Does Kubernetes need a minimum number of replicas in order to carry out a rolling deployment? - kubernetes

Question

Nearly 3 years ago, Kubernetes would not carry out a rolling deployment if you had a single replica (Kubernetes deployment does not perform a rolling update when using a single replica).
Is this still the case? Is there any additional configuration required for this to work?

Answer 1

You are not required to have a minimum number of replicas to rollout an update using Kubernetes Rolling Update anymore.
I tested it on my lab (v1.17.4) and it worked like a charm having only one replica.
You can test it yourself using this Katakoda Lab: Interactive Tutorial - Updating Your App
This lab is setup to create a deployment with 3 replicas. Before starting the lab, edit the deployment and change the number of replicas to one and follow the lab steps.
I created a lab using different example similar to your previous scenario. Here is my deployment:
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 1
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx-container
image: nginx:1.16.1
ports:
- containerPort: 80
Deployment is running with one replica only:
kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-deployment-6c4699c59c-w8clt 1/1 Running 0 5s
Here I edited my nginx-deployment.yaml and changed the version of nginx to nginx:latest and rolled out my deployment running replace:
$ kubectl replace -f nginx-deployment.yaml
deployment.apps/nginx-deployment replaced
Another option is to change the nginx version using the kubectl set image command:
kubectl set image deployment/nginx-deployment nginx-container=nginx:latest --record
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-deployment-566d9f6dfc-hmlf2 0/1 ContainerCreating 0 3s
nginx-deployment-6c4699c59c-w8clt 1/1 Running 0 48s
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-deployment-566d9f6dfc-hmlf2 1/1 Running 0 6s
nginx-deployment-6c4699c59c-w8clt 0/1 Terminating 0 51s
$ kubectl get pods
NAME READY STATUS RESTARTS AGE
nginx-deployment-566d9f6dfc-hmlf2 1/1 Running 0 13s
As you can see, everything worked normally with only one replica.
In the latest version of the documentation we can read:
Deployment ensures that only a certain number of Pods are down while
they are being updated. By default, it ensures that at least 75% of
the desired number of Pods are up (25% max unavailable).
Deployment also ensures that only a certain number of Pods are created
above the desired number of Pods. By default, it ensures that at most
125% of the desired number of Pods are up (25% max surge).