HDFS租约实践

一、租约详解
Why租约
HDFS的读写模式为 “write-once-read-many”，为了实现write-once，需要设计一种互斥机制，租约应运而生
租约本质上是一个有时间约束的锁，即：在一定时间内对租约持有者（也就是客户端）赋予一定的权限

HDFS租约模型

Lease和DFSClient的对应关系为一对一（即：在Hdfs-Server端，为每个DFSClient建立一个Lease），Lease包含的主要信息有：
　　* holder：租约持有者（即：DFSClient）
　　* lastUpdate：上次续约时间
　　* paths：该租约持有的文件路径
　　* softLimit和hardLimit
　　　　> 当前时间减去Lease的lastUpdate超过softLimit，允许其它DFSClient抢占该Client持有的filepath（softLimit的默认值为1min）
　　　　> 当前时间减去Lease的astUpdate超过hardLimit，允许LeaseManager强制将该租约回收销毁（hardLimit的默认值为1hour）

顾名思义，LeaseManager是租约的管理者，运行于HDFS-Server端，其主要功能特性有：
　　* 维护了DFSClient和Lease的映射关系（参见leases属性）
　　* 维护了filePath和Lease的映射关系（参见sortedLeasesByPath属性）
　　* 对租约进行生命周期和状态的管理：
　　　　> 创建租约或正常情况下的销毁租约
　　　　> 赋予（或撤销）FilePath权限给租约（撤销FilePath，如：执行文件流的close方法）
　　　　> 接受续约请求，对租约进行续约处理（即：更新Lease的lastUpdate字段）
　　　　> 对超过hardLimit的租约进行销毁处理（参见：LeaseManager.Monitor类）

顾名思义，LeaseRenewer是租约的续约者，运行于HDFS-Client端，其主要功能特性有：
　　* LeaseRenewer维护了一个DFSClient列表和一个定时线程，循环不断的为各个DFSClient进行续约操作
　　* LeaseRenew本质上是一个heartbeat，方便对超时的DFSClient进行容错处理
　　* 从client到server端续约的主流程如下：LeaseRenewer -> DFSClient -> NameNodeRpcServer -> FSNamesystem -> LeaseManager
　　* 其它细节此处不再阐述，直接看源码即可

LeaseManager用来管理租约，那么，FSNamesystem用来协调租约，其主要功能特性有：
　　* FSNamesystem中和租约相关最核心的一个方法是recoverLeaseInternal，startFile方法、appendFile方法和recoverLease方法都会调用该方法，该方法主要功能有：
　　　　> 验证ReCreate
　　　　　如果待操作的文件path已经存在于该DFSClient的Lease的paths列表中，则抛AlreadyBeingCreatedException，
　　　　　提示 “current leaseholder is trying to recreate file”
　　　　> 验证OtherCreate
　　　　　如果待操作的文件path已经存在于其它DFSClient的Lease的paths列表中，此时有两种策略：如果那个DFSClient的Lease的softLimit已经过期，
　　　　　系统会尝试进行Lease-Recovery，然后把path从那个DFSClient的Lease的paths中remove掉，这样新的Client便获取了该path的占有权限；
　　　　　如果那个DFSClient的Lease的softLimit还未过期，则抛AlreadyBeingCreatedException，提示 “because this file is already being created by … on …”　　　　
　　　　> 验证Recovery
　　　　　这个比较简单，如果待操作的文件还处于租约的Recovery状态，则抛异常RecoveryInProgressException，提示稍后重试
　　　　> ForceRecovery
　　　　　recoverLeaseInternal方法提供了force参数，如果force为true，系统会强制进行Lease-Recovery，具体功能见recoverLease方法的注释即可，如下：
　　　　　 　* Immediately revoke the lease of the current lease holder and start lease
　　　　　 　* recovery so that the file can be forced to be closed.
　　　　　　force recovery的使用场景下文会有介绍

Recovery机制
recovery是一种容错机制，主要分为block的recovery和lease的recovery，此处不详述，具体可参考下面的链接：
http://blog.cloudera.com/blog/2015/02/understanding-hdfs-recovery-processes-part-1/

