Question

A Python web app (gunicorn with sync workers, 4 workers × 8 threads = 32 concurrent slots) fronts three downstream services: auth, billing, and recommendations. At 18:00 the recommendations service (a non-critical sidebar widget) starts taking 10s per call instead of 50ms. Within minutes your *entire* app — including login and checkout, which don't even touch recommendations — becomes unresponsive and starts timing out. Recommendations is only ~5% of your traffic. Triage and explain why a slow non-critical dependency took down everything.