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https://issues.apache.org/jira/browse/HDFS-1578?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12981647#action_12981647
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Todd Lipcon commented on HDFS-1578:
-----------------------------------

bq. Todd, I really like your proposal of including data transfer version # in 
DN's descriptor. Is there a simple way of making this work without breaking 
protocol compatiblity?

Hmm, you're trying to add a compatibility layer that is itself compatible with 
previous versions? Seems tricky, since we'd have to add a field to 
DatanodeInfo... so, no, no good ideas if that is a goal.

> First step towards data transter protocol compatibility: support 
> DatanodeProtocol#getDataTransferProtocolVersion
> ----------------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-1578
>                 URL: https://issues.apache.org/jira/browse/HDFS-1578
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Hairong Kuang
>            Assignee: Hairong Kuang
>             Fix For: 0.23.0
>
>
> HADOOP-6904 allows us to handle RPC changes in a compatible way. However, we 
> have one more protocol to take care of, the data transfer protocol, which a 
> dfs client uses to read data from or write data to a datanode.
>  
> My proposal is to add a new RPC getDataTransferVersion to DatanodeProtocol 
> that returns the data transfer protocol version running on the datanode. A 
> dfs client gets the datanode's version number before it reads from/writes to 
> a datanode. With this, the dfs client could behave differently according to 
> datanode's data transfer version. This provides a base for us to make data 
> transfer protocol changes in a compatible way.

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