Phases allowed: hot, warm, cold.

Makes the index data read-only; disables data write operations against the index.

To use the readonly action in the hot phase, the rollover action must be present. If no rollover action is configured, ILM will reject the policy.

PUT _ilm/policy/my_policy
{
  "policy": {
    "phases": {
      "warm": {
        "actions": {
          "readonly" : { }
        }
      }
    }
  }
}