Categories of [[search engine]] software include web search or full-text search (e.g., [[Lucene]]), database or structured data search such as [[Diesel point]], and mixed or [[enterprise search]] like [[Google Search Appliance]]. The largest online directories, such as [[Google]] and [[Yahoo]], utilize hundreds of thousands of computers to process billions of website documents using web crawlers(spiders), then return results for thousands of searches per second. Processing high volumes of queries requires software to run in a highly distributed environment with a high degree of redundancy.
