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https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898418#action_12898418
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Hairong Kuang commented on HADOOP-6904:
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> But the new client will get the "now such method" exception and will know
> that he is talking to an older client.
Sanjay I think you meant the old client will get the "no such method" exception
and will know he is talking to an older server.
Yes, this adds one more RPC and does not seem to be elegant. Furthermore,
If a protocol has a method x, if you add an improved version of x, called
method y, to the protocol. Later on add method an improved version of y, called
method z, a protocol. Both changes do not bump version #. When a client failed
on calling Z, it does not know it should fail back to y or z.
I think bumping up version number is good, clearly indicating a change are made
to a protocol. It should be the application itself to decide how to deal with
the change in a compatible way.
> 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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