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Deploying a Flask Application With Jenkins to a Kubernetes Cluster

Hakan Bayraktar
8 min readAug 21, 2024

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Introduction

In this guide, you will learn how to automate the deployment of a Flask application using Jenkins, Docker, and Kubernetes in a CI/CD (Continuous Integration and Continuous Deployment) pipeline. This step-by-step tutorial covers everything from setting up Jenkins and Docker to configuring Jenkins, pushing the Dockerized Flask application to DockerHub, and deploying it on Kubernetes

Prerequisites:

  • A server with Ubuntu 24.04
  • Basic knowledge of Linux commands is required
  • Access to a Kubernetes cluster (you can use Minikube, Kind, or DigitalOcean Kubernetes).

Step 1: Installing Jenkins

First, install Jenkins on your Ubuntu server.

Install Java:

sudo apt update -y
sudo apt install openjdk-17-jdk -y

Install Jenkins:

sudo wget -O /usr/share/keyrings/jenkins-keyring.asc \
https://pkg.jenkins.io/debian/jenkins.io-2023.key
echo "deb [signed-by=/usr/share/keyrings/jenkins-keyring.asc]" \
https://pkg.jenkins.io/debian binary/ | sudo tee \
/etc/apt/sources.list.d/jenkins.list > /dev/null
sudo apt-get update -y
sudo apt-get install jenkins -y

Start and enable Jenkins:

sudo systemctl start jenkins
sudo systemctl enable jenkins

Access Jenkins: Open your browser and navigate to http://your-server-ip:8080. Unlock Jenkins using the initial admin password:

You can find your-server-ip with this command:

curl ifconfig.co
sudo cat /var/lib/jenkins/secrets/initialAdminPassword

Complete the setup by installing suggested plugins and creating the first admin user.

The setup prompts to either Install suggested plugins or Select plugins to install. It’s fine to simply install the suggested plugins

The next step is to the Create First Admin User. Enter the credentials you want to use for your Jenkins administrator, then click Save and Continue.

Once you specify the Jenkins URL, click Save and Finish

You should see a page that says Jenkins is ready!

Click Start using Jenkins to open the Jenkins dashboard

Step 2: Installing Docker

Next, we need Docker to build and manage our containerized applications.

Install Docker on Jenkins Server:

sudo apt-get update -y
sudo apt-get install ca-certificates curl -y
sudo install -m 0755 -d /etc/apt/keyrings
sudo curl -fsSL https://download.docker.com/linux/ubuntu/gpg -o /etc/apt/keyrings/docker.asc
sudo chmod a+r /etc/apt/keyrings/docker.asc
echo \
  "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/ubuntu \
  $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
  sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
sudo apt-get update -y
sudo apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin -y

check docker installation

docker — version

Add Jenkins User to Docker Group:

sudo systemctl enable docker
sudo systemctl restart jenkins
sudo usermod -aG docker jenkins

Step 3: Configuring Jenkins for Docker and Kubernetes

Now that Jenkins and Docker are installed, let’s configure Jenkins for Docker and Kubernetes deployments.

Install Required Plugins:

Go to Manage Jenkins > Manage Plugins > Available plugins and install the following plugins:

  • Docker Pipeline
  • Kubernetes Pipeline Devops steps
  • Kubernete CLI
  • Kubernetes

Configure Docker Hub Credentials in Jenkins:

Go to Manage Jenkins > Credentials> System>Global credentials (unrestricted) and Select Add Credentials button
Kind: Username with password
Username: hbayraktar (your dockerhub username)
Password: Dockerhub password
ID: dockerhub-cred

Configure Kubernetes:

Create a kubernetes service account

kubectl create serviceaccount jenkins

Create a role binding based on the permission needed by the application using below

cat <<EOF | kubectl create -f -
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: jenkins-integration
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
- kind: ServiceAccount
  name: jenkins
  namespace: default
EOF

Extract Service account token using kubectl

kubectl get secrets $(kubectl get serviceaccounts jenkins -o jsonpath='{.secrets[].name}') -o jsonpath='{.data.token}' | base64 -d

