[jira] Created: (HADOOP-7158) Reduce RPC packet size for homogeneous arrays, such as the array responses to listStatus() and getBlockLocations()

["Matt Foley (JIRA)" <ji...@apache.org>]

Reduce RPC packet size for homogeneous arrays, such as the array responses to listStatus() and getBlockLocations()
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                 Key: HADOOP-7158
                 URL: https://issues.apache.org/jira/browse/HADOOP-7158
             Project: Hadoop Common
          Issue Type: Improvement
          Components: io
    Affects Versions: 0.22.0
            Reporter: Matt Foley
            Assignee: Matt Foley


While commenting on HADOOP-6949, which proposes a big improvement in the RPC wire format for arrays of primitives, Konstantin Shvachko said:
"Can/should we extend this to arrays of non-primitive types? This should benefit return types for calls like listStatus() and getBlockLocations() on a large directory."

The improvement for primitive arrays is based on not type-labeling every element in the array, so the array in question must be strictly homogenous; it cannot have subtypes of the assignable type.  For instance, it could not be applied to heartbeat responses of DatanodeCommand[], whose array elements carry subtypes of DatanodeCommand, each of which must be type-labeled independently.  However, as Konstantin points out, it could really help lengthy response arrays for things like listStatus() and getBlockLocations().

I will attach a prototype implementation to this Jira, for discussion.  However, since it can't be automatically applied to all arrays passing through RPC, I'll just providing the wrapper type.  By using it, a caller is asserting that the array is strictly homogeneous in the above sense.


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