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From "Hong Tang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4243) Serialization framework use SequenceFile/TFile/Other metadata to instantiate deserializer
Date Tue, 03 Nov 2009 19:56:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4243?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12773152#action_12773152

Hong Tang commented on HADOOP-4243:

TFile does not store objects and is oblivious to Serialization framework.

> Serialization framework use SequenceFile/TFile/Other metadata to instantiate deserializer
> -----------------------------------------------------------------------------------------
>                 Key: HADOOP-4243
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4243
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: contrib/serialization
>            Reporter: Pete Wyckoff
> SequenceFile metadata is useful for storing additional information about the serialized
data, for example, for RecordIO, whether the data is CSV or Binary.  For thrift, the same
thing - Binary, JSON, ...
> For Hive, this may be especially important, because it has a Dynamic generic serializer/deserializer
that takes its DDL at runtime (as opposed to RecordIO and Thrift which require pre-compilation
into a specific class whose name can be stored in the sequence file key or value class). 
 In this case, the class name is like Record.java in RecordIO - it doesn't tell you anything
without the DDL.
> One way to address this could be adding the sequence file metadata to the getDeserializer
call in Serialization interface.  The api would then be something like getDeserializer(Class<?>,
Map<Text, Text> metadata) or Properties metadata.
> But, I am open to proposals.
> This also means that saying a class implements Writable is not enough to necessarily
deserialize it since it may do specific actions based on the metadata - e.g., RecordIO might
determine whether to use CSV rather than the default Binary deserialization.
> There's the other issue of the getSerializer returning the metadata to be written to
the Sequence/T File.

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