Docker containerization in FastAPI - Step-by-Step Execution

Concept Flow - Docker containerization

Write FastAPI app code

↓

Create Dockerfile

↓

Build Docker image

↓

Run Docker container

↓

App runs inside container

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Access app via browser or API client

This flow shows how you write a FastAPI app, create a Dockerfile, build an image, run a container, and then access the app.

Execution Sample

FastAPI

FROM python:3.12-slim
WORKDIR /app
COPY requirements.txt ./
RUN pip install -r requirements.txt
COPY . .
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "80"]

This Dockerfile sets up a FastAPI app environment, installs dependencies, copies code, and runs the app with Uvicorn.

Execution Table

Step	Action	Details	Result
1	Read Dockerfile	Start parsing Dockerfile instructions	Ready to build image
2	FROM python:3.12-slim	Base image with Python 3.12	Base image pulled or cached
3	WORKDIR /app	Set working directory inside container	Working directory set to /app
4	COPY requirements.txt ./	Copy requirements file to /app	requirements.txt available in /app
5	RUN pip install -r requirements.txt	Install Python dependencies	Dependencies installed
6	COPY . .	Copy all app files to /app	App source code copied
7	CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "80"]	Set container start command	Container will run FastAPI app on port 80
8	Build image	Docker builds image with above steps	Image created successfully
9	Run container	Start container from image	FastAPI app running inside container
10	Access app	Open browser or API client to container IP:80	FastAPI app responds to requests
11	Exit	Stop container or close app	Container stops, app no longer running

💡 Container stops or user closes app, ending execution

Variable Tracker

Variable	Start	After Step 4	After Step 6	After Step 9	Final
Working Directory	None	/app	/app	/app	/app
requirements.txt	Not present	Copied	Copied	Copied	Copied
Dependencies Installed	No	No	Yes	Yes	Yes
App Source Code	Not present	Not present	Copied	Copied	Copied
Container State	Not running	Not running	Not running	Running	Stopped

Key Moments - 3 Insights

Why do we set WORKDIR before copying files?

WORKDIR sets the folder where commands run and files copy. See execution_table step 3 and 4 where files copy into /app after setting WORKDIR.

What happens if dependencies are not installed before running the app?

The app will fail to start because required packages are missing. Step 5 installs dependencies before copying app code and running it (steps 6 and 7).

Why do we expose the app on 0.0.0.0 instead of localhost?

0.0.0.0 allows the app to accept connections from outside the container. Step 7 CMD uses this so the app is reachable from the host machine.

Visual Quiz - 3 Questions

Look at the execution_table, what is the working directory after step 4?

A /app

B /

C /root

D Not set yet

💡 Hint

Check the 'Working Directory' variable in variable_tracker after step 4

At which step does the container start running the FastAPI app?

A Step 7

B Step 9

C Step 8

D Step 10

💡 Hint

Look at execution_table rows describing container state and app running

If we skip the RUN pip install step, what will happen when running the container?

A App runs normally

B Container fails to build

C App fails due to missing dependencies

D App runs but with errors logged

💡 Hint

Refer to key_moments about dependency installation importance

Concept Snapshot

Docker containerization with FastAPI:
- Write FastAPI app code
- Create Dockerfile with base image, WORKDIR, copy files
- RUN installs dependencies
- CMD runs app with uvicorn on 0.0.0.0:80
- Build image and run container
- Access app via container IP and port
- Container isolates app environment for easy deployment

Full Transcript

Docker containerization for FastAPI involves writing your app code, then creating a Dockerfile that sets a Python base image, sets the working directory, copies your requirements and app files, installs dependencies, and finally runs the app with Uvicorn. The Docker build process reads these instructions step-by-step, creating an image. Running a container from this image starts the FastAPI app inside an isolated environment. You can then access the app through the container's IP and port. This method ensures your app runs the same everywhere, simplifying deployment and scaling.