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From "Sanjay Radia (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6904) A baby step towards inter-version communications between dfs client and NameNode
Date Fri, 13 Aug 2010 20:56:19 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898416#action_12898416
] 

Sanjay Radia commented on HADOOP-6904:
--------------------------------------

>If we do not bump the version number, old client can continue to talk to a new server,
but we also want a new client to talk to an old server.
But the new client will get the "now such method" exception and will know that he is talking
to an older client.

(BTW I am not at this point suggesting we modify our practice of changing version #s (yet).

For example with Avro or PBs, if you call a old server with a new method you simple get back
a "not such method" exception. 
Correct Doug?


> 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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