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July 25, 2020

A single-node Kubernetes cluster without virtualization or a container registry

kubernetesdockerk3sfedorarecipe

This post is a recipe for setting up a minimal Kubernetes cluster on Fedora without requiring virtualization or a container registry. These two features make the system cloud-agnostic and the cluster entirely self-contained. The post will end with us running a simple Flask app from a local container.

This setup is primarily useful for simple CI environments or application development on Linux. (Docker Desktop has better tooling for development on Mac or Windows.)

Getting Kubernetes

The core of this effort is K3s, a Kubernetes distribution that allows us to run on a single node without virtualization.

But first off, install Docker.

Then install K3s:

$ curl -sfL https://get.k3s.io | sh -

It may prompt you to adjust some SELinux policies like so:

$ sudo dnf install -y container-selinux selinux-policy-base
$ sudo rpm -i https://rpm.rancher.io/k3s-selinux-0.1.1-rc1.el7.noarch.rpm

Swap these out with whatever it prompts and retry the K3s install.

Finally, install kubectl:

$ curl -LO https://storage.googleapis.com/kubernetes-release/release/`curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt`/bin/linux/amd64/kubectl

Now copy the global K3s kubeconfig into ~/.kube/config:

$ sudo cp /etc/rancher/k3s/k3s.yaml ~/.kube/config
$ sudo chown $USER:$GROUP ~/.kube/config

And enable K3s:

$ sudo systemctl enable k3s

If you're on Fedora 31+ you'll need to disable cgroups v2 and reboot:

$ sudo grubby --args="systemd.unified_cgroup_hierarchy=0" --update-kernel=ALL
$ sudo reboot

Finally, you can run kubectl:

$ kubectl get pods
No resources found in default namespace.

A simple application

We'll create a small Flask app, containerize it, and write a Kubernetes deployment and service config for it.

We begin with app.py:

from flask import Flask

app = Flask(__name__)


@app.route('/')
def index():
    return 'Hello World, Flask!'


if __name__ == '__main__':
    app.run(debug=True)

Then a Dockerfile:

FROM python:3-slim

RUN pip install flask
COPY . /app

CMD python3 /app/app.py

Then the deployment in manifest.yaml:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: helloworld
spec:
  selector:
    matchLabels:
      name: helloworld
  template:
    metadata:
      labels:
        name: helloworld
    spec:
      containers:
        - image: helloworld
          name: helloworld

Running in Kubernetes

First we build, save, and import the image into k3s:

$ docker build . -t helloworld
$ docker save helloworld > helloworld.tar
$ sudo k3s ctr image import helloworld.tar
$ kubectl apply -f ./manifest.yaml
$ kubectl port-forward $(kubectl get pods | grep helloworld | cut -d ' ' -f 1) 5000 > log 2>&1 &
$ curl localhost:5000
Hello World, Flask

And that's it!