Analyzing control plane logs is a critical competency for the Certified Kubernetes Administrator (CKA) exam, serving as the primary method for diagnosing cluster failures. The control plane consists of four core components: kube-apiserver, kube-scheduler, kube-controller-manager, and etcd. The appr…Analyzing control plane logs is a critical competency for the Certified Kubernetes Administrator (CKA) exam, serving as the primary method for diagnosing cluster failures. The control plane consists of four core components: kube-apiserver, kube-scheduler, kube-controller-manager, and etcd. The approach to log analysis depends on the deployment method.
In a standard `kubeadm` cluster, these components run as **Static Pods**. If the API server is responsive, you can check logs via `kubectl logs -n kube-system <pod-name>`. However, troubleshooting usually implies the API server is down. In this scenario, you must SSH into the control plane node and bypass kubectl. Use the container runtime CLI (e.g., `crictl ps` or `docker ps`) to identify the failing container's ID, then view logs using `crictl logs <id>`. You are looking for "CrashLoopBackOff" causes, typically resulting from typos in YAML manifests located in `/etc/kubernetes/manifests`, incorrect command-line arguments, or certificate path mismatches.
If the cluster was set up using binaries (systemd), logs are managed by `journalctl`. You would inspect them using commands like `journalctl -u kube-apiserver -f`.
Specific patterns to look for include:
1. **API Server:** Connection refused errors to etcd, indicating database unavailability.
2. **Etcd:** "Database space exceeded" or high disk latency warnings.
3. **Scheduler:** "Failed to schedule" errors indicating resource starvation or taint/toleration conflicts.
Successfully analyzing these logs allows you to pinpoint the exact configuration error—whether it is a syntax error in a static pod manifest or a networking issue—and apply the necessary fix to restore the control plane to a Running state.
Analyzing Control Plane Logs: The CKA Guide
Why it is Important The control plane is the "brain" of a Kubernetes cluster. It consists of critical components like the kube-apiserver, etcd, kube-scheduler, and kube-controller-manager. If any of these fail, the cluster may stop scheduling pods, lose state data, or become entirely unresponsive to kubectl commands. Analyzing these logs is often the only way to determine why a cluster is broken and is a fundamental skill for the CKA exam.
What it is Analyzing control plane logs is the process of retrieving and interpreting the error streams generated by Kubernetes system components. In standard CKA environments (usually created with kubeadm), these components run as Static Pods. This means they are managed directly by the Kubelet on the master node, rather than by the API server deployment controllers.
How it Works Since these components run as containers, their logs are managed by the container runtime (e.g., containerd, CRI-O, or Docker). 1. Standard Scenario: If the API server is running, logs are accessible via standard Kubernetes tools. 2. Failure Scenario: If the API server crashes, the Kubernetes API becomes unavailable. You cannot use kubectl. You must access the logs directly from the host machine's container runtime or file system.
How to Answer Questions regarding Analyzing Control Plane Logs Follow this troubleshooting workflow: 1. Check Pod Status: Run kubectl get pods -n kube-system. Look for pods with status CrashLoopBackOff or Error. 2. If kubectl works: Inspect logs using kubectl logs -n kube-system <pod-name>. 3. If kubectl fails (Connection Refused): SSH into the control plane node immediately. 4. Inspect Runtime: Use crictl ps -a (or docker ps -a) to find the container ID of the exited control plane component. 5. Read Logs: Run crictl logs <container-id> to view the stderr output and identify the specific error (e.g., "typing error in command", "invalid certificate path"). 6. Fix the Manifest: Navigate to /etc/kubernetes/manifests/ and edit the corresponding YAML file to fix the configuration error.
Exam Tips: Answering Questions on Analyzing control plane logs • Know the 'Chicken and Egg' Problem: If the kube-apiserver is down, kubectl logs will not work. You must serve yourself by using crictl or looking at /var/log/pods/ or /var/log/containers/ directly on the node. • Don't Forget the Kubelet: The Kubelet is not a pod; it is a systemd service. If static pods aren't starting at all, check the Kubelet logs using journalctl -u kubelet | grep -i error. • Manifest Watcher: When you edit a file in /etc/kubernetes/manifests/, the Kubelet automatically restarts the pod. You do not need to run kubectl apply or restart the service manually. Just save the file and wait 20 seconds. • Keywords to Grep: When looking at verbose logs, filter for "Fatal", "Error", or "Failure" to save time.