Docker containerization in FastAPI - Mini Project: Build & Apply

Docker containerization

📖 Scenario: You want to share your FastAPI web app easily with others or deploy it on any computer without setup hassle. Docker helps by packaging your app and its environment into a container.

🎯 Goal: Build a simple FastAPI app and create a Docker container to run it anywhere.

📋 What You'll Learn

Create a basic FastAPI app with one route

Add a variable for the port number

Write a Dockerfile to containerize the app

Run the app inside the Docker container

💡 Why This Matters

🌍 Real World

Docker containerization lets developers package their web apps with all dependencies so they run the same everywhere, avoiding setup issues.

💼 Career

Knowing how to containerize apps with Docker is essential for modern backend development, deployment pipelines, and cloud services.

1

Create a basic FastAPI app

Create a file called main.py with a FastAPI app named app. Add a route / that returns a dictionary with {"message": "Hello, Docker!"}.

FastAPI

# Your code here

Hint

Import FastAPI, create app = FastAPI(), then define an async function with @app.get("/") decorator returning the dictionary.

2

Add a port configuration variable

In main.py, add a variable called PORT and set it to 8000.

FastAPI

from fastapi import FastAPI

app = FastAPI()

PORT = 8000

@app.get("/")
async def read_root():
    return {"message": "Hello, Docker!"}

# Your code here

Hint

Just add PORT = 8000 anywhere in main.py.

3

Write a Dockerfile to containerize the app

Create a Dockerfile with these lines exactly: FROM python:3.12-slim, WORKDIR /app, COPY . /app, RUN pip install fastapi uvicorn, and CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"].

FastAPI

from fastapi import FastAPI

app = FastAPI()

PORT = 8000

@app.get("/")
async def read_root():
    return {"message": "Hello, Docker!"}

# Create Dockerfile with required lines

Hint

Write a Dockerfile with the exact lines to set up Python, copy files, install packages, and run the app with uvicorn.

4

Run the app inside the Docker container

Add a docker-compose.yml file with a service named fastapi-app that builds from the current directory, maps port 8000 on the host to 8000 in the container, and restarts always.

FastAPI

from fastapi import FastAPI

app = FastAPI()

PORT = 8000

@app.get("/")
async def read_root():
    return {"message": "Hello, Docker!"}

# Dockerfile content:
# FROM python:3.12-slim
# WORKDIR /app
# COPY . /app
# RUN pip install fastapi uvicorn
# CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

# Create docker-compose.yml with fastapi-app service
# Your code here

Hint

Write a docker-compose.yml file with a service named fastapi-app that builds the Dockerfile, maps port 8000, and restarts automatically.

Practice

1. What is the main purpose of using Docker with a FastAPI application?

easy

A. To package the app and its environment so it runs the same everywhere

B. To write the FastAPI code faster

C. To replace the need for Python in the app

D. To automatically generate API documentation

Solution

1. Step 1: Understand Docker's role

   Docker packages an app with all its dependencies into a container.

2. Step 2: Recognize the benefit for FastAPI

   This container runs the app the same way on any machine, avoiding environment issues.

3. Final Answer:

   To package the app and its environment so it runs the same everywhere -> Option A

4. Quick Check:

   Docker ensures consistent app environment = A [OK]

Hint: Docker ensures your app runs identically on any machine [OK]

Common Mistakes:

• Thinking Docker speeds up coding
• Believing Docker replaces Python
• Confusing Docker with API doc tools

2. Which line correctly sets the base image in a Dockerfile for a FastAPI app using Python 3.12?

easy

A. BASE python:3.12

B. FROM python:3.12-slim

C. IMAGE python:latest

D. START python:3.12

Solution

1. Step 1: Recall Dockerfile syntax for base image

   The correct keyword to set the base image is FROM followed by the image name.

2. Step 2: Match the correct Python version format

   python:3.12-slim is a valid lightweight Python 3.12 image.

3. Final Answer:

   FROM python:3.12-slim -> Option B

4. Quick Check:

   Base image line starts with FROM = A [OK]

Hint: Dockerfile base image always starts with FROM keyword [OK]

Common Mistakes:

• Using BASE or IMAGE instead of FROM
• Omitting the version tag
• Using START which is invalid

3. Given this Dockerfile snippet for FastAPI, what will happen when you run the container?

FROM python:3.12-slim
WORKDIR /app
COPY . /app
RUN pip install fastapi uvicorn
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

medium

A. The container will fail because pip install is missing requirements.txt

B. The app will start but only listen on localhost inside the container

C. The FastAPI app will start and listen on port 8000 inside the container

D. The container will build but not run the app automatically

Solution

1. Step 1: Analyze the CMD command

   The CMD runs uvicorn with host 0.0.0.0 and port 8000, which means it listens on all interfaces inside the container.

2. Step 2: Check the RUN command

   pip installs fastapi and uvicorn directly, so dependencies are present for the app to run.

3. Final Answer:

   The FastAPI app will start and listen on port 8000 inside the container -> Option C

4. Quick Check:

   CMD runs uvicorn on 0.0.0.0:8000 = B [OK]

Hint: uvicorn host 0.0.0.0 means listen on all container interfaces [OK]

Common Mistakes:

• Assuming pip install needs requirements.txt always
• Thinking 0.0.0.0 means localhost only
• Believing CMD does not run automatically

4. You wrote this Dockerfile but your FastAPI app does not start inside the container:

FROM python:3.12-slim
WORKDIR /app
COPY . /app
RUN pip install -r requirements.txt
CMD uvicorn main:app --host 0.0.0.0 --port 8000

What is the likely error?

medium

A. CMD should be a JSON array, not a plain string

B. WORKDIR should be set after COPY

C. pip install command is missing fastapi package

D. COPY command should use absolute paths

Solution

1. Step 1: Check CMD syntax

   CMD should be given as a JSON array to avoid shell parsing issues in Dockerfiles.

2. Step 2: Identify the correct CMD format

   Correct CMD is: ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

3. Final Answer:

   CMD should be a JSON array, not a plain string -> Option A

4. Quick Check:

   CMD must be JSON array for correct execution = C [OK]

Hint: Use JSON array for CMD to run commands properly in Docker [OK]

Common Mistakes:

• Writing CMD as a plain string instead of array
• Changing WORKDIR order unnecessarily
• Assuming pip install misses fastapi if requirements.txt exists

5. You want to optimize your FastAPI Docker image to be smaller and faster to build. Which approach is best?

hard

A. Use the latest Python image without specifying a version

B. Install all packages globally on your host machine before building

C. Copy the entire project including unnecessary files into the image

D. Use a multi-stage Dockerfile with a build stage and a slim final stage

Solution

1. Step 1: Understand multi-stage builds

   Multi-stage builds allow building dependencies in one stage and copying only needed files to a smaller final image.

2. Step 2: Recognize benefits for FastAPI Docker images

   This reduces image size and speeds up build by excluding build tools and caches from the final image.

3. Final Answer:

   Use a multi-stage Dockerfile with a build stage and a slim final stage -> Option D

4. Quick Check:

   Multi-stage builds optimize image size and build speed = D [OK]

Hint: Multi-stage Dockerfiles keep images small and fast to build [OK]

Common Mistakes:

• Installing packages on host instead of inside container
• Copying unnecessary files bloats image
• Using latest tag risks unexpected changes