Same IDs with different _routing values

According to elasticsearch documentation, it is possible to have docs with the same _id indexed with different _routing values. Hence, the documentation states that the uniqueness on _id is not guaranteed because these docs can end up on different shards (which appears to be a feature rather than a bug)

How about the scenario when two docs with the same _id indexed with different routing values end up on the same shard? Consider the query body below:

PUT test-index
{
  "settings": {
    "index": 
    {
      "number_of_shards": 2
      }
  }
}


PUT test-index/_doc/1?routing=user1
{
  "title": "This is document number with routing=user1"
}

PUT test-index/_doc/1?routing=user2
{
  "title": "This is document number with routing=user2"
}

GET test-index/_search

The search queries renders the following result:

{
  "took": 2,
  "timed_out": false,
  "_shards": {
    "total": 2,
    "successful": 2,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 1,
      "relation": "eq"
    },
    "max_score": 1,
    "hits": [
      {
        "_index": "test-index",
        "_id": "1",
        "_score": 1,
        "_routing": "user2",
        "_source": {
          "title": "This is document number with routing=user2"
        }
      }
    ]
  }
}

Why does the search response only shows doc under user2 despite having 2 shards? It is certain that both docs ended up on the same shard because as per formula:

shard_num = (hash(_routing) % num_routing_shards) / routing_factor
where routing_factor = num_routing_shards / num_primary_shards

In my case routing_factor is 1 (i.e. 2 routing shards / 2 primary shards), so the shard ID is basically the hash of the _routing value mod 2.

Using your routing values, we get the following shard IDs (we can experiment murmur3 here):

murmur3("user1") % 2 = 3305849917 % 2 = shard 1
murmur3("user2") % 2 = 4180509323 % 2 = shard 1

However, if both docs with same _id containing different _routing values end up on the same shard, why does it only show one doc?

Tldr

Because they share the same ID on the same shard, the second query is not an insert, it is an update.

Evidences:

If you play the following commands in order:

PUT 76349386
{
  "settings": {
    "index": 
    {
      "number_of_shards": 2
      }
  }
}

Then

PUT 76349386/_doc/1?routing=user1
{
  "title": "This is document number with routing=user1"
}

Will give you:

{
  "_index": "76349386",
  "_id": "1",
  "_version": 1,
  "result": "created",  <= Here it says the result of the operation was a creation
  "_shards": {
    "total": 2,
    "successful": 1,
    "failed": 0
  },
  "_seq_no": 0,
  "_primary_term": 1
}

But then when you will play the second command

PUT 76349386/_doc/1?routing=user2
{
  "title": "This is document number with routing=user2"
}

The response will look a little bit different:

{
  "_index": "76349386",
  "_id": "1",
  "_version": 2,
  "result": "updated", <= it is an update.
  "_shards": {
    "total": 2,
    "successful": 1,
    "failed": 0
  },
  "_seq_no": 1,
  "_primary_term": 1
}

The document with _id 1 has been updated.