Language and Framework Guides
Run a Flask Container App

14min
Back4app Containers is the perfect platform for hosting your Flask applications. With its ability to automatically deploy Dockerized Flask apps, you can effortlessly launch your project in a scalable and flexible environment. 
In this comprehensive guide, we will walk you through the process of preparing and deploying your Flask application on Back4app Containers, covering everything from simple projects to more complex setups.
If you have any questions or comments, feel free to join the conversation in the #Containers channel on the Back4app Community on Slack for discussions specific to Flask on Back4app Containers. Anytime you can also contact us at [email protected].
At anytime if you want to check a sample working Flask project on Back4app containers go to : https://github.com/templates-back4app/containers-python-flask-sample﻿
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1-Prepare your Flask application:
Make sure your Flask application is structured properly and that all necessary files, including templates and static files, are organized in the project directory. For more complex projects, ensure that you have a proper app factory setup and follow Flask best practices.
2-Create a requirements.txt file:
List all required dependencies and their versions in a requirements.txt file. This file will be used by the Dockerfile to install the necessary packages when building the Docker image.
Here's a sample requirements.txt file for a Flask application, including some common dependencies you might use in various projects. Feel free to add or remove packages as needed for your specific application:
requirements.txt
Flask==2.1.1
gunicorn==20.1.0
Werkzeug==2.1.1
Jinja2==3.1.0
MarkupSafe==2.1.1
itsdangerous==2.1.1
click==8.1.2
SQLAlchemy==1.4.29
Flask-SQLAlchemy==2.5.1
Flask-Migrate==3.1.1
Flask-Login==0.5.0
Flask-WTF==1.0.3
WTForms==3.0.2
Flask-Cors==3.1.1
Flask-Mail==0.9.1
Flask-RESTful==0.3.9
Flask-Script==2.0.6
requests==2.27.1

Flask==2.1.1
gunicorn==20.1.0
Werkzeug==2.1.1
Jinja2==3.1.0
MarkupSafe==2.1.1
itsdangerous==2.1.1
click==8.1.2
SQLAlchemy==1.4.29
Flask-SQLAlchemy==2.5.1
Flask-Migrate==3.1.1
Flask-Login==0.5.0
Flask-WTF==1.0.3
WTForms==3.0.2
Flask-Cors==3.1.1
Flask-Mail==0.9.1
Flask-RESTful==0.3.9
Flask-Script==2.0.6
requests==2.27.1
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3-Create a Dockerfile
Write a Dockerfile to define your application's Docker image. Make sure to use a suitable base image (such as python:3.x-slim), install dependencies from the requirements.txt file, copy your project files, expose the correct port, and specify the appropriate command to start your Flask application (e.g., using gunicorn).
Example 1: Basic Flask Application
This Dockerfile is for a simple Flask application using Gunicorn as the WSGI server.
Dockerfile
DockerfileCopy code# Base image
FROM python:3.9-slim

# Working directory
WORKDIR /app

# Copy requirements file and install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the project files
COPY . .

# Expose the server port
EXPOSE 8080

# Command to start the server
CMD ["gunicorn", "-b", "0.0.0.0:8080", "app:app"]
DockerfileCopy code# Base image
FROM python:3.9-slim

# Working directory
WORKDIR /app

# Copy requirements file and install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the project files
COPY . .

# Expose the server port
EXPOSE 8080

# Command to start the server
CMD ["gunicorn", "-b", "0.0.0.0:8080", "app:app"]
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Example 2: Flask Application with Waitress (Alternative WSGI Server)
This Dockerfile uses Waitress, an alternative WSGI server that is suitable for production environments and can be used instead of Gunicorn.
Dockerfile
DockerfileCopy code# Base image
FROM python:3.9-slim

# Working directory
WORKDIR /app

# Copy requirements file and install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the project files
COPY . .

# Expose the server port
EXPOSE 8080

# Command to start the server
CMD ["waitress-serve", "--host=0.0.0.0", "--port=8080", "app:app"]
DockerfileCopy code# Base image
FROM python:3.9-slim

