Apache Cassandra

Apache Cassandra
Original author(s) Avinash Lakshman, Prashant Malik
Developer(s) Apache Software Foundation
Initial release 2008
Stable release 3.4 / March 8, 2016 (2016-03-08)
Development status Active
Written in Java
Operating system Cross-platform
Available in English
Type Database
License Apache License 2.0
Website cassandra.apache.org
Helenos is a graphical user interface for Cassandra

Apache Cassandra is an open source distributed database management system designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure. Cassandra offers robust support for clusters spanning multiple datacenters,[1] with asynchronous masterless replication allowing low latency operations for all clients.

Cassandra also places a high value on performance. In 2012, University of Toronto researchers studying NoSQL systems concluded that "In terms of scalability, there is a clear winner throughout our experiments. Cassandra achieves the highest throughput for the maximum number of nodes in all experiments" although "this comes at the price of high write and read latencies."[2]

History

Apache Cassandra was initially developed at Facebook to power their Inbox Search feature by Avinash Lakshman (one of the authors of Amazon's Dynamo) and Prashant Malik. It was released as an open source project on Google code in July 2008.[3] In March 2009, it became an Apache Incubator project.[4] On February 17, 2010 it graduated to a top-level project.[5]

It was named after the Greek mythological prophet Cassandra.[6]

Releases after graduation include

Version Original release date Latest version Release date Status[16]
Old version, no longer supported: 0.6 2010-04-12 0.6.13 2011-04-18 No longer supported
Old version, no longer supported: 0.7 2011-01-10 0.7.10 2011-10-31 No longer supported
Old version, no longer supported: 0.8 2011-06-03 0.8.10 2012-02-13 No longer supported
Old version, no longer supported: 1.0 2011-10-18 1.0.12 2012-10-04 No longer supported
Old version, no longer supported: 1.1 2012-04-24 1.1.12 2013-05-27 No longer supported
Old version, no longer supported: 1.2 2013-01-02 1.2.19 2014-09-18 No longer supported
Old version, no longer supported: 2.0 2013-09-03 2.0.17 2015-09-21 No longer supported
Older version, yet still supported: 2.1 2014-09-16 2.1.13 2016-02-08 Still supported
Current stable version: 2.2 2015-07-20 2.2.5 2016-02-08 Most stable release
Latest preview version of a future release: 3.0 2015-11-09 3.0.5 2016-04-11 Stable release
Latest preview version of a future release: 3.2 2015-11-09 3.2.1 2016-01-18 Latest release
Legend:
Old version
Older version, still supported
Latest version
Latest preview version
Future release

Licensing and support

Apache Cassandra is an Apache Software Foundation project, so it has an Apache License (version 2.0).

Main features

Decentralized
Every node in the cluster has the same role. There is no single point of failure. Data is distributed across the cluster (so each node contains different data), but there is no master as every node can service any request.
Supports replication and multi data center replication
Replication strategies are configurable.[17] Cassandra is designed as a distributed system, for deployment of large numbers of nodes across multiple data centers. Key features of Cassandra’s distributed architecture are specifically tailored for multiple-data center deployment, for redundancy, for failover and disaster recovery.
Scalability
Read and write throughput both increase linearly as new machines are added, with no downtime or interruption to applications.
Fault-tolerant
Data is automatically replicated to multiple nodes for fault-tolerance. Replication across multiple data centers is supported. Failed nodes can be replaced with no downtime.
Tunable consistency
Writes and reads offer a tunable level of consistency, all the way from "writes never fail" to "block for all replicas to be readable", with the quorum level in the middle.[18]
MapReduce support
Cassandra has Hadoop integration, with MapReduce support. There is support also for Apache Pig and Apache Hive.[19]
Query language
Cassandra introduces CQL (Cassandra Query Language), a SQL-like alternative to the traditional RPC interface. CQL is a simple API meant for accessing Cassandra. CQL adds an abstraction layer that hides implementation details of this structure and provides native syntaxes for collections and other common encodings.[20] Language drivers are available for Java (JDBC), Python (DBAPI2), Node.JS (Helenus), Go (gocql) and C++.[21]

