This content originally appeared on DEV Community and was authored by Lloyd Rivers
A Note from Me
In this project, I wanted to apply Kubernetes concepts like init containers and sidecar containers, but I didn’t want to just follow along with a tutorial.
My goal was to build something memorable.
After some brainstorming, I present to you the Get Me App!
What Get Me App Does
The Get Me App is designed to dynamically fetch and display content from various GitHub repositories. The application runs within a Kubernetes pod and comprises two primary components:
Nginx Container: This serves as the main application, hosting the dynamically fetched content. It's lightweight and efficient, making it ideal for serving static web pages.
Sidecar Container: This runs alongside the primary nginx container. It is responsible for refreshing the content every 5 seconds by fetching the latest HTML from a randomly selected GitHub page. This ensures that the content served by Nginx is always up to date.
The nginx container then serves the updated HTML webpage, making it accessible through a browser via a NodePort service.
Prerequisites
Before we start, ensure you have a configuration file to create the Kubernetes cluster. Refer to the Kind Quick Start guide for detailed instructions on setting up your Kind cluster. We covered this in previous lessons, so I won't go into detail here.
Cluster Configuration (config.yml)
Create a file named config.yml with the following content to define your Kind cluster:
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
name: cka-cluster
nodes:
- role: control-plane
extraPortMappings:
- containerPort: 30001 # Change this to match the NodePort
hostPort: 30001
listenAddress: "0.0.0.0"
protocol: tcp
- role: worker
- role: worker
Run the following command to create the cluster:
kind create cluster --name kind-cka-cluster --config config.yml
Use the following command to set the context to the new cluster:
kubectl config use-context kind-kind-cka-cluster
Step 1: Create the Project Directory and File
First, set up the directory and file structure for the project.
mkdir get-me-app
cd get-me-app
nano get-me-app.yml
Step 2: Define the Kubernetes Pod Specification
In the get-me-app.yml file, we’ll define a Kubernetes pod that includes the nginx container, a sidecar container for content refreshing, and an init container for initial data fetch.
apiVersion: v1
kind: Pod
metadata:
name: get-me-app
labels:
app: get-me-app
spec:
initContainers:
- name: init-myservice
image: busybox:1.28
command: ['sh', '-c', "until nslookup get-me-app-service.$(cat /var/run/secrets/kubernetes.io/serviceaccount/namespace).svc.cluster.local; do echo waiting for get-me-app-service; sleep 2; done"]
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
volumeMounts:
- name: workdir
mountPath: /usr/share/nginx/html
- name: content-refresher
image: busybox
command: ["sh", "-c", "while true; do url=$(shuf -n 1 -e https://github.com/piyushsachdeva/CKA-2024 https://github.com/kubernetes https://github.com/jenkinsci https://github.com/techiescamp/kubernetes-learning-path https://github.com/NotHarshhaa/kubernetes-learning-path); wget -O /work-dir/index.html \"$url\"; sleep 5; done"]
volumeMounts:
- name: workdir
mountPath: "/work-dir"
volumes:
- name: workdir
emptyDir: {}
What Each Part of This Pod Specification Does
nginx container: This is the primary container, serving content on port 80. The
volumeMountmakes the/usr/share/nginx/htmldirectory available in the pod’sworkdirvolume.Sidecar container (
content-refresher): This container runs awhile trueloop, downloading the latest version of the webpage every 5 seconds. This ensures that the content in theworkdirvolume stays updated.Init container (
install): This initializes the content by downloading the clock page to theworkdirvolume when the pod starts. The init container runs only once during initialization and does not restart after completion.Volumes: The
workdirvolume (anemptyDirtype) is shared among the containers, allowing the init container, sidecar, and nginx to access and serve the same content.
Step 3: Test Locally with NodePort (Optional)
To make the app accessible through a browser on your local machine, configure a NodePort service to expose the pod’s port 80.
Add this service definition in get-me-app-service.yml:
apiVersion: v1
kind: Service
metadata:
name: get-me-app-service
spec:
type: NodePort
selector:
app: get-me-app
ports:
- port: 80
targetPort: 80
nodePort: 30001
Deploy the setup with:
kubectl apply -f get-me-app.yml
kubectl apply -f get-me-app-service.yml
Access it by visiting http://localhost:30001 in your browser, and you should the github page. Refresh the page after 5 seconds and you should see a different github page.
Key Takeaways
This project helped me understand how init containers can initialize shared resources and how sidecar containers keep those resources updated for the main application. It's an engaging way to experiment with and learn about real-time data handling in Kubernetes.
This content originally appeared on DEV Community and was authored by Lloyd Rivers
Lloyd Rivers | Sciencx (2024-10-29T09:19:11+00:00) CKA Full Course 2024: Day 11/40 Multi Container Pod Kubernetes – Sidecar vs Init Container. Retrieved from https://www.scien.cx/2024/10/29/cka-full-course-2024-day-11-40-multi-container-pod-kubernetes-sidecar-vs-init-container/
Please log in to upload a file.
There are no updates yet.
Click the Upload button above to add an update.