Hadoop Facebook使用Mapreduce的共同朋友

Question

我正在嘗試使用hadoop（Java版）中的mapreduce程序，以從json文件中找到共同的朋友列表。 json文件內容具有以下模式：

{"name":"abc","id":123} [{"name":"xyz","id":124},{"name":"def","id":125},{"name":"cxf","id":155}]
{"name":"cxf","id":155} [{"name":"xyz","id":124},{"name":"abc","id":123},{"name":"yyy","id":129}]

模式解釋如下：

朋友json標簽，由相關朋友json的數組分隔

因此abc擁有xyz，def和cxf作為朋友cxf擁有xyz abc和yyy作為朋友。

鑒於以上所述，abc和cxf之間的共同朋友是xyz。

嘗試通過創建自定義可寫對象使用mapreduce來實現相同的功能，而映射器發出以下鍵值，鍵是一對朋友，而值是該鍵中第一個朋友（即一對朋友）的相關朋友

K->V
(abc,xyz) -> [xyz,def,cxf]
(abc,def) -> [xyz,def,cxf]
(abc,cxf) -> [xyz,def,cxf]
(cxf,xyz) -> [xyz,abc,yyy]
(cxf,abc) -> [xyz,abc,yyy]
(cxf,yyy) -> [xyz,abc,yyy]

這里的鍵實際上是Custom可寫的，創建了一個擴展WritableComparable的類，並且我重寫了compareTo方法，以使這兩個對（a，b）和（b，a）相同。 但是我面臨的問題是，並非對所有組合都調用compareTo方法，因此reducer邏輯失敗了。

根據以上示例，映射器發出了6 K，V對。 但是compareTo僅被調用5次key1.compareTo（key2），key2.compareTo（key3），key3.compareTo（key4），key4.compareTo（key5），key5.compareTo（key6）。

知道為什么會這樣嗎？

以下是根據f11ler建議的邏輯的代碼

駕駛員等級：

package com.facebook.updated;

import java.io.IOException;

import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.lib.output.SequenceFileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
import org.apache.log4j.Logger;

public class FacebookMain extends Configured implements Tool

{

    Logger logger = Logger.getLogger(FacebookMain.class);

    public static void main(String[] args) throws Exception {
        System.exit(ToolRunner.run(new FacebookMain(), args));

    }

    @Override
    public int run(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
        logger.info("Running======>");
        Job job = Job.getInstance();

        job.setJarByClass(FacebookMain.class);
        job.setJobName("FBApp");

        job.setMapOutputKeyClass(Friend.class);
        job.setMapOutputValueClass(Friend.class);

        job.setOutputKeyClass(FriendPair.class);
        job.setOutputValueClass(Friend.class);

        job.setMapperClass(FacebookMapper.class);
        job.setReducerClass(FacebookReducer.class);

        job.setInputFormatClass(org.apache.hadoop.mapreduce.lib.input.TextInputFormat.class);
        job.setOutputFormatClass(SequenceFileOutputFormat.class);

        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        boolean val = job.waitForCompletion(true);

        return val ? 0 : 1;

    }

}

customWritables（用於表示朋友和對子）

package com.facebook.updated;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import lombok.Getter;
import lombok.Setter;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;
import org.apache.log4j.Logger;

@Getter
@Setter
public class Friend implements WritableComparable<Friend> {

    Logger logger = Logger.getLogger(Friend.class);

    private IntWritable id;
    private Text name;

    public Friend() {
        this.id = new IntWritable();
        this.name = new Text();
    }

    @Override
    public int compareTo(Friend arg0) {
        int val = getId().compareTo(arg0.getId());
        logger.info("compareTo Friend ======> " + arg0 + " and " + this + " compare is " + val);
        return val;
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        id.readFields(in);
        name.readFields(in);
    }

    @Override
    public void write(DataOutput out) throws IOException {
        id.write(out);
        name.write(out);
    }

    @Override
    public boolean equals(Object obj) {
        Friend f2 = (Friend) obj;
        boolean val = this.getId().equals(f2.getId());
        //logger.info("equals Friend ======> " + obj + " and " + this);
        return val;
    }

