ReplicaSets

ReplicaSets are a k8s construct for maintaining a number of Pods replicas at all times. They are the successor of the ReplicationController construct and are a lower-level concept than a Deployment.

Note

It’s generally advised against using them directly.

They use label selectors¹ to acquire pods for management and ownerReference fields ² for garbage collection³ and to prevent other controllers from trying to manage its pods (dependants). Furthermore, they use pod templates⁴ to create new pods from.

Important

Unlike a Deployment, RSs don’t provide declarative updates to pods with revision control.

Warning

ReplicaSets can acquire pods with a different .spec as long as their labels match its selector. They don’t evaluate whether their template matches the “to-be-acquired” pod’s .spec.

Note

Pods with matching labels, but with an ownerReference that is a controller⁵ will not be acquired by the ReplicaSet. If, however, the pod lacks an owner reference or the reference is not a controller, the RS with a matching selector will acquire the pod immediately.

Sources

• https://kubernetes.io/docs/concepts/workloads/controllers/replicaset/

Footnotes

1. See Labels and Label Selectors in k8s. ↩

2. See Owners and Dependents in k8s. ↩

3. See Garbage Collection in k8s. ↩

4. See Pod Templates. ↩

5. See k8s controllers. ↩