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
