File Uploads & Background Tasks: File Handling, Celery, Job Processing

🎯 Learning Objectives

• Implement secure file upload with validation
• Process uploaded files in the background with Celery
• Track job status for long-running tasks
• Handle concurrent uploads at scale

📖 Core Content

1.1 Secure File Upload

python
Run Inline
# runnable
import os
from flask import request, jsonify
from werkzeug.utils import secure_filename
ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg', 'pdf'}
UPLOAD_FOLDER = '/app/uploads'
def allowed_file(filename):
    return '.' in filename and \
           filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS
@app.route('/upload', methods=['POST'])
def upload_file():
    if 'file' not in request.files:
        return jsonify({"error": "No file"}), 400
    file = request.files['file']
    if file.filename == '' or not allowed_file(file.filename):
        return jsonify({"error": "Invalid file"}), 400
    # Secure the filename and save
    filename = secure_filename(file.filename)
    filepath = os.path.join(UPLOAD_FOLDER, filename)
    file.save(filepath)
    return jsonify({"message": "File uploaded", "filename": filename})

1.2 Background Processing Flow

1.3 Why This Matters

2. 📝 Practice Questions

Q1: 100 users simultaneously upload 10MB images. The Flask server times out. Describe the solution using background tasks.

Problem: Flask's synchronous request handling blocks until the file is fully saved. With 100 concurrent uploads, the server runs out of worker threads.

Solution:

1. Accept upload quickly: Save the file to disk, return 202 Accepted immediately
2. Queue processing: Enqueue a Celery task for image processing (e.g., resize, thumbnail generation)
3. Return a task ID: The client receives {"task_id": "abc123"} immediately
4. Client polls status: The frontend polls /status/abc123 to check progress
5. Webhook notification (optional): The Celery task sends a webhook when done

This keeps the web server responsive regardless of processing time. Multiple Celery workers process tasks concurrently from the Redis queue. Join Discord Previous7.1 Authentication & AuthorizationNextTesting