If you have 4 nodes, is it possible to set the number of replicas to 5 or 6?

Yes, it is possible. The number of replicas in a Kubernetes cluster is not limited by the physical number of nodes. Kubernetes will try to distribute pods across available nodes, and if resources are insufficient, some pods will remain in the Pending state. However, such deployment will be suboptimal, as some replicas will not be able to start.

Let's consider an example: we have 4 nodes, and we want to run 5 replicas of an application. Kubernetes will attempt to distribute these 5 pods across 4 nodes. It might happen that one node hosts 2 pods, and the others host one each, or other distributions depending on the scheduler and available resources. If resources on all 4 nodes are sufficient to run all 5 pods, they will start. If not, one or more pods will remain in the Pending state until resources become available.

Now, consider 6 replicas on 4 nodes. This situation is similar to the previous one. Some nodes may host more than one pod. If the total resource requirement (CPU, memory) for 6 replicas exceeds the available resources on 4 nodes, some pods will remain in Pending.

It is important to understand that although Kubernetes allows setting a number of replicas greater than the number of nodes, this is not always efficient and can lead to:

• Resource shortage: Pods may not start due to lack of CPU, memory, or other resources on nodes.
• Uneven load distribution: The load may be unevenly distributed among nodes.
• Availability issues: If nodes fail, the number of running replicas may decrease significantly.

For optimal operation, it is recommended to have enough nodes to host all replicas considering their resource needs and to plan cluster scaling as the number of replicas increases.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  replicas: 5 # Set 5 replicas
  selector:
    matchLabels:
      app: my-app
  template:
    metadata:
      labels:
        app: my-app
    spec:
      containers:
      - name: my-container
        image: nginx:latest
        resources:
          limits:
            memory: "128Mi"
            cpu: "500m"
          requests:
            memory: "64Mi"
            cpu: "250m"

In this example, we create a Deployment with 5 replicas. Kubernetes will try to place them on available nodes. If the total resources on 4 nodes (2500m CPU and 320Mi memory according to requests) are sufficient, all 5 pods will start. If resources are insufficient, the pods will remain pending.