二、场景介绍

如上图所示，我们的应用场景介绍如下：
* Worker：一个Worker是一个进程，每个Worker负责运行一批Task
* Task：Task负责把抓取到的数据源源不断的实时同步到Hdfs，每个Task负责管理Hdfs中的N个文件（如上图，Task-1在hdfs中对应了Task-1-file-1和Task-1-file-2）
* (Re-)balance：Task和Worker之间的关系是动态的（即：Task在Worker上是平均分配的），当新Worker加入，现有Worker退出、新增Task和删除Task的时候，会触发Rebalance，Task重新分配。比如上图中，增加一个Worker-3，Reblance完成之后的结果为：worker-1运行Task-1和Task-2，worker-2运行Task-3和Task-4，worker-3运行Task-5

三、设计方案
结合租约的特点和我们的场景需求，需要进行针对性的设计，才能避免触发【租约异常】，下面以问答的形式阐述核心设计方案

一个进程内如何同时访问多个hadoop集群？

若要一个进程内同时访问多个hadoop集群，那就需要针对每个集群分别创建各自的FileSystem实例，需要做的有两点：
* 其一：保证针对这多个集群的Configuration实例中的 “fs.defaultFS” 的配置是不同的
* 其二：HADOOP_USER_NAME属性不能通过System.setProperty方法设置，应该调用FileSystem的get方法时动态传入

对文件流应该怎样管理？

Ps：DFSClient是一个进程级实例，即对应同一个hadoop集群，在一个worker进程中只有一个DFSClient
一个文件对应了一个文件流，创建文件流时FSNamesystem会把流对应的path放到Lease中，关闭文件流时FSNamesystem会把流对应的path从Lease中移除，对文件流的管理需要保证以下几个原则
* 其一：流的生命周期应该和Task保持一致，Task运行过程中流随用随创建，Task关闭时把其占有的所有流也关闭，这样才能保证在发生Reblance后，不会出现租约被其它DFSClient占用的问题
* 其二：超时不用的流要及时清理，保证其它使用者有机会获取权限，比如发生日切之后，所有的数据都写到新文件中了，前一天的文件不会再有写入操作，那么应该及时关闭前一天的文件流

如何解决Other-Create问题？

何时会触发Other-Create问题?
其一：Worker宕机，其负责的Task漂移到其它Worker，漂移后的Task便会收到Other-Create异常，只有当超过softLimit之后，异常才会解除，即恢复时间需要1分钟
其二：其它程序原因，如：在发生Reblance时，Task会先被关闭再漂移，如果Task在关闭的过程中流关闭的有问题（比如触发了超时），也可能会触发Other-Create异常
如何应对？
Other-Create异常中包含了other-Dfsclient的IP信息，我们可以调用other-worker提供的接口，远程关闭出问题的流，如果关闭失败或者访问出现超时（宕机的时候会超时），再进行force recovery操作

如何解决Re-Create问题？

流关闭时可能会出现异常，如果出现异常，需要进行force recovery操作，否则的话租约将一直不可释放，一直报Re-Create异常

四、源码解析
【设计方案】部分的描述比较抽象，下面我们结合源码进行更详细的介绍，所有的关键描述都放到了源码注释里，直接看注释即可

package com.ucar.hdfs.lease.demo;

import com.ucar.hdfs.lease.demo.stream.FileStreamHolder;
import com.ucar.hdfs.lease.demo.stream.FileStreamToken;
import com.ucar.hdfs.lease.demo.util.FileLockUtils;
import com.ucar.hdfs.lease.demo.util.HdfsConfig;
import com.ucar.hdfs.lease.demo.util.RemoteUtil;
import org.apache.hadoop.fs.FSDataOutputStream;

import java.text.MessageFormat;
import java.util.List;
import java.util.concurrent.locks.ReentrantLock;

/**
* 一个Demo类，可以自己写一些单元测试类，验证租约相关的原理
*/
public class TaskDemo {

private final FileStreamHolder fileStreamHolder;
private final HdfsConfig hdfsConfig;
private final String filePathPrefix;

public TaskDemo(FileStreamHolder fileStreamHolder, HdfsConfig hdfsConfig, String filePathPrefix) {
    this.fileStreamHolder = fileStreamHolder;
    this.hdfsConfig = hdfsConfig;
    this.filePathPrefix = filePathPrefix;
}

public void start() {
    fileStreamHolder.start();
}

public void stop() {
    fileStreamHolder.close();
}

public void writeData(String fileName, List content) throws Exception {
    String hdfsFilePath = this.hdfsConfig.getHdfsAddress() + filePathPrefix + fileName + ".txt";

