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 + 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.client.HBaseAdmin;
public class AddColoumn{
public 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/lib/*: 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
Instantiate the HBaseAdmin class.
// Instantiating configuration object Configuration conf = HBaseConfiguration.create(); // Instantiating HBaseAdmin class HBaseAdmin admin = new HBaseAdmin(conf);
Step2
Add the column family using deleteColumn() method. Pass the table name and the column family name as parameters to this method.
// Deleting column family
admin.deleteColumn("employee", "contactDetails");
Given below is the complete program to delete a column family from an existing table.
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.HBaseConfiguration;
import org.apache.hadoop.hbase.MasterNotRunningException;
import org.apache.hadoop.hbase.client.HBaseAdmin;
public class DeleteColoumn{
public static void main(String args[]) throws MasterNotRunningException, IOException{
// Instantiating configuration class.
Configuration conf = HBaseConfiguration.create();
// Instantiating HBaseAdmin class.
HBaseAdmin admin = new HBaseAdmin(conf);
// Deleting a column family
admin.deleteColumn("employee","contactDetails");
System.out.println("coloumn deleted");
}
}
Compile and execute the above program as shown below.
$javac DeleteColumn.java $java DeleteColumn
The following should be the output:
column deleted