原书：Elasticsearch: the Definitive Guide source
https://github.com/elastic/elasticsearch
It's a distributed search engine built on Lucene, with a Java API and a JSON REST API. It's an indexed document-oriented database. You can do reasonably sophisticated searches with a query DSL.
Life inside a Cluster 原文
每一个cluster有一个master node，用来管理加减node和产生或是删除index。用户可以跟任意一个node沟通，所有的node都知道每一个文件住在哪儿并且能够将用户的请求转发到正确的node要到该要的回复。
An index is just a logical namespace that points to one or more physical shards.
A shard is a low-level worker unit that holds just a slice of all the data in the index--a single instance of Lucene.
Documents are stored in shards, and shards are allocated to nodes in your cluster, auto-balanced by Elasticsearch.
