What?

 - ElasticSearch is a search engine (lucene based )
 - Which stores JSON data (using the inverse index data structure) 
 - Processes it
 - Supports full text based search using REST APIs

Why?

How?

 - A document is like a row in a relational database, representing a given entity — the thing you’re searching for. 
 For example, a document can represent an encyclopedia article or log entries from a web server.   
 - An index is a collection of documents that have similar characteristics.
 - An index in Elasticsearch is actually what’s called an inverted index.
 - Instead of searching the text directly it searches the index.
 - An inverted index lists every unique word that appears in any text being searched and identifies all of the saved document for each word occur. 
 - The term “name” occurs in document A and B, so it is mapped to that document.