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https://issues.apache.org/jira/browse/HADOOP-6120?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12733331#action_12733331
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Doug Cutting commented on HADOOP-6120:
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If, e.g., a class both implements Writable and also either implements 
AvroReflectSerializeable or whose package is listed in the config, then which 
serialization is used is determined by the order of the serializers in the 
config, which could change.  Similarly for the Serializeable interface and 
JavaSerialization.  So its not safe to assume that the Class->Serialization map 
is fixed, and we should really be storing at least the serialization's name in 
container metadata, and probably also a version (e.g. serialVersionUID for 
JavaSerialization).  And once we have a mechanism to support that, we can also 
store other metadata, like the schema, so that we can read older versions and 
generic data.

+1 for removing JavaSerialization from the defaults.

+1 for a separate Jira on serialization metadata.  But it would be best to have 
such metadata by the 0.21 freeze, in two weeks.


> Add support for Avro types in hadoop
> ------------------------------------
>
>                 Key: HADOOP-6120
>                 URL: https://issues.apache.org/jira/browse/HADOOP-6120
>             Project: Hadoop Common
>          Issue Type: New Feature
>            Reporter: Sharad Agarwal
>            Assignee: Sharad Agarwal
>         Attachments: 6120_v1.patch, 6120_v2.patch, 6120_v3.patch, 
> 6120_v4.patch, 6120_v5.patch, HADOOP-6120.patch
>
>
> Support to serialize and deserialize Avro types in Hadoop.

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