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Why can’t I scale StatefulSets directly?

When creating Pulsar CRs, the Pulsar Operators create a group of resources, including the StatefulSets for Pulsar brokers, proxies (optional), bookies, and ZooKeeper. The Pulsar Operators check whether these StatefulSets work as expected according to the CRs and make any change to the StatefulSets if necessary. To increase or decrease the number of Pods controlled by the StatefulSets, you should not edit the StatefulSets directly. For example, if you run the kubectl scale statefulsets brokers-broker --replicas=5 -n pulsar command to scale the replicas of Pulsar brokers from the default value 2 to 5, the StatefulSet controller first adds 3 new Pods but it deletes them immediately. This is because the Pulsar Operators detect the difference between the configured replicas (2) in the Pulsar broker CR and the replicas (5) that are actually running. For the Pulsar Operators, this is not the desired state so they force the StatefulSet controller to delete the newly-created 3 Pods. To see how the Pulsar Operators work on the broker StatefulSet, run the following command: 
kubectl -n pulsar describe statefulsets brokers-broker
Navigate to the Events field and you may see the following result: 
Events:
  Type    Reason            Age                  From                    Message
  ----    ------            ----                 ----                    -------
  Normal  SuccessfulCreate  30s                  statefulset-controller  create Pod brokers-broker-0 in StatefulSet brokers-broker successful
  Normal  SuccessfulCreate  30s                  statefulset-controller  create Pod brokers-broker-1 in StatefulSet brokers-broker successful
  Normal  SuccessfulDelete  47s                  statefulset-controller  create Pod brokers-broker-2 in StatefulSet brokers-broker successful
  Normal  SuccessfulDelete  47s                  statefulset-controller  create Pod brokers-broker-3 in StatefulSet brokers-broker successful
  Normal  SuccessfulDelete  47s                  statefulset-controller  create Pod brokers-broker-4 in StatefulSet brokers-broker successful
  Normal  SuccessfulCreate  48s                  statefulset-controller  delete Pod brokers-broker-4 in StatefulSet brokers-broker successful
  Normal  SuccessfulCreate  48s                  statefulset-controller  delete Pod brokers-broker-3 in StatefulSet brokers-broker successful
  Normal  SuccessfulCreate  48s                  statefulset-controller  delete Pod brokers-broker-2 in StatefulSet brokers-broker successful
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