how to save pig bag in json format

Question

I'm running Pig

example$ pig --version
Apache Pig version 0.8.1-cdh3u1 (rexported) 
compiled Jul 18 2011, 08:29:40

on very simple dataset

example$ hadoop fs -cat /user/pavel/trivial.log
1   one
2   two
3   three

I'm trying to save the bag format as json by using the following script:

REGISTER ./pig.jar;
A = LOAD 'trivial.log' USING PigStorage('\t') AS (mynum: int, mynumstr: chararray);
B = GROUP A BY mynum;
DUMP B;
STORE B into 'trivial_json.out' USING JsonStorage();

and I get an error: Backend error message

---------------------
java.lang.NullPointerException
    at org.apache.pig.ResourceSchema.<init>(ResourceSchema.java:239)
    at org.apache.pig.builtin.JsonStorage.prepareToWrite(JsonStorage.java:129)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigOutputFormat$PigRecordWriter.<init>(PigOutputFormat.java:124)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.PigOutputFormat.getRecordWriter(PigOutputFormat.java:85)
    at org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:559)
    at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:414)
    at org.apache.hadoop.mapred.Child$4.run(Child.java:270)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1127)
    at org.apache.hadoop.mapred.Child.main(Child.java:264)

Pig Stack Trace
---------------
ERROR 2997: Unable to recreate exception from backed error: java.lang.NullPointerException

org.apache.pig.backend.executionengine.ExecException: ERROR 2997: Unable to recreate exception from backed error: java.lang.NullPointerException
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.Launcher.getErrorMessages(Launcher.java:221)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.Launcher.getStats(Launcher.java:154)
    at org.apache.pig.backend.hadoop.executionengine.mapReduceLayer.MapReduceLauncher.launchPig(MapReduceLauncher.java:337)
    at org.apache.pig.backend.hadoop.executionengine.HExecutionEngine.execute(HExecutionEngine.java:382)
    at org.apache.pig.PigServer.executeCompiledLogicalPlan(PigServer.java:1209)
    at org.apache.pig.PigServer.execute(PigServer.java:1201)
    at org.apache.pig.PigServer.access$100(PigServer.java:129)
    at org.apache.pig.PigServer$Graph.execute(PigServer.java:1528)
    at org.apache.pig.PigServer.executeBatchEx(PigServer.java:373)
    at org.apache.pig.PigServer.executeBatch(PigServer.java:340)
    at org.apache.pig.tools.grunt.GruntParser.executeBatch(GruntParser.java:115)
    at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:172)
    at org.apache.pig.tools.grunt.GruntParser.parseStopOnError(GruntParser.java:144)
    at org.apache.pig.tools.grunt.Grunt.exec(Grunt.java:90)
    at org.apache.pig.Main.run(Main.java:396)
    at org.apache.pig.Main.main(Main.java:107)
================================================================================

I'm not strong enough in Java to debug in minutes, can somebody suggest what might be going on?

Thanks much! -Pavel

Answer 1

Got some questions regarding this by email, thought this might be useful for others:

It turned out that the JsonStorage class wasn't included in my Pig installation. In fact it's not even out in a stable branch yet, you won't find it in 0.9.2. But if you get the latest trunk

http://svn.apache.org/repos/asf/pig/trunk/

then trunk/test/org/apache/pig/test/TestJsonLoaderStorage.java shows how it works. If you are not averse to unreleased versions then you could try it. If you go this way you should also take a look at Avro (binary format with json metadata). It's not officially out either.

I was trying to run a streaming python job and was trying to avoid manually specifying the schema. I ended up passing Pig bags and parsing them by hand.

Hope this helps. -Pavel