Cassandra - Quick Guide

Cassandra - Introduction

Apache Cassandra is a highly scalable, high-performance distributed database designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure. It is a type of NoSQL database. Let us first understand what a NoSQL database does.

NoSQLDatabase

A NoSQL database (sometimes called as Not Only SQL) is a database that provides a mechanism to store and retrieve data other than the tabular relations used in relational databases. These databases are schema-free, support easy replication, have simple API, eventually consistent, and can handle huge amounts of data.

The primary objective of a NoSQL database is to have

simplicity of design,
horizontal scaling, and
finer control over availability.

NoSql databases use different data structures compared to relational databases. It makes some operations faster in NoSQL. The suitability of a given NoSQL database depends on the problem it must solve.

NoSQL vs. Relational Database

The following table lists the points that differentiate a relational database from a NoSQL database.

Relational Database	NoSql Database
Supports powerful query language.	Supports very simple query language.
It has a fixed schema.	No fixed schema.
Follows ACID (Atomicity, Consistency, Isolation, and Durability).	It is only “eventually consistent”.
Supports transactions.	Does not support transactions.

Besides Cassandra, we have the following NoSQL databases that are quite popular −

Apache HBase − HBase is an open source, non-relational, distributed database modeled after Google’s BigTable and is written in Java. It is developed as a part of Apache Hadoop project and runs on top of HDFS, providing BigTable-like capabilities for Hadoop.
MongoDB − MongoDB is a cross-platform document-oriented database system that avoids using the traditional table-based relational database structure in favor of JSON-like documents with dynamic schemas making the integration of data in certain types of applications easier and faster.

What is Apache Cassandra?

Apache Cassandra is an open source, distributed and decentralized/distributed storage system (database), for managing very large amounts of structured data spread out across the world. It provides highly available service with no single point of failure.

Listed below are some of the notable points of Apache Cassandra −

It is scalable, fault-tolerant, and consistent.
It is a column-oriented database.
Its distribution design is based on Amazon’s Dynamo and its data model on Google’s Bigtable.
Created at Facebook, it differs sharply from relational database management systems.
Cassandra implements a Dynamo-style replication model with no single point of failure, but adds a more powerful “column family” data model.
Cassandra is being used by some of the biggest companies such as Facebook, Twitter, Cisco, Rackspace, ebay, Twitter, Netflix, and more.

Features of Cassandra

Cassandra has become so popular because of its outstanding technical features. Given below are some of the features of Cassandra:

Elastic scalability − Cassandra is highly scalable; it allows to add more hardware to accommodate more customers and more data as per requirement.
Always on architecture − Cassandra has no single point of failure and it is continuously available for business-critical applications that cannot afford a failure.
Fast linear-scale performance − Cassandra is linearly scalable, i.e., it increases your throughput as you increase the number of nodes in the cluster. Therefore it maintains a quick response time.
Flexible data storage − Cassandra accommodates all possible data formats including: structured, semi-structured, and unstructured. It can dynamically accommodate changes to your data structures according to your need.
Easy data distribution − Cassandra provides the flexibility to distribute data where you need by replicating data across multiple data centers.
Transaction support − Cassandra supports properties like Atomicity, Consistency, Isolation, and Durability (ACID).
Fast writes − Cassandra was designed to run on cheap commodity hardware. It performs blazingly fast writes and can store hundreds of terabytes of data, without sacrificing the read efficiency.

History of Cassandra

Cassandra was developed at Facebook for inbox search.
It was open-sourced by Facebook in July 2008.
Cassandra was accepted into Apache Incubator in March 2009.
It was made an Apache top-level project since February 2010.

Cassandra - Architecture

The design goal of Cassandra is to handle big data workloads across multiple nodes without any single point of failure. Cassandra has peer-to-peer distributed system across its nodes, and data is distributed among all the nodes in a cluster.

All the nodes in a cluster play the same role. Each node is independent and at the same time interconnected to other nodes.
Each node in a cluster can accept read and write requests, regardless of where the data is actually located in the cluster.
When a node goes down, read/write requests can be served from other nodes in the network.

Data Replication in Cassandra

In Cassandra, one or more of the nodes in a cluster act as replicas for a given piece of data. If it is detected that some of the nodes responded with an out-of-date value, Cassandra will return the most recent value to the client. After returning the most recent value, Cassandra performs a read repair in the background to update the stale values.

The following figure shows a schematic view of how Cassandra uses data replication among the nodes in a cluster to ensure no single point of failure.

Note − Cassandra uses the Gossip Protocol in the background to allow the nodes to communicate with each other and detect any faulty nodes in the cluster.

Components of Cassandra

The key components of Cassandra are as follows −

Node − It is the place where data is stored.
Data center − It is a collection of related nodes.
Cluster − A cluster is a component that contains one or more data centers.
Commit log − The commit log is a crash-recovery mechanism in Cassandra. Every write operation is written to the commit log.
Mem-table − A mem-table is a memory-resident data structure. After commit log, the data will be written to the mem-table. Sometimes, for a single-column family, there will be multiple mem-tables.
SSTable − It is a disk file to which the data is flushed from the mem-table when its contents reach a threshold value.
Bloom filter − These are nothing but quick, nondeterministic, algorithms for testing whether an element is a member of a set. It is a special kind of cache. Bloom filters are accessed after every query.

Cassandra Query Language

Users can access Cassandra through its nodes using Cassandra Query Language (CQL). CQL treats the database (Keyspace) as a container of tables. Programmers use cqlsh: a prompt to work with CQL or separate application language drivers.

Clients approach any of the nodes for their read-write operations. That node (coordinator) plays a proxy between the client and the nodes holding the data.

Write Operations

Every write activity of nodes is captured by the commit logs written in the nodes. Later the data will be captured and stored in the mem-table. Whenever the mem-table is full, data will be written into the SStable data file. All writes are automatically partitioned and replicated throughout the cluster. Cassandra periodically consolidates the SSTables, discarding unnecessary data.

Read Operations

During read operations, Cassandra gets values from the mem-table and checks the bloom filter to find the appropriate SSTable that holds the required data.

Cassandra - Data Model

The data model of Cassandra is significantly different from what we normally see in an RDBMS. This chapter provides an overview of how Cassandra stores its data.

Cluster

Cassandra database is distributed over several machines that operate together. The outermost container is known as the Cluster. For failure handling, every node contains a replica, and in case of a failure, the replica takes charge. Cassandra arranges the nodes in a cluster, in a ring format, and assigns data to them.

Keyspace

Keyspace is the outermost container for data in Cassandra. The basic attributes of a Keyspace in Cassandra are −

Replication factor − It is the number of machines in the cluster that will receive copies of the same data.
Replica placement strategy − It is nothing but the strategy to place replicas in the ring. We have strategies such as simple strategy (rack-aware strategy), old network topology strategy (rack-aware strategy), and network topology strategy (datacenter-shared strategy).
Column families − Keyspace is a container for a list of one or more column families. A column family, in turn, is a container of a collection of rows. Each row contains ordered columns. Column families represent the structure of your data. Each keyspace has at least one and often many column families.

The syntax of creating a Keyspace is as follows −

CREATE KEYSPACE Keyspace name
WITH replication = {'class': 'SimpleStrategy', 'replication_factor' : 3};

The following illustration shows a schematic view of a Keyspace.

Column Family

A column family is a container for an ordered collection of rows. Each row, in turn, is an ordered collection of columns. The following table lists the points that differentiate a column family from a table of relational databases.

Relational Table	Cassandra column Family
A schema in a relational model is fixed. Once we define certain columns for a table, while inserting data, in every row all the columns must be filled at least with a null value.	In Cassandra, although the column families are defined, the columns are not. You can freely add any column to any column family at any time.
Relational tables define only columns and the user fills in the table with values.	In Cassandra, a table contains columns, or can be defined as a super column family.

A Cassandra column family has the following attributes −

keys_cached − It represents the number of locations to keep cached per SSTable.
rows_cached − It represents the number of rows whose entire contents will be cached in memory.
preload_row_cache − It specifies whether you want to pre-populate the row cache.

Note − Unlike relational tables where a column family’s schema is not fixed, Cassandra does not force individual rows to have all the columns.

The following figure shows an example of a Cassandra column family.

Column

A column is the basic data structure of Cassandra with three values, namely key or column name, value, and a time stamp. Given below is the structure of a column.

SuperColumn

A super column is a special column, therefore, it is also a key-value pair. But a super column stores a map of sub-columns.

Generally column families are stored on disk in individual files. Therefore, to optimize performance, it is important to keep columns that you are likely to query together in the same column family, and a super column can be helpful here.Given below is the structure of a super column.

Data Models of Cassandra and RDBMS

The following table lists down the points that differentiate the data model of Cassandra from that of an RDBMS.

