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From Josh Wills <jwi...@cloudera.com>
Subject Re: crunch 0.8.2+6-cdh4.4.0
Date Fri, 30 May 2014 21:00:13 GMT
One thought would be to use an Option-style type for the value of the
PTable, so that you would have PTable<Key, Optional<T>>, using Guava's
Optional wrapper class, described here:

https://code.google.com/p/guava-libraries/wiki/UsingAndAvoidingNullExplained

You would need to create a derived PType for Optionals, perhaps using
PTypeFamily.collections(ptype) (so that a zero-element collection would be
Optional.absent, and a single-element collection would map to
Optional.of(value). If you're going to have null values in your
PCollections, null checking your code is the sort of thing you're certainly
doing anyway, so adding a bit of boilerplate to handle the null-values
officially is probably a good idea.

Unfortunately, the org.apache.avro.mapred.Pair stuff is part of the
underlying Avro MR framework, it isn't something we directly control in
Crunch.


On Fri, May 30, 2014 at 1:07 PM, Surbhi Mungre <mungre.surbhi@gmail.com>
wrote:

>  In our codebase there are various places where we are using PTables with
> null values because we were under the impression that PTables supports null
> values like PCollections. Based on Josh's reply below, we will have to
> replace our PTables with PCollections of Pair so that null values can be
> supported. If we make this change, then we will have to convert PCollection
> back to PTable where ever we want to do an operation supported only by
> PTables like groupByKey. Replacing all PTables with PCollections is going
> to add lot of unnecessary complexity. Does anyone know of a better
> workaround for this problem?
>
> Also, a PTable and a PCollection of Pair should essentially behave in same
> manner. In this case, functionally a PTable does not seem like an extension
> of a PCollection. So, is it possible for PTables to use
> "org.apache.crunch.Pair" under the covers instead of using
> "org.apache.avro.mapred.Pair"?
>
> -Surbhi
>
>>
>> *From: *Josh Wills <jwills@cloudera.com>
>>
>> *Reply-To: *"user@crunch.apache.org" <user@crunch.apache.org>
>>
>> *Date: *Thursday, February 6, 2014 2:10 PM
>>
>> *To: *"user@crunch.apache.org" <user@crunch.apache.org>
>>
>> *Subject: *Re: crunch 0.8.2+6-cdh4.4.0
>>
>>
>> Hey Stephen-
>>
>>
>> Thanks for putting this together; I had an inkling of what the issue was
>> when you sent the stack trace and I managed to verify it using your unit
>> test. The org.apache.avro.mapred.Pair schema that is used in PTable isn't
>> defined by Crunch and doesn't allow null values. If you change your example
>> code to use Avros.pairs() instead of tableOf(), the test works properly,
>> b/c Crunch tuples do permit nulls.
>>
>>
>> I'm unsure of the best way to fix this if we want to. We could force
>> AvroFileTarget to convert its inputs to a Crunch tuple and not allow it to
>> write out an avro Pair schema, but I'd be concerned that this would break
>> other use cases that I'm not aware of.
>>
>>
>> J
>>
>>
>>
>> On Thu, Feb 6, 2014 at 9:35 AM, Stephen Durfey <sjdurfey@gmail.com>
>> wrote:
>>
>> Attached is a unit test that emits a null avro record that causes this
>> stack trace to occur. The file that is being read in doesn't matter as the
>> contents are ignored. I just needed to read in something to use the
>> MRPipeline. Also, this test fails on both 0.8.2+6-cdh4.4.0 and
>> 0.8.0-cdh4.3.0 (I haven't tried versions older than this), which leads me
>> to believe that this test isn't completely representative of the complexity
>> in my code base. Nevertheless, it is the exact same exception with the same
>> emit behavior.
>>
>>
>> Side note: In MemPipeline, when writing a PTable as a text file, there is
>> an assumption that all key and value pairs are non-null. This assumption
>> could be elsewhere in MemPipeline#write as well, but I haven't tried the
>> other flows.
>>
>>
>>
>> On Wed, Jan 29, 2014 at 4:46 PM, Stephen Durfey <sjdurfey@gmail.com>
>> wrote:
>>
>> This is the full stack trace. I removed some parts of the stack trace
>> that emits the null values:
>>
>>
>> org.apache.crunch.CrunchRuntimeException:
>> org.apache.avro.file.DataFileWriter$AppendWriteException:
>> java.lang.NullPointerException: null of <specific avro model name> in field
>> value of org.apache.avro.mapred.Pair
>>
>>     at
>> org.apache.crunch.impl.mr.emit.MultipleOutputEmitter.emit(MultipleOutputEmitter.java:45)
>>
>>     at org.apache.crunch.MapFn.process(MapFn.java:34)
>>
>>     at org.apache.crunch.impl.mr.run.RTNode.process(RTNode.java:99)
>>
>>     at
>> org.apache.crunch.impl.mr.emit.IntermediateEmitter.emit(IntermediateEmitter.java:56)
>>
>>     at org.apache.crunch.MapFn.process(MapFn.java:34)
>>
>>     at org.apache.crunch.impl.mr.run.RTNode.process(RTNode.java:99)
>>
>>     at
>> org.apache.crunch.impl.mr.emit.IntermediateEmitter.emit(IntermediateEmitter.java:56)
>>
>>     <code that emits the null value>
>>
>>     at org.apache.crunch.impl.mr.run.RTNode.cleanup(RTNode.java:118)
>>
>>     at org.apache.crunch.impl.mr.run.RTNode.cleanup(RTNode.java:121)
>>
>>     at org.apache.crunch.impl.mr.run.RTNode.cleanup(RTNode.java:121)
>>
>>     at org.apache.crunch.impl.mr.run.RTNode.cleanup(RTNode.java:121)
>>
>>     at