# Working directory
WORKDIR /app

# Copy requirements file and install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the project files
COPY . .

# Expose the server port
EXPOSE 8080

# Command to start the server
CMD ["waitress-serve", "--host=0.0.0.0", "--port=8080", "app:app"]
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Make sure to add waitress to your requirements.txt file.
Example 3: Flask Application with Gunicorn and Worker Optimization
This Dockerfile uses Gunicorn with worker optimization for better performance. It adjusts the number of Gunicorn worker processes based on the available CPU cores.
Dockerfile
DockerfileCopy code# Base image
FROM python:3.9-slim

# Install build dependencies and curl
RUN apt-get update && apt-get install -y --no-install-recommends \
    build-essential \
    curl \
    && rm -rf /var/lib/apt/lists/*

# Working directory
WORKDIR /app

# Copy requirements file and install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the project files
COPY . .

# Expose the server port
EXPOSE 8080

# Calculate the number of worker processes based on the number of CPU cores
CMD ["sh", "-c", "gunicorn -b 0.0.0.0:8080 --workers $(($(nproc --all) * 2 + 1)) app:app"]
DockerfileCopy code# Base image
FROM python:3.9-slim

# Install build dependencies and curl
RUN apt-get update && apt-get install -y --no-install-recommends \
    build-essential \
    curl \
    && rm -rf /var/lib/apt/lists/*

# Working directory
WORKDIR /app

# Copy requirements file and install dependencies
COPY requirements.txt requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Copy the rest of the project files
COPY . .

# Expose the server port
EXPOSE 8080

# Calculate the number of worker processes based on the number of CPU cores
CMD ["sh", "-c", "gunicorn -b 0.0.0.0:8080 --workers $(($(nproc --all) * 2 + 1)) app:app"]
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4- Test your application locally
Build and run the Docker image locally to ensure that your Flask application works as expected. Address any issues or errors before deploying to Back4app Containers.
Text
docker build -t your-app-name .
docker run -p 8080:8080 your-app-name
docker build -t your-app-name .
docker run -p 8080:8080 your-app-name
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5-Push your project to a Git repository
Create a .gitignore file to exclude unnecessary or sensitive files from your repository (e.g., __pycache__, .env, *.pyc, etc.). Initialize a Git repository, commit your project files, and push them to a remote repository (e.g., on GitHub).
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6-Deploy your application on Back4app Containers
After creating your Back4app account you can follow the steps listed on the Docs:
﻿1- Connect you GitHub repo with Back4app﻿
﻿2- Prepare your project for deployment﻿
In summary containers will follow the instructions detailed on your Dockerfile and start to create your App.
7-Monitor deployment
Keep an eye on the deployment logs and status on the Back4app Containers dashboard. Address any errors or issues that arise during deployment. In case of more complex projects, ensure that all necessary services (such as databases or external APIs) are correctly configured and accessible.
Notice that you also have a section called Logs which lists all the running logs for your app and its also important to keep an eye on it to check if you have any problem with your App.
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8-Troubleshooting
Common deployments errors are listed here. Other possible common errors when deploying Flask Applications are:
Incorrect host configuration:
Flask applications should be configured to run on host 0.0.0.0 when deployed on Back4app Containers. If your application is using localhost or 127.0.0.1, it might not be accessible externally.
Incompatible or missing dependencies:
Ensure that all required dependencies are listed in your requirements.txt file and that their versions are compatible with each other and your application code. Missing or incompatible dependencies can lead to runtime errors.
Database connection issues:
If your Flask application relies on a database, ensure that the connection string and credentials are correctly set up in the application configuration. Also, verify that the database is reachable from the Back4app Containers environment.
Application crashes or unhandled exceptions:
Unhandled exceptions or crashes in your Flask application code can cause deployment failures or unexpected behavior. Examine your application logs for any error messages, and address any issues in your code.
Incorrectly configured WSGI server:
Make sure that your WSGI server (e.g., Gunicorn or Waitress) is correctly configured in your Dockerfile and that it starts your Flask application using the right entry point (e.g., app:app).
Invalid environment variables or configuration:
Check if your application relies on specific environment variables or configuration files, and ensure they are correctly set up in the Back4app Containers environment.
8-Scale your application
For more complex projects that require additional resources or horizontal/vertical scaling, consider upgrading your Back4app Containers plan to handle increased traffic and load.
9-Sample Application
For a sample Flask application project you can go to : https://github.com/templates-back4app/containers-python-flask-sample﻿
Updated 10 May 2023
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