Job search for a regional job board

Design search for a regional job board with about 25,000 open postings from 3,000 employers. Job seekers type keywords, such as "night shift nurse", and filter by city or remote, salary range, job type and date posted. Results are sorted by relevance, with newer postings first among similar matches, 20 per page. Employers post, edit and close jobs throughout the day, and a closed job must disappear from results within a minute. The site handles about 300,000 searches a day, peaking at 20 a second. Describe the search index, how it stays in sync with the postings database, and how a query runs.