Merge pull request #43095 from pacoxu/kubeadm-etcd-learner-mode

add a new blog for etcd learner mode usage in kubeadm join

Author: k8s-ci-robot

content/en/blog/_posts/2023-09-25-kubeadm-use-etcd-learner-mode.md

@@ -0,0 +1,107 @@
+---
+layout: blog
+title: 'kubeadm: Use etcd Learner to Join a Control Plane Node Safely'
+date: 2023-09-25
+slug: kubeadm-use-etcd-learner-mode
+---
+
+**Author:** Paco Xu (DaoCloud)
+
+The [`kubeadm`](/docs/reference/setup-tools/kubeadm/) tool now supports etcd learner mode, which
+allows you to enhance the resilience and stability
+of your Kubernetes clusters by leveraging the [learner mode](https://etcd.io/docs/v3.4/learning/design-learner/#appendix-learner-implementation-in-v34) 
+feature introduced in etcd version 3.4.
+This guide will walk you through using etcd learner mode with kubeadm. By default, kubeadm runs
+a local etcd instance on each control plane node.
+
+In v1.27, kubeadm introduced a new feature gate `EtcdLearnerMode`. With this feature gate enabled,
+when joining a new control plane node, a new etcd member will be created as a learner and
+promoted to a voting member only after the etcd data are fully aligned.
+
+## What are the advantages of using etcd learner mode?
+
+etcd learner mode offers several compelling reasons to consider its adoption
+in Kubernetes clusters:
+
+ 1. **Enhanced Resilience**: etcd learner nodes are non-voting members that catch up with
+    the leader's logs before becoming fully operational. This prevents new cluster members
+    from disrupting the quorum or causing leader elections, making the cluster more resilient
+    during membership changes.
+ 2. **Reduced Cluster Unavailability**: Traditional approaches to adding new members often
+    result in cluster unavailability periods, especially in slow infrastructure or misconfigurations.
+    etcd learner mode minimizes such disruptions.
+ 3. **Simplified Maintenance**: Learner nodes provide a safer and reversible way to add or replace
+    cluster members. This reduces the risk of accidental cluster outages due to misconfigurations or
+    missteps during member additions.
+ 4. **Improved Network Tolerance**: In scenarios involving network partitions, learner mode allows
+    for more graceful handling. Depending on the partition a new member lands, it can seamlessly
+    integrate with the existing cluster without causing disruptions.
+
+In summary, the etcd learner mode improves the reliability and manageability of Kubernetes clusters
+during member additions and changes, making it a valuable feature for cluster operators.
+
+## How nodes join a cluster that's using the new mode
+
+### Create a Kubernetes cluster backed by etcd in learner mode {#create-K8s-cluster-etcd-learner-mode}
+
+For a general explanation about creating highly available clusters with kubeadm, you can refer to
+[Creating Highly Available Clusters with kubeadm](/docs/setup/production-environment/tools/kubeadm/high-availability/).
+
+To create a Kubernetes cluster, backed by etcd in learner mode, using kubeadm, follow these steps:
+
+```shell
+# kubeadm init --feature-gates=EtcdLearnerMode=true ...
+kubeadm init --config=kubeadm-config.yaml
+```
+
+The kubeadm configuration file is like below:
+
+```yaml
+apiVersion: kubeadm.k8s.io/v1beta3
+kind: ClusterConfiguration
+featureGates:
+  EtcdLearnerMode: true
+```
+
+The kubeadm tool deploys a single-node Kubernetes cluster with etcd set to use learner mode.
+
+### Join nodes to the Kubernetes cluster
+
+Before joining a control-plane node to the new Kubernetes cluster, ensure that the existing control plane nodes
+and all etcd members are healthy.
+
+Check the cluster health with `etcdctl`. If `etcdctl` isn't available, you can run this tool inside a container image.
+You would do that directly with your container runtime using a tool such as `crictl run` and not through Kubernetes
+
+Here is an example on a client command that uses secure communication to check the cluster health of the etcd cluster:
+
+```shell
+ETCDCTL_API=3 etcdctl --endpoints 127.0.0.1:2379 \
+  --cert=/etc/kubernetes/pki/etcd/server.crt \
+  --key=/etc/kubernetes/pki/etcd/server.key \
+  --cacert=/etc/kubernetes/pki/etcd/ca.crt \
+   member list
+...
+dc543c4d307fadb9, started, node1, https://10.6.177.40:2380, https://10.6.177.40:2379, false
+```
+
+To check if the Kubernetes control plane is healthy, run `kubectl get node -l node-role.kubernetes.io/control-plane=`
+and check if the nodes are ready.
+
+Note: It is recommended to have an odd number of members in a etcd cluster.
+
+Before joining a worker node to the new Kubernetes cluster, ensure that the control plane nodes are healthy.
+
+## What's next
+
+- The feature gate `EtcdLearnerMode` is alpha in v1.27 and we expect it to graduate to beta in the next
+  minor release of Kubernetes  (v1.29).
+- etcd has an open issue that may make the process more automatic: 
+  [Support auto-promoting a learner member to a voting member](https://github.com/etcd-io/etcd/issues/15107).
+- Learn more about the kubeadm [configuration format](/docs/reference/config-api/kubeadm-config.v1beta3/) here.
+
+## Feedback
+
+Was this guide helpful? If you have any feedback or encounter any issues, please let us know.
+Your feedback is always welcome! Join the bi-weekly [SIG Cluster Lifecycle meeting](https://docs.google.com/document/d/1Gmc7LyCIL_148a9Tft7pdhdee0NBHdOfHS1SAF0duI4/edit)
+or weekly [kubeadm office hours](https://docs.google.com/document/d/130_kiXjG7graFNSnIAgtMS1G8zPDwpkshgfRYS0nggo/edit). Or reach us via [Slack](https://slack.k8s.io/) (channel **#kubeadm**), or the [SIG's mailing list](https://groups.google.com/g/kubernetes-sig-cluster-lifecycle).