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/fastap... ⏭ Next up: FastAPI Error Handling and Custom Exception Handlers #FastAPI #Python