HDFS中使用append出错

1.文件append的问题

hadoop的版本1.0.4以后，API中已经有了追加写入的功能，但不建议在生产环境中使用，原因如下：Does HDFS allow appends to files? This is currently set to false because there are bugs in the "append code" and is not supported in any prodction cluster. 如果要测试的话，需要把dfs.support.appen的参数设置为true，不然客户端写入的时候会报错：

Exception in thread "main" org.apache.hadoop.ipc.RemoteException: java.io.IOException: Append to hdfs not supported. Please refer to dfs.support.append configuration parameter.

解决：修改namenode节点上的hdfs-site.xml。

    dfs.support.append
    true
 

2.在Hadoop-1.0.4和Hadoop-2.2的使用append时，需求：追加写入文件，如果文件不存在，需求先创建。

异常：

Exception in thread "main" org.apache.hadoop.ipc.RemoteException: org.apache.hadoop.hdfs.protocol.AlreadyBeingCreatedException: failed to create file /huangq/dailyRolling/mommy-dailyRolling for DFSClient_-1456545217 on client 10.1.85.243 because current leaseholder is trying to recreate file.
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.recoverLeaseInternal(FSNamesystem.java:1374)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.startFileInternal(FSNamesystem.java:1246)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.appendFile(FSNamesystem.java:1426)
at org.apache.hadoop.hdfs.server.namenode.NameNode.append(NameNode.java:643)
at sun.reflect.GeneratedMethodAccessor25.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)

出现上述报错的代码：

FileSystem fs = FileSystem.get(conf);
Path dstPath = new Path(dst);
if (!fs.exists(dstPath)) {
    fs.create(dstPath);
} 
FSDataOutputStream fsout = fs.append(dstPath);
BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(fsout));

异常的原因：fs句柄的原因，改成下面就ok了。

解决：创建完文件之后，关闭流fs，再次获得一次新的fs。

FileSystem fs = FileSystem.get(conf);
Path dstPath = new Path(dst);
if (!fs.exists(dstPath)) {
    fs.create(dstPath);
    fs.close();
    fs = FileSystem.get(conf);
} 
FSDataOutputStream fsout = fs.append(dstPath);