hscloud/doc/codelabs/getting-started/building-flask-web-app.md

# Building your own Flask web service

In this chapter, we will build our own web service using the Flask framework, dockerize it and register it in the registry.

## Requirements

You will require `docker` in this example. You can find installation instructions [here](https://docs.docker.com/engine/install/). For example, on Ubuntu you can install it with:

```bash
curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh
```

To create, build and register docker image you don't need a compiled hscloud environment, howerver you will need it to deploy the image to kubernetes.

## Steps

We create a simple Flask application:

```py
# app.py

from flask import Flask

app = Flask(__name__)

@app.route('/')

def hello_world():
    return 'Hello, World!'

if __name__ == '__main__':
    app.run()
```

We can check if the application works by running it. First we start the virtual environment and install Flask:

```bash
# for Linux and macOS environments
python3 -m venv .venv
source .venv/bin/activate
```

```bash
# for Windows environments
py -m venv .venv
.venv\bin\Activate.bat
```

After that:
```bash
pip3 install flask
python3 app.py
```

Now we create `Dockerfile` file for our application:

```Dockerfile
# Use an official Python runtime as a base image
FROM python:3.12.0-bullseye

# Set the working directory in docker
WORKDIR /app

# Copy the current directory contents into the container at /app
COPY . /app

# Install any needed packages specified in requirements.txt
RUN pip install --upgrade pip && \
    pip install flask gunicorn

# Set environment variables
ENV FLASK_APP=app.py
ENV FLASK_RUN_HOST=0.0.0.0

# Expose port 5000 for the Flask app to listen on
EXPOSE 5000

# Define command to run the application using gunicorn
CMD ["gunicorn", "-w", "4", "-b", ":5000", "app:app"]
```

We can test if the application works in Docker:

```bash
docker build -t myflask:2137 .   # Build the image
docker run -p 5000:5000 myflask:2137  # Run the container
```

We can check if the application works by opening the browser at `http://localhost:5000`.

## Pushing the image to the registry

First, we need to authenticate to the registry. That will require visiting the link in a web browser:

```
https://registry.k0.hswaw.net
```

You should get command similar to:

```
docker login -u $hs_username -p *************** https://registry.k0.hswaw.net
```

Run it in your environment.

Now we need to create local server for our docker image:

```bash
# create local server for our docker image. Replace $hs_username with your sso username
docker tag myflask:2137 registry.k0.hswaw.net/$hs_username/myflask:2137
docker push registry.k0.hswaw.net/$hs_username/myflask:2137
```

To remove the image from the registry, you can use:

```bash
# remove image from registry. Replace $hs_username with your sso username
docker rmi registry.k0.hswaw.net/$hs_username/myflask:2137
```

## Next step

[Deploying Flask web app](deploying-flask-web-app.md)