Avro
You are right. A minor correction, you can't use new NullWritable() and
create an object. NullWritable is immutable and Singleton unlike the other
Writables in hadoop. When you nedd to use NullWritable instance you can give
NullWritable.get(), which would do the job.
Ie
output.collect ( NullWritable.get(), new Text(output_string) );
Regards
Bejoy K S
-----Original Message-----
From: Arko Provo Mukherjee <arkoprovomukher...@gmail.com>
Date: Thu, 15 Sep 2011 00:16:27
To: <mapreduce-user@hadoop.apache.org>
Reply-To: mapreduce-user@hadoop.apache.org
Subject: Re: Reducers without output files
Hello,
Many thanks for your reply!
So to clarify, I should do the following:
public static class Reduce extends MapReduceBase implements
Reducer<IntWritable, Text, NullWritable, Text> {
reduce () { // Pseudo reduce funtion - ignoring the proper syntax
// The processing goes here.
output.collect ( new NullWritable(), new Text(output_string) );
}
}
Finally in the main method of the Driver Class:
// For the Map Class
jobconf.setMapOutputKeyClass(IntWritable.class);
jobconf.setMapOutputValueClass(Text.class);
// For the Reduce Class
jobconf.setOutputKeyClass(NullWritable.class);
jobconf.setOutputValueClass(Text.class);
Please do correct me if my understanding is wrong.
Thanks again for your help!
Warm Regards
Arko
On Wed, Sep 14, 2011 at 11:12 PM, <bejoy.had...@gmail.com> wrote:
> Akro
> To add on, if you want to ignore the key part then substitute key with
> NullWritable. And do the desired modification in driver class in place of
> output key type.
>
> Hope it helps
> Regards
> Bejoy K S
>
> -----Original Message-----
> From: bejoy.had...@gmail.com
> Date: Thu, 15 Sep 2011 04:09:12
> To: <mapreduce-user@hadoop.apache.org>
> Reply-To: bejoy.had...@gmail.com
> Subject: Re: Reducers without output files
>
> Hi Akro
> You can achieve the same within the existing mapreduce frame work
> itself. Give a NullWritable in place of reducer output value in reduce
> function. In your driver class as well mention the output value type as
> NullWritable.
>
> ------Original Message------
> From: Arko Provo Mukherjee
> To: mapreduce-user@hadoop.apache.org
> ReplyTo: mapreduce-user@hadoop.apache.org
> Subject: Reducers without output files
> Sent: Sep 15, 2011 08:56
>
> Hello Everyone,
>
> I have a small issue with my Reducer that I am trying to figure out
> and wanted some advice.
>
> In the reducer, when writing to the output file as declared in
> FileOutputFormat.setOutputPath() I want to write only the key and not
> the value when I am calling output.collect().
>
> Is there a way I can ignore the key part?
>
> Else,
>
> Can I write a Reducer function that doesn't do a output.collect()??
>
> Say I omit the FileOutputFormat.setOutputPath() in the Driver Class.
>
> I can then manually write the output to HDFS in the format I like.
>
> Is this legal way to do stuff?
>
> Many thanks in advance!
> Warm Regards
> Arko
>
>
> Regards
> Bejoy K S
