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--- courses:mapreduce-tutorial:step-24 [2012/01/27 20:40]
straka
+++ courses:mapreduce-tutorial:step-24 [2012/01/27 21:52]
straka
@@ Line 1: / Line 1: @@
 ====== MapReduce Tutorial : Mappers, running Java Hadoop jobs ======
-We start by exploring a simple Hadoop job with Mapper only. The Mapper outputs only keys starting with ''A''.
+We start by going through a simple Hadoop job with Mapper only.
+A mapper which processes (key, value) pairs of types (Kin, Vin) and produces (key, value) pairs of types (Kout, Vout) must be a subclass of [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/Mapper.html|Mapper<Kin, Vin, Kout, Vout>]]. In our case, the mapper is subclass of ''Mapper<Text, Text, Text, Text>''.
+The mapper must define a ''map'' method and may provide ''setup'' and ''context'' method:
+<code java>
+  public static class TheMapper extends Mapper<Text, Text, Text, Text>{
+    public void setup(Context context) throws IOException, InterruptedException {}
+    public void map(Text key, Text value, Context context) throws IOException, InterruptedException {}
+    public void cleanup(Context context) throws IOException, InterruptedException {}
+  }
+</code>
+Outputting (key, value) pairs is performed using the [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/MapContext.html|MapContext<Kin, Vin, Kout, Vout>]] object (the ''Context'' is an abbreviation for this type), with the method ''context.write(Kout key, Vout value)''.
+Here is the source of the whole Hadoop job:
+<file java MapperOnlyHadoopJob.java>
+import java.io.IOException;
+import org.apache.hadoop.conf.*;
+import org.apache.hadoop.fs.Path;
+import org.apache.hadoop.io.*;
+import org.apache.hadoop.mapreduce.*;
+import org.apache.hadoop.mapreduce.lib.input.*;
+import org.apache.hadoop.mapreduce.lib.output.*;
+import org.apache.hadoop.util.*;
+public class MapperOnlyHadoopJob extends Configured implements Tool {
+  // Mapper
+  public static class TheMapper extends Mapper<Text, Text, Text, Text>{
+    public void setup(Context context) {
+    }
+    public void map(Text key, Text value, Context context) throws IOException, InterruptedException {
+      if (key.getLength() > 0 && Character.toUpperCase(key.charAt(0)) == 'A') {
+        context.write(key, value);
+      }
+    }
+    public void cleanup(Context context) {
+    }
+  }
+  // Job configuration
+  public int run(String[] args) throws Exception {
+    if (args.length < 2) {
+      System.err.printf("Usage: %s.jar in-path out-path", this.getClass().getName());
+      return 1;
+    }
+    Job job = new Job(getConf(), this.getClass().getName());    // Create class representing Hadoop job.
+    job.setJarByClass(this.getClass());                         // Use jar containing current class.
+    job.setMapperClass(TheMapper.class);                        // The mapper of the job.
+    job.setOutputKeyClass(Text.class);                          // Type of the output keys.
+    job.setOutputValueClass(Text.class);                        // Type of the output values.
+    job.setInputFormatClass(KeyValueTextInputFormat.class);     // Input format.
+                                                                // Output format is the default -- TextOutputFormat
+    FileInputFormat.addInputPath(job, new Path(args[0]));       // Input path is on command line.
+    FileOutputFormat.setOutputPath(job, new Path(args[1]));     // Output path is on command line too.
+    return job.waitForCompletion(true) ? 0 : 1;
+  }
+  // Main method
+  public static void main(String[] args) throws Exception {
+    int res = ToolRunner.run(new MapperOnlyHadoopJob(), args);
+    System.exit(res);
+  }
+}
+</file>
+===== Running the job =====
+Download the source and compile it.
+The official way of running Hadoop jobs is to use the ''/SGE/HADOOP/active/bin/hadoop'' script. Jobs submitted through this script can be configured using Hadoop properties only. Therefore a wrapper script is provided, with similar options as the Perl API runner:
+  * ''net/projects/hadoop/bin/hadoop [-r number_of_reducers] job.jar [generic Hadoop properties] input_path output_path'' -- executes the given job locally in a single thread. It is useful for debugging.
+  * ''net/projects/hadoop/bin/hadoop -jt cluster_master [-r number_of_reducers] job.jar [generic Hadoop properties] input_path output_path'' -- submits the job to given ''cluster_master''.
+  * ''net/projects/hadoop/bin/hadoop -c number_of_machines [-w secs_to_wait_after_job_finishes] [-r number_of_reducers] job.jar [generic Hadoop properties] input_path output_path'' -- creates a new cluster with specified number of machines, which executes given job, and then waits for specified number of seconds before it stops.

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