Kubernetes node affinity - Assigning a pod to a specific node?

Question

The k8s node affinity documentation explains how to deploy a pod to a specific node by first tagging the node with a label and using nodeSelector to pick the node.

However, I have a use case where I have 40-50 deployments in a cluster and I want to add a new node to the cluster and set that node dedicated to one of those deployments/pods without altering all those deployments which don't have any nodeSelector specified

For example, lets say I have 3 deployments, with no nodeSelector defined and 3 worker nodes. This means k8s decides where the pods will be deployed and it could be deployed in one of those 3 nodes. Now I have to create a 4th deployment, and add 4th server and I want to dedicate the 4th deployment to the 4th server and also want to make sure that k8s won't schedule the first 3 deployments to this 4th node. How can I do this without going through all those 3 deployment scheme and apply a nodeSelector filter to not deploy on 4th node? (It would be ok to perform this change on 3 deployments, but I am talking about 50s of deployments in real life scenario)

The only thing that I can think of is to taint the node but if I do it, none of the pods will be scheduled there.

Is there a better approach here to achieve this goal that I am not aware of?

Answer 1

If you taint the 4th node, the 4th deployment with the toleration will be deployed on that node, while the other three won't.

Answer 2

You could use an admission controller to dynamically add NodeSelector into a pod spec at runtime rather than modifying existing deployment yamls. You could write any custom logic in the mutating web-hook to cater to your use case. For example you could have a label/annotation in the new deployment spec and based on the existence of that label/annotation you could dynamically add a NodeSelector.

PodNodeSelector admission web-hook is a good example to refer to.