Find why a shard is unassigned

Command

GET /_cluster/allocation/explain
{
  "index": "customer",
  "shard": 0,
  "primary": true
}

What it does

Explains in detail why a specific shard could not be allocated (unassigned).

When to use it

When looking for the reason behind unassigned shards in a yellow/red cluster.

Caution

Read-only diagnostic command; no risk.