Attaching Traffic With Services

Goal

You create the five Service types and an Ingress yourself, and you create and fix a situation where a mismatched selector leaves the endpoints empty.

Why it matters

In the exam, the problem "I created the Service but can't connect" is mostly one of three things: the selector, the port, or Ready. Once you understand that a Service object does not carry traffic but is a declaration that builds a backend list from a selector, the diagnostic order sets itself. You first check whether the endpoints have an IP. If they do, it is a data plane or port problem; if not, it is a selector or Pod Ready problem.

It is also important to distinguish targetPort from port. port is the port the Service opens, and targetPort is the port on the Pod side. If they are the same you can omit one, which makes them easy to confuse, but the mistake shows up the moment you set them differently.

Steps

1. Create the namespace cka-net and the Deployment web (2 replicas, image nginx:1.27, Pod label app=web). Then create the ClusterIP Service web-svc. Port 80, targetPort 80, selector app=web.
2. Create the Deployment api (2 replicas, image nginx:1.27, Pod label app=api). Create the Service api-svc with the wrong selector app=api-server, save the endpoint query result to /root/cka-net/endpoints-before.txt, and then fix the selector to app=api.
3. Create the NodePort Service web-np. Port 80, selector app=web, and set nodePort explicitly to 30080.
4. Create the Deployment db (1 replica, image nginx:1.27, Pod label app=db) and the headless Service db-headless. clusterIP None, port 5432, selector app=db.
5. Create the Service multi-svc. Selector app=web, two ports — name http (port 80, targetPort 8080) and name metrics (port 9090).
6. Create the Service ext-db. Type ExternalName, externalName nas.homelab.internal.
7. Create the Ingress web-ing. ingressClassName nginx, host cka.homelab.internal; send path / (pathType Prefix) to port 80 of web-svc, and path /api (pathType Prefix) to port 80 of api-svc.

Reference

• You can also see the endpoints with kubectl get endpointslice -n cka-net -l kubernetes.io/service-name=web-svc.
• In step 2, the order is graded. Be sure to save the result to the file before you fix it.
• Common mistake 1: trying to fix clusterIP: None later. This field is immutable.
• Common mistake 2: omitting pathType in the Ingress. It is a required field in v1, so the request is rejected.

ClusterIP Service and endpoints

Create the namespace cka-net and the Deployment web (2 replicas, image nginx:1.27, Pod label app=web). Then create the ClusterIP Service web-svc. Port 80, targetPort 80, selector app=web.

kubectl expose is the fastest, but check what the selector ends up as. Endpoints are filled in only when the Pods are Ready.

A selector typo leaves the endpoints empty

Create the Deployment api (2 replicas, image nginx:1.27, Pod label app=api). Create the Service api-svc with the wrong selector app=api-server, save the endpoint query result to /root/cka-net/endpoints-before.txt, and then fix the selector to app=api.

First create it with the wrong selector, see with your own eyes that the endpoints are empty, leave that in the file, and then fix it. If you do not keep the order, no evidence remains.

Specify the NodePort number directly

Create the NodePort Service web-np. Port 80, selector app=web, and set nodePort explicitly to 30080.

nodePort goes inside the ports array. The default range is 30000–32767, and values outside it are rejected.

Headless Service

Create the Deployment db (1 replica, image nginx:1.27, Pod label app=db) and the headless Service db-headless. clusterIP None, port 5432, selector app=db.

If you set clusterIP to None, no VIP is assigned. This field cannot be changed after creation, so if you created it wrongly, create it again.

Expose two ports

Create the Service multi-svc. Selector app=web, two ports — name http (port 80, targetPort 8080) and name metrics (port 9090).

If there is more than one port, each port must have a name. port and targetPort can be different values.

Point to an external system with ExternalName

Create the Service ext-db. Type ExternalName, externalName nas.homelab.internal.

An ExternalName Service has neither a selector nor ports, and it does not get a ClusterIP. If you created it with another type first, it is cleaner to create it again.

Putting it together: combine two Services with an Ingress

Create the Ingress web-ing. ingressClassName nginx, host cka.homelab.internal; send path / (pathType Prefix) to port 80 of web-svc, and path /api (pathType Prefix) to port 80 of api-svc.

ingressClassName is a field of spec, not an annotation. Each path requires a pathType, and the backend is identified with service.name and service.port.number.