If a Deployment uses ReplicaSets to scale Pods up and down, and StatefulSets don't have ReplicaSets...
So, how does it manage to scale Pods up and down? I mean, what resource is responsible? What requests does a StatefulSet make in order to scale?
In short StatefulSet Controller handles statefulset replicas.
A StatefulSet is a Kubernetes API object for managing stateful application workloads. StatefulSets handle the deployment and scaling of sets of Kubernetes pods, providing guarantees about their uniqueness and ordering.
Similar to deployments, StatefulSets manage pods with identical container specifications. They differ in terms of maintaining a persistent identity for each pod. While the pods are all created based on the same spec, they are not interchangeable, so each pod is given a persistent identifier that is maintained through rescheduling.
Benefits of a StatefulSet deployment include:
Unique identifiers—every pod in the StatefulSet is assigned a unique, stable network identity, consisting of a hostname based on the application name and instance number. For example, a StatefulSet for a web application with three instances may have pods labeled web1, web2 and web3.
Persistent storage—every pod has its own stable, persistent volume, either by default or as defined per storage class. When the pods in a cluster are scaled down or deleted, their associated volumes are not lost, and the data persists. Unneeded resources can be purged by scaling down the StatefulSet to 0 before deleting the unused pods.
Ordered deployment and scaling—the pods in a StatefulSet are created and deployed in order, according to their increments. Pods are also shut down in (reverse) order, ensuring that the deployment and runtime are reliable and repeatable. The StatefulSet won’t scale until all every required pod is running, so if a pod fails, it will recreate the pod before it attempts to add more instances as per the scaling requirements.
Automated, ordered updates—a StatefulSets can handle rolling updates, shutting down each node and rebuilding it according to the original order, until every node has been replaced and the older versions cleaned up. The persistent volumes can be reused, so data is migrated to the new version automatically.