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HBase - Describe & Alter

describe

This command returns the description of the table. Its syntax is as follows:

hbase> describe  table name

Given below is the output of the describe command on the emp table.

hbase(main):006:0> describe  emp 
   DESCRIPTION
      ENABLED
      
 emp , {NAME ⇒  READONLY , DATA_BLOCK_ENCODING ⇒  NONE , BLOOMFILTER
⇒  ROW , REPLICATION_SCOPE ⇒  0 , COMPRESSION ⇒  NONE , VERSIONS ⇒
 1 , TTL true

⇒  FOREVER , MIN_VERSIONS ⇒  0 , KEEP_DELETED_CELLS ⇒  false ,
BLOCKSIZE ⇒  65536 , IN_MEMORY ⇒  false , BLOCKCACHE ⇒  true }, {NAME
⇒  personal

data , DATA_BLOCK_ENCODING ⇒  NONE , BLOOMFILTER ⇒  ROW ,
REPLICATION_SCOPE ⇒  0 , VERSIONS ⇒  5 , COMPRESSION ⇒  NONE ,
MIN_VERSIONS ⇒  0 , TTL

⇒  FOREVER , KEEP_DELETED_CELLS ⇒  false , BLOCKSIZE ⇒  65536 ,
IN_MEMORY ⇒  false , BLOCKCACHE ⇒  true }, {NAME ⇒  professional
data , DATA_BLO

CK_ENCODING ⇒  NONE , BLOOMFILTER ⇒  ROW , REPLICATION_SCOPE ⇒  0 ,
VERSIONS ⇒  1 , COMPRESSION ⇒  NONE , MIN_VERSIONS ⇒  0 , TTL ⇒
 FOREVER , K

EEP_DELETED_CELLS ⇒  false , BLOCKSIZE ⇒  65536 , IN_MEMORY ⇒
 false , BLOCKCACHE ⇒  true }, {NAME ⇒  table_att_unset ,
DATA_BLOCK_ENCODING ⇒  NO 

NE , BLOOMFILTER ⇒  ROW , REPLICATION_SCOPE ⇒  0 , COMPRESSION ⇒
 NONE , VERSIONS ⇒  1 , TTL ⇒  FOREVER , MIN_VERSIONS ⇒  0 ,
KEEP_DELETED_CELLS

⇒  false , BLOCKSIZE ⇒  6

alter

Alter is the command used to make changes to an existing table. Using this command, you can change the maximum number of cells of a column family, set and delete table scope operators, and delete a column family from a table.

Changing the Maximum Number of Cells of a Column Family

Given below is the syntax to change the maximum number of cells of a column family.

hbase> alter  t1 , NAME ⇒  f1 , VERSIONS ⇒ 5

In the following example, the maximum number of cells is set to 5.

hbase(main):003:0> alter  emp , NAME ⇒  personal data , VERSIONS ⇒ 5
Updating all regions with the new schema...
0/1 regions updated.
1/1 regions updated.
Done.
0 row(s) in 2.3050 seconds

Table Scope Operators

Using alter, you can set and remove table scope operators such as MAX_FILESIZE, READONLY, MEMSTORE_FLUSHSIZE, DEFERRED_LOG_FLUSH, etc.

Setting Read Only

Below given is the syntax to make a table read only.

hbase>alter  t1 , READONLY(option)

In the following example, we have made the emp table read only.

hbase(main):006:0> alter  emp , READONLY
Updating all regions with the new schema...
0/1 regions updated.
1/1 regions updated.
Done.
0 row(s) in 2.2140 seconds

Removing Table Scope Operators

We can also remove the table scope operators. Given below is the syntax to remove ‘MAX_FILESIZE’ from emp table.

hbase> alter  t1 , METHOD ⇒  table_att_unset , NAME ⇒  MAX_FILESIZE

Deleting a Column Family

Using alter, you can also delete a column family. Given below is the syntax to delete a column family using alter.

hbase> alter ‘ table name ’, ‘delete’ ⇒ ‘ column family ’

Given below is an example to delete a column family from the ‘emp’ table.

Assume there is a table named employee in HBase. It contains the following data:

hbase(main):006:0> scan  employee 

   ROW                   COLUMN+CELL

row1 column = personal:city, timestamp = 1418193767, value = hyderabad

row1 column = personal:name, timestamp = 1418193806767, value = raju

row1 column = professional:designation, timestamp = 1418193767, value = manager

row1 column = professional:salary, timestamp = 1418193806767, value = 50000

1 row(s) in 0.0160 seconds

Now let us delete the column family named professional using the alter command.

hbase(main):007:0> alter  employee , delete ⇒ professional 
Updating all regions with the new schema...
0/1 regions updated.
1/1 regions updated.
Done.
0 row(s) in 2.2380 seconds

Now verify the data in the table after alteration. Observe the column family ‘professional’ is no more, since we have deleted it.

hbase(main):003:0> scan  employee 
   ROW             COLUMN &plus; CELL
row1 column = personal:city, timestamp = 14181936767, value = hyderabad

row1 column = personal:name, timestamp = 1418193806767, value = raju

1 row(s) in 0.0830 seconds

Adding a Column Family Using Java API

You can add a column family to a table using the method addColumn() of HBAseAdmin class. Follow the steps given below to add a column family to a table.

Step 1

Instantiate the HBaseAdmin class.

// Instantiating configuration object
Configuration conf = HBaseConfiguration.create();

// Instantiating HBaseAdmin class
HBaseAdmin admin = new HBaseAdmin(conf);

Step 2

The addColumn() method requires a table name and an object of HColumnDescriptor class. Therefore instantiate the HColumnDescriptor class. The constructor of HColumnDescriptor in turn requires a column family name that is to be added. Here we are adding a column family named “contactDetails” to the existing “employee” table.

// Instantiating columnDescriptor object

HColumnDescriptor columnDescriptor = new
HColumnDescriptor("contactDetails");

Step 3

Add the column family using addColumn method. Pass the table name and the HColumnDescriptor class object as parameters to this method.

// Adding column family
admin.addColumn("employee", new HColumnDescriptor("columnDescriptor"));

Given below is the complete program to add a column family to an existing table.

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;

import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.HColumnDescriptor;
import org.apache.hadoop.hbase.MasterNotRunningException;
import org.apache.hadoop.hbase.cpent.HBaseAdmin;

pubpc class AddColoumn{

   pubpc static void main(String args[]) throws MasterNotRunningException, IOException{

      // Instantiating configuration class.
      Configuration conf = HBaseConfiguration.create();

      // Instantiating HBaseAdmin class.
      HBaseAdmin admin = new HBaseAdmin(conf);

      // Instantiating columnDescriptor class
      HColumnDescriptor columnDescriptor = new HColumnDescriptor("contactDetails");
      
      // Adding column family
      admin.addColumn("employee", columnDescriptor);
      System.out.println("coloumn added");
   }
}

Compile and execute the above program as shown below.

$javac AddColumn.java
$java AddColumn

The above compilation works only if you have set the classpath in “ .bashrc ”. If you haven t, follow the procedure given below to compile your .java file.

//if "/home/home/hadoop/hbase " is your Hbase home folder then.

$javac -cp /home/hadoop/hbase/pb/*: Demo.java

If everything goes well, it will produce the following output:

column added

Deleting a Column Family Using Java API

You can delete a column family from a table using the method deleteColumn() of HBAseAdmin class. Follow the steps given below to add a column family to a table.

Step1