[Adaptive Server Selector] support for strictReplicaGroup routing strategy

[MeihanLi]

It seems currently Adaptive Server Selector is partially supported for strictReplicaGroup. But it breaks the assumption that by default, for strictReplicaGroup routing, queries are routed to servers from 1 replica group only.

I tested and figured out that for an upsert table with 2 replica groups, queries can be routed to servers from 2 replica groups depending on the server performance.

For example, a tenant has 4 servers, the table has 2 partitions, 2 replicas, 2 replica groups. For select count(*) query, it can pick serverA and serverD from 2 different RGs or serverA and serverB from 1 RG to serve the query.

RG assignment:
RG 0: serverA, serverB
RG 1: serverC, serverD

segments are assigned as below:
partition 0     serverA              serverC
partition 1     serverB              serverD

Are there any other concerns on turning on adaptive server selector for strictReplicaGroup routing tables except that? Also I was wondering if anyone is looking into supporting numReplicaGroupsToQuery with Adaptive Server Selection. What's the plan for it?

[MeihanLi]

cc: @vvivekiyer @ankitsultana

[vvivekiyer]

AdaptiveServerSelection is not support for strictReplicaGroup.
It's only supported in balanced and ReplicaGroup.

The current solution ranks servers and picks the best set of servers. For strictReplicaGroup, we might need a solution that ranks replicaGroups before picking the best one.

[ankitsultana]

@vvivekiyer if one enables adaptive server selector on a broker that serves Upsert tables, what would happen? Would it lead to incorrect results?

[timothy-e]

This could actually lead to correctness issues, if adaptive routing is enabled on a cluster that has upsert tables. I have a fix up in #18947.