Mailing-List: contact core-dev-help@hadoop.apache.org; run by ezmlm
Precedence: bulk
Reply-To: core-dev@hadoop.apache.org
Message-ID: <1069476496.1207194744510.JavaMail.jira@brutus>
Date: Wed, 2 Apr 2008 20:52:24 -0700 (PDT)
From: "Alejandro Abdelnur (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-3149) supporting multiple outputs for M/R
 jobs
In-Reply-To: <1166162754.1207040907546.JavaMail.jira@brutus>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


    [ https://issues.apache.org/jira/browse/HADOOP-3149?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12584975#action_12584975 ] 

Alejandro Abdelnur commented on HADOOP-3149:
--------------------------------------------

findbugs, fixed one, the second one  IS is a false positive (explained that in previous comment)

fixing checksytle warnings in next version of patch.

* It is not always desirable the OutputFormat/Key/Value classes of all outputs to be the same, we have several use cases where they are different (I've point this as a limitation of the Runping's patch).
* Limiting the names to [a-zA-Z0-9] has a purpose, the names are used to create the files under the output dir, you don't want to get funny characters in the leafname.

Now  I understand what you have in mind with the KeyValue object and not having MultipleOutput* classes. I have the following comments on that:

# first bullet item above.
# By piggybacking on the current collector to write to multiple outputs, while you avoid introducing new classes, is not obvious and I would say confusing for the developer, I'd rather have explicit methods/classes for handling multiple outputs.

I would rather refactor code to:

* get rid of the {{MultipleOutput Task/Map/Reduce/Collector}} classes.
* Don't introduce a {{KeyValue}} class.

Add a {{ collect(String namedOutput, WritableComparable key, Writable value) }} method to the MultipleOutputs and the usage pattern would be

{{noformat}}
public class MyReducer implements Reducer {
  private MultipleOutputs mos;

  public void configure(JobConf conf) {
    mos = new MultipleOutputs(conf);
  }

  public void reduce(WritableComparable key, Iterator<Writable> values, OutputCollector collector, Reporter reporter) throws IOException {
    Writable value = values.next();
    collector.collect(key,value);
    mos.collect("aa", key, value);
  }

  public void close() throws IOException {
    mos.close();
  }

}
{{noformat}}

configuration of the job prior to dispatching would remain the same.


> supporting multiple outputs for M/R jobs
> ----------------------------------------
>
>                 Key: HADOOP-3149
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3149
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>         Environment: all
>            Reporter: Alejandro Abdelnur
>            Assignee: Alejandro Abdelnur
>             Fix For: 0.17.0
>
>         Attachments: patch3149.txt, patch3149.txt, patch3149.txt
>
>
> The outputcollector supports writing data to a single output, the 'part' files in the output path.
> We found quite common that our M/R jobs have to write data to different output. For example when classifying data as NEW, UPDATE, DELETE, NO-CHANGE to later do different processing on it.
> Handling the initialization of additional outputs from within the M/R code complicates the code and is counter intuitive with the notion of job configuration.
> It would be desirable to:
> # Configure the additional outputs in the jobconf, potentially specifying different outputformats, key and value classes for each one.
> # Write to the additional outputs in a similar way as data is written to the outputcollector.
> # Support the speculative execution semantics for the output files, only visible in the final output for promoted tasks.
> To support multiple outputs the following classes would be added to mapred/lib:
> * {{MOJobConf}} : extends {{JobConf}} adding methods to define named outputs (name, outputformat, key class, value class)
> * {{MOOutputCollector}} : extends {{OutputCollector}} adding a {{collect(String outputName, WritableComparable key, Writable value)}} method.
> * {{MOMapper}} and {{MOReducer}}: implement {{Mapper}} and {{Reducer}} adding a new {{configure}}, {{map}} and {{reduce}} signature that take the corresponding {{MO}} classes and performs the proper initialization.
> The data flow behavior would be: key/values written to the default (unnamed) output (using the original OutputCollector {{collect}} signature) take part of the shuffle/sort/reduce processing phases. key/values written to a named output from within a map don't.
> The named output files would be named using the task type and task ID to avoid collision among tasks (i.e. 'new-m-00002' and 'new-r-00001').
> Together with the setInputPathFilter feature introduced by HADOOP-2055 it would become very easy to chain jobs working on particular named outputs within a single directory.
> We are using heavily this pattern and it greatly simplified our M/R code as well as chaining different M/R. 
> We wanted to contribute this back to Hadoop as we think is a generic feature many could benefit from.

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.