Map side join using Distributed cache in Hadoop

     You want to populate an associative array in order to perform a map-side join. You’ve decided to put this information in a text file, place that file into the DistributedCache and read it in your Mapper before any records are processed. Indentify which method in the Mapper you should use to implement code for reading the file and populating the associative array?

Answer is, Configure method used inside Mapper method.

Explanation: 
See 3) below. Here is an illustrative example on how to use the DistributedCache: 
// Setting up the cache for the application 
1. Copy the requisite files to the FileSystem: 
$ bin/hadoop fs -copyFromLocal lookup.dat /myapp/lookup.dat 
$ bin/hadoop fs -copyFromLocal map.zip /myapp/map.zip 
$ bin/hadoop fs -copyFromLocal mylib.jar /myapp/mylib.jar 
$ bin/hadoop fs -copyFromLocal mytar.tar /myapp/mytar.tar 
$ bin/hadoop fs -copyFromLocal mytgz.tgz /myapp/mytgz.tgz 
$ bin/hadoop fs -copyFromLocal mytargz.tar.gz /myapp/mytargz.tar.gz 

2. Setup the application's JobConf: 
JobConf job = new JobConf(); 
DistributedCache.addCacheFile(new URI("/myapp/lookup.dat#lookup.dat"), job); 
DistributedCache.addCacheArchive(new URI("/myapp/map.zip", job); 
DistributedCache.addFileToClassPath(new Path("/myapp/mylib.jar"), job); 
DistributedCache.addCacheArchive(new URI("/myapp/mytar.tar", job); 
DistributedCache.addCacheArchive(new URI("/myapp/mytgz.tgz", job); 
DistributedCache.addCacheArchive(new URI("/myapp/mytargz.tar.gz", job); 

3. Use the cached files in theMapper orReducer: public static class MapClass 
extends MapReduceBase implements Mapper { 
private Path[] localArchives; 
private Path[] localFiles; 
public void configure(JobConf job) { 
// Get the cached archives/files 
localArchives = DistributedCache.getLocalCacheArchives(job); 
localFiles = DistributedCache.getLocalCacheFiles(job); 
} 
public void map(K key, V value, OutputCollector output, Reporter reporter) 
throws IOException { 
// Use data from the cached archives/files here 
// ... 
// ... output.collect(k, v); 
} 

}