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Elasticsearch is a NoSQL distributed database that is good for, well, searching. Elasticsearch's percolate query is an inverse search that returns a subset of the queries that match the document. Percolate queries are perfect for when you have an ever changing set of criteria (probably created by users) that many documents need to be checked against. For example, a user can create a query that triggers when a twitter post mentions their name, or when a temperature in a city is above a certain threshold.