Operator dropping wrong replicas when removing older replicas from cluster layout

[jannikbend]

Description

When removing replicas from a cluster that are not the ones that were added most recently the operator tries to remove the wrong replicas from the cluster (using SYSTEM DROP REPLICA '...'). This only affects the replicas in ch and not the STS, Pods and other k8s resources as these are remove correctly.

So given a cluster config looks like:

clusters:
      - name: replicated
        layout:
          shards:
            - name: "0"
              replicas:
                - name: "0-0" # Old
                - name: "0-1" # Old
                - name: "0-2" # Old
                # New replicas on dedicated nodes
                - name: "0-0-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated
                - name: "0-1-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated
                - name: "0-2-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated

When we now try to remove the old replicas "0-0", "0-1" and "0-2" the operator will try to remove the replicas "0-0-dedicated" etc. However it fails in doing so (E1024 08:42:53.714011 1 connection.go:194] Exec():FAILED Exec(http://operator/:***@chi-analytics-replicated-0-0.clickhouse.svc.cluster.local:8123/) doRequest: transport failed to send a request to ClickHouse: dial tcp: lookup chi-analytics-replicated-0-0.clickhouse.svc.cluster.local on 172.17.0.10:53: no such host for SQL: SYSTEM DROP REPLICA 'chi-analytics-replicated-0-0-dedicated') because it tries to execute the SQL statements on the already removed pods.

The same behaviour can be observed when adding replicas: If we try to add a new replicas by not appending it to the end of the replicas array there will be no schema migration.

See this related slack post as well: https://altinitydbworkspace.slack.com/archives/C02K1MWEK2L/p1729859400145679

Reproduction

1. Create cluster wit replicas

clusters:
      - name: replicated
        layout:
          shards:
            - name: "0"
              replicas:
                - name: "0-0"
                - name: "0-1"
                - name: "0-2"

2. Add a new replicas

clusters:
      - name: replicated
        layout:
          shards:
            - name: "0"
              replicas:
                - name: "0-0" # Old
                - name: "0-1" # Old
                - name: "0-2" # Old
                # New replicas on dedicated nodes
                - name: "0-0-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated
                - name: "0-1-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated
                - name: "0-2-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated

3. Remove the created replicas in step 1

clusters:
      - name: replicated
        layout:
          shards:
            - name: "0"
              replicas:
                # New replicas on dedicated nodes
                - name: "0-0-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated
                - name: "0-1-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated
                - name: "0-2-dedicated"
                  templates:
                    podTemplate: clickhouse-dedicated

[hodgesrm]

Thanks @jannikbend for posting. This behavior is at least surprising, I would be interested to hear from @Slach and @sunsingerus what they think about the case.

[sunsingerus]

Need to check in details, adding/deleting replicas is quite common scenario, but may be devil is in some details...

[vprus]

Hi, is there any update here?

I find myself in a situation where I need to remove a specific replica, because it's in the wrong AZ, and the above approach would be useful.