RDBMS	Cassandra
RDBMS deals with structured data.	Cassandra deals with unstructured data.
It has a fixed schema.	Cassandra has a flexible schema.
In RDBMS, a table is an array of arrays. (ROW x COLUMN)	In Cassandra, a table is a list of “nested key-value pairs”. (ROW x COLUMN key x COLUMN value)
Database is the outermost container that contains data corresponding to an application.	Keyspace is the outermost container that contains data corresponding to an application.
Tables are the entities of a database.	Tables or column families are the entity of a keyspace.
Row is an individual record in RDBMS.	Row is a unit of replication in Cassandra.
Column represents the attributes of a relation.	Column is a unit of storage in Cassandra.
RDBMS supports the concepts of foreign keys, joins.	Relationships are represented using collections.

Cassandra - Installation

Cassandra can be accessed using cqlsh as well as drivers of different languages. This chapter explains how to set up both cqlsh and java environments to work with Cassandra.

Pre-Installation Setup

Before installing Cassandra in Linux environment, we require to set up Linux using ssh (Secure Shell). Follow the steps given below for setting up Linux environment.

Create a User

At the beginning, it is recommended to create a separate user for Hadoop to isolate Hadoop file system from Unix file system. Follow the steps given below to create a user.

Open root using the command “su”.
Create a user from the root account using the command “useradd username”.
Now you can open an existing user account using the command “su username”.

Open the Linux terminal and type the following commands to create a user.

$ su
password:
# useradd hadoop
# passwd hadoop
New passwd:
Retype new passwd

SSH Setup and Key Generation

SSH setup is required to perform different operations on a cluster such as starting, stopping, and distributed daemon shell operations. To authenticate different users of Hadoop, it is required to provide public/private key pair for a Hadoop user and share it with different users.

The following commands are used for generating a key value pair using SSH −

copy the public keys form id_rsa.pub to authorized_keys,
and provide owner,
read and write permissions to authorized_keys file respectively.

$ ssh-keygen -t rsa
$ cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys
$ chmod 0600 ~/.ssh/authorized_keys

Verify ssh:

ssh localhost

Installing Java

Java is the main prerequisite for Cassandra. First of all, you should verify the existence of Java in your system using the following command −

$ java -version

If everything works fine it will give you the following output.

java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b13)
Java HotSpot(TM) Client VM (build 25.0-b02, mixed mode)

If you don’t have Java in your system, then follow the steps given below for installing Java.

Step 1

Download java (JDK <latest version> - X64.tar.gz) from the following link:

Then jdk-7u71-linux-x64.tar.gz will be downloaded onto your system.

Step 2

Generally you will find the downloaded java file in the Downloads folder. Verify it and extract the jdk-7u71-linux-x64.gz file using the following commands.

$ cd Downloads/
$ ls
jdk-7u71-linux-x64.gz
$ tar zxf jdk-7u71-linux-x64.gz
$ ls
jdk1.7.0_71 jdk-7u71-linux-x64.gz

Step 3

To make Java available to all users, you have to move it to the location “/usr/local/”. Open root, and type the following commands.

$ su
password:
# mv jdk1.7.0_71 /usr/local/
# exit

Step 4

For setting up PATH and JAVA_HOME variables, add the following commands to ~/.bashrc file.

export JAVA_HOME = /usr/local/jdk1.7.0_71
export PATH = $PATH:$JAVA_HOME/bin

Now apply all the changes into the current running system.

$ source ~/.bashrc

Step 5

Use the following commands to configure java alternatives.

# alternatives --install /usr/bin/java java usr/local/java/bin/java 2
# alternatives --install /usr/bin/javac javac usr/local/java/bin/javac 2
# alternatives --install /usr/bin/jar jar usr/local/java/bin/jar 2

# alternatives --set java usr/local/java/bin/java
# alternatives --set javac usr/local/java/bin/javac
# alternatives --set jar usr/local/java/bin/jar

Now use the java -version command from the terminal as explained above.

Setting the Path

Set the path of Cassandra path in “/.bashrc” as shown below.

[hadoop@linux ~]$ gedit ~/.bashrc

export CASSANDRA_HOME = ~/cassandra
export PATH = $PATH:$CASSANDRA_HOME/bin

Download Cassandra

Apache Cassandra is available at Download Link Cassandra using the following command.

$ wget http://supergsego.com/apache/cassandra/2.1.2/apache-cassandra-2.1.2-bin.tar.gz

Unzip Cassandra using the command zxvf as shown below.

$ tar zxvf apache-cassandra-2.1.2-bin.tar.gz.

Create a new directory named cassandra and move the contents of the downloaded file to it as shown below.

$ mkdir Cassandra
$ mv apache-cassandra-2.1.2/* cassandra.

Configure Cassandra

Open the cassandra.yaml: file, which will be available in the bin directory of Cassandra.

$ gedit cassandra.yaml

Note − If you have installed Cassandra from a deb or rpm package, the configuration files will be located in /etc/cassandra directory of Cassandra.

The above command opens the cassandra.yaml file. Verify the following configurations. By default, these values will be set to the specified directories.

data_file_directories “/var/lib/cassandra/data”
commitlog_directory “/var/lib/cassandra/commitlog”
saved_caches_directory “/var/lib/cassandra/saved_caches”

Make sure these directories exist and can be written to, as shown below.

Create Directories

As super-user, create the two directories /var/lib/cassandra and /var./log/cassandra into which Cassandra writes its data.

[root@linux cassandra]# mkdir /var/lib/cassandra
[root@linux cassandra]# mkdir /var/log/cassandra

Give Permissions to Folders

Give read-write permissions to the newly created folders as shown below.

[root@linux /]# chmod 777 /var/lib/cassandra
[root@linux /]# chmod 777 /var/log/cassandra

Start Cassandra

To start Cassandra, open the terminal window, navigate to Cassandra home directory/home, where you unpacked Cassandra, and run the following command to start your Cassandra server.

$ cd $CASSANDRA_HOME
$./bin/cassandra -f

Using the –f option tells Cassandra to stay in the foreground instead of running as a background process. If everything goes fine, you can see the Cassandra server starting.

Programming Environment

To set up Cassandra programmatically, download the following jar files −

slf4j-api-1.7.5.jar
cassandra-driver-core-2.0.2.jar
guava-16.0.1.jar
metrics-core-3.0.2.jar
netty-3.9.0.Final.jar

Place them in a separate folder. For example, we are downloading these jars to a folder named “Cassandra_jars”.

Set the classpath for this folder in “.bashrc”file as shown below.

[hadoop@linux ~]$ gedit ~/.bashrc

//Set the following class path in the .bashrc file.

export CLASSPATH = $CLASSPATH:/home/hadoop/Cassandra_jars/*

Eclipse Environment

Open Eclipse and create a new project called Cassandra _Examples.

Right click on the project, select Build Path→Configure Build Path as shown below.

It will open the properties window. Under Libraries tab, select Add External JARs. Navigate to the directory where you saved your jar files. Select all the five jar files and click OK as shown below.

Under Referenced Libraries, you can see all the required jars added as shown below −

Maven Dependencies

Given below is the pom.xml for building a Cassandra project using maven.

<project xmlns = "http://maven.apache.org/POM/4.0.0" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"  
   xsi:schemaLocation = "http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <build>
      <sourceDirectory>src</sourceDirectory>
      <plugins>
         <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.1</version>
				
               <configuration>
                  <source>1.7</source>
                  <target>1.7</target>
               </configuration>
					
         </plugin>
      </plugins>
   </build> 

   <dependencies>
      <dependency>
         <groupId>org.slf4j</groupId>
         <artifactId>slf4j-api</artifactId>
         <version>1.7.5</version>
      </dependency>
 
      <dependency>
         <groupId>com.datastax.cassandra</groupId>
         <artifactId>cassandra-driver-core</artifactId>
         <version>2.0.2</version>
      </dependency>
 
      <dependency>
         <groupId>com.google.guava</groupId>
         <artifactId>guava</artifactId>
         <version>16.0.1</version>
      </dependency>
 
      <dependency>
         <groupId>com.codahale.metrics</groupId>
         <artifactId>metrics-core</artifactId>
         <version>3.0.2</version>
      </dependency>
 
      <dependency>
         <groupId>io.netty</groupId>
         <artifactId>netty</artifactId>
         <version>3.9.0.Final</version>
      </dependency>
   </dependencies>

</project>

Cassandra - Referenced Api

This chapter covers all the important classes in Cassandra.

Cluster

This class is the main entry point of the driver. It belongs to com.datastax.driver.core package.

Methods

S. No.	Methods and Description
1	Session connect() It creates a new session on the current cluster and initializes it.
2	void close() It is used to close the cluster instance.
3	static Cluster.Builder builder() It is used to create a new Cluster.Builder instance.

S. No.

Methods and Description

Session connect()

It creates a new session on the current cluster and initializes it.

void close()

It is used to close the cluster instance.

static Cluster.Builder builder()

It is used to create a new Cluster.Builder instance.

