Meaning of context.getconfiguration in hadoop - hadoop

Question

I have this doubt in code of searching by argument.
what is the meaning of context.getConfiguration().get("Uid2Search");
package SearchTxnByArg;
// This is the Mapper Program for SearchTxnByArg
import java.io.IOException;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
public class MyMap extends Mapper<LongWritable, Text, NullWritable, Text>{
public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException {
String Txn = value.toString();
String TxnParts[] = Txn.split(",");
String Uid = TxnParts[2];
String Uid2Search = context.getConfiguration().get("Uid2Search");
if(Uid.equals(Uid2Search))
{
context.write(null, value);
}
}
}
Driver Program
package SearchTxnByArg;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class MyDriver {
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
conf.set("Uid2Search", args[0]);
Job job = new Job(conf, "Map Reduce Search Txn by Arg");
job.setJarByClass(MyDriver.class);
job.setMapperClass(MyMap.class);
job.setMapOutputKeyClass(NullWritable.class);
job.setMapOutputValueClass(Text.class);
job.setNumReduceTasks(0);
FileInputFormat.addInputPath(job, new Path(args[1]));
FileOutputFormat.setOutputPath(job, new Path(args[2]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}

Answer 1

I don't know how you have written your driver program. But in my experience,
If you are trying to get system property either by using -D option from the command line or by System.setproperty method by default these values will be set to context configuration.
As per documentation,
Configurations are specified by resources. A resource contains a set
of name/value pairs as XML data. Each resource is named by either a
String or by a Path. If named by a String, then the classpath is
examined for a file with that name. If named by a Path, then the local
filesystem is examined directly, without referring to the classpath.
Unless explicitly turned off, Hadoop by default specifies two
resources, loaded in-order from the classpath: core-default.xml :
Read-only defaults for hadoop. core-site.xml: Site-specific
configuration for a given hadoop installation. Applications may add
additional resources, which are loaded subsequent to these resources
in the order they are added.
Please see this answer as well
Context object: allows the Mapper/Reducer to interact with the rest of the Hadoop system. It includes configuration data for the job as well as interfaces which allow it to emit output.
Applications can use the Context:
to report progress
to set application-level status messages
update Counters
indicate they are alive
to get the values that are stored in job configuration across map/reduce phase.