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https://issues.apache.org/jira/browse/HDFS-3453?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13281620#comment-13281620
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Kihwal Lee commented on HDFS-3453:
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A bit more on the incompatibility:
ClientProtocol and namenode still have the original method and implementation.
So pre-1.0 clients can still talk to 1.0.x namenodes. The 1.0.x client, on the
other hand, internally calls only the new method; The original create method is
never called. Since pre-1.0 namenode does not understand this new method, 1.0
clients cannot create files on pre-1.0 clusters. The patch makes client call
the new method only when createParent is set to false. All others will be
fulfilled by the original method.
The protocol-level incompatibility still exists, but that only manifests when
the new file system level method, FileSystem#createNonRecursive(), is
explicitly called. Normal use cases within hadoop do not use this method.
Currently only SequenceFile#createWriter() calls it when caller specifically
asks for it. This was added to support HBase.
In summary, 1.0.x and pre-1.0 are incompatible, but become interoperable with
this patch for most use cases.
> HDFS does not use ClientProtocol in a backward-compatible way
> -------------------------------------------------------------
>
> Key: HDFS-3453
> URL: https://issues.apache.org/jira/browse/HDFS-3453
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs client
> Affects Versions: 1.0.0
> Reporter: Kihwal Lee
> Assignee: Kihwal Lee
> Fix For: 1.1.0
>
> Attachments: hdfs-3453-branch-1.patch
>
>
> HDFS-617 was brought into branch-0.20-security/branch-1 to support
> non-recursive create, along with HADOOP-6840 and HADOOP-6886. However, the
> changes in HDFS was done in an incompatible way, making the client unusable
> against older clusters, even when plain old create() is called. This is
> because DFS now internally calls create() through the newly introduced
> method. By simply changing how the methods are wired internally, we can
> remove this limitation. We may eventually switch back to the approach in
> HDFS-617 when the majority of users adopt branch-1 based releases.
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