    @Override
    public String toString() {
        return id + ":" + name + " ";
    }
}

package com.facebook.updated;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

import lombok.Getter;
import lombok.Setter;

import org.apache.hadoop.io.WritableComparable;
import org.apache.log4j.Logger;

@Getter
@Setter
public class FriendPair implements WritableComparable<FriendPair> {

    Logger logger = Logger.getLogger(FriendPair.class);

    private Friend first;
    private Friend second;

    public FriendPair() {
        this.first = new Friend();
        this.second = new Friend();
    }

    public FriendPair(Friend f1, Friend f2) {
        this.first = f1;
        this.second = f2;
    }

    @Override
    public int compareTo(FriendPair o) {

        logger.info("compareTo FriendPair ======> " + o + " and " + this);
        FriendPair pair2 = o;
        int cmp = -1;

        if (getFirst().compareTo(pair2.getFirst()) == 0 || getFirst().compareTo(pair2.getSecond()) == 0) {
            cmp = 0;
        }
        if (cmp != 0) {
            // logger.info("compareTo FriendPair ======> " + o + " and " + this
            // + " comparison is " + cmp);
            return cmp;
        }
        cmp = -1;
        if (getSecond().compareTo(pair2.getFirst()) == 0 || getSecond().compareTo(pair2.getSecond()) == 0) {
            cmp = 0;
        }

        // logger.info("compareTo FriendPair ======> " + o + " and " + this +
        // " comparison is " + cmp);

        // logger.info("getFirst() " + getFirst());
        // logger.info("pair2.getFirst() " + pair2.getFirst());
        // logger.info("getFirst().compareTo(pair2.getFirst()) " +
        // getFirst().compareTo(pair2.getFirst()));
        // logger.info("getFirst().compareTo(pair2.getSecond()) " +
        // getFirst().compareTo(pair2.getSecond()));
        // logger.info("getSecond().compareTo(pair2.getFirst()) " +
        // getSecond().compareTo(pair2.getFirst()));
        // logger.info("getSecond().compareTo(pair2.getSecond()) " +
        // getSecond().compareTo(pair2.getSecond()));
        // logger.info("pair2.getSecond() " + pair2.getSecond());
        // logger.info("getSecond() " + getSecond());
        // logger.info("pair2.getFirst() " + pair2.getFirst());
        // logger.info("pair2.getSecond() " + pair2.getSecond());

        return cmp;
    }

    @Override
    public boolean equals(Object obj) {

        FriendPair pair1 = this;
        FriendPair pair2 = (FriendPair) obj;

        boolean eq = false;

        logger.info("equals FriendPair ======> " + obj + " and " + this);

        if (pair1.getFirst().equals(pair2.getFirst()) || pair1.getFirst().equals(pair2.getSecond()))
            eq = true;

        if (!eq) {
            // logger.info("equals FriendPair ======> " + obj + " and " + this +
            // " equality is " + eq);
            return false;
        }
        if (pair1.getSecond().equals(pair2.getFirst()) || pair1.getSecond().equals(pair2.getSecond()))
            eq = true;

        // logger.info("equals FriendPair ======> " + obj + " and " + this +
        // " equality is " + eq);

        return eq;
    }

    @Override
    public void readFields(DataInput in) throws IOException {
        first.readFields(in);
        second.readFields(in);
    }

    @Override
    public void write(DataOutput out) throws IOException {
        first.write(out);
        second.write(out);
    }

    @Override
    public String toString() {
        return "[" + first + ";" + second + "]";
    }

    @Override
    public int hashCode() {
        logger.info("hashCode FriendPair ======> " + this);
        return first.getId().hashCode() + second.getId().hashCode();
    }
}

映射器和減速器

package com.facebook.updated;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.log4j.Logger;

import com.mongodb.BasicDBList;
import com.mongodb.BasicDBObject;
import com.mongodb.util.JSON;

public class FacebookMapper extends Mapper<LongWritable, Text, Friend, Friend> {

    Logger log = Logger.getLogger(FacebookMapper.class);

