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Apache Cassandra

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Apache Cassandra
Original author(s)Avinash Lakshman, Prashant Malik
Developer(s)Apache Software Foundation
Initial release2008
Stable release
0.8.0 / June 2, 2011 (2011-06-02)
Repository
Written inJava
Operating systemCross-platform
Available inEnglish
TypeColumn-oriented DBMS
LicenseApache License 2
Websitehttp://cassandra.apache.org/

Cassandra is an open source distributed database management system. It is an Apache Software Foundation top-level project[1] 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 powers their Inbox Search feature.[2] 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.[3]

Cassandra provides a structured key-value store with tunable consistency.[4] 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. This makes Cassandra a hybrid data management system between a column-oriented DBMS and a row-oriented store.[5] Also, besides using the way of modeling of BigTable, it has properties like eventual consistency, the Gossip protocol, a master-master way of serving the read and write requests that are inspired by Amazon's Dynamo.[6]

History

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.[3] In March 2009, it became an Apache Incubator project.[7] On February 17, 2010 it graduated to a top-level project.[1]

Data model

A table in Cassandra is a distributed multidimensional map indexed by a key. The value is an object that is highly structured. The row key in a table is a string with no size restrictions, although typically 16 to 36 bytes long. Every operation under a single row key is atomic per replica no matter how many columns are being read or written into. Columns are grouped together into sets called column families very much similar to what happens in the BigTable system. Cassandra exposes two kinds of column families: Simple and Super. Super column families can be visualized as a column family within a column family. The top dimension in Cassandra is called Keyspace.

Therefore, Cassandra can handle maps with four or five dimensions:

Map with four dimensions:

  1. Keyspace -> Column Family
  2. Column Family -> Column Family Row
  3. Column Family Row -> Columns
  4. Column -> Data value

Map with five dimensions:

  1. Keyspace -> Super Column Family
  2. Super Column Family -> Super Column Family Row
  3. Super Column Family Row -> Super Columns
  4. Super Column -> Columns
  5. Column -> Data value

The first dimension - Keyspace -> (Super) Column Family - is limited to a (small) set of schema-predefined keys. Cassandra documentation mentions similarities of this dimension with the table definition inside a relational database.

Furthermore, applications can specify the sort order of columns within a Super Column or Simple Column family. The system allows columns to be sorted either by time or by name. Time sorting of columns is exploited by applications like Facebook Inbox Search, where the results are always displayed in time-sorted order. Any column within a column family is accessed using the convention column_family : column, and any column within a column family that is of type super is accessed using the convention column_family : super_column : column.

Clustering

When the cluster for Apache Cassandra is designed, an important point is to select the right partitioner. Two partitioners exist:[8]

  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.

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.

Elasticity

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.

Prominent users

  • Cisco's WebEx uses Cassandra to store user feed and activity in near real time.[9]
  • Cloudkick uses Cassandra to store the server metrics of their users.[10]
  • Digg, a large social news website, announced on Sep 9th, 2009 that it is rolling out its use of Cassandra[11] and confirmed this on March 8, 2010.[12] TechCrunch has since linked Cassandra to Digg v4 reliability criticisms and recent company struggles.[13] Lead engineers at Digg later rebuked these criticisms as red herring and blamed a lack of load testing.[14]
  • Facebook used Cassandra to power Inbox Search, with over 200 nodes deployed.[2] This was abandoned in late 2010.[15]
  • IBM has done research in building a scalable email system based on Cassandra.[16]
  • Rackspace is known to use Cassandra internally.[17]
  • Reddit switched to Cassandra from memcacheDB on March 12, 2010[18] and experienced some problems with overload handling in Cassandra in May.[19]
  • 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.[20][21] Twitter continues to use it but not for Tweets themselves.[22]
  • Yakaz uses Cassandra on a five-node cluster to store millions of images as well as its social data.[23]

See also

References

  1. ^ a b "Cassandra is an Apache top level project". Mail-archive.com. 2010-02-18. Retrieved 2010-03-29.
  2. ^ a b "Niet compatibele browser". Facebook. Retrieved 2010-03-29.
  3. ^ a b James Hamilton (July 12, 2008). "Facebook Releases Cassandra as Open Source". Retrieved 2009-06-04.
  4. ^ http://www.cs.cornell.edu/projects/ladis2009/papers/lakshman-ladis2009.pdf
  5. ^ Vishal Shinde (2011-01-19). "Apache Cassandra". http://www.linuxquestions.in/: Linux Questions, Help, Howtos and Tutorials. Retrieved 2010-03-09. The values from a column family for each key are stored together, making Cassandra a hybrid between a column-oriented DBMS and a row-oriented store {{cite web}}: External link in |location= (help)
  6. ^ 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… {{cite web}}: External link in |location= (help)
  7. ^ "Is this the new hotness now?". Mail-archive.com. 2009-03-02. Retrieved 2010-03-29.
  8. ^ 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}}: External link in |location= (help); line feed character in |quote= at position 325 (help)
  9. ^ "Re: Cassandra users survey". Mail-archive.com. 2009-11-21. Retrieved 2010-03-29.
  10. ^ https://www.cloudkick.com/blog/2010/mar/02/4_months_with_cassandra/
  11. ^ Ian Eure. "Looking to the future with Cassandra".
  12. ^ John Quinn. "Saying Yes to NoSQL; Going Steady with Cassandra".
  13. ^ Erick Schonfeld. "As Digg Struggles, VP Of Engineering Is Shown The Door".
  14. ^ "Is Cassandra to Blame for Digg v4's Failures?".
  15. ^ Kannan Muthukkaruppan. "The Underlying Technology of Messages".
  16. ^ "Powered by Google Docs" (PDF). Docs.google.com. Retrieved 2010-03-29.
  17. ^ "Nabble - cassandra-user@incubator.apache.org - Cassandra users survey". N2.nabble.com. Retrieved 2010-03-29.
  18. ^ Posted by david [ketralnis] (2010-03-12). "what's new on reddit: She who entangles men". blog.reddit. Retrieved 2010-03-29.
  19. ^ 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. Retrieved 2010-05-16.
  20. ^ Popescu, Alex. "Cassandra @ Twitter: An Interview with Ryan King". myNoSQL. Retrieved 2010-03-29.
  21. ^ Babcock, Charles. "Twitter Drops MySQL For Cassandra - Cloud databases". InformationWeek. Retrieved 2010-03-29.
  22. ^ "Cassandra at Twitter Today".
  23. ^ "Yakaz Technologies".

Bibliography

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