Cluster.Builder

This class is used to instantiate the Cluster.Builder class.

Methods

S. No	Methods and Description
1	Cluster.Builder addContactPoint(String address) This method adds a contact point to cluster.
2	Cluster build() This method builds the cluster with the given contact points.

S. No

Methods and Description

Cluster.Builder addContactPoint(String address)

This method adds a contact point to cluster.

Cluster build()

This method builds the cluster with the given contact points.

Session

This interface holds the connections to Cassandra cluster. Using this interface, you can execute CQL queries. It belongs to com.datastax.driver.core package.

Methods

S. No.	Methods and Description
1	void close() This method is used to close the current session instance.
2	ResultSet execute(Statement statement) This method is used to execute a query. It requires a statement object.
3	ResultSet execute(String query) This method is used to execute a query. It requires a query in the form of a String object.
4	PreparedStatement prepare(RegularStatement statement) This method prepares the provided query. The query is to be provided in the form of a Statement.
5	PreparedStatement prepare(String query) This method prepares the provided query. The query is to be provided in the form of a String.

Cassandra - Cqlsh

This chapter introduces the Cassandra query language shell and explains how to use its commands.

By default, Cassandra provides a prompt Cassandra query language shell (cqlsh) that allows users to communicate with it. Using this shell, you can execute Cassandra Query Language (CQL).

Using cqlsh, you can

define a schema,
insert data, and
execute a query.

Starting cqlsh

Start cqlsh using the command cqlsh as shown below. It gives the Cassandra cqlsh prompt as output.

[hadoop@linux bin]$ cqlsh
Connected to Test Cluster at 127.0.0.1:9042.
[cqlsh 5.0.1 | Cassandra 2.1.2 | CQL spec 3.2.0 | Native protocol v3]
Use HELP for help.
cqlsh>

Cqlsh − As discussed above, this command is used to start the cqlsh prompt. In addition, it supports a few more options as well. The following table explains all the options of cqlsh and their usage.

Options	Usage
cqlsh --help	Shows help topics about the options of cqlsh commands.
cqlsh --version	Provides the version of the cqlsh you are using.
cqlsh --color	Directs the shell to use colored output.
cqlsh --debug	Shows additional debugging information.
cqlsh --execute cql_statement	Directs the shell to accept and execute a CQL command.
cqlsh --file= “file name”	If you use this option, Cassandra executes the command in the given file and exits.
cqlsh --no-color	Directs Cassandra not to use colored output.
cqlsh -u “user name”	Using this option, you can authenticate a user. The default user name is: cassandra.
cqlsh-p “pass word”	Using this option, you can authenticate a user with a password. The default password is: cassandra.

Cqlsh Commands

Cqlsh has a few commands that allow users to interact with it. The commands are listed below.

Documented Shell Commands

Given below are the Cqlsh documented shell commands. These are the commands used to perform tasks such as displaying help topics, exit from cqlsh, describe,etc.

HELP − Displays help topics for all cqlsh commands.
CAPTURE − Captures the output of a command and adds it to a file.
CONSISTENCY − Shows the current consistency level, or sets a new consistency level.
COPY − Copies data to and from Cassandra.
DESCRIBE − Describes the current cluster of Cassandra and its objects.
EXPAND − Expands the output of a query vertically.
EXIT − Using this command, you can terminate cqlsh.
PAGING − Enables or disables query paging.
SHOW − Displays the details of current cqlsh session such as Cassandra version, host, or data type assumptions.
SOURCE − Executes a file that contains CQL statements.
TRACING − Enables or disables request tracing.

CQL Data Definition Commands

CREATE KEYSPACE − Creates a KeySpace in Cassandra.
USE − Connects to a created KeySpace.
ALTER KEYSPACE − Changes the properties of a KeySpace.
DROP KEYSPACE − Removes a KeySpace
CREATE TABLE − Creates a table in a KeySpace.
ALTER TABLE − Modifies the column properties of a table.
DROP TABLE − Removes a table.
TRUNCATE − Removes all the data from a table.
CREATE INDEX − Defines a new index on a single column of a table.
DROP INDEX − Deletes a named index.

CQL Data Manipulation Commands

INSERT − Adds columns for a row in a table.
UPDATE − Updates a column of a row.
DELETE − Deletes data from a table.
BATCH − Executes multiple DML statements at once.

CQL Clauses

SELECT − This clause reads data from a table
WHERE − The where clause is used along with select to read a specific data.
ORDERBY − The orderby clause is used along with select to read a specific data in a specific order.

Cassandra - Shell Commands

Cassandra provides documented shell commands in addition to CQL commands. Given below are the Cassandra documented shell commands.

Help

The HELP command displays a synopsis and a brief description of all cqlsh commands. Given below is the usage of help command.

cqlsh> help

Documented shell commands:
===========================
CAPTURE COPY DESCRIBE EXPAND PAGING SOURCE
CONSISTENCY DESC EXIT HELP SHOW TRACING.

CQL help topics:
================
ALTER           CREATE_TABLE_OPTIONS       SELECT
ALTER_ADD       CREATE_TABLE_TYPES         SELECT_COLUMNFAMILY
ALTER_ALTER     CREATE_USER                SELECT_EXPR
ALTER_DROP      DELETE                     SELECT_LIMIT
ALTER_RENAME    DELETE_COLUMNS             SELECT_TABLE

Capture

This command captures the output of a command and adds it to a file. For example, take a look at the following code that captures the output to a file named Outputfile.

cqlsh> CAPTURE '/home/hadoop/CassandraProgs/Outputfile'

When we type any command in the terminal, the output will be captured by the file given. Given below is the command used and the snapshot of the output file.

cqlsh:howcodex> select * from emp;

You can turn capturing off using the following command.

cqlsh:howcodex> capture off;

Consistency

This command shows the current consistency level, or sets a new consistency level.

cqlsh:howcodex> CONSISTENCY
Current consistency level is 1.

Copy

This command copies data to and from Cassandra to a file. Given below is an example to copy the table named emp to the file myfile.

cqlsh:howcodex> COPY emp (emp_id, emp_city, emp_name, emp_phone,emp_sal) TO ‘myfile’;
4 rows exported in 0.034 seconds.

If you open and verify the file given, you can find the copied data as shown below.

Describe

This command describes the current cluster of Cassandra and its objects. The variants of this command are explained below.

Describe cluster − This command provides information about the cluster.

cqlsh:howcodex> describe cluster;

Cluster: Test Cluster
Partitioner: Murmur3Partitioner

Range ownership:
                  -658380912249644557 [127.0.0.1]
                  -2833890865268921414 [127.0.0.1]
                  -6792159006375935836 [127.0.0.1]

Describe Keyspaces − This command lists all the keyspaces in a cluster. Given below is the usage of this command.

cqlsh:howcodex> describe keyspaces;

system_traces system tp howcodex

Describe tables − This command lists all the tables in a keyspace. Given below is the usage of this command.

cqlsh:howcodex> describe tables;
emp

Describe table − This command provides the description of a table. Given below is the usage of this command.

cqlsh:howcodex> describe table emp;

CREATE TABLE howcodex.emp (
   emp_id int PRIMARY KEY,
   emp_city text,
   emp_name text,
   emp_phone varint,
   emp_sal varint
) WITH bloom_filter_fp_chance = 0.01
   AND caching = '{"keys":"ALL", "rows_per_partition":"NONE"}'
   AND comment = ''
   AND compaction = {'min_threshold': '4', 'class':
   'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy',
   'max_threshold': '32'}
	
   AND compression = {'sstable_compression':
   'org.apache.cassandra.io.compress.LZ4Compressor'}
	
   AND dclocal_read_repair_chance = 0.1
   AND default_time_to_live = 0
   AND gc_grace_seconds = 864000
   AND max_index_interval = 2048
   AND memtable_flush_period_in_ms = 0
   AND min_index_interval = 128
   AND read_repair_chance = 0.0
   AND speculative_retry = '99.0PERCENTILE';
CREATE INDEX emp_emp_sal_idx ON howcodex.emp (emp_sal);

Describe Type

This command is used to describe a user-defined data type. Given below is the usage of this command.

cqlsh:howcodex> describe type card_details;

CREATE TYPE howcodex.card_details (
   num int,
   pin int,
   name text,
   cvv int,
   phone set<int>,
   mail text
);

Describe Types

This command lists all the user-defined data types. Given below is the usage of this command. Assume there are two user-defined data types: card and card_details.

cqlsh:howcodex> DESCRIBE TYPES;

card_details card

Expand

This command is used to expand the output. Before using this command, you have to turn the expand command on. Given below is the usage of this command.

cqlsh:howcodex> expand on;
cqlsh:howcodex> select * from emp;

@ Row 1
-----------+------------
    emp_id | 1
  emp_city | Hyderabad
  emp_name | ram
 emp_phone | 9848022338
   emp_sal | 50000
  
@ Row 2
-----------+------------
    emp_id | 2
  emp_city | Delhi
  emp_name | robin
 emp_phone | 9848022339
   emp_sal | 50000
  
@ Row 3
-----------+------------
    emp_id | 4
  emp_city | Pune
  emp_name | rajeev
 emp_phone | 9848022331
   emp_sal | 30000
  
@ Row 4
-----------+------------
    emp_id | 3
  emp_city | Chennai
  emp_name | rahman
 emp_phone | 9848022330
   emp_sal | 50000
(4 rows)

Note − You can turn the expand option off using the following command.

cqlsh:howcodex> expand off;
Disabled Expanded output.

