throws IOException {
long fileLength, blockSize;
int replication;
DatanodeDescriptor clientNode = null;
Block newBlock = null;
......
//为新的block选择DataNode
DatanodeDescriptor targets[] = replicator.chooseTarget(replication,
clientNode,
null,
blockSize);
......
//得到文件路径中所有path的INode，其中最后一个是新添加的文件对的INode，状态为under construction
INode[] pathINodes = dir.getExistingPathINodes(src);
int inodesLen = pathINodes.length;
INodeFileUnderConstruction pendingFile = (INodeFileUnderConstruction)
pathINodes[inodesLen - 1];
//为文件分配block, 并设置在那写DataNode上
newBlock = allocateBlock(src, pathINodes);
pendingFile.setTargets(targets);
......
return new LocatedBlock(newBlock, targets, fileLength);
}
3.5、客户端
在分配了DataNode和block以后，createBlockOutputStream开始写入数据。
private boolean createBlockOutputStream(DatanodeInfo[] nodes, String client,
boolean recoveryFlag) {
//创建一个socket，链接DataNode
InetSocketAddress target = NetUtils.createSocketAddr(nodes[0].getName());
s = socketFactory.createSocket();
int timeoutValue = 3000 * nodes.length + socketTimeout;
s.connect(target, timeoutValue);
s.setSoTimeout(timeoutValue);
s.setSendBufferSize(DEFAULT_DATA_SOCKET_SIZE);
long writeTimeout = HdfsConstants.WRITE_TIMEOUT_EXTENSION * nodes.length +
datanodeWriteTimeout;
DataOutputStream out = new DataOutputStream(
new BufferedOutputStream(NetUtils.getOutputStream(s, writeTimeout),
DataNode.SMALL_BUFFER_SIZE));
blockReplyStream = new DataInputStream(NetUtils.getInputStream(s));
//写入指令
out.writeShort( DataTransferProtocol.DATA_TRANSFER_VERSION );
out.write( DataTransferProtocol.OP_WRITE_BLOCK );
out.writeLong( block.getBlockId() );
out.writeLong( block.getGenerationStamp() );
out.writeInt( nodes.length );
out.writeBoolean( recoveryFlag );
Text.writeString( out, client );
out.writeBoolean(false);
out.writeInt( nodes.length - 1 );
//注意，次循环从1开始，而非从0开始。将除了第一个DataNode以外的另外两个DataNode的信息发送给第一个DataNode, 第一个DataNode可以根据此信息将数据写给另两个DataNode
for (int i = 1; i < nodes.length; i++) {
nodes.write(out);
}
checksum.writeHeader( out );
out.flush();
firstBadLink = Text.readString(blockReplyStream);
if (firstBadLink.length() != 0) {
throw new IOException("Bad connect ack with firstBadLink " + firstBadLink);
}
blockStream = out;
}
客户端在DataStreamer的run函数中创建了写入流后，调用blockStream.write将数据写入DataNode
3.6、DataNode
DataNode的DataXceiver中，收到指令DataTransferProtocol.OP_WRITE_BLOCK则调用writeBlock函数：
private void writeBlock(DataInputStream in) throws IOException {
DatanodeInfo srcDataNode = null;
//读入头信息
Block block = new Block(in.readLong(),
dataXceiverServer.estimateBlockSize, in.readLong());
int pipelineSize = in.readInt(); // num of datanodes in entire pipeline
boolean isRecovery = in.readBoolean(); // is this part of recovery?
String client = Text.readString(in); // working on behalf of this client
boolean hasSrcDataNode = in.readBoolean(); // is src node info present
if (hasSrcDataNode) {
srcDataNode = new DatanodeInfo();
srcDataNode.readFields(in);
}
int numTargets = in.readInt();
if (numTargets < 0) {
throw new IOException("Mislabelled incoming datastream.");
}
//读入剩下的DataNode列表，如果当前是第一个DataNode，则此列表中收到的是第二个，第三个DataNode的信息，如果当前是第二个DataNode，则受到的是第三个DataNode的信息
DatanodeInfo targets[] = new DatanodeInfo[numTargets];
for (int i = 0; i < targets.length; i++) {
DatanodeInfo tmp = new DatanodeInfo();
tmp.readFields(in);
targets = tmp;
}
DataOutputStream mirrorOut = null; // stream to next target
DataInputStream mirrorIn = null; // reply from next target
DataOutputStream replyOut = null; // stream to prev target
Socket mirrorSock = null; // socket to next target
BlockReceiver blockReceiver = null; // responsible for data handling
String mirrorNode = null; // the name:port of next target
String firstBadLink = ""; // first datanode that failed in connection setup
try {
//生成一个BlockReceiver, 其有成员变量DataInputStream in为从客户端或者上一个DataNode读取数据，还有成员变量DataOutputStream mirrorOut，用于向下一个DataNode写入数据，还有成员变量OutputStream out用于将数据写入本地。
blockReceiver = new BlockReceiver(block, in,
s.getRemoteSocketAddress().toString(),