    ReentrantLock lock = FileLockUtils.getLock(hdfsFilePath);
    FileStreamToken fileStreamToken = null;
    try {
        lock.lock();
        fileStreamToken = fileStreamHolder.getStreamToken(hdfsFilePath, this.hdfsConfig);
        writeDataInternal(fileStreamToken.getFileStream(), content);
    } catch (Exception e) {
        if (fileStreamToken != null) {
            // 出现异常的时候，必须把流回收一下，否则的话异常会持续不断的报，并且无法自动恢复
            // 我们曾经遇到过的根本无法自动恢复的异常有：

            /*
            Caused by: java.net.ConnectException: Connection timed out
            at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method) ~[na:1.8.0_121]
            at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:717) ~[na:1.8.0_121]
            at org.apache.hadoop.net.SocketIOWithTimeout.connect(SocketIOWithTimeout.java:206) ~[na:na]
            at org.apache.hadoop.net.NetUtils.connect(NetUtils.java:530) ~[na:na]
            at org.apache.hadoop.hdfs.DFSOutputStream.createSocketForPipeline(DFSOutputStream.java:1610) ~[na:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.transfer(DFSOutputStream.java:1123) ~[na:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.addDatanode2ExistingPipeline(DFSOutputStream.java:1112) ~[na:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.setupPipelineForAppendOrRecovery(DFSOutputStream.java:1253) ~[na:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:594) ~[na:na]
            */

            /*
            Caused by: java.io.IOException: Failed to replace a bad datanode on the existing pipeline due to no more good datanodes being available to try. (Nodes: current=[xxx:50010, xxx:50010], original=[xxx:50010, xxx:50010]). The current failed datanode replacement policy is DEFAULT, and a client may configure this via 'dfs.client.block.write.replace-datanode-on-failure.policy' in its configuration.
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.findNewDatanode(DFSOutputStream.java:1040) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.addDatanode2ExistingPipeline(DFSOutputStream.java:1106) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.setupPipelineForAppendOrRecovery(DFSOutputStream.java:1253) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.processDatanodeError(DFSOutputStream.java:1004) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:548) ~[hadoop-hdfs-2.6.3.jar:na]
            */

            /*
            Caused by: java.net.SocketTimeoutException: 70000 millis timeout while waiting for channel to be ready for read. ch : java.nio.channels.SocketChannel[connected local=/xxx:36061 remote=/xxx:50010]
            at org.apache.hadoop.net.SocketIOWithTimeout.doIO(SocketIOWithTimeout.java:164) ~[hadoop-common-2.6.3.jar:na]
            at org.apache.hadoop.net.SocketInputStream.read(SocketInputStream.java:161) ~[hadoop-common-2.6.3.jar:na]
            at org.apache.hadoop.net.SocketInputStream.read(SocketInputStream.java:131) ~[hadoop-common-2.6.3.jar:na]
            at org.apache.hadoop.net.SocketInputStream.read(SocketInputStream.java:118) ~[hadoop-common-2.6.3.jar:na]
            at java.io.FilterInputStream.read(FilterInputStream.java:83) ~[na:1.8.0_121]
            at java.io.FilterInputStream.read(FilterInputStream.java:83) ~[na:1.8.0_121]
            at org.apache.hadoop.hdfs.protocolPB.PBHelper.vintPrefixed(PBHelper.java:2205) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.transfer(DFSOutputStream.java:1142) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.addDatanode2ExistingPipeline(DFSOutputStream.java:1112) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.setupPipelineForAppendOrRecovery(DFSOutputStream.java:1253) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.processDatanodeError(DFSOutputStream.java:1004) ~[hadoop-hdfs-2.6.3.jar:na]
            at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:548) ~[hadoop-hdfs-2.6.3.jar:na]
            */
            fileStreamHolder.close(hdfsFilePath);
        } else {
            //可能是Other-Create异常，尝试远程关闭
            RemoteUtil.tryRemoteClose(hdfsFilePath, e);
        }
        throw new RuntimeException(MessageFormat.format("Data Append failed for file - {0}.", hdfsFilePath), e);
    } finally {
        if (fileStreamToken != null) {
            fileStreamToken.setLastUpdateTime(System.currentTimeMillis());
        }
        if (lock != null) {
            lock.unlock();
        }
    }
}

private void writeDataInternal(FSDataOutputStream fsOut, List transferData) throws Exception {
    StringBuffer sb = new StringBuffer();
    for (String row : transferData) {
        sb.append(row);
        sb.append("\n");
    }

    byte[] bytes = sb.toString().getBytes("UTF-8");
    fsOut.write(bytes);
    fsOut.hsync();
}

