Background file processing in FastAPI - Mini Project: Build & Apply

Background File Processing with FastAPI

📖 Scenario: You are building a simple web service that accepts file uploads. To keep the service fast and responsive, you want to process the uploaded files in the background without making the user wait.

🎯 Goal: Create a FastAPI app that accepts a file upload and processes the file content in the background using FastAPI's BackgroundTasks. The app should immediately respond to the user while the file processing happens asynchronously.

📋 What You'll Learn

Create a FastAPI app instance named app

Create a POST endpoint /upload/ that accepts a file upload with parameter file of type UploadFile

Use BackgroundTasks to run a function called process_file in the background

The process_file function should read the file content and simulate processing by writing the content length to a file named processed.txt

Return a JSON response immediately confirming the file upload

💡 Why This Matters

🌍 Real World

Web applications often need to handle file uploads without making users wait for long processing times. Background file processing allows the app to stay responsive while handling heavy tasks.

💼 Career

Understanding background tasks in FastAPI is valuable for backend developers building scalable and user-friendly APIs that handle file uploads or other time-consuming operations.

1

Set up FastAPI app and import modules

Import FastAPI, UploadFile, and BackgroundTasks from fastapi. Create a FastAPI app instance called app.

FastAPI

from fastapi import FastAPI, UploadFile, BackgroundTasks

# Create FastAPI app instance
# Your code here

Need a hint?

Use app = FastAPI() to create the app instance.

2

Create the background processing function

Define a function called process_file that takes one parameter file of type UploadFile. Inside the function, read the file content asynchronously using await file.read() and write the length of the content to a file named processed.txt.

FastAPI

from fastapi import FastAPI, UploadFile, BackgroundTasks

app = FastAPI()

# Define background file processing function
# Your code here

Need a hint?

Use async def and await file.read() to read the file content asynchronously.

3

Create the file upload endpoint with background task

Create a POST endpoint /upload/ using @app.post. The endpoint function called upload_file should accept parameters: file of type UploadFile and background_tasks of type BackgroundTasks. Inside the function, add the background task to call process_file with the uploaded file. Return a JSON response with key message and value "File upload received".

FastAPI

from fastapi import FastAPI, UploadFile, BackgroundTasks

app = FastAPI()

async def process_file(file: UploadFile):
    content = await file.read()
    with open('processed.txt', 'w') as f:
        f.write(f'File content length: {len(content)}')

# Create POST endpoint /upload/ with background task
# Your code here

Need a hint?

Use background_tasks.add_task(process_file, file) to run the file processing in the background.

4

Add file upload parameter type and run the app

Ensure the file parameter in the upload_file endpoint is annotated as UploadFile. Add the necessary import for UploadFile if missing. This completes the FastAPI app for background file processing.

FastAPI

from fastapi import FastAPI, UploadFile, BackgroundTasks

app = FastAPI()

async def process_file(file: UploadFile):
    content = await file.read()
    with open('processed.txt', 'w') as f:
        f.write(f'File content length: {len(content)}')

@app.post('/upload/')
async def upload_file(file: UploadFile, background_tasks: BackgroundTasks):
    background_tasks.add_task(process_file, file)
    return {"message": "File upload received"}

# Confirm UploadFile type annotation and imports
# Your code here

Need a hint?

Make sure the file parameter is typed as UploadFile for FastAPI to handle file uploads correctly.