    @Override
    protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, Friend, Friend>.Context context)
            throws IOException, InterruptedException {

        String line = value.toString();
        StringTokenizer st = new StringTokenizer(line, "\t");
        String person = st.nextToken();
        String friends = st.nextToken();

        BasicDBObject personObj = (BasicDBObject) JSON.parse(person);
        BasicDBList friendsList = (BasicDBList) JSON.parse(friends);

        List<Friend> frndJavaList = new ArrayList<>();

        for (Object frndObj : friendsList) {
            frndJavaList.add(getFriend((BasicDBObject) frndObj));
        }

        Friend frnd = getFriend(personObj);
        Friend[] array = frndJavaList.toArray(new Friend[frndJavaList.size()]);
        for (Friend f : array) {
            log.info("Map output is " + f + " and " + frnd);
            context.write(f, frnd);
        }
    }

    private static Friend getFriend(BasicDBObject personObj) {
        Friend frnd = new Friend();
        frnd.setId(new IntWritable(personObj.getInt("id")));
        frnd.setName(new Text(personObj.getString("name")));
        frnd.setHomeTown(new Text(personObj.getString("homeTown")));
        return frnd;
    }
}

package com.facebook.updated;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.hadoop.mapreduce.Reducer;
import org.apache.log4j.Logger;

public class FacebookReducer extends Reducer<Friend, Friend, FriendPair, Friend> {

    Logger log = Logger.getLogger(FacebookReducer.class);

    @Override
    protected void reduce(Friend friend, Iterable<Friend> vals,
            Reducer<Friend, Friend, FriendPair, Friend>.Context context) throws IOException, InterruptedException {
        List<Friend> friends = new ArrayList<>();
        for (Friend frnd : vals) {
            friends.add(frnd);
        }
        log.info("Reducer output is " + friend + " and values are " + friends);
        if (friends.size() == 2) {
            FriendPair key = new FriendPair(friends.get(0), friends.get(1));
            context.write(key, friend);
        } else {
            //log.info("Size of friends is not 2 key is " + friend + " and values are " + friends);
        }

    }
}

輸入包含2行的json文件

{"name":"abc","id":123} [{"name":"xyz","id":124},{"name":"def","id":125},{"name":"cxf","id":155}]
{"name":"cxf","id":155} [{"name":"xyz","id":124},{"name":"abc","id":123},{"name":"yyy","id":129}]

減速器的輸出 （abc，abc）-> xyz

Answer 1

compareTo方法是排序所必需的，此關系應該是可傳遞的。 這意味着如果a> b且b> c，則a> c。 可能這對您的實現不正確。

為什么在mapper中生成此類記錄？ 如果“成為朋友”是對稱關系，則可以使用此邏輯（偽代碼）簡單地執行僅映射器的工作：

for(int i = 0; i < values.length; ++i)
    for(int j = 0; j < values.length; ++j)
        if (i ==j)
            continue
        emmit (values[i], values[j]), key

更新：如果這不是對稱的（這意味着“ xyz有朋友abc”不跟在“ abc有朋友xyz”之后），那么我們需要反向記錄：

映射器：

for(int i = 0; i < values.length; ++i)
    emmit values[i], key

減速器（與之前的映射器相同）：

for(int i = 0; i < values.length; ++i)
    for(int j = 0; j < values.length; ++j)
        if (i ==j)
            continue
        emmit (values[i], values[j]), key

更新2：

讓我們看看該算法如何與您的示例配合使用：

映射器的結果：

xyz -> abc
def -> abc
cxf -> abc
xyz -> cxf
abc -> cxf
yyy -> cxf

Mapreduce將按鍵將此值分組，所以化簡器的輸入為：

xyz -> [abc,cxf]
def -> [abc]
cxf -> [abc]
abc -> [cxf]
yyy -> [cxf]

在reducer中，我們按值進行嵌套循環，但是跳過與self的比較。 結果：

(abc, cxf) -> xyz

這就是我們想要得到的。