}

package com.ucar.hdfs.lease.demo.util;

import org.apache.hadoop.hdfs.protocol.AlreadyBeingCreatedException;
import org.apache.hadoop.ipc.RemoteException;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RemoteUtil {
private static final Logger logger = LoggerFactory.getLogger(RemoteUtil.class);
private static final String RECREATE_IDENTIFIER = “because this file is already being created by”;

public static void tryRemoteClose(String hdfsFilePath, Exception e) {
    try {
        if (e instanceof RemoteException) {
            RemoteException re = (RemoteException) e;
            String className = re.getClassName();
            if (className.equals(AlreadyBeingCreatedException.class.getName()) && e.getMessage().contains(RECREATE_IDENTIFIER)) {
                logger.info("stream remote close begin for file : " + hdfsFilePath);
                colseInternal(hdfsFilePath, parseIp(e.getMessage()));
                logger.info("stream remote close end for file : " + hdfsFilePath);
            }
        }
    } catch (Exception ex) {
        logger.error("stream remote close failed for file : " + hdfsFilePath, ex);
    }
}

static void colseInternal(String hdfsFilePath, String address) {
    //TODO
    // 远程调用其它进程，进行流关闭操作
    // 此处应该进行更细致的处理
    // 1. 如果流关闭失败了，我们也应该执行一下FileSystem的recoverLease方法
    // 2. 如果访问超时了，那么应该出现了宕机的情况，虽然通过softLimit会自动恢复，但是如果实时性要求高，应该也执行一下FileSystem的recoverLease方法
    // 3. 或者根据自己的场景，压根儿就不远程关闭，强制执行recoverLease方法也行
}

private static String parseIp(String message) {
    Pattern p = Pattern.compile("\\[.*?\\]");
    Matcher m = p.matcher(message);
    String ip = null;
    while (m.find()) {
        ip = m.group().replace("[", "").replace("]", "");
    }
    return ip;
}

}

package com.ucar.hdfs.lease.demo.util;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.HBaseConfiguration;

import java.net.URI;

/**
* e.g.
*
* hdfsAddress -> hdfs://hadoop2cluster
* zkUrl -> 192.168.0.1,192.168.0.2,192.168.0.3
* zkPort -> 2181
* hadoopUser -> hadoop
* haNameNode1 -> namenode01.10101111.com:8020
* haNameNode2 -> namenode02.10101111.com:8020
* hdfsPacketSize -> 20971520
*
*/
public class HdfsConfig {
private volatile String hdfsAddress;
private volatile URI hdfsUri;
private volatile String zkUrl;
private volatile String zkPort;
private volatile String hadoopUser;
private volatile String haNameNode1;
private volatile String haNameNode2;
private volatile Long hdfsPacketSize;
private volatile Configuration configuration;

public HdfsConfig(String hdfsAddress, String zkUrl, String zkPort, String hadoopUser, String haNameNode1, String haNameNode2, long hdfsPacketSize) {
    this.hdfsAddress = hdfsAddress;
    this.zkUrl = zkUrl;
    this.zkPort = zkPort;
    this.hadoopUser = hadoopUser;
    this.haNameNode1 = haNameNode1;
    this.haNameNode2 = haNameNode2;
    this.hdfsPacketSize = hdfsPacketSize;
    this.hdfsUri = URI.create(this.hdfsAddress);
    this.buildConfiguration();
}

private void buildConfiguration() {
    this.configuration = HBaseConfiguration.create();
    this.configuration.set("fs.defaultFS", this.hdfsAddress);
    this.configuration.set("dfs.support.append", "true");
    this.configuration.set("hbase.zookeeper.quorum", this.zkUrl);
    this.configuration.set("hbase.zookeeper.property.clientPort", this.zkPort);
    this.configuration.set("dfs.client-write-packet-size", String.valueOf(hdfsPacketSize));

