Contents
- Replication Controller
- ReplicaSets
- Deployments
Replication Controller
- Kubernetes encourages Desired State deployment.
- Controllers are responsible for managing the pod lifecycle and ensuring that the specified number of pod replicas are running at any given time. They handle node failures.
- It is best practice to use the replication controller to manage the pod lifecycle rather than creating a pod repeatedly.
Replication Controller Example
Replication.yaml:
apiVersion: v1
kind: ReplicationController
metadata:
name: nginx-rc
spec:
replicas: 3
selector:
app: nginx-app
template:
metadata:
name: nginx-pod
labels:
app: nginx-app
spec:
containers:
- name: mynginx-con
image: nginx:1.18.0
ports:
- containerPort: 80
Commands
Run the following commands:
kubectl apply -f Replication.yaml
kubectl get all
kubectl delete pod/nginx-rc-<random-value> kubectl get all # New Pod created
Note that another pod is automatically created.
kubectl scale replicationcontroller/nginx-rc –replicas=5 kubectl get pods # 2 new pods will be created kubectl delete -f Replication.yaml
ReplicaSet
- It is associated with a pod and indicates how many instances of that pod should be running within the cluster.
- It can be considered a replacement for a replication controller. The key difference is that the replication controller only supports name equality-based selectors, whereas the replica set supports set-based selectors, allowing for more flexible grouping of pods.
- It is recommended to use replica sets along with higher-level controllers such as deployments.
ReplicaSet Example
Replica-set.yaml:
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: nginx-rs
spec:
replicas: 3
selector:
matchLabels:
app: nginx-app
template:
metadata:
name: nginx-pod
labels:
app: nginx-app
spec:
containers:
- name: mynginx-con
image: nginx
ports:
- containerPort: 80
Note that the section below template is what we have earlier used for the pod.
Commands
Execute the following commands:
kubectl apply -f replica-set.yaml
kubectl get replicaset nginx-rs
kubectl scale –replicas=2 replicaset/nginx-rs kubectl get pods kubectl delete pod nginx-rs-<random-value> kubectl get pods
Note that the existing pod is deleted, and immediately a new pod is created.
kubectl label pod <pod-name> app-
kubectl get pods
kubectl delete -f replica-set.yaml
Note that deleting the replica set automatically deletes all the pods.
Key Differences: ReplicaSet vs. Replication Controller
- ReplicaSet: Supports the new set-based selector, providing more flexibility (e.g., environment in (production, QA)).
- Replication Controller: Only supports equality-based selector (e.g., environment=production).
Limitation of ReplicaSet:
If you change the image in the pod template, the ReplicaSet will not automatically destroy the old pods and replace them with new pods.
Deployments
- The deployment controller wraps around and extends the ReplicaSet controller.
- The deployment instructs Kubernetes on how to create and update instances of your application. Once created, the Kubernetes master schedules the application instances onto individual nodes in the cluster.
- Once the application instances are created, a Kubernetes Deployment Controller continuously monitors those instances. If the node hosting an instance goes down or is deleted, the Deployment Controller replaces it, providing a self-healing mechanism.
- Deployments are an upgraded version of the replication controller. They manage the deployment of ReplicaSets, also an upgraded version of the replication controller. Deployments can update ReplicaSets and roll back to previous versions.
Creating a Deployment
Run the following command to create a deployment:
kubectl apply deployment nginx-deployment –image nginx:1.16.0 kubectl annotate deployment nginx-deployment kubernetes.io/change-cause=’nginx:1.16.0′
Or use the deployment.yaml file:
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
annotations:
kubernetes.io/change-cause: ‘nginx:1.16.0’
spec:
replicas: 2
selector:
matchLabels:
app: nginx-app
template:
metadata:
labels:
app: nginx-app
spec:
containers:
- name: nginx-con
image: nginx:1.16.0
ports:
- containerPort: 80
imagePullPolicy: Always
Commands
Deploy using the deployment.yaml file:
kubectl apply -f deployment.yaml
kubectl scale deployment nginx-deployment –replicas=3 kubectl get pods
To update the deployment, change the image in deployment.yaml to nginx:1.17.0:
kubectl apply -f deployment.yaml
Note that the old pods are destroyed, and new pods are created with the new image.
kubectl get pods
kubectl describe pod <pod-name>
#Note the version of the container image is nginx:1.17.0
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