kubernetes-hands-on/08-service

Accessing my first application: service

Introduction

In this section you will learn how to access your application from outside your cluster, and do service discovery.

Prerequisites

If it's not already done install the minikube addon ingress:

$ minikube addons enable ingress
✅  ingress was successfully enabled

First service

You are able to deploy an image with multiple replicas, but it is not very convenient to access it. You need to know the IP of a pod to be able to target your application. And it's not accessible from the outside of the cluster.

What we need is a service. It'll allow us to access our pods internally or externally.

First apply the deployment:

$ kubectl apply -f 08-service/01-simple-deployment.yml
deployment.apps "simple-deployment" created

Now start another container. We will use it to see what we can access internally inside Kubernetes:

Apply the pod:

$ kubectl apply -f 08-service/02-bash.yml
pod "bash" created

And connect to it:

$ kubectl exec -it bash -- /bin/bash
root@bash:/#

Install dnsutils & curl in the container, you will need them:

root@bash:/# apt update && apt install dnsutils curl
[...]

You now have a shell inside a Kubernetes pod running in your cluster. Let this console open so you can type commands. Try to curl one of the pods created by the deployment above. How can you access the deployment without targeting a specific pod?

Ok, now let's create our first service 03-internal-service.yml:

apiVersion: v1
kind: Service
metadata:
  name: simple-internal-service
spec:
  ports:
  - port: 80
    targetPort: 9876
  selector:
    app: simple-deployment

Let's have a look at the manifest:

• kind: A service has the kind Service
• spec:
  • ports: the list of ports to expose. Here we export port 80, but redirect internally all traffic to the targetPort 9876
  • selector: which pods to give access to

The selector part

selector:
  app: simple-deployment

is central to Kubernetes. It is with those fields that you will tell Kubernetes which pods to give access through this service.

Apply the service:

$ kubectl apply -f 08-service/03-internal-service.yml
service "simple-service" created

Your service is now accessible internally, try this in your bash container:

root@bash:/# nslookup simple-internal-service
Server:   10.96.0.10
Address:  10.96.0.10#53

Name: simple-internal-service.default.svc.cluster.local
Address: 10.96.31.244

Try to curl the /health url, remember the ports we choose in the service.

Can you access this service from the outside of Kubernetes?

The answer is no, it's not possible. To do this you need an ingress. Ingress means "entering into".

Ingress

You need to connect internet to the ingress that'll connect it to a service:

 internet
     |
[ ingress ]
--|-----|--
[ service ]

Let's create our ingress:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: simple-ingress
  annotations:
    nginx.ingress.kubernetes.io/ssl-redirect: "false"
spec:
  backend:
    serviceName: simple-internal-service
    servicePort: 80

Let's have a look at the manifest:

• kind: Ingress
• metadata:
  • annotations: some annotations specific to the ingress
    • nginx.ingress.kubernetes.io/ssl-redirect: To fix a redirect, see this. This is only used if you use the nginx ingress
• spec:
  • backend: the default backend to redirect all the requests to
    • serviceName: the name of the Kubernetes traffic to redirect to
    • servicePort: the port of the service

Apply the ingress:

$ kubectl apply -f 08-service/04-ingress.yml
ingress.extensions "simple-ingress" created

Get the IP of your minikube cluster:

$ minikube ip
192.168.99.100

Open a browser on http://192.168.99.100/info

With this manifest we have a deployment that manages pods. A service that gives access to the pods, and an ingress that gives access to the pod to the external world.

Global overview

You have seen a lot different kind of Kubernetes, let's take a step back and see how each kind interact with each other:

         +----------------------------------------------------------------------------------+
         |                                                                                  |
         |                                        +-----------------------------+           |
         |                                        |                             |           |
         |                                        |            +-------------+  |           |
         |                                        |            |             |  |           |
         |                                        |           ->     Pod     |  |           |
         |                                        |        --/ |             |  |           |
+-------------+    +-------------+                |    ---/    +-------------+  |           |
|        |    |    |             |                | --/                         |           |
|   Ingress   ----->   Service   ------------------\                            |           |
|        |    |    |             |                | --\        +-------------+  |           |
+-------------+    +-------------+                |    ---\    |             |  |           |
         |                                        |        --\ |     Pod     |  |           |
         |                                        |           ->             |  |           |
         |                                        |            +-------------+  |           |
         |                                        |                             |           |
         |                                        |                  Deployment |           |
         |                                        +-----------------------------+           |
         |                                                                                  |
         |                                                                                  |
         |                                                                                  |
         |                                                                       Kubernetes |
         +----------------------------------------------------------------------------------+

Exercises

1. Deploy an nginx and expose it internally
2. Read this and modify the ingress to have:
   • /simple that goes to the simple-service
   • /nginx that goes to your nginx deployment
3. Change the selector in your simple-service look at what is happening

Clean up

kubectl delete ingress,service,deployment,rs,pod --all

Answers

For 2), you need to add the metadata nginx.ingress.kubernetes.io/rewrite-target: / to the ingress:

• It is possible to capture groups from paths into numbered placeholders and pass them to the rewrite target: https://github.com/kubernetes/ingress-nginx/blob/master/docs/examples/rewrite/README.md#rewrite-target
• Don't forget to create 2 deployments and 2 services.
• You can either change your /etc/hosts to add the name resolution for foo.bar.com, or use curl http://YOUR-IP -H "Host: foo.bar.com"

Links

• https://kubernetes.io/docs/concepts/services-networking/service/
• https://kubernetes.io/docs/concepts/services-networking/dns-pod-service/
• https://kubernetes.io/docs/concepts/services-networking/connect-applications-service/
• https://kubernetes.io/docs/concepts/services-networking/ingress/
• https://kubernetes.io/docs/concepts/services-networking/ingress-controllers/