[Remote Store] With remote translog, revisit write request response - _shard.successful/_shard.total

[ashking94]

Is your feature request related to a problem? Please describe.
Currently in logical (document) replication, a request gets fanned out to replication target from the primary, if replicas are configured and active. With remote translog (on top of segment replication), the translog generated locally would be uploaded to remote store. The replicas would be used for primary term validation as mentioned in #3706.

So, if, lets say, there are 2 replica shards, then a write response would contain a node on _shards-

{
  "_index": "test1_segment",
  "_id": "SSaWyIQBb4oiQ9uHCvge",
  "_version": 1,
  "result": "created",
  "_shards": {
    "total": 3,
    "successful": 3,
    "failed": 0
  },
  "_seq_no": 1,
  "_primary_term": 1
}

Here, it means the request was replicated in 3 (1 primary + 2 replicas) shards in total.

With remote store, since the request is not replicated across replicas (unless the primary is relocating), we have to revisit if we should consider replicas (without local translog) in _shards calculation.

Following points has to be considered -

• how is the _shards field used today by the clients?
• replication definition of write requests

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[ashking94]

Taking the discussion from #5282 to here.
cc @sachinpkale @andrross @dblock @mch2 @Bukhtawar

[saratvemulapalli]

@ashking94 this is labelled 2.5, code freeze is 1/10 do you think you can make it?

[ashking94]

@saratvemulapalli not tracking this for 2.5 release.