Elasticsearch does not impose a strict structure on your data; you can store any JSON documents. JSON documents are first-class citizens in Elasticsearch as opposed to rows and columns in a relational database. A document is roughly equivalent to a record in a relational database table. Traditional relational databases require a schema to be defined beforehand to specify a fixed set of columns and their data types and sizes. Often the nature of data is very dynamic, requiring support for new or dynamic columns. JSON documents naturally support this type of data. For example, take a look at the following document:

{
 "name": "John Smith",
 "address": "121 John Street, NY, 10010",
 "age": 40
}

This document may represent a customer's record. Here the record has the name, address, and age fields of the customer. Another record may look like the following:

{
 "name": "John Doe",
 "age": 38,
 "email": "[email protected]"
}

Note that the second customer doesn't have the address field but, instead, has an email address. In fact, other customer documents may have completely different sets of fields. This provides a tremendous amount of flexibility in terms of what can be stored.