Scaling Flask for High-Concurrency I/O: From Thread Starvation to Async Mastery

Scaling Flask for High-Concurrency I/O: From Thread Starvation to Async Mastery The Queue That Never Empties You've built a solid Flask application. It works great for your typical request-response cycle. But then you hit that dreaded moment: your application gets 50 simultaneous requests, each waiting on an external API call, and suddenly everything grinds to a halt. New requests pile up in a queue, users see timeouts, and you're left wondering why Node.js applications seem to handle this scenario effortlessly. I've been there, and I know the frustration. The fundamental issue isn't that Flask is broken—it's that the traditional WSGI model, which Flask uses, wasn't designed for this exact problem. Let me walk you through what's happening under the hood and show you practical solutions that work with your existing codebase. Understanding the Root Cause: WSGI's Threading Limitation Here's the core issue: Flask runs on WSGI, which uses a thread-per-request model by default. When you run