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The Functional Depth of Docker and Docker Compose

Lisa Kudrow

Feb 08, 2025 am 09:06 AM

The Functional Depth of Docker and Docker Compose

Build multi-container Flask application using Docker Compose and Vultr container registry

Docker Compose allows users to run and define multi-container applications using a single configuration file. It simplifies the process of setting up and managing multiple containers, making it easier to develop, test, and deploy applications. This article will guide you to create a Flask application with two containers, manage Docker images of your application using the Vultr Container Registry (VCR), and manage multiple containers with the multi-container functionality of Docker Compose.

Creation of sample application

The following steps will guide you to create a sample application:

Deploy a Vultr compute instance using the Docker market application through the Vultr customer portal.
Use SSH to securely access the server with non-root sudo users.
Update the server.

Create a new project directory and enter it:

mkdir flask-redis-example
cd flask-redis-example

Create a new file named app.py:
```
nano app.py
```

Add the following code:

from flask import Flask, render_template
import redis

app = Flask(__name__)
redis_client = redis.Redis(host='redis', port=6379)

@app.route('/')
def hello():
    count = redis_client.incr('hits')
    return render_template('index.html', count=count)

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

Save and exit the file. This Flask code connects to the Redis database and increments the counter every time the root URL is accessed.

Allow incoming connections to port 5000 and reloading the firewall:
```
sudo ufw allow 5000
sudo ufw reload
```
Create a new file named requirements.txt:
```
nano requirements.txt
```
Add the following package:
```
<code>flask
redis</code>
```
Save and close the file.
Create another directory in the flask-redis-example directory and enter it:
```
 mkdir static
 cd static
```
Create a new file named styles.css:
```
 nano styles.css
```

Add the following code:

 body {
     font-family: Arial, sans-serif;
     text-align: center;
     background-color: #f4f4f4;
     margin: 0;
     padding: 0;
 }

 h1 {
     color: #333;
     margin-top: 50px;
 }

 p {
     font-size: 18px;
     color: #666;
 }

Save and exit the file.

Create another directory in the flask-redis-example directory and enter it:
```
 mkdir templates
 cd templates
```
Create a new file named index.html:
```
 nano index.html
```

Add the following code:

 <!DOCTYPE html>
 <html>
 <head>
     <title>Flask App</title>
     <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
 </head>
 <body>
     <h1 id="Hello-World">Hello, World!</h1>
     <p>I have been seen {{ count }} times.</p>
 </body>
 </html>

Save and exit the file.

Using Vultr Container Registry

In this section, you will create a Vultr container registry, upload your Docker image to the registry, and set up a Docker Compose file to set up the services of the Flask and Redis databases.

Deploy a Vultr container registry.
Create a Docker manifest in the flask-redis-example directory:
```
nano Dockerfile.flask
```

Add the following configuration:

FROM python:3.9-slim

WORKDIR /app

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY app.py .
COPY static/ ./static/
COPY templates/ ./templates/

EXPOSE 5000

CMD ["python", "app.py"]

Save and exit the file.

Build Docker image:
```
docker build -t flask-app .
```
Login to your Vultr container registry:
```
docker login <url> -u <user> -p <password>
```
Be sure to replace <url></url>, <user></user> and <password></password>, which are provided in the Overview section of your Vultr Container Registry.

Tag Docker image:

mkdir flask-redis-example
cd flask-redis-example

Pusing the mirror to the Vultr container registry:
```
nano app.py
```
After pushing the Docker image, verify that the image exists in the "Repository" section of the Vultr container registry on the Vultr dashboard.
Create a new file named docker-compose.yaml:
```
from flask import Flask, render_template
import redis

app = Flask(__name__)
redis_client = redis.Redis(host='redis', port=6379)

@app.route('/')
def hello():
    count = redis_client.incr('hits')
    return render_template('index.html', count=count)

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)
```
Save and exit the file. The above YAML configuration defines two services web and redis. web Service builds the Flask application from the current directory (.) and maps the container's port 5000 to the host's port 5000. It also specifies that the web service depends on the redis service. redisThe service uses the official Redis Docker image from Docker Hub.
Build Docker Compose file:
```
sudo ufw allow 5000
sudo ufw reload
```
After the build process is completed, visit the Flask application on http://:5000. Try refreshing the website multiple times and observe whether the count of page visits increases.

More use of Vultr container registry

Vultr Container Registry with Docker
Vultr Container Registry with Kubernetes
Build vLLM container image
Build Llama.cpp container image
Build PyTorch container image

Best Practice

Keep docker-compose.yamlThe files are well organized and well documented.
Use named volumes to persist data instead of binding to host directory.
Use environment variables to store sensitive data such as passwords and API keys.
Use the built-in commands of Docker Compose, such as docker-compose up, docker-compose down, and docker-compose ps to manage containers.

Conclusion

In this article, you created a Flask application with two containers, used the Vultr container registry to manage the Docker image of the application, and took advantage of the multi-container functionality of Docker Compose to manage multiple containers.

This article is sponsored by Vultr. Vultr is the world's largest private cloud computing platform. Favorite among developers, Vultr has provided flexible and scalable global cloud computing, cloud GPU, bare metal and cloud storage solutions to more than 1.5 million customers in 185 countries. Learn more about Vultr

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