YaCy (pronounced "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, , so-called YaCy-peers. Each YaCy-peer independently crawls through the Internet, analyzes 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. It is a free search engine that everyone can use to build a search portal for their intranet and to help search the public internet clearly.
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 the user's privacy). Access to the search functions is made by a locally running web server which provides a search box to enter search terms, and returns search results in a similar format to other popular search engines.
In October 2015, after 11 years since the project launched, a large logic structure of YaCy which provides for all distributed ranking algorithms was declared by its core developers to have always been faulty, effectively impairing any ranking capabilities.
YaCy is available on Windows, Mac and GNU/Linux.