|Original author(s)||Avinash Lakshman, Prashant Malik|
|Developer(s)||Apache Software Foundation|
|Stable release||1.2.5 / May 19, 2013|
|License||Apache License 2.0|
Apache Cassandra is an open source distributed database management system. It is an Apache Software Foundation top-level project designed to handle very large amounts of data spread out across many commodity servers while providing a highly available service with no single point of failure. It is a NoSQL solution that was initially developed by Facebook and powered their Inbox Search feature until late 2010. Jeff Hammerbacher, who led the Facebook Data team at the time, has described Cassandra as a BigTable data model running on an Amazon Dynamo-like infrastructure.
Cassandra provides a structured key-value store with tunable consistency. Keys map to multiple values, which are grouped into column families. The column families are fixed when a Cassandra database is created, but columns can be added to a family at any time. Furthermore, columns are added only to specified keys, so different keys can have different numbers of columns in any given family. The values from a column family for each key are stored together.
Apache Cassandra was 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. In March 2009, it became an Apache Incubator project. On February 17, 2010 it graduated to a top-level project.
Releases after graduation include
- 0.6, released Apr 12 2010, added support for integrated caching, and Apache Hadoop MapReduce
- 0.7, released Jan 08 2011, added secondary indexes and online schema changes
- 0.8, released Jun 2 2011, added the Cassandra Query Language (CQL), self-tuning memtables, and support for zero-downtime upgrades
- 1.0, released Oct 17 2011, added integrated compression, leveled compaction, and improved read performance
- 1.1, released Apr 23 2012, added self-tuning caches, row-level isolation, and support for mixed ssd/spinning disk deployments
- 1.2, released Jan 2 2013, added clustering across virtual nodes, inter-node communication, atomic batches, and request tracing
Licensing and support 
Apache Cassandra is an Apache Software Foundation project, so it has an Apache License (version 2.0).
There is professional grade support available from a few companies. In the official wiki of Apache Cassandra's project the following ones, which collaborate with developers of the project, are mentioned
Main features 
- 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. 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.
- Read and write throughput both increase linearly as new machines are added, with no downtime or interruption to applications.
- 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.
- MapReduce support
- Cassandra has Hadoop integration, with MapReduce support. There is support also for Apache Pig and Apache Hive.
- Query language
- CQL (Cassandra Query Language) was introduced, an SQL-like alternative to the traditional RPC interface. Language drivers are available for Java (JDBC), Python (DBAPI2) and Node.JS (Helenus).
Data model 
|This section requires expansion with: informational details and clarification. (September 2012)|
Cassandra is essentially a hybrid between a key-value and a row-oriented (or tabular) database.
- A column family 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.
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. Also, each column family can be grouped in super 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.
When the cluster for Apache Cassandra is designed, an important point is to select the right partitioner. Two partitioners exist:
- 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.
- 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.
Prominent users 
- AppScale uses Cassandra as a back-end for Google App Engine applications
- Cisco's WebEx uses Cassandra to store user feed and activity in near real time.
- The CERN ATLAS experiment uses Cassandra to archive its online DAQ system's monitoring information
- Cloudkick uses Cassandra to store the server metrics of their users.
- Constant Contact uses Cassandra in their social media marketing application.
- Digg, a large social news website, announced on Sep 9th, 2009 that it is rolling out its use of Cassandra and confirmed this on March 8, 2010. TechCrunch has since linked Cassandra to Digg v4 reliability criticisms and recent company struggles. Lead engineers at Digg later rebuked these criticisms as red herring and blamed a lack of load testing.
- Facebook used Cassandra to power Inbox Search, with over 200 nodes deployed. This was abandoned in late 2010 when they built Facebook Messaging platform on HBase.
- IBM has done research in building a scalable email system based on Cassandra.
- InWorldz has researched and developed a scalable high-performance storage system for user inventory items Cassandra.
- Netflix uses Cassandra as their back-end database for their streaming services
- Formspring uses Cassandra to count responses, as well as store Social Graph data (followers, following, blockers, blocking) for 26 Million accounts with 10 million responses a day
- Mahalo.com uses Cassandra to record user activity logs and topics for their Q&A website
- Ooyala Built a scalable, flexible, real-time analytics engine using Cassandra
- At Openwave, Cassandra acts as a distributed database and serves as a distributed storage mechanism for Openwave’s next generation messaging platform
- OpenX is running over 130 nodes on Cassandra for their OpenX Enterprise product to store and replicate advertisements and targeting data for ad delivery
- Plaxo has "reviewed 3 billion contacts in [their] database, compared them with publicly available data sources, and identified approximately 600 million unique people with contact info."
- PostRank uses Cassandra as their backend database
- Rackspace is known to use Cassandra internally.
