How port names are resolved in kubernetes-CodePudding

There's a working k8s configuration which uses the same port name in a pod and in a service. Here's a config part of the pod:

ports:
- containerPort: 8000
  name: app-port
  protocol: TCP

Here's a config part of the service:

  ports:
  - name: app-port
    nodePort: 32000
    port: 8000
    protocol: TCP
    targetPort: app-port
  type: NodePort

How is the name app-port resolved?

CodePudding user response：

When you create the Service, it is associated with Pods selected by the label selector defined in the Service spec.selector.

When a request is made to the Service, the Control Plane retrieves its spec.ports[*].targetPort value:

If it does not exist, the value of spec.ports[*].port field is used instead to forward traffic to the Pods.
If it does exist and it is a number, the targetPort value is used to forward traffic to the Pods.
If it is a string, the Control Plane looks up the port by name in spec.ports[*].name of the Pod, and uses the port with the matching name as the target port.

The relevant piece of code in Kubernetes is this:

// FindPort locates the container port for the given pod and portName.  If the
// targetPort is a number, use that.  If the targetPort is a string, look that
// string up in all named ports in all containers in the target pod.  If no
// match is found, fail.
func FindPort(pod *v1.Pod, svcPort *v1.ServicePort) (int, error) {
    portName := svcPort.TargetPort
    switch portName.Type {
    case intstr.String:
        name := portName.StrVal
        for _, container := range pod.Spec.Containers {
            for _, port := range container.Ports {
                if port.Name == name && port.Protocol == svcPort.Protocol {
                    return int(port.ContainerPort), nil
                }
            }
        }
    case intstr.Int:
        return portName.IntValue(), nil
    }

    return 0, fmt.Errorf("no suitable port for manifest: %s", pod.UID)
}

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