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Avro >> mail # user >> Hadoop 0.23, Avro Specific 1.6.3 and "org.apache.avro.generic.GenericData$Record cannot be cast to "

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Re: Hadoop 0.23, Avro Specific 1.6.3 and "org.apache.avro.generic.GenericData$Record cannot be cast to "
Hi Jacob,

On May 13, 2012, at 2:03pm, Jacob Metcalf wrote:

> Ken, thanks for getting back to me.
> 1) The Avro specific classes are generated and packed in the same JAR as the mapper and reducer. Attached is my examplehttp://markmail.org/download.xqy?id=m6te4atgmyrrqyv5&number=1 which in parallel I am also getting working on MRUnit so am discussing on that forum. If you want to build it you will need to build odagio-avro.
> I agree and cannot comprehend how if the mapper can serialize, the reducer cannot deserialize. My only guess is that the reducer is running in a separate JVM and it is only this which has classpath issues. Logically the mapper output would be deserialized before my reducer is instantiated. I noticed that the JAR does get exploded so my only thought is that there is something going wrong in the Cygwin/Hadoop layer at reduction.
> 2) Yes the latest version of avro is in my Job Jar. However I am again not sure how to manipulate the Hadoop classpath to ensure it is first. This is possibly more a topic for the Hadoop list.

Two comments…

1. Your pom.xml doesn't look like it's set up to build a proper Hadoop job jar.

After running "mvn assembly:assembly" you should have a job jar that has a lib subdirectory, and inside of that sub-dir you'll have all fo the jars (NOT the classes) for your dependent jars such as avro.

See http://exported.wordpress.com/2010/01/30/building-hadoop-job-jar-with-maven/

After running mvn assembly:assembly in your example directory I get a target/hadoop-example.jar file that's got Hadoop classes (and a bunch of others) all jammed inside it.

And your job jar shouldn't have Hadoop classes or jars inside it - those should be provided.

2. I would suggest using Hadoop 0.20.2 if you're on Cygwin.

That version avoids issues with Hadoop not being able to set permissions on local file system directories.


-- Ken

> Subject: Re: Hadoop 0.23, Avro Specific 1.6.3 and "org.apache.avro.generic.GenericData$Record cannot be cast to "
> Date: Sun, 13 May 2012 11:18:13 -0700
> Hi Jacob,
> On May 13, 2012, at 4:48am, Jacob Metcalf wrote:
> I have just spent several frustrating hours on getting an example MR job using Avro working with Hadoop and after finally getting it working I thought I would share my findings with everyone.
> I wrote an example job trying to use Avro MR 1.6.3 to serialize between Map and Reduce then attempted to deploy and run. I am setting up a development cluster with Hadoop 0.23 running pseudo-distributed under cygwin. I ran my job and it failed with:
> "org.apache.avro.generic.GenericData$Record cannot be cast to net.jacobmetcalf.avro.Room"
> Where Room is an Avro generated class. I found two problems. The first I have partly solved, the second one is more to do with Hadoop and is as yet unsolved:
> 1) Why when I am using Avro Specific does it end up going Generic?
> When deserializing SpecificDatumReader.java attempts to instantiate your target class through reflection. If it fails to create your class it defaults to a GenericData.Record. This Doug has explained here: http://mail-archives.apache.org/mod_mbox/avro-user/201101.mbox/%[EMAIL PROTECTED]%3E
> But why it is doing it was a little harder to work out. Debugging I saw the SpecificDatumReader could not find my class in its classpath. However in my Job Runner I had done:
> job.setJarByClass(HouseAssemblyJob.class); // This should ensure the JAR is distributed around the cluster
> I expected with this Hadoop would distribute my Jar around the cluster. It may be doing the distribution but it definitely did not add it to the Reducers classpath. So to get round this I have now set HADOOP_CLASSPATH to the directory I am running from. This is not going to work in a real cluster where the Job Runner is on a different machine to where the Reducer so I am keen to figure out whether the problem is Hadoop 0.23, my environment variables or the fact I am running under Cygwin.

Ken Krugler
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Hadoop, Cascading, Mahout & Solr