GitHub link - https://github.com/SUBHAM-NANDI/40DaysKubernetes/tree/main/Day%208

Understanding Kubernetes Replication Controller, ReplicaSet, and Deployment

1. Replication Controller: Ensuring Pod Availability

A Replication Controller is responsible for ensuring that a specified number of pod replicas are running at any given time. It continuously monitors the pods' state and spins up new ones if any fail or are terminated.

Pod Crashes and Auto-Healing: When a pod crashes or becomes unavailable, the replication controller detects this and automatically spins up a new pod to replace the failed one. This ensures that the application is always available to the user, even during failures.
Multiple Replicas for High Availability: A key benefit of using a replication controller is high availability. Instead of relying on a single pod, multiple identical pods can be deployed. If one pod fails, traffic can still be served by the remaining pods.
Scaling the Application: If traffic to an application increases, the replication controller can be updated to increase the number of replicas. For example, if the original configuration had two replicas, this can be scaled to three or more to handle increased demand.

2. ReplicaSet: A More Flexible Approach

The ReplicaSet is the next evolution of the replication controller, designed with greater flexibility in mind. It provides the same core functionality—ensuring the desired number of pod replicas are running—but adds more advanced label selectors for managing pods.

Improved Label Selector Support: ReplicaSets allow more expressive label selectors, which can be useful for grouping pods. This enables more dynamic and granular control over which pods are managed.
Managing Pods Across Nodes: Like the replication controller, ReplicaSets can manage pods across multiple nodes in a Kubernetes cluster. This ensures better load balancing and resource optimization across the cluster.

3. Deployment: The Preferred Method for Managing Updates

A Deployment is built on top of ReplicaSets and provides additional capabilities, making it the preferred method for managing application updates in Kubernetes. Deployments allow for rolling updates, rollback, and scaling, ensuring seamless transitions during updates.

Rolling Updates: When deploying a new version of an application, a deployment ensures that the transition happens gradually. New pods are created with the updated version, and old pods are terminated. This avoids downtime during the update process.
Rollback Capabilities: If something goes wrong during an update, a deployment allows you to easily roll back to a previous version. This adds an extra layer of safety and ensures that any issues in production can be quickly reverted.
Scaling Applications: Deployments can easily scale the number of replicas by simply updating the deployment configuration. This flexibility makes deployments the ideal choice for managing modern, scalable applications in Kubernetes.

Setting Up the Environment

Creating the Replication Controller Configuration

Begin by creating a configuration file for the Replication Controller in YAML format. This file is crucial for defining the desired state of the pods managed by the Replication Controller.

   apiVersion: v1
   kind: ReplicationController
   metadata:
     name: nginx-pyon-rc
     labels:
       env: demo
   spec:
     replicas: 3
     selector:
       matchLabels:
         env: demo
     template:
       metadata:
         labels:
           env: demo
       spec:
         containers:
         - name: nginx
           image: nginx:latest
           ports:
           - containerPort: 80

apiVersion: Specifies the API version for the object, in this case, v1.
kind: Defines the type of object, here ReplicationController.
metadata: Contains metadata such as name and labels.
spec: Defines the desired state, including replicas, selector, and template.

Applying the Configuration

Apply the configuration to create the Replication Controller and manage the pods:

   kubectl apply -f rc.yaml

Verify the created pods:

   kubectl get pods

You should see three pods running, managed by the Replication Controller.

Deleting and Recreating the Replication Controller

If needed, delete the Replication Controller and apply changes to observe the effects:

   kubectl delete rc nginx-pyon-rc
   kubectl apply -f rc.yaml

Transitioning to Replica Sets

Creating a Replica Set Configuration

The Replica Set is an evolution of the Replication Controller, providing improved functionality and flexibility:

   apiVersion: apps/v1
   kind: ReplicaSet
   metadata:
     name: nginx-pyon-rs
   spec:
     replicas: 3
     selector:
       matchLabels:
         env: demo
     template:
       metadata:
         labels:
           env: demo
       spec:
         containers:
         - name: nginx
           image: nginx:latest
           ports:
           - containerPort: 80

apiVersion: Updated to apps/v1.
selector: Defines which pods to manage using matchLabels.

Apply the configuration:

   kubectl apply -f rs.yaml

Verify the Replica Set:

   kubectl get rs

You should see the Replica Set managing the desired number of pods.

Scaling the Replica Set

Scaling the number of replicas can be done either by updating the YAML file or using commands:

   kubectl scale rs nginx-pyon-rs --replicas=5

Leveraging Deployments for Rolling Updates

Creating a Deployment Configuration

Deployments offer advanced features like rolling updates and rollbacks, making them ideal for production environments:

   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: nginx-deployment
   spec:
     replicas: 3
     selector:
       matchLabels:
         app: nginx
     template:
       metadata:
         labels:
           app: nginx
       spec:
         containers:
         - name: nginx
           image: nginx:latest
           ports:
           - containerPort: 80

apiVersion: apps/v1 for Deployments.
selector: matchLabels helps in managing the pods associated with this deployment.

Apply the configuration:

   kubectl apply -f deployment.yaml

Updating the Deployment

To update the image version, use:

   kubectl set image deployment/nginx-deployment nginx=nginx:1.9.1

Check the rollout status:

   kubectl rollout status deployment/nginx-deployment

Rollback if needed:

   kubectl rollout undo deployment/nginx-deployment

Checking Deployment Status

Get detailed information about the Deployment and its status:

   kubectl describe deployment nginx-deployment

40 Days Of Kubernetes (8/40)