What is the reason that when changing the Nginx version, services launched with the replica set do not restart?

Changing the version of Nginx used as part of a service launched through a Replica Set in Kubernetes does not automatically lead to pod restarts for several reasons:

1. Replica Set manages the desired number of replicas: The Replica Set (or its common successor - Deployment) maintains the specified number of pod replicas. It is not aware of the contents of the containers inside the pods and does not track changes in application versions inside them. Its main task is to ensure the correct number of identical pods are running.
2. Changing the Nginx image does not update the pod specification in the Replica Set: Even if you update the Nginx image in the container registry, the Replica Set will continue to run pods from the specification that was used at its creation or last update. The pod specification includes a reference to a specific image (with a standard tag like :latest or a version number).
3. Lack of a trigger for restart: Kubernetes itself does not track changes in image contents (unless you use the :latest tag and explicitly restart the deployment). There is no built-in mechanism that automatically initiates a pod restart when the image content with the same tag changes.

To apply a new version of Nginx, you need to update the pod specification in the Deployment (which manages the Replica Set). This is usually done by changing the image field in the manifest.

Example of updating the Deployment to use a new Nginx image version:

# deployment.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-nginx-deployment
spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      containers:
      - name: nginx
        image: nginx:1.25.3 # Initially was nginx:1.25.2
        ports:
        - containerPort: 80

After changing image: nginx:1.25.3 and applying this manifest (kubectl apply -f deployment.yaml), the Deployment will notice the change in the pod template specification. It will create a new Replica Set with the updated specification and start gradually rolling out new pods with the new Nginx version, while terminating the old ones. This is the standard "Rolling Update" mechanism.

Using the :latest tag can lead to the Deployment using the latest image version when deploying new pods, but it will not automatically update already running pods unless you explicitly restart the Deployment (e.g., via kubectl rollout restart deployment my-nginx-deployment). This is not recommended for production environments due to potential unpredictability.