What is Elasticsearch (ES)? The short answer is: it's a search server. According to Wikipedia, this is the complete definition:
Elasticsearch is a search server based on Lucene. It provides a distributed, multitenant-capable full-text search engine with a RESTful web interface and schema-free JSON documents. Elasticsearch is developed in Java and is released as open source under the terms of the Apache License. Elasticsearch is the second most popular enterprise search engine.
If you need full-text search, real-time analytics of structured data, or a combination of the two, Elasticsearch is a very powerful tool, fit for you. All the big players use it. We will use ES in front of MySQL to store and search for articles. In this way we will reduce the traffic to MySQL and avoid querying it too often. We are not going to discuss ES in detail, so please spend a few minutes reading about its basic operation at http://www.elastic.co/guide/.