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OpenShift and Kubernetes

Overview

You write YAML files that describe what you'd like to have.

You apply those YAML files to Kubernetes.

Kubernetes stores your data in a database called etcd - this stores the desired state of your application.

The Kubernetes control plane looks at etcd, looks at the nodes, then makes the nodes match the desired state.

Project Setup

Create the config directory

cd into the project you created in the docker lesson and make a k8s folder:

cd ~/static-site
mkdir k8s
code .

Create the OpenShift Project

Make sure you are logged into the cluster, either with icc or via Copy Login Command.

Then run the following command, replacing <NAME> with your name, all lowercase, no spaces:

oc new-project <NAME>

For example oc new-project john-smith

Kubernetes Objects

Kubernetes organizes objects by namespace. In OpenShift, namespaces are called Projects.

There are 4 Kubernetes objects you need to know about to make an application available to the internet:

  1. Deployments - specify which image you want to run, and how many pods to create
  2. Pods - run containers
  3. Services - expose your containers to traffic inside the cluster
  4. Routes - expose your services to traffic from outside the cluster

Deployments

The most basic Kubernetes object you'll work with is a Deployment, which tells Kubernetes:

  • which image you would like to run
  • how many replicas (instances) you would like to run

Create the file

  1. Create a file named k8s/deployment.yaml
  2. Paste the following YAML into the file
  3. Replace the word <USERNAME> with your quay.io username
apiVersion: apps/v1
kind: Deployment
metadata:
  name: static-site
  labels:
    app: static-site
spec:
  replicas: 1
  selector:
    matchLabels:
      app: static-site
  template:
    metadata:
      labels:
        app: static-site
    spec:
      containers:
        - name: static-site
          image: quay.io/<USERNAME>/static-site:v1
          ports:
            - containerPort: 8080

Apply the file

oc apply -f k8s/deployment.yaml

Verify the deployment works

oc get deploy

You should see the following:

NAME          READY   UP-TO-DATE   AVAILABLE   AGE
static-site   1/1     1            1           24s

What just happened?

When you applied the file, Kubernetes stored your config in etcd (the "YAML database").

The control plane saw that you wanted 1 replica of static-site:v1 running. It looked at the nodes and couldn't find any.

So the scheduler picked a node, and ran one instance of static-site:v1.

Pods

A Deployment creates Pods. You can see pods by running:

oc get pods

Services

Services allow pods within a Kubernetes cluster to access the pods from your deployment.

Create the file

Create a file named k8s/service.yaml

Paste the following YAML into it:

apiVersion: v1
kind: Service
metadata:
  name: static-site
spec:
  selector:
    app: static-site
  ports:
    - protocol: TCP
      port: 80
      targetPort: 8080

Apply the file

oc apply -f k8s/service.yaml

Verify 

oc get svc

You should see something like this (the IP will be different):

NAME          TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)   AGE
static-site   ClusterIP   172.21.141.7   <none>        80/TCP    2m6s

Routes

Routes are an OpenShift concept. They serve a similar purpose to Kubernetes Ingresses.

Create the file

Create a file named k8s/route.yaml and paste the following contents:

apiVersion: route.openshift.io/v1
kind: Route
metadata:
  name: static-site
spec:
  to:
    kind: Service
    name: static-site

Apply the file

oc apply -f k8s/route.yaml

Verify

oc get routes

This will print something like this:

NAME          HOST/PORT                       PATH  SERVICES      PORT    TERMINATION   WILDCARD
static-site   static-site-...appdomain.cloud        static-site   <all>   edge          None

Copy the HOST/PORT and open it in a browser. You should be able to see your site! 🎉

Additional Challenges

Deploy a new version:

  • Make a change to index.html
  • Rebuild and push a new version of your docker image to quay.io (for example v3)
  • Change k8s/deployment.yaml to point to this new version and apply it

Scale the app:

  • Make a change to k8s/deployment.yaml to increase the number of replicas
  • Apply the file
  • Use oc to list the pods to make sure they are all there
  • Repeat the process, but reduce the replicas

UI

Inspect the app in the UI

Cleanup

Run the following command to delete your project, replacing <NAME> with your project name:

oc project default
oc delete project <NAME>

References