How documents is retrieved if we are not using _routing

Elastic Stack Elasticsearch
1

I have an index with 3 primary and 1 replicas,total 6 shards for an index.
I sent 5 messages to Elasticsearch cluster and these messages is distributed among 5 different shards.

As a client i am curious how a client can retrieve particular message using document id.
I've used GET test_index/_doc/1

But how this document is routed.When i sent GET request how my client know that document id is stored in this shards and nodes.

2

The shard id is computed from the document id.
It's basically "a modulo the number of shards".

3

are you saying this formula shard_id=hash(document_id) % no_of_shards

4

Yes. See _routing field | Elasticsearch Guide [8.1] | Elastic

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

If no _routing is provided, then the _id is used.

5

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