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https://issues.apache.org/jira/browse/HDFS-7007?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14354427#comment-14354427
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Konstantin Shvachko commented on HDFS-7007:
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h3. Possible approaches
# My original idea was to subclass certain NameNode classes to enable sending
proposals for the write RPC calls. The advantages of this approach include
#- that the entire ConsensusNode implementation can be kept completely separate
as a different subproject, like hadoop-hdfs-nfs or hadoop-kms,
#- and allow code reuse from the NameNode
#- While the argument against is too tight integration with NN, as per
[~tlipcon] and [~atm]
# [~stevel] proposed to make NameNode use {{CompositeService}}. I see how that
helps adding components to the NameNode and maintaining their starts and stops,
but I don't see how it helps enabling coordination or avoiding subclassing.
# [~szetszwo] proposed to introduce an interface for {{FSNamesystem}} similar
to DN's {{FsDatasetSpi}}, as in HDFS-5194. I do not have a proposal for that,
but if this is what people want, I can think of one. I see one potential
disadvantage of this approach, which was mentioned for {{FsDatasetSpi}} in
HDFS-5751, that there is code duplication in implementations.
In the end CNode must create a directory same as NN. It would be good to have
them call the same mkdirs() method rather than duplicating it in both.
# Somebody, probably [~sanjay.radia], mentioned "in-line" approach. Akin to
Standby NN, where HAState distinguishes active from standby with execution
depending on respective if statements.
# A rather low-level approach is to invoke coordination in the {{RpcEngine}}
layer. That is, when RPC call arrives we either (writes) submit a proposal to
CE or (reads) call {{NameNodeRpcServer}} directly as in regular NN. We can
annotate NN protocol methods as writes or reads and RpcEngine will invoke
coordination depending on the annotation.
The advantage here is that it allows submitting proposals without fully
deserializing the RPC call. There will be still changes to FSNamesystem, since
some write calls need or can be optimized with extra checks before proposing,
like safeMode and permissions for mkdirs(), or choosing target locations before
{{addBlock()}}.
We will probably end up using a combination of approaches. My favourite as of
today is the last one. It seems to be minimally invasive.
> Interfaces to plugin ConsensusNode.
> -----------------------------------
>
> Key: HDFS-7007
> URL: https://issues.apache.org/jira/browse/HDFS-7007
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: namenode
> Affects Versions: 3.0.0
> Reporter: Konstantin Shvachko
>
> This is to introduce interfaces in NameNode and namesystem, which are needed
> to plugin ConsensusNode.
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