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From "Tony Wu (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-9290) DFSClient#callAppend() is not backward compatible for slightly older NameNodes
Date Thu, 22 Oct 2015 20:01:27 GMT

     [ https://issues.apache.org/jira/browse/HDFS-9290?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Tony Wu updated HDFS-9290:
    Attachment: HDFS-9290.001.patch

In this patch:
* Detect that DFSClient is talking to an older NameNode and make an extra {{getFileInfo()}}
RPC call to request file info.

> 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
>         Attachments: HDFS-9290.001.patch
> 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

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