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我試圖實現不同模式：定製的可寫的Hadoop MapReduce的不同模式產生重複鍵
map(key, record):
emit record,null
reduce(key, records):
emit key
我的鑰匙是一個複雜的，定製Writable。如果我發出減少鍵及其散列碼：我收到以下輸出
context.write(key, new IntWtitable(key.hashCode());
：
key1 -1808937256
key2 -768063202
key3 906064410
key2 -768063202
key3 906064410
從理論上講，輸出應該只包含key1，key2和key3因爲我使用HashPartitioner：具有相同散列碼的密鑰被組合到相同的分區中。這顯然不是這種情況。
如果我將我的複雜Writable成Text對象（並相應地調整映射/減速類），並在Mapper發出：
context.write(new Text(key.toString()), NullWritable.get());
...如預期的輸出：
key1 1013632023
key2 762485389
key3 -1193948769
好吧，這裏是一個最小的工作示例，說明行爲。
輸入：
A A A A A
B B B B B
C C C C C
A A A A A
B B B B B
的MapReduce工作：
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.ArrayWritable;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;
public class DistinctPattern extends Configured implements Tool {
public static class DistinctMapper extends Mapper<Object, Text, ComplexObject, NullWritable> {
public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
ComplexObject o = new ComplexObject(value.toString());
context.write(o, NullWritable.get());
}
}
public static class DistinctReducer extends Reducer<ComplexObject, NullWritable, ComplexObject, IntWritable> {
public void reduce(ComplexObject key, Iterable<NullWritable> values, Context context)
throws IOException, InterruptedException {
context.write(key, new IntWritable(key.hashCode()));
}
}
public static class MyArrayWritable extends ArrayWritable {
public MyArrayWritable(Writable[] values) {
super(DatumObject.class, values);
}
public MyArrayWritable() {
super(DatumObject.class);
}
@Override
public String toString() {
return Arrays.toString(get());
}
}
public static class DatumObject implements Writable {
private String datum;
public DatumObject() {}
public DatumObject(String d) {
datum = d;
}
@Override
public void readFields(DataInput in) throws IOException {
datum = in.readUTF();
}
@Override
public void write(DataOutput out) throws IOException {
out.writeUTF(datum);
}
@Override
public String toString() {
return datum;
}
@Override
public int hashCode() {
return 31 * datum.hashCode();
}
}
public static class ComplexObject implements WritableComparable<ComplexObject> {
private List<DatumObject> data = new ArrayList<>();
public ComplexObject() {}
public ComplexObject(String d) {
String[] elements = d.split(" ");
for(int i = 0; i < elements.length; i++)
data.add(new DatumObject(elements[i]));
}
public int size() {
return data.size();
}
@Override
public void readFields(DataInput in) throws IOException {
data.clear();
MyArrayWritable m = new MyArrayWritable();
m.readFields(in);
Writable[] w = m.get();
for(int i = 0; i < w.length; i++)
data.add((DatumObject) w[i]);
}
@Override
public void write(DataOutput out) throws IOException {
MyArrayWritable m = new MyArrayWritable(data.toArray(new DatumObject[data.size()]));
m.write(out);
}
@Override
public int compareTo(ComplexObject o) {
if(this.equals(o))
return 0;
if(o.size() < this.size())
return -1;
return 1;
}
@Override
public boolean equals(Object obj) {
if(!(obj instanceof ComplexObject))
return false;
ComplexObject other = (ComplexObject) obj;
return other.data.equals(data);
}
@Override
public int hashCode() {
return 31 * data.hashCode();
}
@Override
public String toString() {
StringBuilder s= new StringBuilder();
data.forEach(entry -> {
s.append(entry);
s.append(" ");
});
return s.toString();
}
}
@Override
public int run(String[] args) throws Exception {
Job job = Job.getInstance();
job.setJar("distinct.jar");
job.setJarByClass(DistinctPattern.class);
job.setMapperClass(DistinctMapper.class);
job.setReducerClass(DistinctReducer.class);
job.setMapOutputKeyClass(ComplexObject.class);
job.setMapOutputValueClass(NullWritable.class);
job.setOutputKeyClass(ComplexObject.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(args[0]));
FileOutputFormat.setOutputPath(job, new Path(args[1]));
return job.waitForCompletion(true) ? 0 : 1;
}
public static void main(String[] args) throws Exception {
int exitCode = ToolRunner.run(new DistinctPattern(), args);
System.exit(exitCode);
}
}
預期輸出：
A A A A A 368623362
B B B B B 1285710467
C C C C C -2092169724
實際輸出：
A A A A A 368623362
B B B B B 1285710467
C C C C C -2092169724
A A A A A 368623362
B B B B B 1285710467
我錯過了什麼？
PS：Hadoop的2.7.3