- Reddit switched to Cassandra from memcacheDB on March 12, 2010 and experienced some problems with overload handling in Cassandra in May.
- RockYou uses Cassandra to record every single click for 50 million Monthly Active Users in real-time for their online games
- SoundCloud uses Cassandra to store user account information
- Talentica Software uses Cassandra as a back-end for Analytics Application with Cassandra cluster of 30 nodes and inserting around 200GB data on daily basis.
- Twitter announced it is planning to use Cassandra because it can be run on large server clusters and is capable of taking in very large amounts of data at a time. Twitter continues to use it but not for Tweets themselves.
- Urban Airship uses Cassandra with the mobile service hosting for over 160 million application installs across 80 million unique devices
- @WalmartLabs (previously Kosmix) uses Cassandra with SSD
- Yakaz uses Cassandra on a five-node cluster to store millions of images as well as its social data.
- Zoho uses Cassandra for generating the inbox preview in their Zoho#Zoho_Mail service
Ironically, Facebook moved off its early Cassandra deployment in late 2010 when they replaced Inbox Search with the Facebook Messaging platform. Facebook never deployed an Apache Cassandra release.
Cassandra is the most popular wide column store.
Tools for Cassandra 
Cassandra has built in tools for accessing Cassandra from the direct download, such as cassandra-cli and node-tool.
There are third party tools available, as the following:
- chiton, a GTK data browser.
- cassandra-gui, a Swing data browser.
- Toad for Cloud Databases, an Eclipse plug-in data browser
- OpsCenter, OpsCenter is a tool for management and monitoring of a Cassandra cluster. The Community Edition of OpsCenter is free for anyone to download and use. There is also an Enterprise Edition of OpsCenter that includes additional features.
- Cassandra Cluster Admin, Cassandra Cluster Admin is a GUI tool to help people administrate their Apache Cassandra cluster and modify its content, similar to PHPMyAdmin for MySQL administration.
Client interfaces and language Support 
Cassandra has a lot of high-level client libraries for Python, Java, .Net, Ruby, PHP, Perl, C++, etc.
For a detailed list of client software go to "Client Options" article on Cassandra Wiki
Integration with other tools 
See also 
- Berkeley DB
- BigTable - Original distributed database by Google
- Distributed database
- Distributed hash table (DHT)
- Dynamo (storage system) - Cassandra borrows many elements from Dynamo
- Apache HBase - Apache Hadoop based distributed database. Very similar to BigTable
- Hypertable - Apache Hadoop based distributed database. Very similar to BigTable
- "Cassandra is an Apache top level project". Mail-archive.com. 2010-02-18. Archived from the original on 28 March 2010. Retrieved 2010-03-29.
- "Niet compatibele browser". Facebook. Retrieved 2010-03-29.
- Kannan Muthukkaruppan. "The Underlying Technology of Messages".
- James Hamilton (July 12, 2008). "Facebook Releases Cassandra as Open Source". Retrieved 2009-06-04.
- Olivier Mallassi (2010-06-09). "Let’s play with Cassandra… (Part 1/3)". http://blog.octo.com/: OCTO Talks. Retrieved 2010-03-22. "Hybrid firstly because Cassandra uses a column-oriented way of modeling data (inspired by the BigTable) and permit to use Hadoop Map/Reduce jobs and secondly because it uses patterns inspired by Dynamo like Eventually Consistent, Gossip protocols, a master-master way of serving both read and write requests…"
- "Is this the new hotness now?". Mail-archive.com. 2009-03-02. Archived from the original on 25 April 2010. Retrieved 2010-03-29.
- The Apache Software Foundation Announces Apache Cassandra Release 0.6 : The Apache Software Foundation Blog
- The Apache Software Foundation Announces Apache Cassandra 0.7 : The Apache Software Foundation Blog
- [Cassandra-user] [RELEASE] 0.8.0 - Grokbase
- Cassandra 1.0.0. Is Ready for the Enterprise
- The Apache Software Foundation Announces Apache Cassandra™ v1.1 : The Apache Software Foundation Blog
- The Apache Software Foundation Announces Apache Cassandra™ v1.2
- "Third Party Support" article on Apache Cassandra's wiki
- "Deploying Cassandra across Multiple Data Centers" article on Datastax Cassandra Developer Center
- "Hadoop Support" article on Cassandra's wiki
- DataStax. "Apache Cassandra 0.7 Documentation - Column Families". Apache Cassandra 0.7 Documentation. Retrieved 29 October 2012.
