### ConfigMap
* Volumes and environment variables populated from ConfigMap and Secret objects can now tolerate the named source object or specific keys being missing, by adding `optional: true` to the volume or environment variable source specifications. ([#39981](https://github.com/kubernetes/kubernetes/pull/39981), [@fraenkel](https://github.com/fraenkel))

are adding Windows Server Virtual Machines as nodes to your Kubernetes environment, there is a compatibility issue with certain virtualization products. Specifically the Windows version of the kubelet.exe calls `GetPhysicallyInstalledSystemMemory` to get the physical memory installed on Windows machines and reports it as part of node metrics to heapster. This API call fails for VMware and VirtualBox virtualization environments. This issue is not present in bare metal Windows deployments, in Hyper-V,...

enabled by default in all Kubernetes clients to fix persistently broken connections (https://github.com/kubernetes/client-go/issues/374). If needed, users can tune the feature via the HTTP2_READ_IDLE_TIMEOUT_SECONDS and HTTP2_PING_TIMEOUT_SECONDS environment variables. The feature is disabled if HTTP2_READ_IDLE_TIMEOUT_SECONDS is set to 0. ([#100376](https://github.com/kubernetes/kubernetes/pull/100376), [@liggitt](https://github.com/liggitt)) [SIG API Machinery, CLI, Cloud Provider, Cluster Lifecycle,...

* `hack/local-up-cluster.sh` now enables the Node authorizer by default. Authorization modes can be overridden with the `AUTHORIZATION_MODE` environment variable, and the `ENABLE_RBAC` environment variable is no longer used. ([#49812](https://github.com/kubernetes/kubernetes/pull/49812), [@liggitt](https://github.com/liggitt))

- The certificate signer no longer accepts ca.key passwords via the `CFSSL_CA_PK_PASSWORD` environment variable. This capability was not prompted by user request, never advertised, and recommended against in the security audit. ([#84677](https://github.com/kubernetes/kubernetes/pull/84677), [@mikedanese](https://github.com/mikedanese))

    * If you are running E2E tests which require SSH keys and you utilize environment variables to override their location, you may need to modify the environment variable set. On all providers the environment variable override can now be either an absolute path to the key or a relative path (relative to ~/.ssh). Specifically the changes are:

to "systemd" unless the user is explicit about it. This requires existing machine setups to configure the container runtime to use the "systemd" driver. Documentation on this topic can be found here: https://kubernetes.io/docs/setup/production-environment/container-runtimes/. When upgrading existing clusters / nodes using "kubeadm upgrade" the old cgroupDriver value is preserved, but in 1.22 this change will also apply to "upgrade". For more information on migrating to the "systemd" driver or remaining...

or `/etc/sysconfig/kubelet`, depending on the system...

information for OpenStack cloud 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....

efficiently. kubeadm has promoted high availability (HA) capability to beta, allowing users to use the familiar `kubeadm init` and `kubeadm join` commands to [configure and deploy an HA control plane](https://kubernetes.io/docs/setup/production-environment/tools/kubeadm/high-availability/). An entire new test suite has been created specifically for ensuring these features will stay stable over time.

Certificate management has become more robust in 1.15, with kubeadm now seamlessly rotating...