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 computers, as of September 2006[update], 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.
- Sciencenet, a search engine for scientific knowledge, based on YaCy