    // 高可用设置
    String key = hdfsUri.getAuthority();
    this.configuration.set("dfs.nameservices", key);
    this.configuration.set(String.format("dfs.ha.namenodes.%s", key), "nn1,nn2");
    this.configuration.set(String.format("dfs.namenode.rpc-address.%s.nn1", key), this.haNameNode1);
    this.configuration.set(String.format("dfs.namenode.rpc-address.%s.nn2", key), this.haNameNode2);
    this.configuration.set(String.format("dfs.client.failover.proxy.provider.%s", key), "org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider");
}

public String getHdfsAddress() {
    return hdfsAddress;
}

public URI getHdfsUri() {
    return hdfsUri;
}

public String getZkUrl() {
    return zkUrl;
}

public String getZkPort() {
    return zkPort;
}

public String getHadoopUser() {
    return hadoopUser;
}

public String getHaNameNode1() {
    return haNameNode1;
}

public String getHaNameNode2() {
    return haNameNode2;
}

public Configuration getConfiguration() {
    return configuration;
}

public long getHdfsPacketSize() {
    return hdfsPacketSize;
}

}

package com.ucar.hdfs.lease.demo.util;

import com.google.common.cache.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

public class FileLockUtils {

private static final Logger logger = LoggerFactory.getLogger(FileLockUtils.class);

private static final LoadingCache lockCache = CacheBuilder
        .newBuilder()
        .expireAfterAccess(24, TimeUnit.HOURS)
        .removalListener(new RemovalListener() {
            @Override
            public void onRemoval(RemovalNotification notification) {
                logger.info(String.format("Lock for [%s] was removed , cause is [%s]",
                                notification.getKey(),
                                notification.getCause())
                );
            }
        })
        .build(new CacheLoader() {
            @Override
            public ReentrantLock load(String key) throws Exception {
                return new ReentrantLock();
            }
        });

public static ReentrantLock getLock(String hdfsFilePath) {
    return lockCache.getUnchecked(hdfsFilePath);
}

}

package com.ucar.hdfs.lease.demo.stream;

import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.ucar.hdfs.lease.demo.util.HdfsConfig;
import org.apache.hadoop.fs.FileSystem;

import java.util.concurrent.TimeUnit;

public class FileSystemManager {

private static final LoadingCache fileSystemCache = CacheBuilder
        .newBuilder()
        .expireAfterAccess(24, TimeUnit.HOURS)
        .build(new CacheLoader() {
            @Override
            public FileSystem load(HdfsConfig hdfsConfig) throws Exception {
                // FileSystem一共有两个get方法，需要调用这个get方法才能支持"一个进程内同时访问多个hadoop集群"
                return FileSystem.get(
                        hdfsConfig.getHdfsUri(),
                        hdfsConfig.getConfiguration(),
                        hdfsConfig.getHadoopUser());
            }
        });

public static FileSystem getFileSystem(HdfsConfig hdfsConfig) {
    return fileSystemCache.getUnchecked(hdfsConfig);
}

}

package com.ucar.hdfs.lease.demo.stream;

import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hdfs.DistributedFileSystem;

public class FileStreamToken {

private volatile String pathString;
private volatile Path path;
private volatile DistributedFileSystem fileSystem;
private volatile FSDataOutputStream fileStream;
private volatile long lastUpdateTime;

public FileStreamToken(String pathString, Path path, DistributedFileSystem fileSystem, FSDataOutputStream fileStream) {
    this.pathString = pathString;
    this.path = path;
    this.fileSystem = fileSystem;
    this.fileStream = fileStream;
    this.lastUpdateTime = System.currentTimeMillis();
}

public String getPathString() {
    return pathString;
}

public void setPathString(String pathString) {
    this.pathString = pathString;
}

public Path getPath() {
    return path;
}

public void setPath(Path path) {
    this.path = path;
}

public DistributedFileSystem getFileSystem() {
    return fileSystem;
}

public void setFileSystem(DistributedFileSystem fileSystem) {
    this.fileSystem = fileSystem;
}

public FSDataOutputStream getFileStream() {
    return fileStream;
}

public void setFileStream(FSDataOutputStream fileStream) {
    this.fileStream = fileStream;
}

public long getLastUpdateTime() {
    return lastUpdateTime;
}

public void setLastUpdateTime(long lastUpdateTime) {
    this.lastUpdateTime = lastUpdateTime;
}

