What is the percolator in Elasticsearch?

The percolator is a feature that inverts the normal search flow: instead of storing documents and querying them, you store queries in an index and then check which stored queries match a given document. This is called "reverse search" or "prospective search." Real-world use cases include: alert systems (notify a user when a new product matches their saved search), content tagging (automatically classify incoming articles), and news filtering (route new articles to subscribers based on their interests). You index queries using the percolate field type and then use the percolate query to find all stored queries that match a given document.