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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6904) A baby step towards inter-version RPC communications
Date Sat, 08 Jan 2011 00:25:48 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12979033#action_12979033
] 

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

I just remembered that Class#getMethods() does not return methods in any particular order,
and I believe that Arrays#hashCode() is order-dependent.  So the methods need to be sorted
by name before they are hashed.

Note that Java Serialization defines a good hashing method as the default definition for serialVersionUID:

http://download.oracle.com/javase/6/docs/platform/serialization/spec/class.html#4100

This can be accessed with ObjectStreamClass.lookup(iface).getSerialVersionUID().  Unfortunately
the protocol has to extend java.io.Serializeable, and we'd need to check that explicitly,
as this method returns 0 for non-Serializeable interfaces, but this still might be a good
way to get a well designed hash-function for interfaces.

> A baby step towards inter-version RPC communications
> ----------------------------------------------------
>
>                 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, rpcCompatible-trunk.patch,
rpcCompatible-trunk1.patch, rpcCompatible-trunk2.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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