elastic search

save document as json format.

used for create own serach system or analysce data.

also used in cluster database system.

* type here is table

* index called here is database

* document here is row

* shard - divide data into many partition. It can be divide into many server. 
          concept of cluster database. shard divide data autometaclly when node add.
          Must be in same network.
          
* node - where shard are hosted. hosting server.
          
* replica - copy of shard. If shard failed replica will response.