- Kubeadm: added the experimental (alpha) feature gate `EtcdLearnerMode` that allows etcd members to be joined as learner and only then promoted as voting members ([#113318](https://github.com/kubernetes/kubernetes/pull/113318), [@pacoxu](https://github.com/pacoxu))

### Instrumentation

In v1.9 we focused on improving stability of the components owned by the SIG, including Heapster, Custom Metrics API adapters for Prometheus, and Stackdriver.

### Network

- Kubeadm: during join when a check is performed that a Node with the same name already exists in the cluster, make sure the NodeReady condition is properly validated ([#89602](https://github.com/kubernetes/kubernetes/pull/89602), [@kvaps](https://github.com/kvaps)) [SIG Cluster Lifecycle]

* `--feature-gates` argument has been removed from the `kubeadm join` command. Feature gates will be retrieved from the cluster configuration during the join process. ([#70755](https://github.com/kubernetes/kubernetes/pull/70755), [@ereslibre](https://github.com/ereslibre))

* `kubeadm join` is now blocking on the kubelet performing the TLS Bootstrap properly. Earlier, `kubeadm join` only did the discovery part and exited successfully without checking that the kubelet actually started properly and performed the TLS bootstrap correctly. Now, as kubeadm runs some post-join steps (for example, annotating the Node API object with the CRISocket), `kubeadm join` is now waiting for the kubelet to perform the TLS Bootstrap,...

- Kubeadm increased to 5 minutes its timeout for the TLS bootstrapping process to complete upon join ([#89735](https://github.com/kubernetes/kubernetes/pull/89735), [@rosti](https://github.com/rosti)) [SIG Cluster Lifecycle]

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- kubeadm: Improved resiliency when it comes to updating the `kubeadm-config` configmap upon new control plane joins or resets. This allows for safe multiple control plane joins and/or resets. ([#76821](https://github.com/kubernetes/kubernetes/pull/76821), [@ereslibre](https://github.com/ereslibre))

### Juju
* The kubernetes-master, kubernetes-worker and kubeapi-load-balancer charms have gained an nrpe-external-master relation, allowing the integration of their monitoring in an external Nagios server. ([#41923](https://github.com/kubernetes/kubernetes/pull/41923), [@Cynerva](https://github.com/Cynerva))

- kubeadm: added the `WaitForAllControlPlaneComponents` feature gate. It could be used to tell kubeadm to wait for all control plane components to be ready when running "kubeadm init" or "kubeadm join --control-plane". Previously, kubeadm only waited for the kube-apiserver. The "kubeadm join" workflow now includes a new experimental phase called "wait-control-plane". This phase was marked as non-experimental when WaitForAllControlPlaneComponents became GA. Accordingly, a "kubeadm init" phase...