Create a secret for service account jenkins

cat <<EOF | kubectl create -f -
apiVersion: v1
kind: Secret
type: kubernetes.io/service-account-token
metadata:
  name: jenkins
  annotations:
    kubernetes.io/service-account.name: jenkins
EOF
kubectl get secrets jenkins -o jsonpath='{.data.token}' | base64 -d

Add Kubernetes service account secret in Jenkins Credentials

Manage Jenkins> Credentials > then click on global and click on add credentials

Under Kind, scroll on the drop-down list and then choose Secret text Secret: copy the Kubernetes token we generated earlier and paste it there. ID: SECRET_TOKEN
Click on the Create button

Integrate Remote Kubernetes Cluster with Jenkins

Go to Manage Jenkins > Clouds > Select New cloud

cloud name: kubernetes-cloud

Select Type: kubernetes and click on the create button

cat ~/.kube/config

Name: kubernetes-cred
Kubernetes URL: https://db25c80a-3584-4759-9e48-baa6c16374a8.k8s.ondigitalocean.com (Open your config file and enter server URL)
Kubernetes server certificate key: You need to generate this key using this command

echo -n <contents_of_the_certificate-authority-data_entry_of_my_kubeconfig_file> | base64 — decode

kubernetes server certification key: Copy to echo result

Kubernetes Namespace:default

Credentials: Select secret_token(before we created step 3 )
Press the Test Connection button for check kubernetes cluster connection
If your connection is successfully you should press the Save button

Step 4: Flask-monitoring Application Structure

You can fork or clone this repository https://github.com/hakanbayraktar/flask-monitoring.git

git clone https://github.com/hakanbayraktar/flask-monitoring.git

flask-monitoring/
├── templates
├──index.html
├── app.py
├── Dockerfile
├── requirements.txt
├── Jenkinsfile
├── deploymen.yaml
├── jenkins.sh
├── service.yaml
└── README.md

Dockerfile:

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
ENV FLASK_RUN_HOST=0.0.0.0
EXPOSE 5000
CMD [ "flask", "run"]

app.py

import psutil
from flask import Flask, render_template

app = Flask(__name__)

@app.route("/")
def index():
    cpu_metric = psutil.cpu_percent()
    mem_metric = psutil.virtual_memory().percent
    Message = None
    if cpu_metric > 70 or mem_metric > 70:
        Message = "High CPU or Memory Detected, scale up!!!"
    return render_template("index.html", cpu_metric=cpu_metric, mem_metric=mem_metric, message=Message)

if __name__=='__main__':
    app.run(debug=True, host = '0.0.0.0')

requirements.txt

Flask==2.2.3
MarkupSafe==2.1.2
Werkzeug==2.2.3
itsdangerous==2.1.2
psutil==6.0.0
plotly==5.5.0
tenacity==8.0.1
boto3==1.34.151
kubernetes==10.0.1

index.html

<!DOCTYPE html>
<html>
<head>
    <title>System Monitoring</title>
    <script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
    <style>
        .plotly-graph-div {
            margin: auto;
            width: 50%;
            background-color: rgba(151, 128, 128, 0.688);
            padding: 20px;
        }
    </style>
</head>
<body>
    <div class="container">
        <h1>System Monitoring</h1>
        <div id="cpu-gauge"></div>
        <div id="mem-gauge"></div>
        {% if message %}
        <div class="alert alert-danger">{{ message }}</div>
        {% endif %}
    </div>
    <script>
        var cpuGauge = {
            type: "indicator",
            mode: "gauge+number",
            value: {{ cpu_metric }},
            gauge: {
                axis: { range: [null, 100] },
                bar: { color: "#1f77b4" },
                bgcolor: "white",
                borderwidth: 2,
                bordercolor: "#ccc",
                steps: [
                    { range: [0, 50], color: "#d9f0a3" },
                    { range: [50, 85], color: "#ffeb84" },
                    { range: [85, 100], color: "#ff5f5f" }
                ],
                threshold: {
                    line: { color: "red", width: 4 },
                    thickness: 0.75,
                    value: {{ cpu_metric }}
                }
            }
        };
        
        var memGauge = {
            type: "indicator",
            mode: "gauge+number",
            value: {{ mem_metric }},
            gauge: {
                axis: { range: [null, 100] },
                bar: { color: "#1f77b4" },
                bgcolor: "white",
                borderwidth: 2,
                bordercolor: "#ccc",
                steps: [
                    { range: [0, 50], color: "#d9f0a3" },
                    { range: [50, 85], color: "#ffeb84" },
                    { range: [85, 100], color: "#ff5f5f" }
                ],
                threshold: {
                    line: { color: "red", width: 4 },
                    thickness: 0.75,
                    value: {{ mem_metric }}
                }
            }
        };
        
        var cpuGaugeLayout = { title: "CPU Utilization" };
        var memGaugeLayout = { title: "Memory Utilization" };
        
