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由多个客户端对同一个文件发起并发追加操作后，只有最后发起的那个客户端抢得追加的锁，其他客户端报错：
at org.apache.Hadoop.hdfs.server.namenode.FSNamesystem.appendFile(FSNamesystem.java:2347)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.append(NameNodeRpcServer.java:508)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.append(ClientNamenodeProtocolServerSideTranslatorPB.java:320)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java:59572)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:585)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:928)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2048)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2044)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1491)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2042)
at org.apache.hadoop.ipc.Client.call(Client.java:1347)
at org.apache.hadoop.ipc.Client.call(Client.java:1300)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:206)
at com.sun.proxy.$Proxy9.append(Unknown Source)
at sun.reflect.GeneratedMethodAccessor10.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:186)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:102)
at com.sun.proxy.$Proxy9.append(Unknown Source)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolTranslatorPB.append(ClientNamenodeProtocolTranslatorPB.java:245)
at org.apache.hadoop.hdfs.DFSClient.callAppend(DFSClient.java:1480)
at org.apache.hadoop.hdfs.DFSClient.append(DFSClient.java:1520)
at org.apache.hadoop.hdfs.DFSClient.append(DFSClient.java:1508)
at org.apache.hadoop.hdfs.DistributedFileSystem$4.doCall(DistributedFileSystem.java:310)
at org.apache.hadoop.hdfs.DistributedFileSystem$4.doCall(DistributedFileSystem.java:306)
at org.apache.hadoop.fs.FileSystemLinkResolver.resolve(FileSystemLinkResolver.java:81)
at org.apache.hadoop.hdfs.DistributedFileSystem.append(DistributedFileSystem.java:306)
at org.apache.hadoop.fs.FileSystem.append(FileSystem.java:1160)
at com.goojje.cart.etl.CartCommon.write(CartCommon.java:84)
at com.goojje.cart.CartClientMain.pushData(CartClientMain.java:63)
at com.goojje.cart.CartClientMain.main(CartClientMain.java:29)
org.apache.hadoop.ipc.RemoteException(java.io.IOException): java.lang.NullPointerException
at com.goojje.cart.index.CartIndexer.buildIndex(CartIndexer.java:49)
at com.goojje.cart.ipc.server.CartProtocolImpl.saveIndex(CartProtocolImpl.java:49)
at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at org.apache.hadoop.ipc.WritableRpcEngine$Server$WritableRpcInvoker.call(WritableRpcEngine.java:481)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:928)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2048)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2044)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:415)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1491)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2042)
at org.apache.hadoop.ipc.Client.call(Client.java:1347)
at org.apache.hadoop.ipc.Client.call(Client.java:1300)
at org.apache.hadoop.ipc.WritableRpcEngine$Invoker.invoke(WritableRpcEngine.java:231)
at com.sun.proxy.$Proxy10.saveIndex(Unknown Source)
at com.goojje.cart.CartClientMain.pushData(CartClientMain.java:64)
at com.goojje.cart.CartClientMain.main(CartClientMain.java:29)
分析可知，当前 Hadoop 并不支持并发写操作。下面链接是对这个问题的理论诠释：http://www.linuxidc.com/Linux/2014-01/95941.htm
相关阅读：
将本地文件拷到 HDFS 中 http://www.linuxidc.com/Linux/2013-05/83866.htm
从 HDFS 下载文件到本地 http://www.linuxidc.com/Linux/2012-11/74214.htm
将本地文件上传至 HDFS http://www.linuxidc.com/Linux/2012-11/74213.htm
HDFS 基本文件常用命令 http://www.linuxidc.com/Linux/2013-09/89658.htm
Hadoop 中 HDFS 和 MapReduce 节点基本简介 http://www.linuxidc.com/Linux/2013-09/89653.htm
《Hadoop 实战》中文版 + 英文文字版 + 源码【PDF】http://www.linuxidc.com/Linux/2012-10/71901.htm
Hadoop: The Definitive Guide【PDF 版】http://www.linuxidc.com/Linux/2012-01/51182.htm
更多 Hadoop 相关信息见 Hadoop 专题页面 http://www.linuxidc.com/topicnews.aspx?tid=13