Exit

This command is used to terminate the cql shell.

Show

This command displays the details of current cqlsh session such as Cassandra version, host, or data type assumptions. Given below is the usage of this command.

cqlsh:howcodex> show host;
Connected to Test Cluster at 127.0.0.1:9042.

cqlsh:howcodex> show version;
[cqlsh 5.0.1 | Cassandra 2.1.2 | CQL spec 3.2.0 | Native protocol v3]

Source

Using this command, you can execute the commands in a file. Suppose our input file is as follows −

Then you can execute the file containing the commands as shown below.

cqlsh:howcodex> source '/home/hadoop/CassandraProgs/inputfile';

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |   ram    | 9848022338 | 50000
      2 | Delhi     |   robin  | 9848022339 | 50000
      3 | Pune      |   rajeev | 9848022331 | 30000
      4 | Chennai   |   rahman | 9848022330 | 50000
(4 rows)

Cassandra - Create Keyspace

Creating a Keyspace using Cqlsh

A keyspace in Cassandra is a namespace that defines data replication on nodes. A cluster contains one keyspace per node. Given below is the syntax for creating a keyspace using the statement CREATE KEYSPACE.

Syntax

CREATE KEYSPACE <identifier> WITH <properties>

i.e.

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

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

AND durable_writes = ‘Boolean value’;

The CREATE KEYSPACE statement has two properties: replication and durable_writes.

Replication

The replication option is to specify the Replica Placement strategy and the number of replicas wanted. The following table lists all the replica placement strategies.

Strategy name	Description
Simple Strategy'	Specifies a simple replication factor for the cluster.
Network Topology Strategy	Using this option, you can set the replication factor for each data-center independently.
Old Network Topology Strategy	This is a legacy replication strategy.

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 creating a KeySpace.

Here we are creating a KeySpace named Howcodex.
We are using the first replica placement strategy, i.e.., Simple Strategy.
And we are choosing the replication factor to 1 replica.

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

Verification

You can verify whether the table is created or not using the command Describe. If you use this command over keyspaces, it will display all the keyspaces created as shown below.

cqlsh> DESCRIBE keyspaces;

howcodex system system_traces

Here you can observe the newly created KeySpace howcodex.

Durable_writes

By default, the durable_writes properties of a table is set to true, however it can be set to false. You cannot set this property to simplex strategy.

Example

Given below is the example demonstrating the usage of durable writes property.

cqlsh> CREATE KEYSPACE test
... WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy', 'datacenter1' : 3 }
... AND DURABLE_WRITES = false;

Verification

You can verify whether the durable_writes property of test KeySpace was set to false by querying the System Keyspace. This query gives you all the KeySpaces along with their properties.

cqlsh> 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)

Here you can observe the durable_writes property of test KeySpace was set to false.

Using a Keyspace

You can use a created KeySpace using the keyword USE. Its syntax is as follows −

Syntax:USE <identifier>

Example

In the following example, we are using the KeySpace howcodex.

cqlsh> USE howcodex;
cqlsh:howcodex>

Creating a Keyspace using Java API

You can create a Keyspace using the execute() method of Session class. Follow the steps given below to create 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 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 a cluster object in 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 Cluster class 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 creating a KeySpace named tp. We are using the first replica placement strategy, i.e., Simple Strategy, and we are choosing the replication factor to 1 replica.

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

String query = "CREATE KEYSPACE tp WITH replication "
   + "= {'class':'SimpleStrategy', 'replication_factor':1}; ";
session.execute(query);

Step4 : Use the KeySpace

You can use a created KeySpace using the execute() method as shown below.

execute(“ USE tp ” );

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 Create_KeySpace {

   public static void main(String args[]){

      //Query
      String query = "CREATE KEYSPACE tp WITH replication "
         + "= {'class':'SimpleStrategy', 'replication_factor':1};";
                    
      //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);
     
      //using the KeySpace
      session.execute("USE tp");
      System.out.println("Keyspace created"); 
   }
}

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 Create_KeySpace.java
$java Create_KeySpace

Under normal conditions, it will produce the following output −

Keyspace created

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

Cassandra - Drop Keyspace

Dropping a Keyspace

You can drop a KeySpace using the command DROP KEYSPACE. Given below is the syntax for dropping a KeySpace.

Syntax

DROP KEYSPACE <identifier>

i.e.

DROP KEYSPACE “KeySpace name”

Example

The following code deletes the keyspace howcodex.

cqlsh> DROP KEYSPACE howcodex;

Verification

Verify the keyspaces using the command Describe and check whether the table is dropped as shown below.

cqlsh> DESCRIBE keyspaces;

system system_traces

Since we have deleted the keyspace howcodex, you will not find it in the keyspaces list.

Dropping a Keyspace using Java API

You can create a keyspace using the execute() method of Session class. Follow the steps given below to drop 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 a 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 Cluster class 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 the following example, we are deleting a keyspace named tp. You have to store the query in a string variable and pass it to the execute() method as shown below.

String query = "DROP KEYSPACE tp; ";

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 Drop_KeySpace {

   public static void main(String args[]){

      //Query
      String query = "Drop KEYSPACE tp";

      //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 deleted");
   }
}

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 Delete_KeySpace.java
$java Delete_KeySpace

Under normal conditions, it should produce the following output −

Keyspace deleted

Cassandra - Create Table

Creating a Table

You can create a table using the command CREATE TABLE. Given below is the syntax for creating a table.

Syntax

CREATE (TABLE | COLUMNFAMILY) <tablename>
('<column-definition>' , '<column-definition>')
(WITH <option> AND <option>)

Defining a Column

You can define a column as shown below.

column name1 data type,
column name2 data type,

example:

age int,
name text

Primary Key

The primary key is a column that is used to uniquely identify a row. Therefore,defining a primary key is mandatory while creating a table. A primary key is made of one or more columns of a table. You can define a primary key of a table as shown below.

CREATE TABLE tablename(
   column1 name datatype PRIMARYKEY,
   column2 name data type,
   column3 name data type.
   )

CREATE TABLE tablename(
   column1 name datatype PRIMARYKEY,
   column2 name data type,
   column3 name data type,
   PRIMARY KEY (column1)
   )

Example

Given below is an example to create a table in Cassandra using cqlsh. Here we are −

Using the keyspace howcodex
Creating a table named emp

It will have details such as employee name, id, city, salary, and phone number. Employee id is the primary key.

cqlsh> USE howcodex;
cqlsh:howcodex>; CREATE TABLE emp(
   emp_id int PRIMARY KEY,
   emp_name text,
   emp_city text,
   emp_sal varint,
   emp_phone varint
   );

Verification

The select statement will give you the schema. Verify the table using the select statement as shown below.

cqlsh:howcodex> select * from emp;

 emp_id | emp_city | emp_name | emp_phone | emp_sal
--------+----------+----------+-----------+---------

(0 rows)

Here you can observe the table created with the given columns. Since we have deleted the keyspace howcodex, you will not find it in the keyspaces list.

Creating a Table using Java API

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

Step1: Create a Cluster Object

First of all, create an instance of the 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 a 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 Cluster class 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 ” );

Here we are using the keyspace named tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“ tp” );

Step 3: Execute Query

In the following example, we are creating a table named emp. You have to store the query in a string variable and pass it to the execute() method as shown below.

//Query
String query = "CREATE TABLE emp(emp_id int PRIMARY KEY, "
   + "emp_name text, "
   + "emp_city text, "
   + "emp_sal varint, "
   + "emp_phone varint );";
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 Create_Table {

   public static void main(String args[]){

      //Query
      String query = "CREATE TABLE emp(emp_id int PRIMARY KEY, "
         + "emp_name text, "
         + "emp_city text, "
         + "emp_sal varint, "
         + "emp_phone varint );";
		
      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
   
      //Creating Session object
      Session session = cluster.connect("tp");
 
      //Executing the query
      session.execute(query);
 
      System.out.println("Table created");
   }
}

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 Create_Table.java
$java Create_Table

Under normal conditions, it should produce the following output −

Table created

Cassandra - Alter Table

Altering a Table

You can alter a table using the command ALTER TABLE. Given below is the syntax for creating a table.

