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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6904) A baby step towards inter-version communications between dfs client and NameNode
Date Fri, 03 Dec 2010 17:13:14 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12966588#action_12966588
] 

Doug Cutting commented on HADOOP-6904:
--------------------------------------

Hairong> Major numbers should be updated if we change the (de)serialization format.

Okay, I think I get it.  The "major version" is updated whenever a parameter list changes
or the serialization of a parameter changes.  But adding or removing a method would not require
a change of this value.  If that's right, then perhaps we should call it something like "format
version"?  With Avro, this could be the hash of the protocol, but with Writable it would have
to be the hash of the read and write method bytecodes of each parameter, which would be too
precise, so we instead declare it manually.  Do I have this right?  Thanks!

> 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,
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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