Below an example of keyspace creation, including a column family in CQL 3.0:[22]

CREATE KEYSPACE MyKeySpace
  WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 3 };

USE MyKeySpace;

CREATE COLUMNFAMILY MyColumns (id text, Last text, First text, PRIMARY KEY(id));

INSERT INTO MyColumns (id, Last, First) VALUES ('1', 'Doe', 'John');

SELECT * FROM MyColumns;

Which gives:

 id | first | last
----+-------+------
  1 |  John |  Doe

(1 rows)

Data model

Cassandra is essentially a hybrid between a key-value and a column-oriented (or tabular) database management system. Its data model is a partitioned row store with tunable consistency.[18] Rows are organized into tables; the first component of a table's primary key is the partition key; within a partition, rows are clustered by the remaining columns of the key.[23] Other columns may be indexed separately from the primary key.[24]

Tables may be created, dropped, and altered at run-time without blocking updates and queries.[25]

Cassandra cannot do joins or subqueries. Rather, Cassandra emphasizes denormalization through features like collections.[26]

A column family (called "table" since CQL 3) resembles a table in an RDBMS. Column families contain rows and columns. Each row is uniquely identified by a row key. Each row has multiple columns, each of which has a name, value, and a timestamp. Unlike a table in an RDBMS, different rows in the same column family do not have to share the same set of columns, and a column may be added to one or multiple rows at any time.[27]

Each key in Cassandra corresponds to a value which is an object. Each key has values as columns, and columns are grouped together into sets called column families. Thus, each key identifies a row of a variable number of elements. These column families could be considered then as tables. A table in Cassandra is a distributed multi dimensional map indexed by a key. Furthermore, applications can specify the sort order of columns within a Super Column or Simple Column family.

Clustering

When the cluster for Apache Cassandra is designed, an important point is to select the right partitioner. Two partitioners exist:[28]

  1. RandomPartitioner (RP): This partitioner randomly distributes the key-value pairs over the network, resulting in a good load balancing. Compared to OPP, more nodes have to be accessed to get a number of keys.
  2. OrderPreservingPartitioner (OPP): This partitioner distributes the key-value pairs in a natural way so that similar keys are not far away. The advantage is that fewer nodes have to be accessed. The drawback is the uneven distribution of the key-value pairs.

Management and monitoring

Cassandra is a Java-based system that can be managed and monitored via Java Management Extensions (JMX). The JMX-compliant nodetool utility, for instance, can be used to manage a Cassandra cluster (adding nodes to a ring, draining nodes, decommissioning nodes, and so on).[29] Nodetool also offers a number of commands to return Cassandra metrics pertaining to disk usage, latency, compaction, garbage collection, and more.[30] Additional metrics are available via JMX tools such as JConsole and via pluggable metrics reporters for external monitoring tools, which became available with Cassandra version 2.0.2.[31]

Prominent users

Facebook moved off its pre-Apache Cassandra deployment in late 2010 when they replaced Inbox Search with the Facebook Messaging platform.[46] In 2012, Facebook began using Apache Cassandra in its Instagram unit.[70]

Cassandra is the most popular wide column store,[71] and in September 2014 surpassed Sybase to become the 9th most popular database, close behind Microsoft Access and SQLite.[72]

See also

Academic background

Commercial companies

Alternatives

References

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  2. Rabl, Tilmann; Sadoghi, Mohammad; Jacobsen, Hans-Arno; Villamor, Sergio Gomez-; Mulero -, Victor Muntes; Mankovskii, Serge (2012-08-27). "Solving Big Data Challenges for Enterprise Application Performance Management" (PDF). VLDB. Retrieved 2013-07-25. In terms of scalability, there is a clear winner throughout our experiments. Cassandra achieves the highest throughput for the maximum number of nodes in all experiments... this comes at the price of high write and read latencies
  3. Hamilton, James (July 12, 2008). "Facebook Releases Cassandra as Open Source". Retrieved 2009-06-04.
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Bibliography

External links

Wikimedia Commons has media related to Apache Cassandra.
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