s.getLocalSocketAddress().toString(),
isRecovery, client, srcDataNode, datanode);
// get a connection back to the previous target
replyOut = new DataOutputStream(
NetUtils.getOutputStream(s, datanode.socketWriteTimeout));
//如果当前不是最后一个DataNode，则同下一个DataNode建立socket连接
if (targets.length > 0) {
InetSocketAddress mirrorTarget = null;
// Connect to backup machine
mirrorNode = targets[0].getName();
mirrorTarget = NetUtils.createSocketAddr(mirrorNode);
mirrorSock = datanode.newSocket();
int timeoutValue = numTargets * datanode.socketTimeout;
int writeTimeout = datanode.socketWriteTimeout +
(HdfsConstants.WRITE_TIMEOUT_EXTENSION * numTargets);
mirrorSock.connect(mirrorTarget, timeoutValue);
mirrorSock.setSoTimeout(timeoutValue);
mirrorSock.setSendBufferSize(DEFAULT_DATA_SOCKET_SIZE);
//创建向下一个DataNode写入数据的流
mirrorOut = new DataOutputStream(
new BufferedOutputStream(
NetUtils.getOutputStream(mirrorSock, writeTimeout),
SMALL_BUFFER_SIZE));
mirrorIn = new DataInputStream(NetUtils.getInputStream(mirrorSock));
mirrorOut.writeShort( DataTransferProtocol.DATA_TRANSFER_VERSION );
mirrorOut.write( DataTransferProtocol.OP_WRITE_BLOCK );
mirrorOut.writeLong( block.getBlockId() );
mirrorOut.writeLong( block.getGenerationStamp() );
mirrorOut.writeInt( pipelineSize );
mirrorOut.writeBoolean( isRecovery );
Text.writeString( mirrorOut, client );
mirrorOut.writeBoolean(hasSrcDataNode);
if (hasSrcDataNode) { // pass src node information
srcDataNode.write(mirrorOut);
}
mirrorOut.writeInt( targets.length - 1 );
//此出也是从1开始，将除了下一个DataNode的其他DataNode信息发送给下一个DataNode
for ( int i = 1; i < targets.length; i++ ) {
targets.write( mirrorOut );
}
blockReceiver.writeChecksumHeader(mirrorOut);
mirrorOut.flush();
}
//使用BlockReceiver接受block
String mirrorAddr = (mirrorSock == null) ? null : mirrorNode;
blockReceiver.receiveBlock(mirrorOut, mirrorIn, replyOut,
mirrorAddr, null, targets.length);
......
} finally {
// close all opened streams
IOUtils.closeStream(mirrorOut);
IOUtils.closeStream(mirrorIn);
IOUtils.closeStream(replyOut);
IOUtils.closeSocket(mirrorSock);
IOUtils.closeStream(blockReceiver);
}
}
BlockReceiver的receiveBlock函数中，一段重要的逻辑如下：
void receiveBlock(
DataOutputStream mirrOut, // output to next datanode
DataInputStream mirrIn, // input from next datanode
DataOutputStream replyOut, // output to previous datanode
String mirrAddr, BlockTransferThrottler throttlerArg,
int numTargets) throws IOException {
......
//不断的接受package，直到结束
while (receivePacket() > 0) {}
if (mirrorOut != null) {
try {
mirrorOut.writeInt(0); // mark the end of the block
mirrorOut.flush();
} catch (IOException e) {
handleMirrorOutError(e);
}
}
......
}
BlockReceiver的receivePacket函数如下：
private int receivePacket() throws IOException {
//从客户端或者上一个节点接收一个package
int payloadLen = readNextPacket();
buf.mark();
//read the header
buf.getInt(); // packet length
offsetInBlock = buf.getLong(); // get offset of packet in block
long seqno = buf.getLong(); // get seqno
boolean lastPacketInBlock = (buf.get() != 0);
int endOfHeader = buf.position();
buf.reset();
setBlockPosition(offsetInBlock);
//将package写入下一个DataNode
if (mirrorOut != null) {
try {
mirrorOut.write(buf.array(), buf.position(), buf.remaining());
mirrorOut.flush();
} catch (IOException e) {
handleMirrorOutError(e);
}
}
buf.position(endOfHeader);
int len = buf.getInt();
offsetInBlock += len;
int checksumLen = ((len + bytesPerChecksum - 1)/bytesPerChecksum)*
checksumSize;
int checksumOff = buf.position();
int dataOff = checksumOff + checksumLen;
byte pktBuf[] = buf.array();
buf.position(buf.limit()); // move to the end of the data.
......
//将数据写入本地的block
out.write(pktBuf, dataOff, len);
/// flush entire packet before sending ack
flush();
// put in queue for pending acks
if (responder != null) {
((PacketResponder)responder.getRunnable()).enqueue(seqno,
lastPacketInBlock);
}
return payloadLen;
}
---------------------
【转载】
作者：popsuper1982
原文：https://blog.csdn.net/popsuper1982/article/details/51615285
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