Elasticsearch is a search engine based on the Lucene library. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents. Elasticsearch is developed in Java.

Where to Learn ???

Here are some links so you can learn Elasticsearch free,

  1.   Codemy School (YouTube)
  2.   Hans Schenker bei (YouTube)
  3.   Jayendra Singh Collection (YouTube)
  4.   Media.com
  5.  Techie Life Style (YouTube)

If you know other sites that are free, Fell free to add to the comment section.