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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6904) A baby step towards inter-version communications between dfs client and NameNode
Date Wed, 01 Dec 2010 11:53:14 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12965671#action_12965671

dhruba borthakur commented on HADOOP-6904:

> The real difference is the following: if you check during getProx() and a method is missing
then how does one assert that it is still okay to connect because the missing method is actually
a new method and the call can return a non-implemented exception;

I like the idea of not checking anything during getProxy. Like you said, maybe a method is
missing from the server, but it is also likely that the client is not even interested in making
that call. Instead, we should check at every call (as you explained via the code piece in

Hairong seems to say that she will make the server send method names over and above M-m numbers,
I am ok with that idea.

> 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: majorMinorVersion.patch, majorMinorVersion1.patch, rpcVersion.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
> 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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