Syntax

ALTER (TABLE | COLUMNFAMILY) <tablename> <instruction>

Using ALTER command, you can perform the following operations −

Add a column
Drop a column

Adding a Column

Using ALTER command, you can add a column to a table. While adding columns, you have to take care that the column name is not conflicting with the existing column names and that the table is not defined with compact storage option. Given below is the syntax to add a column to a table.

ALTER TABLE table name
ADD  new column datatype;

Example

Given below is an example to add a column to an existing table. Here we are adding a column called emp_email of text datatype to the table named emp.

cqlsh:howcodex> ALTER TABLE emp
   ... ADD emp_email text;

Verification

Use the SELECT statement to verify whether the column is added or not. Here you can observe the newly added column emp_email.

cqlsh:howcodex> select * from emp;

 emp_id | emp_city | emp_email | emp_name | emp_phone | emp_sal
--------+----------+-----------+----------+-----------+---------

Dropping a Column

Using ALTER command, you can delete a column from a table. Before dropping a column from a table, check that the table is not defined with compact storage option. Given below is the syntax to delete a column from a table using ALTER command.

ALTER table name
DROP column name;

Example

Given below is an example to drop a column from a table. Here we are deleting the column named emp_email.

cqlsh:howcodex> ALTER TABLE emp DROP emp_email;

Verification

Verify whether the column is deleted using the select statement, as shown below.

cqlsh:howcodex> select * from emp;

 emp_id | emp_city | emp_name | emp_phone | emp_sal
--------+----------+----------+-----------+---------
(0 rows)

Since emp_email column has been deleted, you cannot find it anymore.

Altering a Table using Java API

You can create a table using the execute() method of Session class. Follow the steps given below to alter a table 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 a 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 Cluster class 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 ” );
Session session = cluster.connect(“ tp” );

Here we are using the KeySpace named tp. Therefore, create the session object as shown below.

Step 3: Execute Query

In the following example, we are adding a column to a table named emp. To do so, you have to store the query in a string variable and pass it to the execute() method as shown below.

//Query
String query1 = "ALTER TABLE emp ADD emp_email text";
session.execute(query);

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

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

public class Add_column {

   public static void main(String args[]){

      //Query
      String query = "ALTER TABLE emp ADD emp_email text";

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

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 Add_Column.java
$java Add_Column

Under normal conditions, it should produce the following output −

Column added

Deleting a Column

Given below is the complete program to delete a column from an existing table.

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

public class Delete_Column {

   public static void main(String args[]){

      //Query
      String query = "ALTER TABLE emp DROP emp_email;";

      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
 
      //Creating Session object
      Session session = cluster.connect("tp");
 
      //executing the query
      session.execute(query);
    
      System.out.println("Column deleted");
   }
 }

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 Delete_Column.java
$java Delete_Column

Under normal conditions, it should produce the following output −

Column deleted

Cassandra - Drop Table

Dropping a Table

You can drop a table using the command Drop Table. Its syntax is as follows −

Syntax

DROP TABLE <tablename>

Example

The following code drops an existing table from a KeySpace.

cqlsh:howcodex> DROP TABLE emp;

Verification

Use the Describe command to verify whether the table is deleted or not. Since the emp table has been deleted, you will not find it in the column families list.

cqlsh:howcodex> DESCRIBE COLUMNFAMILIES;
employee

Deleting a Table using Java API

You can delete a table using the execute() method of Session class. Follow the steps given below to delete a table 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 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 a 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 Cluster class 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”);

Here we are using the keyspace named tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“tp”);

Step 3: Execute Query

You can execute CQL queries using 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 the following example, we are deleting a table named emp. You have to store the query in a string variable and pass it to the execute() method as shown below.

// Query

String query = "DROP TABLE emp1;”;
session.execute(query);

Given below is the complete program to drop a table in Cassandra using Java API.

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Session;
 
public class Drop_Table {

   public static void main(String args[]){

      //Query
      String query = "DROP TABLE emp1;";
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
    
      //Creating Session object
      Session session = cluster.connect("tp");
   
      //Executing the query
      session.execute(query);
    
      System.out.println("Table dropped");
   }
}

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 Drop_Table.java
$java Drop_Table

Under normal conditions, it should produce the following output −

Table dropped

Cassandra - Truncate Table

Truncating a Table

You can truncate a table using the TRUNCATE command. When you truncate a table, all the rows of the table are deleted permanently. Given below is the syntax of this command.

Syntax

TRUNCATE <tablename>

Example

Let us assume there is a table called student with the following data.

s_id	s_name	s_branch	s_aggregate
1	ram	IT	70
2	rahman	EEE	75
3	robbin	Mech	72

When you execute the select statement to get the table student, it will give you the following output.

cqlsh:tp> select * from student;

 s_id | s_aggregate | s_branch | s_name
------+-------------+----------+--------
    1 |          70 |       IT | ram
    2 |          75 |      EEE | rahman
    3 |          72 |     MECH | robbin

(3 rows)

Now truncate the table using the TRUNCATE command.

cqlsh:tp> TRUNCATE student;

Verification

Verify whether the table is truncated by executing the select statement. Given below is the output of the select statement on the student table after truncating.

cqlsh:tp> select * from student;

 s_id | s_aggregate | s_branch | s_name
------+-------------+----------+--------

(0 rows)

Truncating a Table using Java API

You can truncate a table using the execute() method of Session class. Follow the steps given below to truncate a table.

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 a cluster object using a single line of code as shown below.

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

Step 2: Creating a Session Object

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

Session session = cluster.connect( );

This method creates a new session and initializes it. If you already have a keyspace, then 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 ” );
Session session = cluster.connect(“ tp” );

Here we are using the keyspace named tp. Therefore, create the session object as shown below.

Step 3: Execute Query

In the following example, we are truncating a table named emp. You have to store the query in a string variable and pass it to the execute() method as shown below.

//Query
String query = "TRUNCATE emp;;”;
session.execute(query);

Given below is the complete program to truncate a table in Cassandra using Java API.

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

public class Truncate_Table {

   public static void main(String args[]){
   
      //Query
      String query = "Truncate student;";
   
      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
   
      //Creating Session object
      Session session = cluster.connect("tp");
   
      //Executing the query
      session.execute(query);
      System.out.println("Table truncated");
   }
}

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 Truncate_Table.java
$java Truncate_Table

Under normal conditions, it should produce the following output −

Table truncated

Cassandra - Create Index

Creating an Index using Cqlsh

You can create an index in Cassandra using the command CREATE INDEX. Its syntax is as follows −

CREATE INDEX <identifier> ON <tablename>

Given below is an example to create an index to a column. Here we are creating an index to a column ‘emp_name’ in a table named emp.

cqlsh:howcodex> CREATE INDEX name ON emp1 (emp_name);

Creating an Index using Java API

You can create an index to a column of a table using the execute() method of Session class. Follow the steps given below to create an index to a column in a table.

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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace called tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“ tp” );

Step 3: Execute Query

In the following example, we are creating an index to a column called emp_name, in a table named emp. You have to store the query in a string variable and pass it to the execute() method as shown below.

//Query
String query = "CREATE INDEX name ON emp1 (emp_name);";
session.execute(query);

Given below is the complete program to create an index of a column in a table in Cassandra using Java API.

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

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

      //Query
      String query = "CREATE INDEX name ON emp1 (emp_name);";
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
       
      //Creating Session object
      Session session = cluster.connect("tp");
 
      //Executing the query
      session.execute(query);
      System.out.println("Index created");
   }
}

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 Create_Index.java
$java Create_Index

Under normal conditions, it should produce the following output −

Index created

Cassandra - Drop Index

Dropping an Index

You can drop an index using the command DROP INDEX. Its syntax is as follows −

DROP INDEX <identifier>

Given below is an example to drop an index of a column in a table. Here we are dropping the index of the column name in the table emp.

cqlsh:tp> drop index name;

Dropping an Index using Java API

You can drop an index of a table using the execute() method of Session class. Follow the steps given below to drop an index from a table.

Step1: Create a Cluster Object

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 a 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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace named tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“ tp” );

Step 3: Execute Query

In the following example, we are dropping an index “name” of emp table. You have to store the query in a string variable and pass it to the execute() method as shown below.

//Query
String query = "DROP INDEX user_name;";
session.execute(query);

Given below is the complete program to drop an index in Cassandra using Java API.

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

public class Drop_Index {

   public static void main(String args[]){
     
      //Query
      String query = "DROP INDEX user_name;";
 
      //Creating cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();.
 
