#### Extensibility

- The community has been asking for continuing support of extensibility, so this cycle features more work around CRDs and API Machinery. Most of the enhancements in this cycle were from SIG API Machinery and related areas.

### Extensibility around core Kubernetes APIs

#### CustomResourceDefinitions Pruning

Note that snapshot support will require Kubernetes distributors to bundle the Snapshot controller, Snapshot CRDs, and validation webhook. In addition, a CSI driver supporting the snapshot functionality must also be deployed on the cluster.

### Non-recursive Volume Ownership (FSGroup) graduates to Beta

* Transition CSINodeInfo and CSIDriver alpha CRDs to in-tree CSINode and CSIDriver core storage v1beta1 APIs. ([#74283](https://github.com/kubernetes/kubernetes/pull/74283), [@xing-yang](https://github.com/xing-yang))
    * ACTION REQUIRED: the alpha CRDs are no longer used and drivers will need to be updated to use the beta APIs.

The main themes of this release are:

- **Custom resources:** CRDs are in widespread use as a way to extend Kubernetes to persist and serve new resource types, and have been available in beta since the 1.7 release. The 1.16 release marks the graduation of CRDs to general availability (GA).

- The example API server has renamed its `wardle.k8s.io` API group to `wardle.example.com` ([#81670](https://github.com/kubernetes/kubernetes/pull/81670), [@liggitt](https://github.com/liggitt))
- CRDs defaulting is promoted to GA. Note: the feature gate CustomResourceDefaulting will be removed in 1.18. ([#84713](https://github.com/kubernetes/kubernetes/pull/84713), [@sttts](https://github.com/sttts))

- CRD supports multi-version Schema, Subresources and AdditionalPrintColumns (NOTE that CRDs created prior to 1.13 populated the top-level additionalPrinterColumns field by default. To apply an updated that changes to per-version additionalPrinterColumns, the top-level additionalPrinterColumns field must be explicitly set to null). ([#702...

* Added Establishing Controller on CRDs to avoid race between Established condition and CRs actually served. In HA setups, the Established condition is delayed by 5 seconds. ([#63068](https://github.com/kubernetes/kubernetes/pull/63068), [@xmudrii](https://github.com/xmudrii))

- Revised `OpenAPI v2` fetching for CustomResourceDefinitions. CRDs are now aggregated lazily,
  which improves resource usage during installation of many CRDs. As a result, the first request
  to fetch the OpenAPI may be slower. ([#118808](https://github.com/kubernetes/kubernetes/pull/118808), [@Jefftree](https://github.com/Jefftree))

    - [API Machinery](#api-machinery)
      - [kube-apiserver](#kube-apiserver)
      - [Dynamic Admission Control](#dynamic-admission-control)
      - [Custom Resource Definitions (CRDs)](#custom-resource-definitions-crds)
      - [Garbage Collector](#garbage-collector)
      - [Monitoring/Prometheus](#monitoringprometheus)
      - [Go Client](#go-client)
  - [External Dependencies](#external-dependencies)

- Reduce API server memory when many CRDs are loaded by sharing a single etcd3 client logger across all clients ([#111648](https://github.com/kubernetes/kubernetes/pull/111648), [@negz](https://github.com/negz)) [SIG API Machinery]