YaCy (read "ya see") is a free distributed search engine, built on principles of peer-to-peer (P2P) networks. Its core is a computer program written in Java distributed on several hundred of computers, as of September 2006, so-called YaCy-peers. Each YaCy-peer independently crawls through the Internet, analyses and indexes found web pages and stores indexing results in a common database (so called index) which is shared with other YaCy-peers using principles of P2P networks.

Compared to semi-distributed search engines, the YaCy-network has a decentralised architecture. All YaCy-peers are equal and no central server exists. It can be run either in a crawling mode or as a local proxy server, indexing web pages visited by the person running YaCy on his or her computer. (Several mechanisms are provided to protect user's privacy).

Access to the search functions is made by a locally running web server which provides a search box to enter the query and returns results of the search in form of a web page as usual on other search portals and engines

The program is released under the GPL license.

Architecture

YaCy search engine is based on five elements:

Crawler - a search robot which traverses from web page to web page and analyses their context.

Indexer - creates a Reverse Word Index (RWI) i.e. each word from the RWI has its list of relevant URLs and Ranking information. Words are saved in form of word hashes.

Reverse Word Index Database - AVL tree structure creates ordered URLs and supports efficient table join needed for a search for given combination of words.

Search and Administration interface - made as a web interface provided by a local HTTP servlet with servlet engine.

P2P network - used to store the Reverse Word Index Database utilizing a Distributed Hash Table.

Advantages

As there is no central server, the results can't be censored, and the reliability is (at least theoretically) higher.

Because the engine is not owned by a company, there is no centralised advertising or manipulated ranking.

Because of the design of YaCy, it can be used to index the 'hidden web', like Tor, I2P or Freenet.

Disadvantages

As there is no central server and the YaCy network is open to anyone, malicious peers are (theoretically) able to insert inaccurate or commercially biased search results.

At present, YaCy returns on the average significantly fewer results and is much slower than large commercial search engines.