      //Creating Session object
      Session session = cluster.connect("tp"); 
 
      //Executing the query
      session.execute(query);
      
      System.out.println("Index dropped");
   }
}

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 Drop_index.java
$java Drop_index

Under normal conditions, it should produce the following output −

Index dropped

Cassandra - Batch Statements

Using Batch Statements

Using BATCH, you can execute multiple modification statements (insert, update, delete) simultaneiously. Its syntax is as follows −

BEGIN BATCH
<insert-stmt>/ <update-stmt>/ <delete-stmt>
APPLY BATCH

Example

Assume there is a table in Cassandra called emp having the following data −

emp_id	emp_name	emp_city	emp_phone	emp_sal
1	ram	Hyderabad	9848022338	50000
2	robin	Delhi	9848022339	50000
3	rahman	Chennai	9848022330	45000

In this example, we will perform the following operations −

Insert a new row with the following details (4, rajeev, pune, 9848022331, 30000).
Update the salary of employee with row id 3 to 50000.
Delete city of the employee with row id 2.

To perform the above operations in one go, use the following BATCH command −

cqlsh:howcodex> BEGIN BATCH
... INSERT INTO emp (emp_id, emp_city, emp_name, emp_phone, emp_sal) values(  4,'Pune','rajeev',9848022331, 30000);
... UPDATE emp SET emp_sal = 50000 WHERE emp_id =3;
... DELETE emp_city FROM emp WHERE emp_id = 2;
... APPLY BATCH;

Verification

After making changes, verify the table using the SELECT statement. It should produce the following output −

cqlsh:howcodex> select * from emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad | ram      | 9848022338 | 50000
      2 | null      | robin    | 9848022339 | 50000
      3 | Chennai   | rahman   | 9848022330 | 50000
      4 | Pune      | rajeev   | 9848022331 | 30000
    
(4 rows)

Here you can observe the table with modified data.

Batch Statements using Java API

Batch statements can be written programmatically in a table using the execute() method of Session class. Follow the steps given below to execute multiple statements using batch statement with the help of Java API.

Step1: Create a Cluster Object

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. Use the following code to create the 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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace named tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“tp”);

Step 3: Execute Query

In this example, we will perform the following operations −

Insert a new row with the following details (4, rajeev, pune, 9848022331, 30000).
Update the salary of employee with row id 3 to 50000.
Delete the city of the employee with row id 2.

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

String query1 = ” BEGIN BATCH INSERT INTO emp (emp_id, emp_city, emp_name,   emp_phone, emp_sal) values( 4,'Pune','rajeev',9848022331, 30000);
UPDATE emp SET emp_sal = 50000 WHERE emp_id =3;
DELETE emp_city FROM emp WHERE emp_id = 2;
APPLY BATCH;”;

Given below is the complete program to execute multiple statements simultaneously on a table in Cassandra using Java API.

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

public class Batch {

   public static void main(String args[]){
    
      //query
      String query =" BEGIN BATCH INSERT INTO emp (emp_id, emp_city,
         emp_name, emp_phone, emp_sal) values( 4,'Pune','rajeev',9848022331, 30000);"
    
         + "UPDATE emp SET emp_sal = 50000 WHERE emp_id =3;"
         + "DELETE emp_city FROM emp WHERE emp_id = 2;"
         + "APPLY BATCH;";

      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
 
      //Creating Session object
      Session session = cluster.connect("tp");
 
      //Executing the query
      session.execute(query);

      System.out.println("Changes done");
   }
}

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 Batch.java
$java Batch

Under normal conditions, it should produce the following output −

Changes done

Cassandra - Create Data

Creating Data in a Table

You can insert data into the columns of a row in a table using the command INSERT. Given below is the syntax for creating data in a table.

INSERT INTO <tablename>
(<column1 name>, <column2 name>....)
VALUES (<value1>, <value2>....)
USING <option>

Example

Let us assume there is a table called emp with columns (emp_id, emp_name, emp_city, emp_phone, emp_sal) and you have to insert the following data into the emp table.

emp_id	emp_name	emp_city	emp_phone	emp_sal
1	ram	Hyderabad	9848022338	50000
2	robin	Hyderabad	9848022339	40000
3	rahman	Chennai	9848022330	45000

Use the commands given below to fill the table with required data.

cqlsh:howcodex> INSERT INTO emp (emp_id, emp_name, emp_city,
   emp_phone, emp_sal) VALUES(1,'ram', 'Hyderabad', 9848022338, 50000);

cqlsh:howcodex> INSERT INTO emp (emp_id, emp_name, emp_city,
   emp_phone, emp_sal) VALUES(2,'robin', 'Hyderabad', 9848022339, 40000);

cqlsh:howcodex> INSERT INTO emp (emp_id, emp_name, emp_city,
   emp_phone, emp_sal) VALUES(3,'rahman', 'Chennai', 9848022330, 45000);

Verification

After inserting data, use SELECT statement to verify whether the data has been inserted or not. If you verify the emp table using SELECT statement, it will give you the following output.

cqlsh:howcodex> SELECT * FROM emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |      ram | 9848022338 | 50000
      2 | Hyderabad |    robin | 9848022339 | 40000
      3 |   Chennai |   rahman | 9848022330 | 45000
 
(3 rows)

Here you can observe the table has populated with the data we inserted.

Creating Data using Java API

You can create data in a table using the execute() method of Session class. Follow the steps given below to create data in a table using java API.

Step1: Create a Cluster Object

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 a 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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace called tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“ tp” );

Step 3: Execute Query

In the following example, we are inserting data in a table called emp. You have to store the query in a string variable and pass it to the execute() method as shown below.

String query1 = “INSERT INTO emp (emp_id, emp_name, emp_city, emp_phone, emp_sal)
   VALUES(1,'ram', 'Hyderabad', 9848022338, 50000);” ;
 
String query2 = “INSERT INTO emp (emp_id, emp_name, emp_city, emp_phone, emp_sal)
   VALUES(2,'robin', 'Hyderabad', 9848022339, 40000);” ;
 
String query3 = “INSERT INTO emp (emp_id, emp_name, emp_city, emp_phone, emp_sal)
   VALUES(3,'rahman', 'Chennai', 9848022330, 45000);” ;
 
session.execute(query1);
session.execute(query2);
session.execute(query3);

Given below is the complete program to insert data into a table in Cassandra using Java API.

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

public class Create_Data {

   public static void main(String args[]){

      //queries
      String query1 = "INSERT INTO emp (emp_id, emp_name, emp_city, emp_phone,  emp_sal)"
		
         + " VALUES(1,'ram', 'Hyderabad', 9848022338, 50000);" ;
                             
      String query2 = "INSERT INTO emp (emp_id, emp_name, emp_city,
         emp_phone, emp_sal)"
      
         + " VALUES(2,'robin', 'Hyderabad', 9848022339, 40000);" ;
                             
      String query3 = "INSERT INTO emp (emp_id, emp_name, emp_city, emp_phone, emp_sal)"
       
         + " VALUES(3,'rahman', 'Chennai', 9848022330, 45000);" ;

      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
 
      //Creating Session object
      Session session = cluster.connect("tp");
       
      //Executing the query
      session.execute(query1);
        
      session.execute(query2);
        
      session.execute(query3);
        
      System.out.println("Data created");
   }
}

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 Create_Data.java
$java Create_Data

Under normal conditions, it should produce the following output −

Data created

Cassandra - Update Data

Updating Data in a Table

UPDATE is the command used to update data in a table. The following keywords are used while updating data in a table −

Where − This clause is used to select the row to be updated.
Set − Set the value using this keyword.
Must − Includes all the columns composing the primary key.

While updating rows, if a given row is unavailable, then UPDATE creates a fresh row. Given below is the syntax of UPDATE command −

UPDATE <tablename>
SET <column name> = <new value>
<column name> = <value>....
WHERE <condition>

Example

Assume there is a table named emp. This table stores the details of employees of a certain company, and it has the following details −

emp_id	emp_name	emp_city	emp_phone	emp_sal
1	ram	Hyderabad	9848022338	50000
2	robin	Hyderabad	9848022339	40000
3	rahman	Chennai	9848022330	45000

Let us now update emp_city of robin to Delhi, and his salary to 50000. Given below is the query to perform the required updates.

cqlsh:howcodex> UPDATE emp SET emp_city='Delhi',emp_sal=50000
   WHERE emp_id=2;

Verification

Use SELECT statement to verify whether the data has been updated or not. If you verify the emp table using SELECT statement, it will produce the following output.

cqlsh:howcodex> select * from emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |      ram | 9848022338 | 50000
      2 |     Delhi |    robin | 9848022339 | 50000
      3 |   Chennai |   rahman | 9848022330 | 45000
      
(3 rows)

Here you can observe the table data has got updated.

Updating Data using Java API

You can update data in a table using the execute() method of Session class. Follow the steps given below to update data in a table using Java API.

Step1: Create a Cluster Object

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. Use the following code to create the 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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace named tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“tp”);

Step 3: Execute Query

In the following example, we are updating the emp table. You have to store the query in a string variable and pass it to the execute() method as shown below:

String query = “ UPDATE emp SET emp_city='Delhi',emp_sal=50000
WHERE emp_id = 2;” ;

Given below is the complete program to update data in a table using Java API.

