Editing Distributed hash table (section)

== History ==
DHT research was originally motivated, in part, by [[peer-to-peer]] (P2P) systems such as [[Freenet]], [[Gnutella]], [[BitTorrent]] and [[Napster]], which took advantage of resources distributed across the Internet to provide a single useful application. In particular, they took advantage of increased [[Bandwidth (computing)|bandwidth]] and [[hard disk]] capacity to provide a file-sharing service.<ref>{{cite journal |last1=Liz, Crowcroft |display-authors=et al |title=A survey and comparison of peer-to-peer overlay network schemes |journal=IEEE Communications Surveys & Tutorials |date=2005 |volume=7 |issue=2 |pages=72–93 |doi=10.1109/COMST.2005.1610546 |citeseerx=10.1.1.109.6124 |s2cid=7971188 |url=http://www.cl.cam.ac.uk/teaching/2005/AdvSysTop/survey.pdf |access-date=2019-09-24 |archive-date=2023-10-05 |archive-url=https://web.archive.org/web/20231005035522/https://www.cl.cam.ac.uk/teaching/2005/AdvSysTop/survey.pdf |url-status=live }}</ref>

These systems differed in how they located the data offered by their peers. Napster, the first large-scale P2P content delivery system, required a central index server: each node, upon joining, would send a list of locally held files to the server, which would perform searches and refer the queries to the nodes that held the results. This central component left the system vulnerable to attacks and lawsuits.

Gnutella and similar networks moved to a [[query flooding]] model{{spaced ndash}} in essence, each search would result in a message being broadcast to every machine in the network. While avoiding a [[single point of failure]], this method was significantly less efficient than Napster. Later versions of Gnutella clients moved to a dynamic querying model which vastly improved efficiency.<ref>{{cite journal |last1=Richter, Stevenson|display-authors=et al |title=Analysis of the impact of dynamic querying models on client-server relationships |journal=Trends in Modern Computing |date=2009 |pages=682–701}}</ref>

Freenet is fully distributed, but employs a [[Heuristic (computer science)|heuristic]] [[key-based routing]] in which each file is associated with a key, and files with similar keys tend to cluster on a similar set of nodes. Queries are likely to be routed through the network to such a cluster without needing to visit many peers.<ref>{{citation |url=https://freenetproject.org/papers/lic.pdf |title=Searching in a Small World Chapters 1 & 2 |access-date=2012-01-10 |archive-date=2012-03-16 |archive-url=https://web.archive.org/web/20120316102141/https://freenetproject.org/papers/lic.pdf |url-status=dead }}</ref> However, Freenet does not guarantee that data will be found.

Distributed hash tables use a more structured key-based routing in order to attain both the decentralization of Freenet and Gnutella, and the efficiency and guaranteed results of Napster. One drawback is that, like Freenet, DHTs only directly support exact-match search, rather than keyword search, although Freenet's [[routing algorithm]] can be generalized to any key type where a closeness operation can be defined.<ref>{{citation |chapter-url=https://freenetproject.org/papers/ddisrs.pdf |title=A Distributed Decentralized Information Storage and Retrieval System |chapter=Section 5.2.2 |access-date=2012-01-10 |archive-date=2012-03-16 |archive-url=https://web.archive.org/web/20120316102156/https://freenetproject.org/papers/ddisrs.pdf |url-status=dead }}</ref>

In 2001, four systems&mdash;[[Content addressable network|CAN]],<ref name="Ratnasamy01">{{Cite journal |last1=Ratnasamy |first1=Sylvia |last2=Francis |first2=Paul |last3=Handley |first3=Mark |last4=Karp |first4=Richard |last5=Shenker |first5=Scott |date=2001-08-27 |title=A scalable content-addressable network |url=https://dl.acm.org/doi/10.1145/964723.383072 |journal=SIGCOMM Comput. Commun. Rev. |volume=31 |issue=4 |pages=161–172 |doi=10.1145/964723.383072 |issn=0146-4833}}</ref> [[Chord (peer-to-peer)|Chord]],<ref>[[Hari Balakrishnan]], [[M. Frans Kaashoek]], David Karger, [[Robert Tappan Morris|Robert Morris]], and Ion Stoica. [http://www.cs.berkeley.edu/~istoica/papers/2003/cacm03.pdf Looking up data in P2P systems] {{Webarchive|url=https://web.archive.org/web/20160519125101/http://www.cs.berkeley.edu/~istoica/papers/2003/cacm03.pdf |date=2016-05-19 }}. In [[Communications of the ACM]], February 2003.</ref> [[Pastry (DHT)|Pastry]], and [[Tapestry (DHT)|Tapestry]]&mdash;brought attention to DHTs. 
A project called the Infrastructure for Resilient Internet Systems (Iris) was funded by a $12 million grant from the United States [[National Science Foundation]] in 2002.<ref>{{Cite news |title= New P2P network funded by US government |author= David Cohen |work= New Scientist |date= October 1, 2002 |url= https://www.newscientist.com/article.ns?id=dn2861 |access-date= November 10, 2013 |archive-date= April 6, 2008 |archive-url= https://web.archive.org/web/20080406123915/http://www.newscientist.com/article.ns?id=dn2861 |url-status= live }}</ref>
Researchers included [[Sylvia Ratnasamy]], [[Ion Stoica]], [[Hari Balakrishnan]] and [[Scott Shenker]].<ref>{{Cite news |title= MIT, Berkeley, ICSI, NYU, and Rice Launch the IRIS Project |work= Press release |publisher= MIT |date= September 25, 2002 |url= https://iris.pdos.csail.mit.edu/MITPressRelease1.doc |access-date= November 10, 2013 |url-status= dead |archive-url= https://web.archive.org/web/20150926070618/https://iris.pdos.csail.mit.edu/MITPressRelease1.doc |archive-date= September 26, 2015 }}</ref>
Outside academia, DHT technology has been adopted as a component of BitTorrent and in [[PlanetLab]] projects such as the Coral Content Distribution Network.<ref>{{Cite journal| title=Democratizing content publication with Coral | url=https://www.scs.stanford.edu/~dm/home/papers/freedman:coral.pdf | year=2004 | journal=NSDI | volume=4 | access-date=2024-05-01 }}</ref>