I want to write a Hadoop MapReduce Join in Java - hadoop

Question

I'm completely new in Hadoop Framework and I want to write a "MapReduce" program (HadoopJoin.java) that joins on x attribute between two tables R and S. The structure of the two tables is :
R (tag : char, x : int, y : varchar(30))
and
S (tag : char, x : int, z : varchar(30))
For example we have for R table :
r 10 r-10-0
r 11 r-11-0
r 12 r-12-0
r 21 r-21-0
And for S table :
s 11 s-11-0
s 21 s-41-0
s 21 s-41-1
s 12 s-31-0
s 11 s-31-1
The result should look like :
r 11 r-11-0 s 11 s-11-0
etc.
Can anyone help me please ?

Answer 1

It will be very difficult to describe join in mapreduce for someone who is new to this Framework but here I provide a working implementation for your situation and I definitely recommend you to read section 9 of Hadoop The Definitive Guide 4th Eddition. It has described how to implement Join in mapreduce very well.
First of all you might consider using higher level frameworks such as Pig, Hive and Spark because they provide join operation in their core part of implementation.
Secondly There are many ways to implement mapreduce depending of the nature of your data. This ways include map-side join and reduce-side join. In this answer I have implemented the reduce-side join:
Implementation:
First of all we should have two different mapper for two different datset notice that in your case same mapper can be used for two dataset but in many situation you need different mappers for different dataset and because of that I have defined two mappers to make this solution more general:
I have used TextPair that have two attributes, one of them is the key that is used to join data and the other one is a tag that specify which dataset this record belongs to. If it belongs to the first dataset this tag will be 0. otherwise it will be 1.
I have implemented TextPair.FirstComparator to ensure that for each key(join by key) the record of the first dataset is the first key which is received by reducer. And all the other records in second dataset with that id are received after that. Actually this line of code will do the trick for us:
job.setGroupingComparatorClass(TextPair.FirstComparator.class);
So in reducer the first record that we will receive is the record from dataset1 and after that we receive record from dataset2. The only thing that should be done is that we have to write those records.
Mapper for dataset1:
public class JoinDataSet1Mapper
extends Mapper<LongWritable, Text, TextPair, Text> {
#Override
protected void map(LongWritable key, Text value, Context context)
throws IOException, InterruptedException {
String[] data = value.toString().split(" ");
context.write(new TextPair(data[1], "0"), value);
}
}
Mapper for DataSet2:
public class JoinDataSet2Mapper
extends Mapper<LongWritable, Text, TextPair, Text> {
#Override
protected void map(LongWritable key, Text value, Context context)
throws IOException, InterruptedException {
String[] data = value.toString().split(" ");
context.write(new TextPair(data[1], "1"), value);
}
}
Reducer:
public class JoinReducer extends Reducer<TextPair, Text, NullWritable, Text> {
public static class KeyPartitioner extends Partitioner<TextPair, Text> {
#Override
public int getPartition(TextPair key, Text value, int numPartitions) {
return (key.getFirst().hashCode() & Integer.MAX_VALUE) % numPartitions;
}
}
#Override
protected void reduce(TextPair key, Iterable<Text> values, Context context)
throws IOException, InterruptedException {
Iterator<Text> iter = values.iterator();
Text stationName = new Text(iter.next());
while (iter.hasNext()) {
Text record = iter.next();
Text outValue = new Text(stationName.toString() + "\t" + record.toString());
context.write(NullWritable.get(), outValue);
}
}
}
Custom key:
public class TextPair implements WritableComparable<TextPair> {
private Text first;
private Text second;
public TextPair() {
set(new Text(), new Text());
}
public TextPair(String first, String second) {
set(new Text(first), new Text(second));
}
public TextPair(Text first, Text second) {
set(first, second);
}
public void set(Text first, Text second) {
this.first = first;
this.second = second;
}
public Text getFirst() {
return first;
}
public Text getSecond() {
return second;
}
#Override
public void write(DataOutput out) throws IOException {
first.write(out);
second.write(out);
}
#Override
public void readFields(DataInput in) throws IOException {
first.readFields(in);
second.readFields(in);
}
#Override
public int hashCode() {
return first.hashCode() * 163 + second.hashCode();
}
#Override
public boolean equals(Object o) {
if (o instanceof TextPair) {
TextPair tp = (TextPair) o;
return first.equals(tp.first) && second.equals(tp.second);
}
return false;
}
#Override
public String toString() {
return first + "\t" + second;
}
#Override
public int compareTo(TextPair tp) {
int cmp = first.compareTo(tp.first);
if (cmp != 0) {
return cmp;
}
return second.compareTo(tp.second);
}
public static class FirstComparator extends WritableComparator {
private static final Text.Comparator TEXT_COMPARATOR = new Text.Comparator();
public FirstComparator() {
super(TextPair.class);
}
#Override
public int compare(byte[] b1, int s1, int l1,
byte[] b2, int s2, int l2) {
try {
int firstL1 = WritableUtils.decodeVIntSize(b1[s1]) + readVInt(b1, s1);
int firstL2 = WritableUtils.decodeVIntSize(b2[s2]) + readVInt(b2, s2);
return TEXT_COMPARATOR.compare(b1, s1, firstL1, b2, s2, firstL2);
} catch (IOException e) {
throw new IllegalArgumentException(e);
}
}
#Override
public int compare(WritableComparable a, WritableComparable b) {
if (a instanceof TextPair && b instanceof TextPair) {
return ((TextPair) a).first.compareTo(((TextPair) b).first);
}
return super.compare(a, b);
}
}
}
JobDriver:
public class JoinJob extends Configured implements Tool {
#Override
public int run(String[] args) throws Exception {
Job job = Job.getInstance(getConf(), "Join two DataSet");
job.setJarByClass(getClass());
Path ncdcInputPath = new Path(getConf().get("job.input1.path"));
Path stationInputPath = new Path(getConf().get("job.input2.path"));
Path outputPath = new Path(getConf().get("job.output.path"));
MultipleInputs.addInputPath(job, ncdcInputPath,
TextInputFormat.class, JoinDataSet1Mapper.class);
MultipleInputs.addInputPath(job, stationInputPath,
TextInputFormat.class, JoinDataSet2Mapper.class);
FileOutputFormat.setOutputPath(job, outputPath);
job.setPartitionerClass(JoinReducer.KeyPartitioner.class);
job.setGroupingComparatorClass(TextPair.FirstComparator.class);
job.setMapOutputKeyClass(TextPair.class);
job.setReducerClass(JoinReducer.class);
job.setOutputKeyClass(Text.class);
return job.waitForCompletion(true) ? 0 : 1;
}
public static void main(String[] args) throws Exception {
int exitCode = ToolRunner.run(new JoinJob(), args);
System.exit(exitCode);
}
}