Have you ever wondered what would happen if we reverse the traditional model of using queries to find documents? Does it make sense to find documents matching the queries? It is not a surprise that there is a whole range of solutions where this model is very useful. Whenever you operate on an unbounded stream of input data, where you search for the occurrences of particular events, you can use this approach. This can be used for the detection of failures in a monitoring system or for the 'Tell me when a product with the defined criteria will be available in this shop' functionality. In this section, we will look at how an Elasticsearch percolator works and how it can handle this last example.

curl -XPOST 'localhost:9200/notifier' -d '{
  "mappings": {
    "book" : {
      "properties" : {
        "available" : {
          "type" : "boolean...