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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-6904) A baby step towards inter-version communications between dfs client and NameNode
Date Fri, 13 Aug 2010 21:58:18 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898434#action_12898434
] 

Hairong Kuang commented on HADOOP-6904:
---------------------------------------

> Slight improvement to your design:
> If a new server knows that it is compatible with an older client then it should accept
the client's version.
> If the client is new then the decision can only be made on the client side. 

Sanjay, this is exactly what I did in my patch to HDFS-1335. Here is the algorithm for getProtocolVersion:

If client is old and server is new
  server checks compatibility. if not compatible, throws VersionIncompatibleException. Connection
fails.
  Otherwise, return server version; RPC client will throw a VersionMismatch. DFS client catches
VersionMismatch but ignores it because server is newer. Connection succeeds.

if client is newer and server is old
  server simply returns the server version;
  RPC client throws VersionMismatch. DFS client catches VersionMismatch and then performs
a compatibility check; If not compatible, throws VersionIncompatible and connection fails.
Otherwise, connection succeeds.

> 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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