Design Push Notification System

Mobile Engagement Mandate: Push at Planet Scale

We are designing a push notification platform: applications hand us a message and a target, and we deliver a notification to the right devices across iOS and Android. The fact that defines the architecture is that we do not own the last hop. The actual delivery to a device goes through Apple Push Notification service (APNs) and Firebase Cloud Messaging (FCM); our platform registers device tokens, fans a message out to the right devices, and hands each one to the right provider, but the provider decides whether and when it reaches the phone. So our job is everything up to the provider: token lifecycle, targeting, fan-out, prioritization, and reliable hand-off, plus consuming the provider's feedback to keep tokens healthy.

The spine is therefore: register and maintain device tokens, accept transactional and marketing sends, fan a send out to the matching devices, dispatch each through a pooled connection to APNs or FCM with the right priority, and process provider feedback (delivery receipts, invalid-token signals) to prune dead tokens. State this framing up front, because every decision, prioritization, token hygiene, backpressure, follows from the constraint that the platform delivers to the provider, not to the device.