kubernetes - Does kubectl port-forward ignore loadBalance services?

Question

My Environment: Mac dev machine with latest Minikube/Docker

I built (locally) a simple docker image with a simple Django REST API "hello world".I'm running a deployment with 3 replicas. This is my yaml file for defining it:

apiVersion: v1
kind: Service
metadata:
  name: myproj-app-service
  labels:
    app: myproj-be
spec:
  type: LoadBalancer
  ports:
    - port: 8000
  selector:
    app: myproj-be
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myproj-app-deployment
  labels:
    app: myproj-be
spec:
  replicas: 3
  selector:
    matchLabels:
      app: myproj-be
  template:
    metadata:
      labels:
        app: myproj-be
    spec:
      containers:
        - name: myproj-app-server
          image: myproj-app-server:4
          ports:
            - containerPort: 8000
          env:
            - name: DATABASE_URL
              value: postgres://myname:@10.0.2.2:5432/myproj2
            - name: REDIS_URL
              value: redis://10.0.2.2:6379/1

When I apply this yaml it generates things correctly. - one deployment - one service - three pods

Deployments:

NAME                  READY   UP-TO-DATE   AVAILABLE   AGE
myproj-app-deployment   3/3     3            3           79m

Services:

NAME               TYPE           CLUSTER-IP    EXTERNAL-IP   PORT(S)          AGE
kubernetes         ClusterIP      10.96.0.1     <none>        443/TCP          83m
myproj-app-service   LoadBalancer   10.96.91.44   <pending>     8000:31559/TCP   79m

Pods:

NAME                                   READY   STATUS    RESTARTS   AGE
myproj-app-deployment-77664b5557-97wkx   1/1     Running   0          48m
myproj-app-deployment-77664b5557-ks7kf   1/1     Running   0          49m
myproj-app-deployment-77664b5557-v9889   1/1     Running   0          49m

The interesting thing is that when I SSH into the Minikube, and hit the service using curl 10.96.91.44:8000 it respects the LoadBalancer type of the service and rotates between all three pods as I hit the endpoints time and again. I can see that in the returned results which I have made sure to include the HOSTNAME of the pod.

However, when I try to access the service from my Hosting Mac -- using kubectl port-forward service/myproj-app-service 8000:8000 -- Every time I hit the endpoint, I get the same pod to respond. It doesn't load balance. I can see that clearly when I kubectl logs -f <pod> to all three pods and only one of them is handling the hits, as the other two are idle...

Is this a kubectl port-forward limitation or issue? or am I missing something greater here?

Answer 1

The kubernetes API only provides Pod port forward operations (CREATE and GET). Similar API operations don't exist for Service endpoints.

So I would say kubectl looks up a Pod from the service information provided on the command line and forwards directly to a Pod rather than forwarding to the ClusterIP/Service port and allowing the cluster to load balance the service like regular service traffic.

kubectl code

Here's a little bit of the flow from the kubectl code that seems to back that up (I'll just add that Go isn't my primary language)

The portforward.go Complete function is where kubectl portforward does the first look up for a pod from options via AttachablePodForObjectFn:

The AttachablePodForObjectFn is defined as attachablePodForObject in this interface, then here is the attachablePodForObject function.

To my (inexperienced) Go eyes, it appears the attachablePodForObject is the thing kubectl uses to look up a Pod to from a Service defined on the command line.

Then from there on everything deals with filling in the Pod specific PortForwardOptions (which doesn't include a service) and is passed to the kubernetes API.