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From "Tony Wu (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-9290) DFSClient#callAppend() is not backward compatible for slightly older NameNodes
Date Thu, 22 Oct 2015 19:58:27 GMT
Tony Wu created HDFS-9290:
-----------------------------

             Summary: DFSClient#callAppend() is not backward compatible for slightly older
NameNodes
                 Key: HDFS-9290
                 URL: https://issues.apache.org/jira/browse/HDFS-9290
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 2.7.1
            Reporter: Tony Wu
            Assignee: Tony Wu
            Priority: Minor


HDFS-7210 combined 2 RPC calls used at file append into a single one. Specifically {{getFileInfo()}}
is combined with {{append()}}. While backward compatibility for older client is handled by
the new NameNode (protobuf). Newer client's {{append()}} call does not work with older NameNodes.
One will run into an exception like the following:
{code:java}
java.lang.NullPointerException
        at org.apache.hadoop.hdfs.DFSOutputStream.isLazyPersist(DFSOutputStream.java:1741)
        at org.apache.hadoop.hdfs.DFSOutputStream.getChecksum4Compute(DFSOutputStream.java:1550)
        at org.apache.hadoop.hdfs.DFSOutputStream.<init>(DFSOutputStream.java:1560)
        at org.apache.hadoop.hdfs.DFSOutputStream.<init>(DFSOutputStream.java:1670)
        at org.apache.hadoop.hdfs.DFSOutputStream.newStreamForAppend(DFSOutputStream.java:1717)
        at org.apache.hadoop.hdfs.DFSClient.callAppend(DFSClient.java:1861)
        at org.apache.hadoop.hdfs.DFSClient.append(DFSClient.java:1922)
        at org.apache.hadoop.hdfs.DFSClient.append(DFSClient.java:1892)
        at org.apache.hadoop.hdfs.DistributedFileSystem$4.doCall(DistributedFileSystem.java:340)
        at org.apache.hadoop.hdfs.DistributedFileSystem$4.doCall(DistributedFileSystem.java:336)
        at org.apache.hadoop.fs.FileSystemLinkResolver.resolve(FileSystemLinkResolver.java:81)
        at org.apache.hadoop.hdfs.DistributedFileSystem.append(DistributedFileSystem.java:336)
        at org.apache.hadoop.hdfs.DistributedFileSystem.append(DistributedFileSystem.java:318)
        at org.apache.hadoop.fs.FileSystem.append(FileSystem.java:1164)
{code}

The cause is that the new client code is expecting both the last block and file info in the
same RPC but the old NameNode only replied with the first. The exception itself does not reflect
this cause and one will have to look at the HDFS source code to really understand what happened.

We can have the client detect it's talking to a old NameNode and send an extra {{getFileInfo()}}
RPC. At the very least we can improve the exception being thrown to accurately reflect the
failure.



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