Kubernetes Deployment Tutorial

Deployment implements the features like rolling updates, roll-back of updates. Deployments have the capability to update the the docker image which you are using in the pods and are also capable of rolling back to the previous docker image versions. In this blog post i will clearly explain about how to create kubernetes deployment and how to update the deployment and how to rollback deployments.

If you create a deployment with name 'mydep' it will create a ReplicaSet with name mydep-<replica-set-id>, which will further create a Pod with name mydep-<replica-set->-<pod-id>. so using deployment we can create the repicaset. so no need to create replicaset again manually for this pods.

cat abcd.yml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: mydep
  labels:
    dep: nginxdep
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.16.1
        ports:
        - containerPort: 80

Create Deployment

using kubectl create command we can create the deployment.

master $ kubectl create -f abcd.yml

deployment.apps/mydep created

List the deployments

using kubectl get command we can list the deployments

master $ kubectl get deployments

NAME    READY   UP-TO-DATE   AVAILABLE   AGE
mydep   3/3     3            3           87s

Describe the deployment:

To know more about deployment, deployment metadata, pods status and replicas we can use describe command.

kubectl describe deployment <deployment-name>

master $ kubectl describe deployment mydep

Name:                   mydep
Namespace:              default
CreationTimestamp:      Sat, 07 Sep 2019 18:02:36 +0000
Labels:                 dep=nginxdep
Annotations:            deployment.kubernetes.io/revision: 1
Selector:               app=nginx
Replicas:               3 desired | 3 updated | 3 total | 3 available | 0 unavailable
StrategyType:           RollingUpdate
MinReadySeconds:        0
RollingUpdateStrategy:  25% max unavailable, 25% max surge
Pod Template:
  Labels:  app=nginx
  Containers:
   nginx:
    Image:        nginx:1.16.1
    Port:         80/TCP
    Host Port:    0/TCP
    Environment:  <none>
    Mounts:       <none>
  Volumes:        <none>
Conditions:
  Type           Status  Reason
  ----           ------  ------
  Available      True    MinimumReplicasAvailable
  Progressing    True    NewReplicaSetAvailable
OldReplicaSets:  <none>
NewReplicaSet:   mydep-59777878f8 (3/3 replicas created)
Events:
  Type    Reason             Age   From                   Message
  ----    ------             ----  ----                   -------
  Normal  ScalingReplicaSet  5m1s  deployment-controller  Scaled up replica set mydep-59777878f8 to 3

Update Deployment or Rolling Update

To edit or update existed deployment edit your yml file. Here update means changing the docker images/labels/... not replicas numbers. after updating the deployment yml file execute kubectl apply -f ymlfile command to update your deployments. For every update one replicaset will be created. Rolling updates incrementally replace your resource's Pods with new ones. whenever you update the deployment using RollingUpdate(default) strategy, a new ReplicaSet is created and the Deployment moves the Pods from the old ReplicaSet to the new one.

vi abcd.yml

change the docker image version/docker image

kubectl apply -f abcd.yml

here abcd.yml is my deployment yml file. Here i am updating my deployment from old nginx:1.16.1 to new nginx 1.17.3 version. and using kubectl apply -f command we can update our deployment.

Rollout History

we can verify the roll-out history using kubectl rollout history command. It will give the all the information about your updates/rollouts

kubectl rollout history deployment/mydep

master $ kubectl rollout history deployment/mydep

deployment.extensions/mydep
REVISION  CHANGE-CAUSE
1         <none>
2         <node>

Hear you can see the two versions. we got the first one when we creating the deployment. after changing the image from nginx:1.16.1 to nginx 1.17.3 we got second version.

Rollout status

verify the status of rollout using kubectl rollout status command. it will tell you whether the new update is completed or still in progress. update will take some time to change pods from old version to new versions.

kubectl rollout status deployment/<deploymentname>

master $ kubectl rollout status deployment/mydep

deployment "mydep" successfully rolled out

Rollback deployment

as we discussed in rollout history we have two versions, in first version we are using nginx 1.16.1. in second version we are using ngin 1.17.3. kubernetes deployment provides feature like rollout to previous deployments
using kubectl rollout undo command we can rollback to previous versions.

kubectl rollout undo deployment/mydep --to-revision=1

master $ kubectl rollout undo deployment/mydep --to-revision=1

deployment.extensions/mydep rolled back

so you will get previous version of deployment. you can verify with kubectl describe deployment mydep. if you don't mention revision number it will roll out to recent deployment or last deployment.

Scale the deployment

using kubectl scale command we can upscale or descale your deployment, i,e we can increase or decrease pods running under this deployment.

Upscale:

master $ kubectl scale deployment mydep --replicas=5

deployment.extensions/mydep scaled

Downscale:

master $ kubectl scale deployment mydep --replicas=1

deployment.extensions/mydep scaled

we use same command to upscale and downscale, just we have increase or decrease the number of replicas

verify the number of pods running under this deployment  with kubectl get deployment mydep

master $ kubectl get deployment mydep

NAME    READY   UP-TO-DATE   AVAILABLE   AGE
mydep   1/1     1            1

Here you can see after down scaling to 1, only 1 pod is running under this deployment.

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