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Date: Tue, 10 Aug 2010 19:17:19 -0400 (EDT)
From: "Hairong Kuang (JIRA)" <jira@apache.org>
To: common-issues@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-6904) A baby step towards inter-version
 communications between dfs client and NameNode
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    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12897081#action_12897081 ] 

Hairong Kuang commented on HADOOP-6904:
---------------------------------------

Doug, now that the patch to HDFS-1335 is out there, do you see any issue with this approach? Today I rewrote HDFS-946 to make it backward compatible and tested on my local machine. It worked out very well. 

> A baby step towards inter-version communications between dfs client and NameNode
> --------------------------------------------------------------------------------
>
>                 Key: HADOOP-6904
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6904
>             Project: Hadoop Common
>          Issue Type: New Feature
>          Components: ipc
>    Affects Versions: 0.22.0
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.22.0
>
>         Attachments: rpcVersion.patch, rpcVersion1.patch
>
>
> Currently RPC communications in Hadoop is very strict. If a client has a different version from that of the server, a VersionMismatched exception is thrown and the client can not connect to the server. This force us to update both client and server all at once if a RPC protocol is changed. But sometime different versions do not mean the client & server are not compatible. It would be nice if we could relax this restriction and allows us to support inter-version communications.
> My idea is that DfsClient catches VersionMismatched exception when it connects to NameNode. It then checks if the client & the server is compatible. If yes, it sets the NameNode version in the dfs client and allows the client to continue talking to NameNode. Otherwise, rethrow the VersionMismatch exception.

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