>> org.apache.crunch.impl.mr.run.CrunchReducer.cleanup(CrunchReducer.java:63)
>>
>>     at org.apache.hadoop.mapreduce.Reducer.run(Reducer.java:166)
>>
>>     at
>> org.apache.hadoop.mapred.ReduceTask.runNewReducer(ReduceTask.java:610)
>>
>>     at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:444)
>>
>>     at org.apache.hadoop.mapred.Child$4.run(Child.java:268)
>>
>>     at java.security.AccessController.doPrivileged(Native Method)
>>
>>     at javax.security.auth.Subject.doAs(Subject.java:396)
>>
>>     at
>> org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1408)
>>
>>     at org.apache.hadoop.mapred.Child.main(Child.java:262)
>>
>> Caused by: org.apache.avro.file.DataFileWriter$AppendWriteException:
>> java.lang.NullPointerException: null of <specific avro model name> in field
>> value of org.apache.avro.mapred.Pair
>>
>>     at org.apache.avro.file.DataFileWriter.append(DataFileWriter.java:263)
>>
>>     at
>> org.apache.crunch.types.avro.AvroOutputFormat$1.write(AvroOutputFormat.java:87)
>>
>>     at
>> org.apache.crunch.types.avro.AvroOutputFormat$1.write(AvroOutputFormat.java:84)
>>
>>     at org.apache.crunch.io.CrunchOutputs.write(CrunchOutputs.java:128)
>>
>>     at
>> org.apache.crunch.impl.mr.emit.MultipleOutputEmitter.emit(MultipleOutputEmitter.java:41)
>>
>>     ... 23 more
>>
>> Caused by: java.lang.NullPointerException: null of <specific avro model
>> name> in field value of org.apache.avro.mapred.Pair
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.npe(GenericDatumWriter.java:93)
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:87)
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:58)
>>
>>     at org.apache.avro.file.DataFileWriter.append(DataFileWriter.java:257)
>>
>>     ... 27 more
>>
>> Caused by: java.lang.NullPointerException
>>
>>     at org.apache.avro.generic.GenericData.getField(GenericData.java:537)
>>
>>     at org.apache.avro.generic.GenericData.getField(GenericData.java:552)
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.writeRecord(GenericDatumWriter.java:104)
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:66)
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.writeRecord(GenericDatumWriter.java:106)
>>
>>     at
>> org.apache.avro.generic.GenericDatumWriter.write(GenericDatumWriter.java:66)
>>
>>     ... 29 more
>>
>>
>>
>> On Wed, Jan 29, 2014 at 4:30 PM, Micah Whitacre <mkwhitacre@gmail.com>
>> wrote:
>>
>> That actually seems likely as the persistence of fixed bytes in the
>> Buffered vs Direct encoders differs.
>>
>>
>> Stephen can you include the full stack trace of the NPE which will help
>> to verify if differences in the encoders are at fault.
>>
>>
>>
>> On Wed, Jan 29, 2014 at 4:14 PM, Josh Wills <josh.wills@gmail.com> wrote:
>>
>> Maybe the binary encoder change?
>>
>> On Jan 29, 2014 3:19 PM, "Durfey,Stephen" <Stephen.Durfey@cerner.com>
>> wrote:
>>
>> Sorry for the initial confusion. The exceptions that I am seeing, look
>> like were caused by another exception way up in my console log (that I
>> originally missed). I think the true exception is an Avro exception. It is
>> an Avro PType, and the original NPE is coming from the
>> GenericData#getField,
>>
>> when the GenericDatumWriter is serializing. The exception is for a null
>> value in the org.apache.avro.mapred.Pair (I believe this Pair is created in
>> PairMapFn?)  object when expecting a specific type of Avro. I'm having a
>> difficult time figuring out what changed between versions. Without changing
>>
>> anything in my code, and just changing Crunch versions causes these
>> exceptions to be thrown.
>>
>>
>> - Stephen
>>
>>
>> *From: *Josh Wills <josh.wills@gmail.com>
>>
>> *Reply-To: *"user@crunch.apache.org" <user@crunch.apache.org>
>>
>> *Date: *Tuesday, January 28, 2014 at 4:08 PM
>>
>> *To: *"user@crunch.apache.org" <user@crunch.apache.org>
>>
>> *Subject: *Re: crunch 0.8.2+6-cdh4.4.0
>>
>>
>> Hey Stephen,
>>
>>
>> Slightly confused here, question inlined.
>>
>>
>>
>> On Tue, Jan 28, 2014 at 12:59 PM, Durfey,Stephen <
>> Stephen.Durfey@cerner.com> wrote:
>>
>> This question is specifically about this version maintained by Cloudera.
>>
>>
>> I was looking to update out Crunch version from 0.8.0-cdh4.3.0
>> to 0.8.2+6-cdh4.4.0. In the process some of our tests starting failing from
>> NullPointerExceptions. I’ve discovered why these exceptions are happening,
>> but I’m having trouble tracking down the where.
>>
>>
>> The exceptions occur when we emit a Pair<POJO, null> that uses an Avro
>> PType. Previously this worked just fine, and by the time the CrunchOutputs
>> started writing to a sequence file the value would be an instance of
>> NullWritable, and it would successfully pull off the output type for
>> serialization (in SequenceFile.BlockCompressWriter#append(k, v)). After the
>> version change the value when it got down to write to a sequence file was
>> 'null', rather than NullWritable.
>>
>>
>> It's an AvroType that's getting written to a Sequence File? Is that right?
>>
>>
>>
>>
>> Any thoughts?
>>
>>
>> - Stephen
>>
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>>
>>
>>
>>
>


-- 
Director of Data Science
Cloudera <http://www.cloudera.com>
Twitter: @josh_wills <http://twitter.com/josh_wills>

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