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https://issues.apache.org/jira/browse/HADOOP-1986?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12533874
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Doug Cutting commented on HADOOP-1986:
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> No one was suggesting a serializer per a concrete class [ ... ]
Actually, I thought we might, and would like to preserve that option. I worry
about performance of introspection. And, for very simple objects, the overhead
of having WritableSerializer#serialize(o,out) call o.write(out) rather than
just being o.write(out) may even be significant. Or it may not be. In any
case, if record code is generated from a DDL, then we can implement this either
way, with per-class serializers or per-baseclass serializers. If we discard
the DDL and code-generation, then we're stuck with introspection, no?
I wonder if we might permit both by having the configuration name not
serializers but serializer factories. So one could specify the availability of
a WritableSerializerFactory that would be constructed, cached and used to
construct serializers for Writables. That could then potentially return a
different serializer for each kind of Writable, or the same serializer for all
Writables.
Finally, if we keep the DDL and generate only the class, not its serializers,
then there could theoretically be compatibility issues with other languages.
If, for example, the DDL defines different types that map to the same type in
Java (short versus character?) then using introspection could cause problems.
This is improbable, but another thing to watch out for.
Do I worry too much?
> Add support for a general serialization mechanism for Map Reduce
> ----------------------------------------------------------------
>
> Key: HADOOP-1986
> URL: https://issues.apache.org/jira/browse/HADOOP-1986
> Project: Hadoop
> Issue Type: New Feature
> Components: mapred
> Reporter: Tom White
> Assignee: Tom White
> Fix For: 0.16.0
>
> Attachments: SerializableWritable.java
>
>
> Currently Map Reduce programs have to use WritableComparable-Writable
> key-value pairs. While it's possible to write Writable wrappers for other
> serialization frameworks (such as Thrift), this is not very convenient: it
> would be nicer to be able to use arbitrary types directly, without explicit
> wrapping and unwrapping.
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