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

public class Update_Data {
  
   public static void main(String args[]){
      
      //query
      String query = " UPDATE emp SET emp_city='Delhi',emp_sal=50000"
          
      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
         
      //Creating Session object
      Session session = cluster.connect("tp");
         
      //Executing the query
      session.execute(query);

      System.out.println("Data updated");
   }
 }

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 Update_Data.java
$java Update_Data

Under normal conditions, it should produce the following output −

Data updated

Cassandra - Read Data

Reading Data using Select Clause

SELECT clause is used to read data from a table in Cassandra. Using this clause, you can read a whole table, a single column, or a particular cell. Given below is the syntax of SELECT clause.

SELECT FROM <tablename>

Example

Assume there is a table in the keyspace named emp with the following details −

emp_id	emp_name	emp_city	emp_phone	emp_sal
1	ram	Hyderabad	9848022338	50000
2	robin	null	9848022339	50000
3	rahman	Chennai	9848022330	50000
4	rajeev	Pune	9848022331	30000

The following example shows how to read a whole table using SELECT clause. Here we are reading a table called emp.

cqlsh:howcodex> select * from emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |   ram    | 9848022338 | 50000
      2 | null      |   robin  | 9848022339 | 50000
      3 | Chennai   |   rahman | 9848022330 | 50000
      4 | Pune      |   rajeev | 9848022331 | 30000
		
(4 rows)

Reading Required Columns

The following example shows how to read a particular column in a table.

cqlsh:howcodex> SELECT emp_name, emp_sal from emp;

 emp_name | emp_sal
----------+---------
      ram | 50000
    robin | 50000
   rajeev | 30000
   rahman | 50000 
	
(4 rows)

Where Clause

Using WHERE clause, you can put a constraint on the required columns. Its syntax is as follows −

SELECT FROM <table name> WHERE <condition>;

Note − A WHERE clause can be used only on the columns that are a part of primary key or have a secondary index on them.

In the following example, we are reading the details of an employee whose salary is 50000. First of all, set secondary index to the column emp_sal.

cqlsh:howcodex> CREATE INDEX ON emp(emp_sal);
cqlsh:howcodex> SELECT * FROM emp WHERE emp_sal=50000;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |      ram | 9848022338 | 50000
      2 |      null |    robin | 9848022339 | 50000
      3 |   Chennai |   rahman | 9848022330 | 50000

Reading Data using Java API

You can read data from a table using the execute() method of Session class. Follow the steps given below to execute multiple statements using batch statement with the help of Java API.

Step1:Create a Cluster Object

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. Use the following code to create the 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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace called tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“tp”);

Step 3: Execute Query

In this example, we are retrieving the data from emp table. Store the query in a string and pass it to the execute() method of session class as shown below.

String query = ”SELECT 8 FROM emp”;
session.execute(query);

Execute the query using the execute() method of Session class.

Step 4: Get the ResultSet Object

The select queries will return the result in the form of a ResultSet object, therefore store the result in the object of RESULTSET class as shown below.

ResultSet result = session.execute( );

Given below is the complete program to read data from a table.

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

public class Read_Data {

   public static void main(String args[])throws Exception{
    
      //queries
      String query = "SELECT * FROM emp";

      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
    
      //Creating Session object
      Session session = cluster.connect("howcodex");
    
      //Getting the ResultSet
      ResultSet result = session.execute(query);
    
      System.out.println(result.all());
   }
}

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 Read_Data.java
$java Read_Data

Under normal conditions, it should produce the following output −

[Row[1, Hyderabad, ram, 9848022338, 50000], Row[2, Delhi, robin,
9848022339, 50000], Row[4, Pune, rajeev, 9848022331, 30000], Row[3,
Chennai, rahman, 9848022330, 50000]]

Cassandra - Read Data

Reading Data using Select Clause

SELECT clause is used to read data from a table in Cassandra. Using this clause, you can read a whole table, a single column, or a particular cell. Given below is the syntax of SELECT clause.

SELECT FROM <tablename>

Example

Assume there is a table in the keyspace named emp with the following details −

emp_id	emp_name	emp_city	emp_phone	emp_sal
1	ram	Hyderabad	9848022338	50000
2	robin	null	9848022339	50000
3	rahman	Chennai	9848022330	50000
4	rajeev	Pune	9848022331	30000

The following example shows how to read a whole table using SELECT clause. Here we are reading a table called emp.

cqlsh:howcodex> select * from emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |   ram    | 9848022338 | 50000
      2 | null      |   robin  | 9848022339 | 50000
      3 | Chennai   |   rahman | 9848022330 | 50000
      4 | Pune      |   rajeev | 9848022331 | 30000
		
(4 rows)

Reading Required Columns

The following example shows how to read a particular column in a table.

cqlsh:howcodex> SELECT emp_name, emp_sal from emp;

 emp_name | emp_sal
----------+---------
      ram | 50000
    robin | 50000
   rajeev | 30000
   rahman | 50000 
	
(4 rows)

Where Clause

Using WHERE clause, you can put a constraint on the required columns. Its syntax is as follows −

SELECT FROM <table name> WHERE <condition>;

Note − A WHERE clause can be used only on the columns that are a part of primary key or have a secondary index on them.

In the following example, we are reading the details of an employee whose salary is 50000. First of all, set secondary index to the column emp_sal.

cqlsh:howcodex> CREATE INDEX ON emp(emp_sal);
cqlsh:howcodex> SELECT * FROM emp WHERE emp_sal=50000;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |      ram | 9848022338 | 50000
      2 |      null |    robin | 9848022339 | 50000
      3 |   Chennai |   rahman | 9848022330 | 50000

Reading Data using Java API

You can read data from a table using the execute() method of Session class. Follow the steps given below to execute multiple statements using batch statement with the help of Java API.

Step1:Create a Cluster Object

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. Use the following code to create the 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 Cluster class as shown below.

Session session = cluster.connect( );

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

Here we are using the KeySpace called tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“tp”);

Step 3: Execute Query

In this example, we are retrieving the data from emp table. Store the query in a string and pass it to the execute() method of session class as shown below.

String query = ”SELECT 8 FROM emp”;
session.execute(query);

Execute the query using the execute() method of Session class.

Step 4: Get the ResultSet Object

The select queries will return the result in the form of a ResultSet object, therefore store the result in the object of RESULTSET class as shown below.

ResultSet result = session.execute( );

Given below is the complete program to read data from a table.

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

public class Read_Data {

   public static void main(String args[])throws Exception{
    
      //queries
      String query = "SELECT * FROM emp";

      //Creating Cluster object
      Cluster cluster = Cluster.builder().addContactPoint("127.0.0.1").build();
    
      //Creating Session object
      Session session = cluster.connect("howcodex");
    
      //Getting the ResultSet
      ResultSet result = session.execute(query);
    
      System.out.println(result.all());
   }
}

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 Read_Data.java
$java Read_Data

Under normal conditions, it should produce the following output −

[Row[1, Hyderabad, ram, 9848022338, 50000], Row[2, Delhi, robin,
9848022339, 50000], Row[4, Pune, rajeev, 9848022331, 30000], Row[3,
Chennai, rahman, 9848022330, 50000]]

Cassandra - Delete Data

Deleting Datafrom a Table

You can delete data from a table using the command DELETE. Its syntax is as follows −

DELETE FROM <identifier> WHERE <condition>;

Example

Let us assume there is a table in Cassandra called emp having the following data −

emp_id	emp_name	emp_city	emp_phone	emp_sal
1	ram	Hyderabad	9848022338	50000
2	robin	Hyderabad	9848022339	40000
3	rahman	Chennai	9848022330	45000

The following statement deletes the emp_sal column of last row −

cqlsh:howcodex> DELETE emp_sal FROM emp WHERE emp_id=3;

Verification

Use SELECT statement to verify whether the data has been deleted or not. If you verify the emp table using SELECT, it will produce the following output −

cqlsh:howcodex> select * from emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |      ram | 9848022338 | 50000
      2 |     Delhi |    robin | 9848022339 | 50000
      3 |   Chennai |   rahman | 9848022330 | null
(3 rows)

Since we have deleted the salary of Rahman, you will observe a null value in place of salary.

Deleting an Entire Row

The following command deletes an entire row from a table.

cqlsh:howcodex> DELETE FROM emp WHERE emp_id=3;

Verification

Use SELECT statement to verify whether the data has been deleted or not. If you verify the emp table using SELECT, it will produce the following output −

cqlsh:howcodex> select * from emp;

 emp_id |  emp_city | emp_name |  emp_phone | emp_sal
--------+-----------+----------+------------+---------
      1 | Hyderabad |      ram | 9848022338 | 50000
      2 |     Delhi |    robin | 9848022339 | 50000
 
(2 rows)

Since we have deleted the last row, there are only two rows left in the table.

