PetSet was introduced as an alpha resource in Kubernetes release 1.3, and was renamed to StatefulSet as a beta resource in 1.5.
Here are some notable changes:
* **StatefulSet is the new PetSet**: PetSet is no longer available in Kubernetes release 1.5 or later. It becomes beta StatefulSet. To understand why the name was changed, see this [discussion thread](https://github.com/kubernetes/kubernetes/issues/27430).
* **StatefulSet guards against split brain**: StatefulSets guarantee at most one Pod for a given ordinal index can be running anywhere in a cluster, to guard against split brain scenarios with distributed applications. *TODO: Link to doc about fencing.*
* **Flipped debug annotation behavior**: The default value of the debug annotation (`pod.alpha.kubernetes.io/initialized`) is now `true`. The absence of this annotation will pause PetSet operations, but will NOT pause StatefulSet operations. In most cases, you no longer need this annotation in your StatefulSet manifests.
Now, for every PetSet manifest you have, prepare a corresponding StatefulSet manifest:
1. Change `apiVersion` from `apps/v1alpha1` to `apps/v1beta1`.
2. Change `kind` from `PetSet` to `StatefulSet`.
3. If you have the debug hook annotation `pod.alpha.kubernetes.io/initialized` set to `true`, you can remove it because it's redundant. If you don't have this annotation, you should add one, with the value set to `false`, to pause StatefulSets operations.
It's recommended that you keep both PetSet manifests and StatefulSet manifests, so that you can safely roll back and recreate your PetSets,
If you find existing PetSets in your cluster in the previous step, you need to delete all PetSets *without cascading*. You can do this from `kubectl` with `--cascade=false`.
Note that if the flag isn't set, **cascading deletion will be performed by default**, and all Pods managed by your PetSets will be gone.
Delete those PetSets by specifying file names. This only works when
the files contain only PetSets, but not other resources such as Services:
```shell
# Delete all existing PetSets without cascading
# Note that <pet-set-file> should only contain PetSets that you want to delete, but not any other resources
kubectl delete -f <pet-set-file> --cascade=false
```
Alternatively, delete them by specifying resource names:
```shell
# Alternatively, delete them by name and namespace without cascading
Make sure you've deleted all PetSets in the system:
```shell
# Get all PetSets again to make sure you deleted them all
# This should return nothing
kubectl get petsets --all-namespaces
```
At this moment, you've deleted all PetSets in your cluster, but not their Pods, Persistent Volumes, or Persistent Volume Claims.
However, since the Pods are not managed by PetSets anymore, they will be vulnerable to node failures until you finish the master upgrade and recreate StatefulSets.
You should be running Node version >= 1.1.0 to run StatefulSets safely. Older versions do not support features which allow the StatefulSet to guarantee that at any time, there is **at most** one Pod with a given identity running in a cluster.
