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Services

Services Part 1
Services Part 2

In the ReplicaSet lessons we have encountered service discovery, the challenge of reliably and predictably finding and accessing workloads running in a Kubernetes cluster.

Creating a Service​

Creating the service is simple.

Create a file 30-service-hello-world.yaml:

apiVersion: v1
kind: Service
metadata:
  name: smpl-go-web-s
spec:
  selector:
    app: smpl-go-web-a-cant-work
  ports:
    - port: 8080

Apply it:

kubectl apply -f 30-service-hello-world.yaml

Once, the Service has been created, it can be accessed using kubectl proxy:

kubectl proxy --port 8001

Then navigate a browser to:

http://localhost:8001/api/v1/namespaces/k8s-training/services/http:smpl-go-web-s:8080/proxy/

This assumes the Service to be within the k8s-training namespace, be reachable via http and listen on port 8080. The trailing /proxy is mandatory.

What Does the Service do?​

In the first step of this lesson a ReplicaSet with one (1) replica has been deployed. This lead to the creation of a single Pod with a single container running the desired web application.

By creating a Service matching the ReplicaSet with annotations using selector and label, the created service should discover the smpl-go-web-a app and uses it as an endpoint. Consequently, requests to the service will be routed to the smpl-go-web Pod. This is what a Service does: it's a layer 4 (ISO OSI: transport layer) router and - if multiple endpoints are available - load balancer.

You can verify the connection of the Service with its application by:

kubectl describe service smpl-go-web-s

And (Don't forget to replace the Pod's ID):

kubectl describe pod smpl-go-web-75457966b7-742sr

The endpoints section of the service description should contain the Pod's IP address

The Service therefore has two major purposes:

  1. Service Discovery: Acting as a stable entry point to access your apps.
  2. Load Balancing: Distributing requests among instances of your app in case there are multiple of them.

Error: No Endpoints Available​

In case you are receiving an error such as No endpoints available for service check the match labels of your service:

kubectl describe service smpl-go-web-s

In the output pay attention to the Selector-field. It must match the Pods's labels.

In order to check the labels of your Pod you can obtain this information by executing:

kubectl describe Pod smpl-go-web-rs-kmqrd

Where smpl-go-web-rs-kmqrd must be replaced with your Pod's name.

Look for the field Labels in the output. Only if there is a match between the label and the Service's selector, the Service will "know" to which Pods it belongs.

Fixed Service​

The fixed version of 30-service-hello-world.yaml then looks like this:

apiVersion: v1
kind: Service
metadata:
  name: smpl-go-web-s
spec:
  selector:
    app: smpl-go-web-a
  ports:
    - port: 8080