Deleting Data using Java API

You can delete data in a table using the execute() method of Session class. Follow the steps given below to delete data from a table using java API.

Step1: Create a Cluster Object

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. Use the following code 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 Cluster class as shown below.

Session session = cluster.connect();

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

Here we are using the KeySpace called tp. Therefore, create the session object as shown below.

Session session = cluster.connect(“tp”);

Step 3: Execute Query

In the following example, we are deleting data from a table named emp. You have to store the query in a string variable and pass it to the execute() method as shown below.

String query1 = ”DELETE FROM emp WHERE emp_id=3; ”;
session.execute(query);

Given below is the complete program to delete data from a table in Cassandra using Java API.

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

public class Delete_Data {

   public static void main(String args[]){
   
      //query
      String query = "DELETE FROM emp WHERE emp_id=3;";

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

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 Delete_Data.java
$java Delete_Data

Under normal conditions, it should produce the following output −

Data deleted

Cassandra - CQL Datatypes

CQL provides a rich set of built-in data types, including collection types. Along with these data types, users can also create their own custom data types. The following table provides a list of built-in data types available in CQL.

Data Type	Constants	Description
ascii	strings	Represents ASCII character string
bigint	bigint	Represents 64-bit signed long
blob	blobs	Represents arbitrary bytes
Boolean	booleans	Represents true or false
counter	integers	Represents counter column
decimal	integers, floats	Represents variable-precision decimal
double	integers	Represents 64-bit IEEE-754 floating point
float	integers, floats	Represents 32-bit IEEE-754 floating point
inet	strings	Represents an IP address, IPv4 or IPv6
int	integers	Represents 32-bit signed int
text	strings	Represents UTF8 encoded string
timestamp	integers, strings	Represents a timestamp
timeuuid	uuids	Represents type 1 UUID
uuid	uuids	Represents type 1 or type 4
		UUID
varchar	strings	Represents uTF8 encoded string
varint	integers	Represents arbitrary-precision integer

Collection Types

Cassandra Query Language also provides a collection data types. The following table provides a list of Collections available in CQL.

Collection	Description
list	A list is a collection of one or more ordered elements.
map	A map is a collection of key-value pairs.
set	A set is a collection of one or more elements.

User-defined datatypes

Cqlsh provides users a facility of creating their own data types. Given below are the commands used while dealing with user defined datatypes.

CREATE TYPE − Creates a user-defined datatype.
ALTER TYPE − Modifies a user-defined datatype.
DROP TYPE − Drops a user-defined datatype.
DESCRIBE TYPE − Describes a user-defined datatype.
DESCRIBE TYPES − Describes user-defined datatypes.

Cassandra - CQL Collections

CQL provides the facility of using Collection data types. Using these Collection types, you can store multiple values in a single variable. This chapter explains how to use Collections in Cassandra.

List

List is used in the cases where

the order of the elements is to be maintained, and
a value is to be stored multiple times.

You can get the values of a list data type using the index of the elements in the list.

Creating a Table with List

Given below is an example to create a sample table with two columns, name and email. To store multiple emails, we are using list.

cqlsh:howcodex> CREATE TABLE data(name text PRIMARY KEY, email list<text>);

Inserting Data into a List

While inserting data into the elements in a list, enter all the values separated by comma within square braces [ ] as shown below.

cqlsh:howcodex> INSERT INTO data(name, email) VALUES ('ramu',
['abc@gmail.com','cba@yahoo.com'])

Updating a List

Given below is an example to update the list data type in a table called data. Here we are adding another email to the list.

cqlsh:howcodex> UPDATE data
... SET email = email &plus;['xyz@howcodex.com']
... where name = 'ramu';

Verification

If you verify the table using SELECT statement, you will get the following result −

cqlsh:howcodex> SELECT * FROM data;

 name | email
------+--------------------------------------------------------------
 ramu | ['abc@gmail.com', 'cba@yahoo.com', 'xyz@howcodex.com']

(1 rows)

SET

Set is a data type that is used to store a group of elements. The elements of a set will be returned in a sorted order.

Creating a Table with Set

The following example creates a sample table with two columns, name and phone. For storing multiple phone numbers, we are using set.

cqlsh:howcodex> CREATE TABLE data2 (name text PRIMARY KEY, phone set<varint>);

Inserting Data into a Set

While inserting data into the elements in a set, enter all the values separated by comma within curly braces { } as shown below.

cqlsh:howcodex> INSERT INTO data2(name, phone)VALUES ('rahman',    {9848022338,9848022339});

Updating a Set

The following code shows how to update a set in a table named data2. Here we are adding another phone number to the set.

cqlsh:howcodex> UPDATE data2
   ... SET phone = phone + {9848022330}
   ... where name = 'rahman';

Verification

If you verify the table using SELECT statement, you will get the following result −

cqlsh:howcodex> SELECT * FROM data2;

   name | phone
--------+--------------------------------------
 rahman | {9848022330, 9848022338, 9848022339}

(1 rows)

MAP

Map is a data type that is used to store a key-value pair of elements.

Creating a Table with Map

The following example shows how to create a sample table with two columns, name and address. For storing multiple address values, we are using map.

cqlsh:howcodex> CREATE TABLE data3 (name text PRIMARY KEY, address
map<timestamp, text>);

Inserting Data into a Map

While inserting data into the elements in a map, enter all the key : value pairs separated by comma within curly braces { } as shown below.

cqlsh:howcodex> INSERT INTO data3 (name, address)
   VALUES ('robin', {'home' : 'hyderabad' , 'office' : 'Delhi' } );

Updating a Set

The following code shows how to update the map data type in a table named data3. Here we are changing the value of the key office, that is, we are changing the office address of a person named robin.

cqlsh:howcodex> UPDATE data3
   ... SET address = address+{'office':'mumbai'}
   ... WHERE name = 'robin';

Verification

If you verify the table using SELECT statement, you will get the following result −

cqlsh:howcodex> select * from data3;

  name | address
-------+-------------------------------------------
 robin | {'home': 'hyderabad', 'office': 'mumbai'}

(1 rows)

Cassandra - CQL User Defined Datatypes

CQL provides the facility of creating and using user-defined data types. You can create a data type to handle multiple fields. This chapter explains how to create, alter, and delete a user-defined data type.

Creating a User-defined Data Type

The command CREATE TYPE is used to create a user-defined data type. Its syntax is as follows −

CREATE TYPE <keyspace name>. <data typename>
( variable1, variable2).

Example

Given below is an example for creating a user-defined data type. In this example, we are creating a card_details data type containing the following details.

Field	Field name	Data type
credit card no	num	int
credit card pin	pin	int
name on credit card	name	text
cvv	cvv	int
Contact details of card holder	phone	set

cqlsh:howcodex> CREATE TYPE card_details (
   ... num int,
   ... pin int,
   ... name text,
   ... cvv int,
   ... phone set<int>
... );

Note − The name used for user-defined data type should not coincide with reserved type names.

Verification

Use the DESCRIBE command to verify whether the type created has been created or not.

CREATE TYPE howcodex.card_details (
   num int,
   pin int,
   name text,
   cvv int,
   phone set<int>
   );

Altering a User-defined Data Type

ALTER TYPE − command is used to alter an existing data type. Using ALTER, you can add a new field or rename an existing field.

Adding a Field to a Type

Use the following syntax to add a new field to an existing user-defined data type.

ALTER TYPE typename
ADD field_name field_type;

The following code adds a new field to the Card_details data type. Here we are adding a new field called email.

cqlsh:howcodex> ALTER TYPE card_details ADD email text;

Verification

Use the DESCRIBE command to verify whether the new field is added or not.

cqlsh:howcodex> describe type card_details;
CREATE TYPE howcodex.card_details (
   num int,
   pin int,
   name text,
   cvv int,
   phone set<int>,
   );

Renaming a Field in a Type

Use the following syntax to rename an existing user-defined data type.

ALTER TYPE typename
RENAME existing_name TO new_name;

The following code changes the name of the field in a type. Here we are renaming the field email to mail.

cqlsh:howcodex> ALTER TYPE card_details RENAME email TO mail;

Verification

Use the DESCRIBE command to verify whether the type name changed or not.

cqlsh:howcodex> describe type card_details;
CREATE TYPE howcodex.card_details (
   num int,
   pin int,
   name text,
   cvv int,
   phone set<int>,
   mail text
   );

Deleting a User-defined Data Type

DROP TYPE is the command used to delete a user-defined data type. Given below is an example to delete a user-defined data type.

Example

Before deleting, verify the list of all user-defined data types using DESCRIBE_TYPES command as shown below.

cqlsh:howcodex> DESCRIBE TYPES;
card_details card

From the two types, delete the type named card as shown below.

cqlsh:howcodex> drop type card;

Use the DESCRIBE command to verify whether the data type dropped or not.

cqlsh:howcodex> describe types;

card_details

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