Create Kubernetes API Versioning, K8s Extension , Certification Tip.md

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DevOps Tools 1 Module/Kubernetes API Versioning, K8s Extension , Certification Tip.md

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+## Kubernetes API Versioning, K8s Extension , Certification Tip: Lecture Notes
+
+
+
+##  Admission Controllers
+
+
+### **Purpose:**
+Admission controllers are plugins that enforce policies on objects being created or updated in a Kubernetes cluster. These policies are essential to maintain security, resource limits, and operational efficiency.
+
+### **Types of Admission Controllers:**
+1. **Mutating Admission Controller:**
+   - Modifies requests to enforce certain policies before they are accepted into the cluster.
+   - Example: Automatically adding labels to pods.
+
+2. **Validating Admission Controller:**
+   - Ensures requests meet certain criteria and validates configurations.
+   - Example: Rejecting deployments with insecure configurations.
+
+![image](https://hackmd.io/_uploads/SkOaSVgU1g.png)
+
+### **Commonly Used Admission Controllers:**
+| Controller         | Purpose                                              |
+|--------------------|------------------------------------------------------|
+| Namespace Lifecycle | Prevents operations on resources in non-existent namespaces |
+| Limit Ranger        | Enforces default resource limits                    |
+| Resource Quota      | Enforces quotas on the number of resources used     |
+
+---
+
+##  Kubernetes API Versions
+
+### **Stages of API Versions:**
+1. **Alpha:**
+   - Experimental features.
+   - May change or be removed in future releases.
+   - **Not recommended** for production use.
+
+2. **Beta:**
+   - Features have broader testing.
+   - Guaranteed not to be removed.
+   - May have minor changes.
+
+3. **Stable:**
+   - Fully tested and ready for production.
+   - No further breaking changes.
+   - Example: `v1`.
+
+### **Examples of Kubernetes API Versions:**
+1. **v1:** Core stable API version including resources like:
+   - Pods
+   - Services
+   - ConfigMaps
+
+2. **Other API Groups:**
+   - `apps/v1` for Deployments
+   - `batch/v1` for Jobs and CronJobs
+
+---
+
+##  Kubernetes Resources and Autoscaling
+
+
+### **Core Resources:**
+1. **Pods:**
+   - Smallest deployable units in Kubernetes.
+   - Represents a group of containers with shared storage, network, and specifications.
+
+2. **Services:**
+   - Abstracts and exposes pods as network services.
+   - Ensures stable communication between pods and users.
+
+3. **ConfigMaps:**
+   - Stores non-sensitive configuration data in a key-value format.
+
+4. **Secrets:**
+   - Stores sensitive data like passwords and tokens securely.
+
+5. **Namespaces:**
+   - Provides a mechanism to isolate groups of resources within a cluster.
+
+6. **Persistent Volumes (PVs):**
+   - Provides storage resources for pods, independent of their lifecycle.
+
+### **Workload Resources (apps/v1, batch/v1):**
+1. **Deployment:**
+   - Manages replicated applications.
+   - Supports rolling updates.
+
+2. **ReplicaSet:**
+   - Ensures a specified number of pod replicas are running at any time.
+
+3. **StatefulSet:**
+   - Manages stateful applications with stable network identities and persistent storage.
+
+4. **DaemonSet:**
+   - Ensures a copy of a pod runs on all or selected nodes in the cluster.
+
+5. **Job:**
+   - Manages batch jobs.
+   - Ensures specified tasks are completed successfully.
+
+6. **CronJob:**
+   - Schedules jobs to run at specific times or intervals.
+
+### **Networking Resources (networking.k8s.io):**
+1. **Ingress:**
+   - Manages external access to services within the cluster.
+   - Acts as an HTTP/HTTPS proxy.
+
+2. **Network Policy:**
+   - Controls traffic flow between pods.
+   - Example: Allowing only specific pods to communicate.
+
+3. **Service:**
+   - Exposes a set of pods and ensures stable networking.
+
+### **Storage Resources:**
+1. **Storage Class:**
+   - Defines different types of storage and policies for dynamic provisioning.
+
+### **RBAC & Authorization:**
+Kubernetes provides **Role-Based Access Control (RBAC)** to manage permissions.
+
+**API Group:** `rbac.authorization.k8s.io`
+
+**Resources:**
+1. **Roles:**
+   - Assign permissions within a namespace.
+
+2. **Role Bindings:**
+   - Binds roles to users or groups within a namespace.
+
+3. **Cluster Roles:**
+   - Assign permissions cluster-wide.
+
+4. **Cluster Role Bindings:**
+   - Binds cluster roles to users or groups cluster-wide.
+
+### **Custom Resources:**
+1. **Custom Resource Definition (CRD):**
+   - Extends the Kubernetes API.
+   - Allows users to define and use their own resources.
+
+---
+
+##  Autoscaling
+Kubernetes provides tools to automatically adjust resources based on usage.
+
+1. **Horizontal Pod Autoscaler (HPA):**
+   - Scales pods horizontally based on CPU or custom metrics.
+
+2. **Vertical Pod Autoscaler (VPA):**
+   - Adjusts resource requests and limits for containers based on actual usage.
+
+---
+
+##  Key Commands (kubectl)
+
+
+### **Basic Pod Commands:**
+1. `kubectl get pods -o wide`
+   - Displays detailed pod information in a wide format.
+
+2. `kubectl get pods -o json`
+   - Outputs pod details in JSON format.
+
+3. `kubectl get pods --field-selector=status.phase=Running`
+   - Filters pods based on their running status.
+
+4. `kubectl get pods --watch`
+   - Watches for changes in pod status in real-time.
+
+### **Aliases:**
+1. `alias kgp='kubectl get pods'`
+   - Shortens the command to list pods.
+
+### **Other Commands:**
+1. `kubectl exec`
+   - Executes commands directly within a container.
+
+---
+
+## **Examples for Better Understanding:**
+1. **Mutating Admission Controller Example:**
+   - Automatically adding a label to pods using `MutatingWebhookConfiguration`.
+
+2. **Validating Admission Controller Example:**
+   - Ensuring deployments do not use `latest` tag for images.
+
+3. **API Version Usage Example:**
+   - Using `apps/v1` for deploying a sample application.
+
+---