# Cluster Health, Shard Sizing and Replicas — Elasticsearch

Source: https://www.skillbyai.com/en/elasticsearch/o-health

> Green, yellow, red and how many shards.

## Reading health and sizing shards

`GET _cluster/health` reports **green** (all primary and replica shards allocated), **yellow** (all primaries allocated but some replicas are not, common on a single-node cluster because a replica is never placed on the same node as its primary) or **red** (at least one primary is unassigned, so some data is unavailable). `_cat/indices`, `_cat/shards` and `_cluster/allocation/explain` show which shards are unassigned and why (disk watermarks, missing nodes, allocation rules). Shard sizing: too many small shards waste heap and cluster-state overhead, while huge shards recover slowly. Elastic's guidance commonly suggests shards of roughly 10 to 50 GB and avoiding thousands of tiny shards; check the docs for your version. Plan replicas for availability (at least one) and add more only if you need read throughput.

## Keeping the cluster healthy

Healthy clusters have sensible shard counts, indices managed through aliases and lifecycle policies, and a practised reindex path.

![Three ideas: health and sizing, aliases and lifecycle, reindexing without downtime.](assets/figures/elasticsearch/section-7-map.svg) — Figure 7.1 — Indices rolling from hot to delete behind a stable alias.

## Everyday health checks

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

```http
GET /_cluster/health

GET /_cat/nodes?v&h=name,node.role,heap.percent,disk.used_percent,cpu

GET /_cat/indices?v&s=store.size:desc

GET /_cat/shards?v&h=index,shard,prirep,state,unassigned.reason

# why is a shard not allocated?
GET /_cluster/allocation/explain
{
  "index": "orders",
  "shard": 0,
  "primary": false
}
```

## Watch disk watermarks

When a node passes the high disk watermark, Elasticsearch moves shards away; at the flood-stage watermark it blocks writes to affected indices. Alert on disk usage well before that.

**Quiz:** A single-node cluster with replicas set to 1 is usually what colour?

- [ ] Blue
- [ ] Green
- [ ] Red
- [x] Yellow, because replicas cannot be placed on the node holding the primary

*Answer:* Yellow, because replicas cannot be placed on the node holding the primary. All primaries are assigned but replicas are not.