Architecture and delivery trade-offs of an app push notification pipeline

As a product manager, walk through the technical components needed to build a push notification system for an app, and the main considerations the team must get right. ```hint Follow one notification end to end Trace a single message from the event that triggers it to the moment the user taps it and lands in the app. Each hop along the way is a component, and each one can fail in its own way. ``` ### Clarifying Questions - Which platforms must the first version reach: iOS, Android, web browsers, or all three? - Will the system carry time-sensitive transactional alerts, bulk promotional campaigns, or both? - Are we building the sending layer ourselves on top of the platform push services, or evaluating a third-party messaging provider? - What scale should we plan for, in registered devices and in the largest single send? ### What a Strong Answer Covers - The delivery path: device registration and tokens, a sending service, the platform push gateways and the app's handling of the notification - Permission, opt-in and per-category preference management under each operating system's rules - Targeting, content templates, scheduling and frequency control, with transactional traffic kept apart from marketing traffic - Reliability at scale: queues, retries, priority, expiry, idempotency and cleanup of dead tokens - Security and privacy of payloads and credentials, plus delivery and engagement metrics with guardrails ### Follow-up Questions - A time-sensitive security alert must reach users within seconds while a campaign is going out to millions of devices. How would you keep one from slowing the other? - Push delivery is best-effort. What would you do for a message the user must not miss? - How would you set the cap on promotional pushes per user, and what data would tell you the cap is wrong?