Question about some details for Query Phase

Elastic Stack Elasticsearch
1

Dear All,

Could you let me know how to process "Query Phase" exactly when an index has replicas?
For all I know, The coordinating node sends Query to every shard for Query Phase.
I want to understand the exact meaning of every shard for Query phase.

For example, an Index of 2 shards(P0, P1) and 1 replica(R0, R1) is allocated in 4 Nodes like below.

  • Node_A : P1
  • Node_B : R0
  • Node_C : R1
  • Node_D : P0

Let's suppose that Node_C receives a search request(i.e., Node_C is the coordinating node now).

Q1 : Does Node_C send Query request to all shards(P1, R0, R1, P0) or to just 2 shards(e.g., "P1 and R0" or "R1 and P0")?
If the latter, how does ES decide 2 shards to send Query request?

Q2 : If the query type is DFS_Query_then_Fetch, are the target shards for Pre-Query also same with the shards for Query?

Thanks for your help always.

2

For Q1, the search uses "adaptive replica selection" by default, which sends the query to the "best" shards

3

Thanks for your reply.
Do you mean that the coordinating node sends Query to only 2 shards in case of my example?
(2 shards is selected by "adaptive replica selection")

4

For what I understand, yes

5

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