Deployments in Kubernetes
What is Deployments in Kubernetes?
A deployment in kubernetes is basically a resource objects that
provides declarative updates to applications. You can set
images, life cycle of a pods, how many replicas will be running
etc. We work to match application current state to desired
state. Deployments are a higher-level resource that manages
ReplicaSets and Pods.
A pod is a unit having one or more containers which is
responsible for running entire or a part of applications.
It shares storage and networking resources to run applications.
Deployment is a management tool which controls the behavior of
all running pods. It can decide what you are looking for
running applications by managing pods.
We have replica set which is used to run multiple instances of
pods. It can scale up and scale down the replicas as per
requirement basically it can perform the load balancing which
can be a requirement of an application. While Deployment is a
higher level management tool which can manages replica set and
can provide declarative updates to pods with lot of additional
features as well.
Take an example of Deployment now, Below is the example of
codes.
# cat deploy-varelite.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-varelite
labels:
app: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.14.2
ports:
- containerPort: 80Now apply it to build a deployment.
# kubectl apply -f deploy-varelite.yaml
deployment.apps/nginx-varelite created
# kubectl get deploy
NAME READY UP-TO-DATE AVAILABLE AGE
nginx-varelite 3/3 3 3 18m
# kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-varelite-cbdccf466-nh4xm 1/1 Running 0 18m
nginx-varelite-cbdccf466-trcw7 1/1 Running 0 18m
nginx-varelite-cbdccf466-wmkkp 1/1 Running 0 18m
# kubectl get pods -o wide
NAME READY STATUS RESTARTS AGE IP NODE NOMINATED NODE READINESS GATES
nginx-varelite-cbdccf466-nh4xm 1/1 Running 0 18m 172.16.182.18 worker3 <none> <none>
nginx-varelite-cbdccf466-trcw7 1/1 Running 0 18m 172.16.235.150 worker1 <none> <none>
nginx-varelite-cbdccf466-wmkkp 1/1 Running 0 18m 172.16.189.85 worker2 <none> <none>Deployment Few Key Features
1. Reliability
Let's test it. I'll delete one of any pod from above deployment
and see what it does.
# kubectl delete pod nginx-varelite-cbdccf466-nh4xm
pod "nginx-varelite-cbdccf466-nh4xm" deleted
# kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-varelite-cbdccf466-26svl 1/1 Running 0 2s
nginx-varelite-cbdccf466-trcw7 1/1 Running 0 20m
nginx-varelite-cbdccf466-wmkkp 1/1 Running 0 20mYou can see in above command. A pod's age is 2s. It means the
deleted pod got recreated. So it shows us reliability.
2. Scalability
Let's scale up the deployment by adding one more pod.
# kubectl scale --replicas=4 deploy nginx-varelite
deployment.apps/nginx-varelite scaled
# kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-varelite-cbdccf466-26svl 1/1 Running 0 3m46s
nginx-varelite-cbdccf466-rxqpn 1/1 Running 0 3s
nginx-varelite-cbdccf466-trcw7 1/1 Running 0 24m
nginx-varelite-cbdccf466-wmkkp 1/1 Running 0 24m3. Rolling and Rollback Support
Let's check what is the image version of current application
and we want to upgrade it to latest version we have.
# kubectl describe deploy nginx-varelite | grep -i image
Image: nginx:1.14.2It has running with 4 replicas. I want to upgrade without
impacting current application. We will change it by editing its
yaml. You can do it by command line as well.
# kubectl edit deploy nginx-varelite
deployment.apps/nginx-varelite edited
# kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-varelite-57d98f69f6-kg47b 1/1 Running 0 8s
nginx-varelite-57d98f69f6-nj4gm 1/1 Running 0 6s
nginx-varelite-57d98f69f6-r4clk 1/1 Running 0 8s
nginx-varelite-57d98f69f6-xt6pn 1/1 Running 0 6sYou can see in above command, Pods have been created few
seconds away. Now image has been upgraded.
# kubectl describe deploy nginx-varelite | grep -i image
Image: nginx:1.16Let's go ahead and check its history. So that we can rollback
if we have any fallout during upgrade.
# kubectl rollout history deploy nginx-varelite
deployment.apps/nginx-varelite
REVISION CHANGE-CAUSE
1 <none>
2 <none>
# kubectl rollout undo deploy nginx-varelite --to-revision=1
deployment.apps/nginx-varelite rolled back
# kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-varelite-cbdccf466-6fbfm 1/1 Running 0 4s
nginx-varelite-cbdccf466-b8fhw 1/1 Running 0 4s
nginx-varelite-cbdccf466-mgrfh 1/1 Running 0 3s
nginx-varelite-cbdccf466-nmfzk 1/1 Running 0 2s
# kubectl describe deploy nginx-varelite | grep -i image
Image: nginx:1.14.2