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From Fengyun RAO <raofeng...@gmail.com>
Subject MapReduce: How to output multiplt Avro files?
Date Thu, 06 Mar 2014 08:09:00 GMT
our input is a line of text which may be parsed to e.g. A or B object.
We want all A objects written to "A.avro" files, while all B objects
written to "B.avro".

I looked into AvroMultipleOutputs class:
http://avro.apache.org/docs/1.7.4/api/java/org/apache/avro/mapreduce/AvroMultipleOutputs.html
There is an example, however, it's not quite clear.
For job submission, it uses AvroMultipleOutputs.addNamedOutput to add
schemas for A and B.
In my program looks like:
        AvroMultipleOutputs.addNamedOutput(job, "A",
AvroKeyOutputFormat.class, aSchema, null);
        AvroMultipleOutputs.addNamedOutput(job, "B",
AvroKeyOutputFormat.class, bSchema, null);
I believe this is for Reducer output files.

*My question is* what the Mapper output should be, in specific what
"job.setMapOutputValueClass" should be,
since the Mapper output could be A or B object, with schema aSchema or
bSchema.

In my progam, I simply set it to GenericData, but get error as below:

14/03/06 15:55:34 INFO mapreduce.Job: Task Id :
attempt_1393817780522_0012_m_000010_2, Status : FAILED
Error: java.lang.NullPointerException
        at
org.apache.hadoop.mapred.MapTask$MapOutputBuffer.init(MapTask.java:989)
        at
org.apache.hadoop.mapred.MapTask.createSortingCollector(MapTask.java:390)
        at org.apache.hadoop.mapred.MapTask.access$100(MapTask.java:79)
        at
org.apache.hadoop.mapred.MapTask$NewOutputCollector.<init>(MapTask.java:674)
        at org.apache.hadoop.mapred.MapTask.runNewMapper(MapTask.java:746)
        at org.apache.hadoop.mapred.MapTask.run(MapTask.java:339)
        at org.apache.hadoop.mapred.YarnChild$2.run(YarnChild.java:165)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:415)
        at
org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1491)
        at org.apache.hadoop.mapred.YarnChild.main(YarnChild.java:160)

I have no idea what this means.

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