Hbase mapside join- One of the tables is not getting read? read from hbase and right result into hbase

Question

I am trying to do mapside join of two tables located in Hbase. My aim is to keep record of the small table in hashmap and compare with the big table, and once matched, write record in a table in hbase again. I wrote the similar code for join operation using both Mapper and Reducer and it worked well and both tables are scanned in mapper class. But since reduce side join is not efficient at all, I want to join the tables in mapper side only. In the following code "commented if block" is just to see that it returns false always and first table (small one) is not getting read. Any hints helps are appreciated. I am using sandbox of HDP.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.conf.Configured;
//import org.apache.hadoop.hbase.client.Scan;
import org.apache.hadoop.hbase.mapreduce.TableMapReduceUtil;
import org.apache.hadoop.hbase.mapreduce.TableReducer;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper.Context;
import org.apache.hadoop.util.Tool;
import com.sun.tools.javac.util.Log;
import java.io.IOException;
import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.client.*;
import org.apache.hadoop.hbase.io.ImmutableBytesWritable;
import org.apache.hadoop.hbase.mapred.TableOutputFormat;
import org.apache.hadoop.hbase.mapreduce.TableMapper;
import org.apache.hadoop.hbase.mapreduce.TableSplit;

public class JoinDriver extends Configured implements Tool {

    static int row_index = 0;

        public static class JoinJobMapper extends TableMapper<ImmutableBytesWritable, Put> {
        private static byte[] big_table_bytarr = Bytes.toBytes("big_table");
        private static byte[] small_table_bytarr = Bytes.toBytes("small_table");

        HashMap<String,String> myHashMap = new HashMap<String, String>();

        byte[] c1_value;
        byte[] c2_value;

        String big_table;
        String small_table;

        String big_table_c1;
        String big_table_c2; 

        String small_table_c1; 
        String small_table_c2; 

        Text mapperKeyS;
        Text mapperValueS; 
        Text mapperKeyB;
        Text mapperValueB; 

        public void map(ImmutableBytesWritable rowKey, Result columns, Context context) {
            TableSplit currentSplit = (TableSplit) context.getInputSplit();
            byte[] tableName = currentSplit.getTableName();

            try {
                Put put = new Put(Bytes.toBytes(++row_index));


                // put small table into hashmap - myhashMap
                if (Arrays.equals(tableName, small_table_bytarr)) {

                    c1_value = columns.getValue(Bytes.toBytes("s_cf"), Bytes.toBytes("s_cf_c1"));
                    c2_value = columns.getValue(Bytes.toBytes("s_cf"), Bytes.toBytes("s_cf_c2"));
                    small_table_c1 = new String(c1_value);
                    small_table_c2 = new String(c2_value);

                    mapperKeyS = new Text(small_table_c1);
                    mapperValueS = new Text(small_table_c2);

                    myHashMap.put(small_table_c1,small_table_c2);


                } else if (Arrays.equals(tableName, big_table_bytarr)) {
                    c1_value = columns.getValue(Bytes.toBytes("b_cf"), Bytes.toBytes("b_cf_c1"));
                    c2_value = columns.getValue(Bytes.toBytes("b_cf"), Bytes.toBytes("b_cf_c2"));
                    big_table_c1 = new String(c1_value);
                    big_table_c2 = new String(c2_value);

                    mapperKeyB = new Text(big_table_c1);
                    mapperValueB = new Text(big_table_c2);



            //  if (set.containsKey(big_table_c1)){

                    put.addColumn(Bytes.toBytes("join"), Bytes.toBytes("join_c1"), Bytes.toBytes(big_table_c1));
                    context.write(new ImmutableBytesWritable(mapperKeyB.getBytes()), put );
                    put.addColumn(Bytes.toBytes("join"), Bytes.toBytes("join_c2"), Bytes.toBytes(big_table_c2));
                    context.write(new ImmutableBytesWritable(mapperKeyB.getBytes()), put );
                    put.addColumn(Bytes.toBytes("join"), Bytes.toBytes("join_c3"),Bytes.toBytes((myHashMap.get(big_table_c1))));
                    context.write(new ImmutableBytesWritable(mapperKeyB.getBytes()), put );

            //      }

                }

            } catch (Exception e) {
                // TODO : exception handling logic
                e.printStackTrace();
            }
        }

    }

    public int run(String[] args) throws Exception {

        List<Scan> scans = new ArrayList<Scan>();



        Scan scan1 = new Scan();
        scan1.setAttribute("scan.attributes.table.name", Bytes.toBytes("small_table"));
        System.out.println(scan1.getAttribute("scan.attributes.table.name"));
        scans.add(scan1);

        Scan scan2 = new Scan();
        scan2.setAttribute("scan.attributes.table.name", Bytes.toBytes("big_table"));
        System.out.println(scan2.getAttribute("scan.attributes.table.name"));
        scans.add(scan2);

        Configuration conf = new Configuration();
        Job job = new Job(conf);
        job.setJar("MSJJ.jar");
        job.setJarByClass(JoinDriver.class);

        TableMapReduceUtil.initTableMapperJob(scans, JoinJobMapper.class, ImmutableBytesWritable.class, Put.class, job);
        TableMapReduceUtil.initTableReducerJob("joined_table", null, job);
        job.setNumReduceTasks(0);


        job.waitForCompletion(true);

        return 0;
    }

    public static void main(String[] args) throws Exception {
        JoinDriver runJob = new JoinDriver();
        runJob.run(args);

    }

}

Answer 1

By reading your problem statement I believe you have got some wrong idea about uses of Multiple HBase table input. I suggest you load small table in a HashMap, in setup method of mapper class. Then use map only job on big table, in map method you can fetch corresponding values from the HashMap which you loaded earlier. Let me know how this works out.