Developing, Deploying and Testing Flask Applications on Kubernetes - Part I

• Dockerizing an existing Flask application.
• Deploying it using Docker Compose.
• Deploying to a Kubernetes cluster.
• Inspecting, testing, and scaling 

Requirements

• Docker
• Kubernetes 
• Git

The Flask Application

• “/”:  This is the root route and it basically responds with a static string. This endpoint can be used as a health check for the application.
• “/<city>/<country>”: The API endpoint used to retrieve the weather information for a given city. Both the city and the country should be provided.

from flask import Flask
import requests

app = Flask(__name__)

API_KEY = "b6907d289e10d714a6e88b30761fae22"

@app.route('/')
def index():
    return 'App Works!'

@app.route('/<string:city>/<string:country>/')
def weather_by_city(country, city):

    url = 'https://samples.openweathermap.org/data/2.5/weather'
    params = dict(
        q=city + "," + country,
        appid= API_KEY,
    )

    response = requests.get(url=url, params=params)
    data = response.json()
    return data

if __name__ == '__main__':
    app.run(host="0.0.0.0", port=5000)

Dockerizing our Flask Application

• The requirements files: This file contains the list of all dependencies needed by the application. For instance, one of the dependencies for our application is Flask. This file will be used to install all the defined packages during the Docker build time. Below is the content of the requirements file.

certifi==2019.9.11
chardet==3.0.4
Click==7.0
Flask==1.1.1
idna==2.8
itsdangerous==1.1.0
Jinja2==2.10.3
MarkupSafe==1.1.1
requests==2.22.0
urllib3==1.25.7
Werkzeug==0.16.0

• Use python:3 as a base image for our application.
• Create the working directory inside the image and copy the requirements file. (This step helps in optimizing the Docker build time.)
• Install all the dependencies using pip. 
• Copy the rest of the application files.
• Expose port 5000 and set the default command (CMD) for the image.

FROM python:3
ENV PYTHONUNBUFFERED 1
RUN mkdir /app
WORKDIR /app
COPY requirements.txt /app
RUN pip install --upgrade pip
RUN pip install -r requirements.txt
COPY . /app
EXPOSE 5000
CMD [ "python", "app.py" ]

$> docker build -t weather:v1.0

Running the application 

$> docker run -dit --rm -p 5000:5000 --name weather weather:v1.0

version: '3.6'
services:
  weather:
    build: .
    ports:
      - "5000:5000"
    volumes:
      - .:/app

$> docker-compose up

$> curl http://0.0.0.0:5000/london/uk/

Kubernetes Overview

• It is free: The project is open-source and maintained by the CNCF.
• It is adopted by big companies such as Google, AWS, Microsoft, and many others.
• There are many cloud systems that offer Kubernetes managed services such as AWS, Google Cloud, and DigitalOcean.
• There are many plugins and tools developed by the Kubernetes community to make managing Kubernetes easier and more productive.

Creating a Kubernetes Cluster for your Development Environment

• Docker for mac: If you have a MacBook, you can install Docker for mac and enable Kubernetes from the configuration of the application as shown in the below image. You will have a Kubernetes cluster deployed locally.
• Microk8s : Single-package fully conformant lightweight Kubernetes that work on 42 flavours of Linux. This can be used to run Kubernetes locally on Linux systems including Raspberry Pi. MicroK8s can be installed on CentOS using the Snap command line as shown in the below snippet

$> sudo yum install epel-release
$> sudo yum install snapd
$> sudo systemctl enable --now snapd.socket
$> sudo ln -s /var/lib/snapd/snap /snap
$> sudo snap install microk8s --classic 

In case you have a different Linux distribution you can find the instruction on the following page. 
• Minikube:  implements a local Kubernetes cluster on macOS, Linux, and Windows that supports most of the Kubernetes features. Depending on your operation system, you can select the commands needed for installing Kubernetes from this page. For instance, to install the application and start it on macOS you can use the below commands:

$> brew install minikube
$> minikube start

Once Minikube, Microk8s, or Docker for Mac is installed and running, you can start using the Kubernetes command line to interact with the Kubernetes cluster.
The above tools can be used easily to bootstrap a development environment and test your Kubernetes Deployments locally. However, they do not implement all features supported by Kubernetes, and not all of them are designed to support multi-node clusters.

Conclusion

Minikube, Microk8s, or Docker for Mac are great tools for local development. However, for testing and staging environments, highly available clusters are needed to simulate and test the application in a production-like environment.
On the other hand, Running Kubernetes clusters 24/7 for the testing environment can be very expensive. You should make sure to run your cluster only when needed, shut it down when it’s no longer required, then recreate when it’s needed again.
Using Cloudplex, (Disclaimer: The author is the Founder and CEO at Cloudplex), creating, running, terminating, and recreating clusters is easy as pie. You can deploy your first cluster for free . In a few minutes, your cluster is up and running, you can save its configuration, shut it down and recreate it when needed.
In part II of this series, we are going to how to deploy our application to a Kubernetes testing cluster. We are going to create the Kubernetes Deployment for our Flask application and use Traefik to manage our Ingress and expose our application to external traffic.
(Disclaimer: The author is the Founder and CEO at Cloudplex)