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https://issues.apache.org/jira/browse/HDFS-1123?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12862538#action_12862538
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bc Wong commented on HDFS-1123:
-------------------------------

Doesn't it still apply after moving to Avro? Avroization makes the layout 
documentation easier. It doesn't describes the semantics of the protocol, which 
is the interesting part.

> Need HDFS Protocol Specification
> --------------------------------
>
>                 Key: HDFS-1123
>                 URL: https://issues.apache.org/jira/browse/HDFS-1123
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: documentation
>            Reporter: bc Wong
>
> It'd be great to document (in a spec, not in the code) the HDFS wire protocol:
> * The layout of the different request and reply messages.
> * The semantics of the various calls.
> * The semantics of the various fields.
> For example, I stumbled upon the goldmine of comments around 
> DataNode.java:1150. It looks correct, but the version number of 9 doesn't 
> inspire confidence that it's up-to-date. (It's also a random place to put 
> such an important comment.)
> Having a formal spec is a big step forward for compatibility. It also 
> highlights design decisions and helps with protocol evolution.

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