快速學習mongodb的mapreduce例子

map和reduce是十分有用的操作,特別是在NOSQL中.本文簡單小結下
在mongodb中對mapreduce的操作,以及在JAVA中如何操作.

1 啟動mongodb
   mongo啟動即可

2 建立db
   use test

3 加點記錄
   > book1 = {name : "Understanding JAVA", pages : 100}
> book2 = {name : "Understanding JSON", pages : 200}
   > db.books.save(book1)
> db.books.save(book2)
  繼續加
  > 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  先來做MAP,這里是先歸類,按頁數去劃分分類,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來統計個數

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

6 然后再查看下,結果顯示為:
  > 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 換用JAVA去實現之,注意下載mongodb的驅動,代碼如下:

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();
}
}
}</pre>