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https://issues.apache.org/jira/browse/HADOOP-6904?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898434#action_12898434
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Hairong Kuang commented on HADOOP-6904:
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> 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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