Notification System

Notification systems look deceptively simple: call an API, send a push. The hard part shows up at the edges. One event, like a breaking-news alert, can need to reach millions of people within seconds. The third-party providers you depend on (Apple's APNs, Google's FCM, an SMS gateway, an email service) throttle you the moment you push hard. Your own sender processes crash mid-blast. And every recipient has preferences, opt-outs and quiet hours you must honor, while still guaranteeing that the password-reset code actually arrives. Get any of these wrong and you either drop important messages or spam people into unsubscribing. This guide walks through the design the way you would in a FAANG interview, and it is tuned to the SysGym "Notification System" simulator: a 35-second run that ramps offered load from 10,000 to 18,000 events per second, kills sender workers twice, and throttles a provider, all while grading you on availability (deliver at least 97 percent), sustained throughput (at least 10,000 per second), peak saturation (keep the hottest component at or below 90 percent) and cost (a footprint of 12 or fewer instances). Every number below is chosen so the reasoning and the gym reinforce each other.