Cassandra - Alter Keyspace

Altering a KeySpace

ALTER KEYSPACE can be used to alter properties such as the number of replicas and the durable_writes of a KeySpace. Given below is the syntax of this command.

Syntax

ALTER KEYSPACE <identifier> WITH <properties>

i.e.

ALTER KEYSPACE “KeySpace Name”
WITH replication = {'class': ‘Strategy name’, 'replication_factor' : ‘No.Of  replicas’};

The properties of ALTER KEYSPACE are same as CREATE KEYSPACE. It has two properties: replication and durable_writes.

Replication

The replication option specifies the replica placement strategy and the number of replicas wanted.

Durable_writes

Using this option, you can instruct Cassandra whether to use commitlog for updates on the current KeySpace. This option is not mandatory and by default, it is set to true.

Example

Given below is an example of altering a KeySpace.

  • Here we are altering a KeySpace named Howcodex.

  • We are changing the replication factor from 1 to 3.

cqlsh.> ALTER KEYSPACE howcodex
WITH replication = {'class':'NetworkTopologyStrategy', 'replication_factor' : 3};

Altering Durable_writes

You can also alter the durable_writes property of a KeySpace. Given below is the durable_writes property of the test KeySpace.

SELECT * FROM system.schema_keyspaces;

  keyspace_name | durable_writes |                                       strategy_class | strategy_options
----------------+----------------+------------------------------------------------------+----------------------------
           test |          False | org.apache.cassandra.locator.NetworkTopologyStrategy | {"datacenter1":"3"}

 howcodex |           True |          org.apache.cassandra.locator.SimpleStrategy | {"replication_factor":"4"}

         system |           True |           org.apache.cassandra.locator.LocalStrategy | { }

  system_traces |           True |          org.apache.cassandra.locator.SimpleStrategy | {"replication_factor":"2"}
(4 rows)


ALTER KEYSPACE test
WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'datacenter1' : 3}
AND DURABLE_WRITES = true;

Once again, if you verify the properties of KeySpaces, it will produce the following output.

SELECT * FROM system.schema_keyspaces;
  keyspace_name | durable_writes |                                       strategy_class | strategy_options
----------------+----------------+------------------------------------------------------+----------------------------
           test |           True | org.apache.cassandra.locator.NetworkTopologyStrategy | {"datacenter1":"3"}

 howcodex |           True |          org.apache.cassandra.locator.SimpleStrategy | {"replication_factor":"4"}

         system |           True |           org.apache.cassandra.locator.LocalStrategy | { }

  system_traces |           True |          org.apache.cassandra.locator.SimpleStrategy | {"replication_factor":"2"}

(4 rows)

Altering a Keyspace using Java API

You can alter a keyspace using the execute() method of Session class. Follow the steps given below to alter a keyspace using Java API

Step1: Create a Cluster Object

First of all, create an instance of Cluster.builder class of com.datastax.driver.core package as shown below.

//Creating Cluster.Builder object
Cluster.Builder builder1 = Cluster.builder();

Add a contact point (IP address of the node) using the addContactPoint() method of Cluster.Builder object. This method returns Cluster.Builder.

//Adding contact point to the Cluster.Builder object
Cluster.Builder builder2 = build.addContactPoint( "127.0.0.1" );

Using the new builder object, create a cluster object. To do so, you have a method called build() in the Cluster.Builder class. The following code shows how to create a cluster object.

//Building a cluster

Cluster cluster = builder.build();

You can build the cluster object using a single line of code as shown below.

Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();

Step 2: Create a Session Object

Create an instance of Session object using the connect() method of Clusterclass as shown below.

Session session = cluster.connect( );

This method creates a new session and initializes it. If you already have a keyspace, you can set it to the existing one by passing the keyspace name in string format to this method as shown below.

Session session = cluster.connect(“ Your keyspace name ” );

Step 3: Execute Query

You can execute CQL queries using the execute() method of Session class. Pass the query either in string format or as a Statement class object to the execute() method. Whatever you pass to this method in string format will be executed on the cqlsh.

In this example,

  • We are altering a keyspace named tp. We are altering the replication option from Simple Strategy to Network Topology Strategy.

  • We are altering the durable_writes to false

You have to store the query in a string variable and pass it to the execute() method as shown below.

//Query
String query = "ALTER KEYSPACE tp WITH replication " + "=   {'class':'NetworkTopologyStrategy', 'datacenter1':3}" +" AND DURABLE_WRITES = false;";
session.execute(query);

Given below is the complete program to create and use a keyspace in Cassandra using Java API.

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Session;

public class Alter_KeySpace {
   public static void main(String args[]){

      //Query
      String query = "ALTER KEYSPACE tp WITH replication " + "= {'class':'NetworkTopologyStrategy', 'datacenter1':3}"
         + "AND DURABLE_WRITES = false;";

      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
   
      //Creating Session object
      Session session = cluster.connect();
 
      //Executing the query
      session.execute(query);
 
      System.out.println("Keyspace altered");
   }
}

Save the above program with the class name followed by .java, browse to the location where it is saved. Compile and execute the program as shown below.

$javac Alter_KeySpace.java
$java Alter_KeySpace

Under normal conditions, it produces the following output −

Keyspace Altered
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