Category: Big Data & Analytics

Cassandra – Read Data ”; Previous Next 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:tutorialspoint> 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:tutorialspoint> 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:tutorialspoint> CREATE INDEX ON emp(emp_sal); cqlsh:tutorialspoint> 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( ); 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”); 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 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 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(“tutorialspoint”); //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]] Print Page Previous Next Advertisements ”;

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:tutorialspoint> 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:tutorialspoint> 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:tutorialspoint> ALTER TABLE emp DROP emp_email; Verification Verify whether the column is deleted using the select statement, as shown below. cqlsh:tutorialspoint> 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 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 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 Print Page Previous Next Advertisements ”;

Cassandra – Installation ”; Previous Next 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>

Cassandra – Useful Resources ”; Previous Next The following resources contain additional information on Cassandra. Please use them to get more in-depth knowledge on this topic. Nodejs Course with 10 real-world Projects Most Popular 89 Lectures 17 hours Eduonix Learning Solutions More Detail Real-Time Spark Project For Beginners: Hadoop, Spark, Docker 25 Lectures 6.5 hours Pari Margu More Detail Apache Cassandra for Beginners 28 Lectures 2 hours Navdeep Kaur More Detail Print Page Previous Next Advertisements ”;

Cassandra – Drop Index ”; Previous Next 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( ); 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 ” ); 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 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 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 Print Page Previous Next Advertisements ”;