Kubernetes/Deployment, ReplicaSet and Pod
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Deployment
Deployment kind provides all required application life-cycle for your applications. This is achieved by managing objects like ReplicaSet and Pod.
Example YAMLs below compare Deployment, ReplicaSet and Pod
| Deployment | ReplicaSet (individual manifest) | Pod (manifest) |
|---|---|---|
apiVersion: apps/v1
kind: Deployment
metadata:
name: kubeapp-deployment
# labels: <lines to verify>
# app: kubeapp
# tier: frontend
spec:
replicas: 3
selector:
matchLabels:
app: kubeapp
template:
metadata:
name: kubeapp
labels:
app: kubeapp
spec:
containers:
- image: nginx:1.16.0
name: app
|
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: kubeapp-replicaset
labels:
app: kubeapp
tier: frontend
spec:
replicas: 3
selector:
matchLabels:
tier: frontend
template:
metadata:
labels:
tier: frontend
spec:
containers:
- name: main
image: nginx:1.16.0
|
apiVersion: v1
kind: Pod
metadata:
name: kuebapp-pod1
labels:
tier: frontend
spec:
containers:
- name: main
image: nginx:1.16.0
|
Create Deployment
Pod hash
| Pod template hash | sample-app.yaml |
|---|---|
hash value is 6c5948bf66 picture is not accurate |
apiVersion: apps/v1
kind: Deployment
metadata:
name: sample-deployment
spec:
replicas: 3
selector:
matchLabels:
app: sample-app
template: #hash value is 6c5948bf66
metadata:
labels:
app: sample-app
spec:
containers:
- name: nginx-container
image: nginx:1.12
ports:
- containerPort: 80
|
Note ReplicaSet, SELECTOR label pod-template-hash=6c5948bf66 that also matches pod's names
$ kubectl get all -owide -n sample-app NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES pod/sample-deployment-6c5948bf66-5bqhw 1/1 Running 0 114s 10.244.2.10 ip-10-0-1-102 <none> <none> pod/sample-deployment-6c5948bf66-t5vg5 1/1 Running 0 114s 10.244.1.11 ip-10-0-1-103 <none> <none> pod/sample-deployment-6c5948bf66-xv6cb 1/1 Running 0 114s 10.244.1.10 ip-10-0-1-103 <none> <none> NAME READY UP-TO-DATE AVAILABLE AGE CONTAINERS IMAGES SELECTOR deployment.apps/sample-deployment 3/3 3 3 114s nginx-container nginx:1.12 app=sample-app NAME DESIRED CURRENT READY AGE CONTAINERS IMAGES SELECTOR replicaset.apps/sample-deployment-6c5948bf66 3 3 3 114s nginx-container nginx:1.12 app=sample-app,pod-template-hash=6c5948bf66
Create a deployment
Create a deployment with a record (for rollbacks). Deployment appends a string of numbers to the end of the name, that is a hash of Pod template and the deployment. Deployment creates a ReplicaSet, that manages a number of Pods.
kubectl create -f kubeapp-deployment.yaml --record #records in a revision history, so it's easy to rollback kubectl rollout status deployments kubeapp #Check the status of the rollout deployment "kubeapp" successfully rolled out kubectl get replicasets #name is <deploymentName> with appended <PodTemplateHash> NAME DESIRED CURRENT READY AGE kubeapp-674dd4d9cd 3 3 3 93s kubectl get pods #name is <ReplicaSetName> with appended <generated-podID> NAME READY STATUS RESTARTS AGE kubeapp-674dd4d9cd-f2rk8 1/1 Running 0 3m36s kubeapp-674dd4d9cd-gnq7v 1/1 Running 0 5m51s kubeapp-674dd4d9cd-kc6lt 1/1 Running 0 3m37s
Deployment operations
Scale up your deployment by adding more replicas:
kubectl scale deployment kubeapp --replicas=5
kubectl get pods #notice ReplicaSet has different hash than Pods, as it's different object
NAME READY STATUS RESTARTS AGE
kubeapp-674dd4d9cd-f2rk8 1/1 Running 0 3m36s
kubeapp-674dd4d9cd-gnq7v 1/1 Running 0 5m51s
kubeapp-674dd4d9cd-kc6lt 1/1 Running 0 3m37s
kubeapp-674dd4d9cd-n5wnp 1/1 Running 0 5m51s
kubeapp-674dd4d9cd-ptt26 1/1 Running 0 3m36s
#Expose the deployment and provide it a service
kubectl expose deployment kubeapp --port 80 --target-port 80 --type NodePort