}

package com.ucar.hdfs.lease.demo.stream;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class FileStreamKeeper {
private static final Logger logger = LoggerFactory.getLogger(FileStreamKeeper.class);
private static final long CLOSE_CHECK_PERIOD = 60000;// 单位ms
private static final long STREAM_LEISURE_LIMIT = 60000;//单位ms

private static ScheduledExecutorService executorService;
private static List holders = new ArrayList<>();

//定时关闭不用的流
public static void start() {
    executorService = Executors.newScheduledThreadPool(1);
    executorService.scheduleAtFixedRate(
            FileStreamKeeper::leisureCheck,
            CLOSE_CHECK_PERIOD,
            CLOSE_CHECK_PERIOD,
            TimeUnit.MILLISECONDS
    );
    logger.info("File Stream Keeper is started.");
}

public static void closeStreamLocal(String hdfsFilePath) {
    if (holders != null && !holders.isEmpty()) {
        holders.stream().forEach(h -> {
            Set> set = h.getTokens().entrySet();
            for (Map.Entry entry : set) {
                try {
                    if (hdfsFilePath.equals(entry.getKey())) {
                        h.close(entry.getKey());
                        return;
                    }
                } catch (Throwable t) {
                    logger.error("stream close failed for file : " + entry.getKey());
                }
            }
        });
    }
}

static synchronized void register(FileStreamHolder fileStreamHolder) {
    holders.add(fileStreamHolder);
}

static synchronized void unRegister(FileStreamHolder fileStreamHolder) {
    holders.remove(fileStreamHolder);
}

private static void leisureCheck() {
    try {
        if (holders != null && !holders.isEmpty()) {
            holders.stream().forEach(h -> {

                logger.info("timer stream close begin.");
                Set> set = h.getTokens().entrySet();
                for (Map.Entry entry : set) {
                    try {
                        FileStreamToken vo = entry.getValue();
                        if (vo.getLastUpdateTime() + STREAM_LEISURE_LIMIT < System.currentTimeMillis()) {
                            h.close(entry.getKey());//超时关闭
                        }
                    } catch (Throwable t) {
                        logger.error("timer stream close failed for file : " + entry.getKey());
                    }
                }
                logger.info("timer stream close end.");
            });
        }
    } catch (Throwable t) {
        logger.error("something goes wrong when do leisure check.", t);
    }
}

}

package com.ucar.hdfs.lease.demo.stream;

import com.ucar.hdfs.lease.demo.util.FileLockUtils;
import com.ucar.hdfs.lease.demo.util.HdfsConfig;
import org.apache.hadoop.fs.CommonConfigurationKeysPublic;
import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hdfs.DistributedFileSystem;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class FileStreamHolder {

private static final Logger logger = LoggerFactory.getLogger(FileStreamHolder.class);

private final Map tokens;
private final ReentrantReadWriteLock readWriteLock;
private volatile boolean running;

public FileStreamHolder() {
    this.tokens = new ConcurrentHashMap<>();
    this.readWriteLock = new ReentrantReadWriteLock();
}

public void start() {
    this.running = true;
    FileStreamKeeper.register(this);
    logger.info("FileStreamHolder is started.");
}

public void close() {
    try {
        this.readWriteLock.writeLock().lock();
        this.running = false;
        if (this.tokens.size() > 0) {
            this.tokens.keySet().forEach(this::close);
        }
    } finally {
        FileStreamKeeper.unRegister(this);
        this.readWriteLock.writeLock().unlock();
    }
    logger.info("FileStreamHolder is closed.");
}

public FileStreamToken getStreamToken(String pathString, HdfsConfig hdfsConfig)
        throws Exception {
    try {
        this.readWriteLock.readLock().lock();
        if (!running) {
            throw new RuntimeException("FileStreamHolder has closed, StreamToken gotten failed.");
        }

        return getStreamTokenInternal(pathString, hdfsConfig);
    } finally {
        this.readWriteLock.readLock().unlock();
    }
}

private FileStreamToken getStreamTokenInternal(String pathString, HdfsConfig hdfsConfig)
        throws Exception {
    DistributedFileSystem hadoopFS = (DistributedFileSystem) FileSystemManager.getFileSystem(hdfsConfig);

