An elasticsearch index is composed of one or more shards. Shards can be primary or replica, primary shards are the first involved in a write, replica are created or updated after the primary.

Shards.

Shards are how elasticsearch scales horizontally and distributes load among the nodes of a cluster. A shard is a lucene index, it can hold up to Integer.MAX_VALUE-128 documents (2,147,483,519). You must tune your index to make sure you won’t have shards greater than 20/30GB (or you will have out of memory issues very soon). Changing the number of shards requires to build/re-build a new index, you must set this while creating the index (or through a proper mapping configuration),

see https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-params.html

{ 
  "settings":
   {
    "index.number_of_shards": 10
   }
}

See template API for details: https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-templates.html

Replicas.

They are important to scale reads and lower the pressure (replica are usually involved in the reads as primary target) and make you’re index resilient to failure. A replica can become a primary shard if needed.

The replicas can be set on-the-fly

curl -X PUT "localhost:9200/indexname/_settings" -H 'Content-Type: application/json' -d' {     "index" : {         "number_of_replicas" : 2     } } '

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