// optional specify whether the ConfigMap or its keys must be defined
  // +optional
  optional bool optional = 4;
}

// Adapts a ConfigMap into a volume.
//
// The contents of the target ConfigMap's Data field will be presented in a
// volume as files using the keys in the Data field as the file names, unless
// the items element is populated with specific mappings of keys to paths.

### Other notable changes

* Added new flag to `kubeadm init`: --node-name, that lets you specify the name of the Node object that will be created ([#48594](https://github.com/kubernetes/kubernetes/pull/48594), [@GheRivero](https://github.com/GheRivero))

- consumers of the 'certificatesigningrequests/approval' API must now have permission to 'approve' CSRs for the specific signer requested by the CSR. More information on the new signerName field and the required authorization can be found at https://kubernetes.io/docs/reference/...

#### Storage

* [stable] Mount options

  * The ability to specify mount options for volumes is moved from beta to stable.

  * A new `MountOptions` field in the `PersistentVolume` spec is available to specify mount options. This field replaces an annotation.

provider can now be specified as environment variables. When we convert the OpenStack cloud provider to run in an external process, we can now use the kubernetes Secrets capability to inject the OS_* variables. This way we can specify the cloud configuration as a configmap, and specify secrets for the userid/password information. The configmap is mounted as a file, and the secrets are made available as environment variables. The external controller itself runs as a pod/daemonset. For backward compatibility,...

- Users can now specify azure file share name in the azure file plugin, making it possible to use existing shares or specify a new share name. ([#76988](https://github.com/kubernetes/kubernetes/pull/76988), [@andyzhangx](https://github.com/andyzhangx))

on/assign-pod-node/) and **[beta]** [pod affinity/anti-affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/). Node affinity/anti-affinity allow you to specify rules for restricting which node(s) a pod can schedule onto, based on the labels on the node. Pod affinity/anti-affinity allow you to specify rules for spreading and packing pods relative to one another, across arbitrary topologies (node, zone, etc.) These affinity rules are now be specified in a new-in-1.6 `affinity`...

updating to 1.9, you are strongly recommended to back up your etcd data. Consult the installation procedure you are using (kargo, kops, kube-up, kube-aws, kubeadm etc) for specific advice.

Some upgrade methods might upgrade etcd from 3.0 to 3.1 automatically when you upgrade from Kubernetes 1.8, unless you specify otherwise. Because [etcd does not support downgrading](https://coreos.com/etcd/docs/latest/upgrades/upgrade_3_1.html), you'll need to either remain on etcd 3.1 or restore from...

- Add scheduling support for RuntimeClasses. RuntimeClasses can now specify nodeSelector constraints & tolerations, which are merged into the PodSpec for pods using that RuntimeClass. ([#808...

- Deprecate the default service IP CIDR. The previous default was `10.0.0.0/24` which will be removed in 6 months/2 releases. Cluster admins must specify their own desired value, by using `--service-cluster-ip-range` on kube-apiserver. ([#81668](https://github.com/kubernetes/kubernetes/pull/81668), [@darshanime](https://github.com/darshanime))