# Cluster, Node, Index, Shard and Replica — Elasticsearch

Source: https://www.skillbyai.com/en/elasticsearch/f-concepts

> The building blocks.

## How data is spread out

A **cluster** is a group of **nodes** (Elasticsearch processes) sharing a cluster name; one elected **master** node manages cluster state such as mappings and shard locations. Data is stored in **indices**, each a logical collection of **documents** (JSON objects with an `_id`). An index is split into one or more **primary shards**, each a self-contained Lucene index, and every primary can have **replica shards**: copies on other nodes that provide failover and extra read capacity. Writes go to the primary and are then replicated; searches can be served by any copy. Since 7.0 a new index defaults to one primary and one replica. The number of primaries is fixed at creation (changing it needs split, shrink or reindex); the number of replicas can be changed at any time.

## Creating an index with explicit shard settings

Kibana Dev Tools console syntax; send the same requests with curl or a client library.

```http
PUT /orders
{
  "settings": {
    "number_of_shards": 3,
    "number_of_replicas": 1
  }
}

# replicas can be changed later; primaries cannot
PUT /orders/_settings
{
  "index": { "number_of_replicas": 2 }
}

# see where each shard copy lives
GET /_cat/shards/orders?v
```

## Chapters and photocopies

An index is a book split into chapters (primary shards) handed to different people (nodes). Each chapter also has a photocopy held by someone else (a replica), so losing one person loses no chapter.

**Quiz:** Which index setting can be changed on a live index without reindexing?

- [ ] The index name
- [ ] number_of_shards
- [ ] The type of an existing field
- [x] number_of_replicas

*Answer:* number_of_replicas. Replica count is dynamic; primary count is fixed at creation.