        Plotly.newPlot('cpu-gauge', [cpuGauge], cpuGaugeLayout);
        Plotly.newPlot('mem-gauge', [memGauge], memGaugeLayout);
    </script>
</body>
</html>

Jenkinsfile

pipeline {
    agent any

    stages {
        stage('Git Cloning') {
            steps {
                echo 'Cloning git repo'
                git url: 'https://github.com/hakanbayraktar/flask-monitoring.git',  branch: 'main'
            }
        }
        stage('Build Docker Image') {
            steps {
                echo 'Building the image'
                sh 'docker build -t flask-monitoring .'
            }
        }
        stage('Push to Docker Hub') {
            steps {
                echo 'Pushing to Docker Hub'
                withCredentials([usernamePassword(credentialsId: 'dockerhub-cred', usernameVariable: 'USER', passwordVariable: 'PASS')]) {
                    sh '''
                    echo "${PASS}" | docker login --username "${USER}" --password-stdin
                    docker tag flask-monitoring ${USER}/flask-monitoring:latest
                    docker push ${USER}/flask-monitoring:latest
                    '''
                }
            }
        }
        stage('Integrate Remote kubernetess with Jenkins') {
            steps {
                withCredentials([string(credentialsId: 'secret_token', variable: 'KUBE_TOKEN')]) {
                    sh '''
                    curl -LO "https://storage.googleapis.com/kubernetes-release/release/v1.20.5/bin/linux/amd64/kubectl"
                    chmod u+x ./kubectl
                    export KUBECONFIG=$(mktemp)
                    ./kubectl config set-cluster do-fra1-ibb-tech --server=https://db25c80a-3584-4759-9e48-baa6c16374a8.k8s.ondigitalocean.com --insecure-skip-tls-verify=true
                    ./kubectl config set-credentials jenkins --token=${KUBE_TOKEN}
                    ./kubectl config set-context default --cluster=do-fra1-ibb-tech --user=jenkins --namespace=default
                    ./kubectl config use-context default
                    ./kubectl get nodes
                    ./kubectl apply -f service.yaml
                    ./kubectl apply -f deployment.yaml
                    '''
                }
            }
        }
    }
}

deployment.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: flask-monitoring
  labels:
    app: flask-monitoring
spec:
  replicas: 1
  selector:
    matchLabels:
      app: flask-monitoring
  template:
    metadata:
      labels:
        app: flask-monitoring
    spec:
      containers:
      - name: flask-monitoring
        image: hbayraktar/flask-monitoring:latest
        ports:
        - containerPort: 5000

service.yaml

apiVersion: v1
kind: Service
metadata:
  name: my-service
spec:
  type: LoadBalancer
  selector:
    app: flask-monitoring
  ports:
    - protocol: TCP
      port: 80
      targetPort: 5000

Step 5: Jenkins Pipeline Configuration

Create a new Jenkins job:

Go to New Item and enter flask-monitoring
Select Pipeline and click OK

Under Advanced Project Options, configure the pipeline with:

Definitions: Select pipeline script from SCM
SCM: Git
Repository URL:https://github.com/hakanbayraktar/flask-monitoring.git
Credentials: None

Branch Specifier (blank for ‘any’): */main
Script Path: Jenkinsfile
click on Save button

Step 6: Deploying on Kubernetes

Conclusion

By following this guide, you’ve set up a CI/CD pipeline with Jenkins, Docker, and Kubernetes, fully automating the process of building, testing, and deploying a Flask application. This setup can be easily adapted for other types of applications or services, making it a powerful tool for DevOps practices.

Jenkins
Jenkins Pipeline
Docker
Kubernetes
Ci Cd Pipeline

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Hakan Bayraktar
Hakan Bayraktar

Written by Hakan Bayraktar

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