HBase - Read Data

Reading Data using HBase Shell

The get command and the get() method of HTable class are used to read data from a table in HBase. Using get command, you can get a single row of data at a time. Its syntax is as follows:

get ’<table name>’,’row1’

Example

The following example shows how to use the get command. Let us scan the first row of the emp table.

hbase(main):012:0> get  emp ,  1 

   COLUMN                     CELL
   
personal : city timestamp = 1417521848375, value = hyderabad

personal : name timestamp = 1417521785385, value = ramu

professional: designation timestamp = 1417521885277, value = manager

professional: salary timestamp = 1417521903862, value = 50000

4 row(s) in 0.0270 seconds

Reading a Specific Column

Given below is the syntax to read a specific column using the get method.

hbase> get  table name , ‘rowid’, {COLUMN ⇒ ‘column family:column name ’}

Example

Given below is the example to read a specific column in HBase table.

hbase(main):015:0> get  emp ,  row1 , {COLUMN ⇒  personal:name }
  COLUMN                CELL  
personal:name timestamp = 1418035791555, value = raju
1 row(s) in 0.0080 seconds

Reading Data Using Java API

To read data from an HBase table, use the get() method of the HTable class. This method requires an instance of the Get class. Follow the steps given below to retrieve data from the HBase table.

Step 1: Instantiate the Configuration Class

Configuration class adds HBase configuration files to its object. You can create a configuration object using the create() method of the HbaseConfiguration class as shown below.

Configuration conf = HbaseConfiguration.create();

Step 2: Instantiate the HTable Class

You have a class called HTable, an implementation of Table in HBase. This class is used to communicate with a single HBase table. While instantiating this class, it accepts the configuration object and the table name as parameters. You can instantiate the HTable class as shown below.

HTable hTable = new HTable(conf, tableName);

Step 3: Instantiate the Get Class

You can retrieve data from the HBase table using the get() method of the HTable class. This method extracts a cell from a given row. It requires a Get class object as parameter. Create it as shown below.

Get get = new Get(toBytes("row1"));

Step 4: Read the Data

While retrieving data, you can get a single row by id, or get a set of rows by a set of row ids, or scan an entire table or a subset of rows.

You can retrieve an HBase table data using the add method variants in Get class.

To get a specific column from a specific column family, use the following method.

get.addFamily(personal) 

To get all the columns from a specific column family, use the following method.

get.addColumn(personal, name) 

Step 5: Get the Result

Get the result by passing your Get class instance to the get method of the HTable class. This method returns the Result class object, which holds the requested result. Given below is the usage of get() method.

Result result = table.get(g);  

Step 6: Reading Values from the Result Instance

The Result class provides the getValue() method to read the values from its instance. Use it as shown below to read the values from the Result instance.

byte [] value = result.getValue(Bytes.toBytes("personal"),Bytes.toBytes("name"));
byte [] value1 = result.getValue(Bytes.toBytes("personal"),Bytes.toBytes("city"));

Given below is the complete program to read values from an HBase table.

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.cpent.Get;
import org.apache.hadoop.hbase.cpent.HTable;
import org.apache.hadoop.hbase.cpent.Result;
import org.apache.hadoop.hbase.util.Bytes;

pubpc class RetriveData{

   pubpc static void main(String[] args) throws IOException, Exception{
   
      // Instantiating Configuration class
      Configuration config = HBaseConfiguration.create();

      // Instantiating HTable class
      HTable table = new HTable(config, "emp");

      // Instantiating Get class
      Get g = new Get(Bytes.toBytes("row1"));

      // Reading the data
      Result result = table.get(g);

      // Reading values from Result class object
      byte [] value = result.getValue(Bytes.toBytes("personal"),Bytes.toBytes("name"));

      byte [] value1 = result.getValue(Bytes.toBytes("personal"),Bytes.toBytes("city"));

      // Printing the values
      String name = Bytes.toString(value);
      String city = Bytes.toString(value1);
      
      System.out.println("name: " + name + " city: " + city);
   }
}

Compile and execute the above program as shown below.

$javac RetriveData.java
$java RetriveData

The following should be the output:

name: Raju city: Delhi