Feature blog for StatefulSet Autodelete beta graduation

Author: mattcary

content/en/blog/_posts/2023-05-04-statefulset-autodelete.md

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+---
+layout: blog
+title: 'Kubernetes 1.27: StatefulSet PVC Auto-Deletion (beta)'
+date: 2023-05-04
+slug: kubernetes-1-27-statefulset-pvc-auto-deletion-beta
+---
+
+**Author:** Matthew Cary (Google)
+
+Kubernetes v1.27 graduated to beta a new policy mechanism for
+[`StatefulSets`](/docs/concepts/workloads/controllers/statefulset/) that controls the lifetime of
+their [`PersistentVolumeClaims`](/docs/concepts/storage/persistent-volumes/) (PVCs). The new PVC
+retention policy lets users specify if the PVCs generated from the `StatefulSet` spec template should
+be automatically deleted or retrained when the `StatefulSet` is deleted or replicas in the `StatefulSet`
+are scaled down.
+
+## What problem does this solve?
+
+A `StatefulSet` spec can include `Pod` and PVC templates. When a replica is first created, the
+Kubernetes control plane creates a PVC for that replica if one does not already exist. The behavior
+before the PVC retention policy was that the control plane never cleaned up the PVCs created for
+`StatefulSets` - this was left up to the cluster administrator, or to some add-on automation that
+you’d have to find, check suitability, and deploy. The common pattern for managing PVCs, either
+manually or through tools such as Helm, is that the PVCs are tracked by the tool that manages them,
+with explicit lifecycle. Workflows that use `StatefulSets` must determine on their own what PVCs are
+created by a `StatefulSet` and what their lifecycle should be.
+
+Before this new feature, when a StatefulSet-managed replica disappears, either because the
+`StatefulSet` is reducing its replica count, or because its `StatefulSet` is deleted, the PVC and its
+backing volume remains and must be manually deleted. While this behavior is appropriate when the
+data is critical, in many cases the persistent data in these PVCs is either temporary, or can be
+reconstructed from another source. In those cases, PVCs and their backing volumes remaining after
+their `StatefulSet` or replicas have been deleted are not necessary, incur cost, and require manual
+cleanup.
+
+## The new `StatefulSet` PVC retention policy
+
+The new `StatefulSet` PVC retention policy is used to control if and when PVCs created from a
+`StatefulSet`’s `volumeClaimTemplate` are deleted.  There are two contexts when this may occur.
+
+The first context is when the `StatefulSet` resource is deleted (which implies that all replicas are
+also deleted). This is controlled by the `whenDeleted` policy. The second context, controlled by
+`whenScaled` is when the `StatefulSet` is scaled down, which removes some but not all of the replicas
+in a `StatefulSet`. In both cases the policy can either be `Retain`, where the corresponding PVCs are
+not touched, or `Delete`, which means that PVCs are deleted. The deletion is done with a normal
+[object deletion](/docs/concepts/architecture/garbage-collection/), so that, for example, all
+retention policies for the underlying PV are respected.
+
+This policy forms a matrix with four cases. I’ll walk through and give an example for each one.
+
+  * **`whenDeleted` and `whenScaled` are both `Retain`.** 
+  
+    This matches the existing behavior for `StatefulSets`, where no PVCs are deleted. This is also
+    the default retention policy. It’s appropriate to use when data on `StatefulSet` volumes may be
+    irreplaceable and should only be deleted manually.
+
+  * **`whenDeleted` is `Delete` and `whenScaled` is `Retain`.** 
+  
+    In this case, PVCs are deleted only when the entire `StatefulSet` is deleted. If the
+    `StatefulSet` is scaled down, PVCs are not touched, meaning they are available to be reattached
+    if a scale-up occurs with any data from the previous replica. This might be used for a temporary
+    `StatefulSet`, such as in a CI instance or ETL pipeline, where the data on the `StatefulSet` is
+    needed only during the lifetime of the `StatefulSet` lifetime, but while the task is running the
+    data is not easily reconstructible. Any retained state is needed for any replicas that scale
+    down and then up.
+
+  * **`whenDeleted` and `whenScaled` are both `Delete`.** 
+  
+    PVCs are deleted immediately when their replica is no longer needed. Note this does not include
+    when a `Pod` is deleted and a new version rescheduled, for example when a node is drained and
+    `Pods` need to migrate elsewhere. The PVC is deleted only when the replica is no longer needed
+    as signified by a scale-down or `StatefulSet` deletion. This use case is for when data does not
+    need to live beyond the life of its replica. Perhaps the data is easily reconstructable and the
+    cost savings of deleting unused PVCs is more important than quick scale-up, or perhaps that when
+    a new replica is created, any data from a previous replica is not usable and must be
+    reconstructed anyway.
+
+  * **`whenDeleted` is `Retain` and `whenScaled` is `Delete`.** 
+  
+    This is similar to the previous case, when there is little benefit to keeping PVCs for fast
+    reuse during scale-up. An example of a situation where you might use this is an Elasticsearch
+    cluster. Typically you would scale that workload up and down to match demand, whilst ensuring a
+    minimum number of replicas (for example: 3). When scaling down, data is migrated away from
+    removed replicas and there is no benefit to retaining those PVCs. However, it can be useful to
+    bring the entire Elasticsearch cluster down temporarily for maintenance. If you need to take the
+    Elasticsearch system offline, you can do this by temporarily deleting the `StatefulSet`, and
+    then bringing the Elasticsearch cluster back by recreating the `StatefulSet`. The PVCs holding
+    the Elasticsearch data will still exist and the new replicas will automatically use them.
+
+Visit the
+[documentation](/docs/concepts/workloads/controllers/statefulset/#persistentvolumeclaim-policies) to
+see all the details.
+
+## What’s next?
+
+Try it out! The `StatefulSetAutoDeletePVC` feature gate is beta and enabled by default on
+cluster running Kubernetes 1.27. Create a `StatefulSet` using the new policy, test it out and tell
+us what you think!
+
+I'm very curious to see if this owner reference mechanism works well in practice. For example, I
+realized there is no mechanism in Kubernetes for knowing who set a reference, so it’s possible that
+the `StatefulSet` controller may fight with custom controllers that set their own
+references. Fortunately, maintaining the existing retention behavior does not involve any new owner
+references, so default behavior will be compatible.
+
+Please tag any issues you report with the label `sig/apps` and assign them to Matthew Cary
+([@mattcary](https://github.com/mattcary) at GitHub).
+
+Enjoy!