Cassandra

In Greek mythology, Cassandra was the daughter of King Priam and Queen Hecuba, of Illiad fame. Apollo granted her the power of prophecy in order to seduce her, and cursed her to never be believed when she rejected him.

It's also software by Apache designed to handle large amounts of data in a scalable way. Intended for cloud and cluster solutions/applications. Used by companies like Netflix.

CQL

The Cassandra Query Language (CQL) is a query language specific to Cassandra that looks a lot like SQL.

Link to CQL documentation: https://cassandra.apache.org/doc/latest/cql/index.html#cql

Difference between CQL and SQL

In spirit, CQL and SQL are designed to be similar. They both use similar syntax - "select things where condition". The entire point of adding CQL to Cassandra was to make it easier for developers familiar with SQL to be able to pick up CQL and deal with Cassandra databases more easily.

However, they are fundamentally different because CQL sits on top of Cassandra, a NoSQL "database" that is actually designed as a big key-value map, and SQL sits on top of a traditional relational database.

They are also different in practice - there are some operations that CQL does not support. CQL does not support the following:

• No joins
• No subqueries
• No transactions
• WHERE conditions can only be applied to columns where an index has been created on that column (unless the column is a primary key)

The following operators:

• No OR operator
• No NOT operators
• The AND operator is the only operator available

Types

Cassandra supports several data types:

• native types
• collection types
• user-defined types
• tuple types
• custom types

Using

Command Line Client

Cassandra can be used from the command line via the cqlsh tool, which provides a Cassandra shell from which CQL commands can be executed.

Like MySQL, you start up the command line program, which starts a client that tries to connect to a server instance. To connect to a local instance, specify localhost as the host:

$ bin/cqlsh localhost
Connected to Test Cluster at localhost:9042.
[cqlsh 5.0.1 | Cassandra 3.8 | CQL spec 3.4.2 | Native protocol v4]
Use HELP for help.
cqlsh>

You can specify various command line options (e.g., username/password, script file, startup file, etc.)

A simple hello world query in CQL:

cqlsh> SELECT cluster_name, listen_address FROM system.local;

 cluster_name | listen_address
--------------+----------------
 Test Cluster |      127.0.0.1

(1 rows)

Full cqlsh documentation: https://cassandra.apache.org/doc/latest/tools/cqlsh.html#cqlsh

Using Other Languages

You can interface with Cassandra from a number of different languages (Java, Python, Node, Scala, Go, Rust, etc.). Full list of client drivers: https://cassandra.apache.org/doc/latest/getting_started/drivers.html

DataStax (company that contributes to Cassandra development) provides several good drivers in various languages:

• Java: https://github.com/datastax/java-driver
• Python: https://github.com/datastax/python-driver
• Node.js: https://github.com/datastax/nodejs-driver

Using on Google Cloud

Google Cloud offers several pre-configured virtual machine instances that run Cassandra, including one created by Google, and one created by Bitnami. See Google Cloud/Cassandra

Resources

DataStax is a cloud provider of Cassandra solutions. In addition to helping develop/contribute to Cassandra, they also provide training resources on Cassandra: https://academy.datastax.com/