service/kubeapp exposed
kubectl get service
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
kubeapp NodePort 10.111.72.150 <none> 80:31472/TCP 11m
kubernetes ClusterIP 10.96.0.1 <none> 443/TCP 18d
#Set the minReadySeconds attribute to your deployment, slows down deployment, so can see changes in realtime
kubectl patch deployment kubeapp -p '{"spec": {"minReadySeconds": 10}}'
kubectl edit deployments.apps kubeapp #see the change above
#Use kubectl apply to update a deployment. It modifies an existing object to align to the new YAML, or creates if does not exists
kubectl describe deployments.apps kubeapp | grep Image #Before
Image: nginx:1.16.0
kubectl apply -f kubeapp-deployment.yaml #update the deployment file image from - image: nginx:1.16.0 -> 1.17.1
kubectl describe deployments.apps kubeapp | grep Image #After
Image: nginx:1.17.1
#Use kubectl replace to replace an existing deployment. It updates existing object, the object must exist before hand
kubectl replace -f kubeapp-deployment.yaml
Rolling update and undo
Rolling update to prevent application downtime
#Run this curl look while the update happens: while true; do curl http://<NodeIP>; done #Perform the rolling update: kubectl set image deployments/kubeapp app=nginx:1.17.1 --v 6 #--v verbose output #Find that new ReplicaSet has been created kubectl describe replicasets kubeapp-[hash] kubectl get replicasets NAME DESIRED CURRENT READY AGE kubeapp-674dd4d9cd 0 0 0 43m kubeapp-99c897449 0 0 0 6m19s kubeapp-d79844ffd 3 3 3 14m #new deployment #Look at the rollout history kubectl rollout history deployment kubeapp deployment.extensions/kubeapp REVISION CHANGE-CAUSE 3 kubectl create --filename=kubeapp-deployment.yaml --record=true 4 <none> #this is empty because no --record was set during kubectl execution 5 kubectl create --filename=kubeapp-deployment.yaml --record=true
Note, without --record, file applied will be empty. All deployments are being recorded.
$ kubectl rollout history -n sock-shop deployment deployment.extensions/carts ... deployment.extensions/catalogue-db REVISION CHANGE-CAUSE 1 kubectl apply --filename=. --record=true deployment.extensions/front-end REVISION CHANGE-CAUSE 2 kubectl apply --filename=front-end-dep.yaml --record=true 3 kubectl apply --filename=front-end-dep.yaml --record=true
Undo the rollout and roll back to the previous version. This is possible because deployment keeps a revision-history. The history is stored with underlying RelicaSet.
kubectl rollout undo deployment kubeapp --to-revision=2 #specific version if needed kubectl rollout undo deployments kubeapp #this has been applied kubectl get replicasets NAME DESIRED CURRENT READY AGE kubeapp-674dd4d9cd 0 0 0 46m kubeapp-99c897449 3 3 3 8m52s #rolled back to previous replicaset kubeapp-d79844ffd 0 0 0 16m #Pause the rollout in the middle of a rolling update (canary release) kubectl rollout pause deployment kubeapp #Resume the rollout after the rolling update looks good kubectl rollout resume deployment kubeapp
Statefulset
When each pod is important
| Statefulset | ReplicaSet |
|---|---|
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: kubeweb
spec:
serviceName: "nginx"
replicas: 2
selector:
matchLabels:
app: kubenginx
template:
metadata:
labels:
app: kubenginx
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
name: web
volumeMounts:
- name: www
mountPath: /usr/share/nginx/html
volumeClaimTemplates:
- metadata:
name: www
spec:
accessModes: [ "ReadWriteOnce" ]
resources:
requests:
storage: 0.5Gi
|
apiVersion: apps/v1
kind: ReplicaSet
metadata:
name: kubeapp-replicaset
labels:
app: kubeapp
tier: frontend
spec:
replicas: 3
selector:
matchLabels:
tier: frontend
template:
metadata:
labels:
tier: frontend
spec:
containers:
- name: main
image: nginx:1.16.0
|