Flask Hit 502s at 300 Users — FastAPI vs Flask vs Django, Honestly

Under 300 concurrent users, Flask's sync workers blocked on I/O, latency went 50ms → 12s → 502, and the thread pool was exhausted. Here's an honest FastAPI vs Flask vs Django comparison — data contracts, concurrency models, dev velocity, deployment, the job market, and exactly when it's worth migrating.

⏳ Timestamps:
0:00 - Cold open: latency spiked from 50ms to 12s, then 502s after ~300 simultaneous users
0:20 - Intro
0:28 - What Is FastAPI?
0:46 - What Is Flask?
1:01 - What Is Django?
1:16 - Data Contracts
1:46 - Flask Contract
1:58 - Django DRF Contract
2:10 - Concurrency Models
2:29 - Developer Velocity
2:45 - Deployment Differences
2:59 - When to Migrate
3:12 - Production Reality Check
3:34 - Simple Flask Example
3:42 - Learning Curve
4:09 - Django Auth Example
4:18 - Use Cases
4:44 - Job Prospects
5:08 - latency spiked from 50ms to 12s, then 502s after ~300 simultaneous users
5:27 - The Fix
5:44 - ⚠ Gotcha: Using sync DB drivers with FastAPI
5:59 - ⚠ Gotcha: Choosing Flask for an enterprise app
6:11 - ⚠ Gotcha: Using Django for a simple API
6:25 - Version Compatibility: FastAPI 0.100+
7:06 - Production Caveat: FastAPI Background Tasks
7:10 - Debugging Guide
7:25 - Interview Questions
7:34 - FAQ
7:51 - Key Takeaways
8:09 - Next up
8:25 - Wrap-up

👉 Full article + code: https://thecodeforge.io/python/fastapi-vs-flask-vs-django/
⏭ Next up: FastAPI Error Handling and Custom Exception Handlers

#FastAPI #Python

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