Learn Cassandra In X Minutes

Overview

Apache Cassandra is a massively scalable open source NoSQL database. Cassandra is perfect for managing large amounts of data across multiple data centers and the cloud. Cassandra delivers continuous availability, linear scalability, and operational simplicity across many commodity servers with no single point of failure, along with a powerful data model designed for maximum flexibility and fast response times.

Cassandra has a “masterless” architecture, meaning all nodes are the same. Cassandra provides automatic data distribution across all nodes that participate in a “ring” or database cluster.

Cassandra also provides customizable replication, storing redundant copies of data across nodes that participate in a Cassandra ring.

Installation

Prerequisites

• Java 7

Download

Download apache-cassandra-2.1.2-bin.tar.gz from http://cassandra.apache.org/download/

Configuration

The Cassandra configuration files can be found in the conf folder.

Data Directories

The following settings are in conf/cassandra.yaml.

• data_file_directories: /var/lib/cassandra/data (Default: $CASSANDRA_HOME/data/data)
• commitlog_directory: /var/lib/cassandra/commitlog (Default: $CASSANDRA_HOME/data/commitlog)
• saved_caches_directory: /var/lib/cassandra/saved_caches (Default: $CASSANDRA_HOME/data/saved_caches)

Log

Log config file is conf/logback.xml

Start

Type the following command to start Cassandra:

$ bin/cassandra -f

Cassandra can be run in the background without the "-f" option.

cqlsh

bin/cqlsh is an interactive command line interface for Cassandra.

Type the following command to connect to Cassandra:

$ 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>

In sqlsh, commands are terminated with a semicolon (';'). 'help;' command can access online help.

Create keyspace:

Keyspace is a namespace of tables.

CREATE KEYSPACE test
WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };

After that, use the new keyspace.

USE test;

Create table:

CREATE TABLE user (
  user_id int PRIMARY KEY,
  first_name text,
  last_name text
);

Insert data:

INSERT INTO user (user_id,  first_name, last_name)
  VALUES (1, 'Ming', 'Li');
INSERT INTO user (user_id,  first_name, last_name)
  VALUES (2, 'Hong', 'Zhang');
INSERT INTO user (user_id,  first_name, last_name)
  VALUES (3, 'Lin', 'Li');

Fetch data:

cqlsh:test> SELECT * FROM user;

 user_id | first_name | last_name
---------+------------+-----------
       1 |       Ming |        Li
       2 |       Hong |     Zhang
       3 |        Lin |        Li

Next, fetch users whose last name is 'Li' by creating an index.

cqlsh:test> CREATE INDEX ON user (last_name);
cqlsh:test> SELECT * FROM user WHERE last_name = 'Li';

 user_id | first_name | last_name
---------+------------+-----------
       1 |       Ming |        Li
       3 |        Lin |        Li

Use Cassandra in Application

To user Cassandra, you need a database driver for a specific language. We can get CQL drivers from Data Stax and Cassandra ClientOptions.

Java Example

Maven Dependency

Please refer to DataStax Java Driver for Apache Cassandra for details.

<dependency>
  <groupId>com.datastax.cassandra</groupId>
  <artifactId>cassandra-driver-core</artifactId>
  <version>2.1.3</version>
</dependency>

Connecting to a Cassandra cluster

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Host;
import com.datastax.driver.core.Metadata;

public class SimpleClient {
    private Cluster cluster = null;

    public void connect(String node) {
        cluster = Cluster.builder()
                .addContactPoint(node)
                .build();
        Metadata metadata = cluster.getMetadata();
        System.out.printf("Connected to cluster: %s\n",
                metadata.getClusterName());
        for (Host host : metadata.getAllHosts()) {
            System.out.printf("Datacenter: %s; Host: %s; Rack: %s\n",
                    host.getDatacenter(), host.getAddress(), host.getRack());
        }
    }

    public void close() {
        if (cluster != null) {
            cluster.close();
        }
    }

    public static void main(String[] args) {
        SimpleClient client = new SimpleClient();
        client.connect("127.0.0.1");
        client.close();
    }
}

