When saving a document to Elasticsearch, it is indexed and placed in a user-specified index, making it available for search and analysis. An index is an analogue of a data table in a traditional DBMS.
In Elasticsearch, a document is a set of fields where each field is a
key: value pair. The index stores optimized documents to enable quickly searching documents by field. Optimization is possible because each document field has a specific type. This is how the field data is effectively stored in the index.
Unlike a traditional DBMS, to save the document in the index, Elasticsearch doesn't require the explicit specification of the schema (links between document fields and their types). Even though it's the recommended approach, you can save documents to the index without explicitly specifying the field types. Elasticsearch will try to determine the type automatically for each field in the document. As a result, you can quickly add documents to Elasticsearch storage and start working with them.
To learn more about how indexes work, see the Elasticsearch documentation.
In multihost clusters, index sharding and replication are supported. This makes it easier to scale the cluster and improves fault tolerance.