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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 Tue, 30 Nov 2010 19:16:14 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12965354#action_12965354
] 

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

@Dhruba
>major/minor numbers can be considered an optimization, isn't it?
The M-m# allows one to assert compatibility; the other way is via method name comparison.
I see them as complementary rather than
as an optimization. Without the major minor number one has to do a check on each call or do
a check on the getProxy. 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;
 with  M-m# one can assert this because when the M# has not changed and hence one is assured
that the method was not deleted and  it must be a new method.

If you look at the example of list() call  I gave above, the method names give a minor simplification
of the code.
I have no issues with passing the method names as long as we also optimize by not sending
the list when there is match of version numbers. I also believe we will have to cache the
results across reconnects (BTW we had discussed the same optimization for 
Avro RPC except that it uses the schema checksum).

> 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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