Understanding the internals of an Elasticsearch index

When users want to store data (or documents) on Elasticsearch, they do so in an index. An index on Elasticsearch is a location to store and organize related documents. They don't all have to be the same type of data, but they generally have to be related to one another. In the SQL world, an index would be comparable to a database containing multiple tables (where each table is designed for a single type of data).

An index is made up of primary shards. Primary shards can be replicated into replica shards to achieve high availability. Each shard is an instance of a Lucene index with the ability to handle indexing and search requests. The primary shard can handle both read and write requests, while replica shards are read-only. When a document is indexed into Elasticsearch, it is indexed by the primary shard before being replicated to the replica shard. The indexing request is only acknowledged once the replica shard has been...