BE THE CODER > No SQL > Mongo DB Examples > How to execute Map and Reduce Algorithm

MongoDB is a scalable, high-performance, open source NoSQL database. It offers Ad hoc queries, Indexing, Replication, Load balancing, File storage (GridFS), Aggregation, MapReduce and Server-side JavaScript execution. This requires the library mongo-2.8.0.jar to be present in the classpath. The following example shows how to execute map reduce algorithm in MongoDB.

File Name :	com/bethecoder/tutorials/mongodb/tests/MapReduce.java
Author :	Sudhakar KV
Email :	[email protected]


package com.bethecoder.tutorials.mongodb.tests;



import java.net.UnknownHostException;



import com.mongodb.DB;

import com.mongodb.DBCollection;

import com.mongodb.DBCursor;

import com.mongodb.DBObject;

import com.mongodb.MapReduceCommand;

import com.mongodb.MapReduceOutput;

import com.mongodb.Mongo;

import com.mongodb.MongoException;

/*

 

things - collection



Doc : { "_id" : 1.0 , "tags" : [ "dog" , "cat"]}

Doc : { "_id" : 2.0 , "tags" : [ "cat"]}

Doc : { "_id" : 3.0 , "tags" : [ "mouse" , "cat" , "dog"]}

Doc : { "_id" : 4.0 , "tags" : [ ]}



*/

public class MapReduce {



  /**

   * @param args

   * @throws MongoException 

   * @throws UnknownHostException 

   */

  public static void main(String[] args) throws UnknownHostException, MongoException {

    

    //Connect to mongoDB with given IP and PORT

    Mongo mongo = new Mongo("localhost", 27017);

    

    //Get the database object

    DB db = mongo.getDB("mydb");



    //Get a single collection

    DBCollection collection = db.getCollection("things");

    System.out.println(collection.toString());

    

    // get all available documents

    DBCursor cursor = collection.find();

    while (cursor.hasNext()) {

      System.out.println("Doc : " + cursor.next());

    }

    

    ////////////// MAP & REDUCE ALGORITHM ////////////////

    

    /**

     * STEP 1

     */

    String map = "function(){" +

             "this.tags.forEach(" +

               "function(z){" +

                 "emit( z , { count : 1 } );" +

               "}" +

             ");" +

           "};";

    

    /**

     * STEP 2

     */

    String reduce = "function( key , values ){" +

              "var total = 0;" +

              

              "for ( var i=0; i<values.length; i++ )" +

                "total += values[i].count;" +

              

              "return { count : total };" +

            "};";

    

    

    // Execute the map reduce function

    MapReduceCommand mr = new MapReduceCommand(

        collection, map, reduce, null, 

        MapReduceCommand.OutputType.INLINE, null);



    // Execute the map reduce

    MapReduceOutput out = collection.mapReduce(mr);



    for ( DBObject obj : out.results() ) {

        System.out.println( obj );

    }

  }



}