Using a session to execute CQL statements

1. Modify SimpleClient class

   • Add a Session property

       private Session session;
     

   • Get a session from your cluster

       session = cluster.connect();
     

2. Add a method, query, executing a SELECT statement on user table.

import com.datastax.driver.core.Cluster;
import com.datastax.driver.core.Host;
import com.datastax.driver.core.Metadata;
import com.datastax.driver.core.ResultSet;
import com.datastax.driver.core.Row;
import com.datastax.driver.core.Session;

public class SimpleClient {
    private Cluster cluster = null;
    private Session session = null;

    public void connect(String node) {
        cluster = Cluster.builder()
                .addContactPoint(node)
                .build();
        Metadata metadata = cluster.getMetadata();
        System.out.printf("Connected to cluster: %s\n",
                metadata.getClusterName());
        for (Host host : metadata.getAllHosts()) {
            System.out.printf("Datacenter: %s; Host: %s; Rack: %s\n",
                    host.getDatacenter(), host.getAddress(), host.getRack());
        }
        session = cluster.connect();
    }

    public void query() {
        ResultSet results = session.execute("SELECT * FROM test.user " +
                "WHERE last_name = 'Li';");
        System.out.println(String.format("%-30s\t%-20s\t%-20s\n%s", "user_id", "first_name",
                "last_name",
                "-------------------------------+-----------------------+--------------------"));
        for (Row row : results) {
            System.out.println(String.format("%-30s\t%-20s\t%-20s", row.getInt("user_id"),
                    row.getString("first_name"),  row.getString("last_name")));
        }
        System.out.println();
    }

    public void close() {
        if (cluster != null) {
            cluster.close();
        }
        if (session != null) {
            session.close();
        }
    }

    public static void main(String[] args) {
        SimpleClient client = new SimpleClient();
        client.connect("127.0.0.1");
        client.query();
        client.close();
    }
}

Using bound statements

public void query() {
    PreparedStatement statement = session.prepare(
            "SELECT * FROM test.user WHERE last_name = ?;");
    BoundStatement boundStatement = new BoundStatement(statement);
    ResultSet results = session.execute(boundStatement.bind("Li"));
    System.out.println(String.format("%-30s\t%-20s\t%-20s\n%s", "user_id", "first_name",
            "last_name",
            "-------------------------------+-----------------------+--------------------"));
    for (Row row : results) {
        System.out.println(String.format("%-30s\t%-20s\t%-20s", row.getInt("user_id"),
                row.getString("first_name"),  row.getString("last_name")));
    }
    System.out.println();
}

Data Model

Cassandra is a partitioned row store:

• Model brought from big table, row Key and a lot of columns
• Rows are organized into tables
• Primary key is required
• The first component of a table's primary key is the partition key
• The partition key determines which node stores the data
• Within a partition, rows are clustered by the remaining columns of the key

CQL

Data Types

CQL supports a rich set of data types including collection types.

The following table illustrates the native data types:

Cassandra has three collection types:

• set: set, {'one', 'two'}
• list: list, ['one', 'two']
• map: map, {1: 'one', 2: 'two'}

Functions

CQL3 supports a few functions:

• token()
• uuid()
• now()
• minTimeuuid()
• maxTimeuuid()
• dateOf()
• unixTimestampOf()
• typeAsBlob()
• blobAsType()

Reference

1. Getting Started
2. CQL3
3. DataModel
4. DataStax Documentation
5. DataStax Github
6. Cassandra Java Driver Docs
7. The Data Model is Dead; Long live the Data Model: Slides
8. Become a Super Modeler: Slides
9. The World's Next Top Data Model: Slides
10. Apache Cassandra 2.0: Data Model on Fire: Slides