快速例子学习mongodb的mapreduce

快速例子学习mongodb的mapreduce

博客分类： 
• 数据库相关/oracle

mongodbjavamongomapreduce

map和reduce是十分有用的操作,特别是在NOSQL中.本文简单小结下 
在mongodb中对mapreduce的操作,以及在JAVA中如何操作. 

1 启动mongodb 
   mongo启动即可 

2 建立db 
   use test 

3 加点记录 
   > book1 = {name : "Understanding JAVA", pages : 100} 
> book2 = {name : "Understanding JSON", pages : 200} 
   > db.books.save(book1) 
> db.books.save(book2) 
  继续加 
  > book = {name : "Understanding XML", pages : 300} 
> db.books.save(book) 
> book = {name : "Understanding Web Services", pages : 400} 
> db.books.save(book) 
> book = {name : "Understanding Axis2", pages : 150} 
> db.books.save(book) 

4  先来做MAP,这里是先归类,按页数去划分分类,如下: 
  

> var map = function() {
var category;
if ( this.pages >= 250 )
category = 'Big Books';
else
category = "Small Books";
emit(category, {name: this.name});
};

5 然后再按reduce来统计个数 
 

> var reduce = function(key, values) {
var sum = 0;
values.forEach(function(doc) {
sum += 1;
});
return {books: sum};
};

6 然后再查看下,结果显示为: 
  > var count  = db.books.mapReduce(map, reduce, {out: "book_results"}); 
> db[count.result].find() 

{ "_id" : "Big Books", "value" : { "books" : 2 } } 
{ "_id" : "Small Books", "value" : { "books" : 3 } } 

7 换用JAVA去实现之,注意下载mongodb的驱动,代码如下: 

import com.mongodb.BasicDBObject;
import com.mongodb.DB;
import com.mongodb.DBCollection;
import com.mongodb.DBObject;
import com.mongodb.MapReduceCommand;
import com.mongodb.MapReduceOutput;
import com.mongodb.Mongo;

public class MongoClient {

 /**
  * @param args
  */
 public static void main(String[] args) {

  Mongo mongo;

  try {
   mongo = new Mongo("localhost", 27017);
   DB db = mongo.getDB("library");

   DBCollection books = db.getCollection("books");

   BasicDBObject book = new BasicDBObject();
   book.put("name", "Understanding JAVA");
   book.put("pages", 100);
   books.insert(book);

   book = new BasicDBObject();
   book.put("name", "Understanding JSON");
   book.put("pages", 200);
   books.insert(book);

   book = new BasicDBObject();
   book.put("name", "Understanding XML");
   book.put("pages", 300);
   books.insert(book);

   book = new BasicDBObject();
   book.put("name", "Understanding Web Services");
   book.put("pages", 400);
   books.insert(book);

   book = new BasicDBObject();
   book.put("name", "Understanding Axis2");
   book.put("pages", 150);
   books.insert(book);

   String map = "function() { "+
             "var category; " +
             "if ( this.pages >= 250 ) "+
             "category = 'Big Books'; " +
             "else " +
             "category = 'Small Books'; "+
             "emit(category, {name: this.name});}";

   String reduce = "function(key, values) { " +
                            "var sum = 0; " +
                            "values.forEach(function(doc) { " +
                            "sum += 1; "+
                            "}); " +
                            "return {books: sum};} ";

   MapReduceCommand cmd = new MapReduceCommand(books, map, reduce,
     null, MapReduceCommand.OutputType.INLINE, null);

   MapReduceOutput out = books.mapReduce(cmd);

   for (DBObject o : out.results()) {
    System.out.println(o.toString());
   }
  } catch (Exception e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}