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From "Nandakumar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11395) RequestHedgingProxyProvider#RequestHedgingInvocationHandler hides the Exception thrown from NameNode
Date Wed, 22 Feb 2017 06:33:44 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11395?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15877571#comment-15877571
] 

Nandakumar commented on HDFS-11395:
-----------------------------------

Thanks for the review [~arpitagarwal].

The HDFS clients expect unwrapped remote exception (like FileNotFoundException, AccessControlException,
UnresolvedPathException).
Unwrapping of remote exception is done in {{DFSClient}}, {{DFSOutputStream}}, etc..

For example
{code:title=DFSClient#callGetBlockLocations}
static LocatedBlocks callGetBlockLocations(ClientProtocol namenode,
      String src, long start, long length) 
      throws IOException {
    try {
      return namenode.getBlockLocations(src, start, length);
    } catch(RemoteException re) {
      throw re.unwrapRemoteException(AccessControlException.class,
                                     FileNotFoundException.class,
                                     UnresolvedPathException.class);
    }
{code}

If we throw {{MultiException(RemoteException(Exception1), RemoteException(Exception2))}} from
RequestHedgingInvocationHandler, unwrapping will not be done (by DFSClient, DFSOutputStream,
etc.. the catch block doesn't handle MultiException) and the hdfs clients will directly receive
MultiException.

The hdfs clients (like hbase, hive, user applications) have no idea on how to handle MultiException.


> RequestHedgingProxyProvider#RequestHedgingInvocationHandler hides the Exception thrown
from NameNode
> ----------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-11395
>                 URL: https://issues.apache.org/jira/browse/HDFS-11395
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: ha
>            Reporter: Nandakumar
>            Assignee: Nandakumar
>         Attachments: HDFS-11395.000.patch
>
>
> When using RequestHedgingProxyProvider, in case of Exception (like FileNotFoundException)
from ActiveNameNode, {{RequestHedgingProxyProvider#RequestHedgingInvocationHandler.invoke}}
receives {{ExecutionException}} since we use {{CompletionService}} for the call. The ExecutionException
is put into a map and wrapped with {{MultiException}}.
> So for a FileNotFoundException the client receives {{MultiException(Map(ExecutionException(InvocationTargetException(RemoteException(FileNotFoundException)))))}}
> It will cause problem in clients which are handling RemoteExceptions.



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