Kubernetes — Deployments, ReplicaSets and Pods

Deployment is a way of defining a desired state to kubernetes so we are saying we need 3 pieces of images at all times. So, here we will deploy a small app called my-dep using Nginx.

https://kubernetes.io/docs/concepts/workloads/controllers/deployment/

The Hierarchy

Deployment (my-dep)
└── ReplicaSet (my-dep-b458fdbb4)
    └── Pods (my-dep-b458fdbb4-kt85r, 5lkrc, 2bl5x...)

Check man pages

Check the syntax to create a deployment file

 k create deployment --help

Create the Deployment Manifest with –dry-run

Create a deployment with the specified name.

kubectl create deployment my-dep --image=nginx --replicas=10 --dry-run=client -o yaml > nginx3.yam)

apiVersion: apps/v1
kind: Deployment
metadata:
generation: 1
labels:
app: my-dep
name: my-dep
namespace: default
spec:
progressDeadlineSeconds: 600
replicas: 5
revisionHistoryLimit: 10
selector:
matchLabels:
app: my-dep
template:
metadata:
labels:
app: my-dep
spec:
containers:
- image: nginx
imagePullPolicy: Always
name: nginx

Apply the Deployment Manifest

To apply the manifest to cluster we run

kubectl apply -f nginx3.yaml

Get Deployments

k get deployments.apps

NAME     READY   UP-TO-DATE   AVAILABLE   AGE
my-dep   5/5     5            5           15m

Describe Deployment

k describe deployments.apps my-dep

Name:                   my-dep
Namespace:              default
Replicas:               5 desired | 5 updated | 5 total | 5 available | 0 unavailable
StrategyType:           RollingUpdate
RollingUpdateStrategy:  25% max unavailable, 25% max surge
Containers:
  nginx:
    Image: nginx
Events:
  Normal  ScalingReplicaSet  deployment-controller  Scaled up replica set my-dep-b458fdbb4 from 0 to 10
  Normal  ScalingReplicaSet  deployment-controller  Scaled down replica set my-dep-b458fdbb4 from 10 to 5

Get ReplicaSets

k get replicasets.apps

NAME               DESIRED   CURRENT   READY   AGE
my-dep-b458fdbb4   5         5         5       14m

Describe ReplicaSet

Replica sets are managed by k8s, its purpose is to maintain a stable set of replica Pods running at any given time

k describe replicasets.apps my-dep-b458fdbb4

Name:           my-dep-b458fdbb4
Controlled By:  Deployment/my-dep
Replicas:       5 current / 5 desired
Pods Status:    5 Running / 0 Waiting / 0 Succeeded / 0 Failed
Events:
  Normal  SuccessfulCreate  replicaset-controller  Created pod: my-dep-b458fdbb4-kt85r
  Normal  SuccessfulCreate  replicaset-controller  Created pod: my-dep-b458fdbb4-5lkrc
  Normal  SuccessfulCreate  replicaset-controller  Created pod: my-dep-b458fdbb4-2bl5x
  Normal  SuccessfulCreate  replicaset-controller  Created pod: my-dep-b458fdbb4-lx8j4
  Normal  SuccessfulCreate  replicaset-controller  Created pod: my-dep-b458fdbb4-pcjfw
  Normal  SuccessfulDelete  replicaset-controller  Deleted pod: my-dep-b458fdbb4-58fj5
  Normal  SuccessfulDelete  replicaset-controller  Deleted pod: my-dep-b458fdbb4-x6ggt

Get Pods

kubectl get pods

NAME                     READY   STATUS    RESTARTS   AGE
my-dep-b458fdbb4-2bl5x   1/1     Running   0          32m
my-dep-b458fdbb4-5lkrc   1/1     Running   0          32m
my-dep-b458fdbb4-kt85r   1/1     Running   0          32m
my-dep-b458fdbb4-lx8j4   1/1     Running   0          32m
my-dep-b458fdbb4-pcjfw   1/1     Running   0          32m
nginx                    1/1     Running   0          38m

Get Pods with Labels

kubectl get pods --show-labels

NAME                     READY   STATUS    RESTARTS   AGE   LABELS
my-dep-b458fdbb4-2bl5x   1/1     Running   0          32m   app=my-dep,pod-template-hash=b458fdbb4
my-dep-b458fdbb4-5lkrc   1/1     Running   0          32m   app=my-dep,pod-template-hash=b458fdbb4
my-dep-b458fdbb4-kt85r   1/1     Running   0          32m   app=my-dep,pod-template-hash=b458fdbb4
my-dep-b458fdbb4-lx8j4   1/1     Running   0          32m   app=my-dep,pod-template-hash=b458fdbb4
my-dep-b458fdbb4-pcjfw   1/1     Running   0          32m   app=my-dep,pod-template-hash=b458fdbb4
nginx                    1/1     Running   0          38m   run=nginx

Get Pods Wide (shows which node)

k get pods -o wide

NAME                     READY   STATUS    IP            NODE
my-dep-b458fdbb4-2bl5x   1/1     Running   10.42.1.186   k3s-worker-02
my-dep-b458fdbb4-5lkrc   1/1     Running   10.42.1.184   k3s-worker-02
my-dep-b458fdbb4-kt85r   1/1     Running   10.42.0.104   k3s-cp-01
my-dep-b458fdbb4-lx8j4   1/1     Running   10.42.0.105   k3s-cp-01
my-dep-b458fdbb4-pcjfw   1/1     Running   10.42.1.185   k3s-worker-02