Designing a real-time notification platform

One Friday evening, five failures, and the platform that prevents them: the two halves (providers and a real-time tier), FERNS and the requirements, five classes of notification, why one queue makes an OTP 8 minutes late, the estimate where SMS is 5% of messages and about 89% of the bill, the seven pieces of the architecture, one payment traced from the Send API to Priya's screen, the five records, the channel trade-offs, and how LinkedIn, Uber and Slack split the same problem.