- Dominic Williams. "Cassandra: RandomPartitioner vs OrderPreservingPartitioner". http://wordpress.com/: WordPress.com. Retrieved 2011-03-23. "When building a Cassandra cluster, the “key” question (sorry, that’s weak) is whether to use the RandomPartitioner (RP), or the OrderPreservingPartitioner (OPP). These control how your data is distributed over your nodes. Once you have chosen your partitioner, you cannot change without wiping your data, so think carefully! The problem with OPP: If the distribution of keys used by individual column families is different, their sets of keys will not fall evenly across the ranges assigned to nodes. Thus nodes will end up storing preponderances of keys (and the associated data) corresponding to one column family or another. If as is likely column families store differing quantities of data with their keys, or store data accessed according to differing usage patterns, then some nodes will end up with disproportionately more data than others, or serving more “hot” data than others."
- cite web|url=http://appscale.cs.ucsb.edu/datastores.html#cassandra
- "Re: Cassandra users survey". Mail-archive.com. 2009-11-21. Archived from the original on 17 April 2010. Retrieved 2010-03-29.
- "A Persistent Back-End for the ATLAS Online Information Service (P-BEAST)".
- 4 Months with Cassandra, a love story | Cloudkick, manage servers better
- Klint Finley (2011-02-18). "This Week in Consolidation: HP Buys Vertica, Constant Contact Buys Bantam Live and More". Read Write Enterprise.
- Ian Eure. "Looking to the future with Cassandra".
- John Quinn. "Saying Yes to NoSQL; Going Steady with Cassandra".
- Erick Schonfeld. "As Digg Struggles, VP Of Engineering Is Shown The Door".
- "Is Cassandra to Blame for Digg v4's Failures?".
- "Powered by Google Docs". Docs.google.com. Retrieved 2010-03-29.
- Dexler, Tranquillity. "InWorldz 0.7.0 R1542X". InWorldz LLC. Retrieved 9 February 2012.
- cite web|url=http://www.slideshare.net/adrianco/migrating-netflix-from-oracle-to-global-cassandra
- Yury Izrailevsky (2011-01-28). "NoSQL at Netflix".
- Martin Cozzi (2011-08-31). "Cassandra at Formspring".
- Watch Cassandra at Mahalo.com | DataStax Episodes | Blip
- Ad Serving Technology - Advanced Optimization, Forecasting, & Targeting | OpenX
- Preston Smalley (2011-03-20). "An important milestone - and it's only the beginning!".
- Ilya Grigorik (2011-03-29). "Webpulp TV: Scaling PostRank with Ilya Grigorik".
- "Hadoop and Cassandra (at Rackspace)". Stu Hood. 2010-04-23. Retrieved 2011-09-01.
- Posted by david [ketralnis] (2010-03-12). "what's new on reddit: She who entangles men". blog.reddit. Archived from the original on 25 March 2010. Retrieved 2010-03-29.
- Posted by the reddit admins at (2010-05-11). "blog.reddit -- what's new on reddit: reddit's May 2010 "State of the Servers" report". blog.reddit. Archived from the original on 14 May 2010. Retrieved 2010-05-16.
- Dathan Vance Pattishall (2011-03-23). "Cassandra is my NoSQL Solution but".
- "Cassandra at SoundCloud".
- cite web|url=http://www.talentica.com
- Popescu, Alex. "Cassandra @ Twitter: An Interview with Ryan King". myNoSQL. Archived from the original on 1 March 2010. Retrieved 2010-03-29.
- Babcock, Charles. "Twitter Drops MySQL For Cassandra - Cloud databases". InformationWeek. Archived from the original on 2 April 2010. Retrieved 2010-03-29.
- "Cassandra at Twitter Today".
- Erik Onnen. "From 100s to 100s of Millions".
- Walmart Labs
- Karl Mueller. "Cassandra on SSD".
- "Yakaz Technologies".
- DB-Engines. "DB-Engines Ranking of Wide Column Stores".
- FAQ on Cassandra's wiki
- "Client Options" article on Cassandra Wiki
- https://kb.askmonty.org/en/cassandra-storage-engine/ Cassandra Storage Engine Documentation
- Solandra source at Github
- Cassandra - A Decentralized Structured Storage System, a 2009 paper presenting Cassandra by their creators Avinash Lakshman and Prashant Malik
||This article's use of external links may not follow Wikipedia's policies or guidelines. (January 2012)|
- Avinash Lakshman (25 August 2008). "Cassandra - A structured storage system on a P2P Network". Engineering @ Facebook's Notes. Retrieved 2009-06-04.
- Project Website
- Project Wiki
- Adopting Apache Cassandra presented by Eben Hewitt on December 1, 2010
- LADIS 2009 WhitePaper by the original contributors Avinash Lakshman & Prashant Malik
- Cassandra Articles on NoSQLDatabases.com
- Cassandra News and Articles on myNoSQL
- Cassandra @ Twitter: an Interview with Ryan King
- Presentation on RDBMS vs. Dynamo, BigTable, and Cassandra
- RPM build for the apache cassandra project
- Cassandra by example - the path of read and write requests