    ReentrantLock lock = FileLockUtils.getLock(pathString);
    try {
        lock.lock();
        FileStreamToken token = tokens.get(pathString);
        if (token == null) {
            FSDataOutputStream fileStream;
            Path path = new Path(pathString);

            if (!hadoopFS.exists(path)) {
                //create方法最终会调用server端FSNamesystem的startFile方法
                fileStream = hadoopFS.create(path, false,
                        hdfsConfig.getConfiguration().getInt(CommonConfigurationKeysPublic.IO_FILE_BUFFER_SIZE_KEY,
                                CommonConfigurationKeysPublic.IO_FILE_BUFFER_SIZE_DEFAULT),
                        (short) 3, 64 * 1024 * 1024L);
                logger.info("stream create succeeded for file : " + pathString);
            } else {
                //append方法最终会调用server端FSNamesystem的appendFile方法
                fileStream = hadoopFS.append(path);
                logger.info("stream append succeeded for file : " + pathString);
            }

            token = new FileStreamToken(pathString, path, hadoopFS, fileStream);
            tokens.put(pathString, token);
        }

        return token;
    } finally {
        lock.unlock();
    }
}

public void close(String pathString) {
    ReentrantLock lock = FileLockUtils.getLock(pathString);
    try {
        lock.lock();
        FileStreamToken vo = tokens.get(pathString);
        if (vo != null) {
            try {
                vo.getFileStream().close();
                logger.info("stream close succeeded for file : " + pathString);
            } catch (Throwable e) {
                logger.error("stream close failed for file : " + pathString, e);
                try {
                    //流关闭失败的时候，必须执行一下recoverLease方法（即：force recovery）
                    //出现异常，流肯定不能接着用了，我们也不知道服务端究竟有没有感知到关闭操作
                    //如果没有感知到close动作，租约一直没有被release，将导致Re-Create问题
                    vo.getFileSystem().recoverLease(vo.getPath());
                    logger.info("lease recover succeeded for file : " + pathString);
                } catch (Exception ex) {
                    logger.error("lease recover failed for file : " + pathString, ex);
                }
            } finally {
                //不管有没有异常，我们都需要进行remove
                //没有异常，说明流关闭成功（严格意义上讲，没有异常也不代表流关闭成功了，假设第一次关的时候出异常了，没有进行处理，随后再次执行close，就不会报异常,具体可参考流的close方法），正常remove没有问题
                //有异常，说明流关闭失败，关了一半儿，流已经有问题了，不能再用了，必须remove掉，不remove的话，后续接着用会报ClosedChannelException异常

                /*Caused by: java.nio.channels.ClosedChannelException
                at org.apache.hadoop.hdfs.DFSOutputStream.checkClosed(DFSOutputStream.java:1622)
                at org.apache.hadoop.fs.FSOutputSummer.write(FSOutputSummer.java:104)
                at org.apache.hadoop.fs.FSDataOutputStream$PositionCache.write(FSDataOutputStream.java:58)
                at java.io.DataOutputStream.write(DataOutputStream.java:107)
                at java.io.FilterOutputStream.write(FilterOutputStream.java:97)
                at com.ucar.datalink.writer.hdfs.handle.RdbEventRecordHandler.writeData(RdbEventRecordHandler.java:246)
                at com.ucar.datalink.writer.hdfs.handle.RdbEventRecordHandler.doWriteData(RdbEventRecordHandler.java:221)
                at com.ucar.datalink.writer.hdfs.handle.RdbEventRecordHandler.toWriteData(RdbEventRecordHandler.java:187)*/
                tokens.remove(pathString);
            }
        }
    } finally {
        if (lock != null) {
            lock.unlock();
        }
    }
}

Map getTokens() {
    return tokens;
}

}

五、参考资料
http://blog.csdn.net/androidlushangderen/article/details/48012001

https://www.tuicool.com/articles/meuuaqU

https://www.tuicool.com/articles/IJjq6v

http://blog.cloudera.com/blog/2015/02/understanding-hdfs-recovery-processes-part-1/