When changing the index of an Elasticsearch alias via API, do write operations immediately point to the right index after the response?

Question

Let's say I got the alias car-alias pointing to car-index-1. I now want car-alias to point to car-index-2.

I therefore perform the following POST request to the Aliases API:

{
  "actions": [
    {
      "remove": {
        "index": "car-index-1",
        "alias": "car-alias"
      }
    },
    {
      "add": {
        "index": "car-index-2",
        "alias": "car-alias"
      }
    }
  ]
}

I receive the following response:

{
   "acknowledged": true
}

Can I now immediately index data into the car-alias and it ends up in the car-index-2?

Or does the "acknowledged": true response not guarantee that write operations point to the right index immediately?

Answer 1

Yes, the alias is changed atomically and will point to car-index-2 immediately when the call returns.

As stated in the documentation "...during the swap, the alias has no downtime and never points to both streams at the same time."