Phases allowed: hot, warm, cold.

Makes the index read-only.

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" : { }
        }
      }
    }
  }
}