Design Mobile Push Notifications: APNs, FCM, and Background Wakeup

Updated July 26, 2026 · techinterview.org

Push notifications look simple from the user side — a banner appears, you tap it, the app opens. From an engineering perspective, designing a push system at scale (millions of users, billions of notifications per day) is a classic mobile system design question because it touches three platforms (iOS, Android, web), several backend services, and serious correctness concerns around dedup, ordering, and delivery guarantees.

Functional requirements

• Send notifications to specific users on iOS and Android. This is the targeted case: resolve a user ID to that user’s active device tokens and send only to those. Interviewers probe how you keep the token-to-device mapping fresh when someone logs in on multiple devices, reinstalls, or switches phones.
• Support broadcast (e.g., “new episode of your podcast”). A single logical send fans out to millions of tokens, so the real question is how you avoid a thundering herd — batch the token list, spread the work across many workers, and stagger delivery instead of hammering APNs/FCM in one burst.
• Personalized content (badge counts, custom images). Badge counts have to be computed per user at send time, and rich images are pulled by the client through a notification service extension on iOS or by URL in the FCM payload. Mind the APNs payload limit (4KB) — put media behind a URL, never inline.
• Silent / data-only pushes that wake the app for background sync. These wake the app without showing a banner, typically to prefetch data before the user opens it. Interviewers want to hear that you throttle them, because iOS budgets silent pushes and will drop or delay them when you send too many.
• Delivery receipt where possible. True end-to-end receipts require the client to ack back to your server on receipt, since neither APNs nor FCM confirms per-message delivery. Be ready to explain that “accepted by APNs” is not the same as “shown to the user.”

Architecture

Three layers: application (your service), fanout, delivery (APNs, FCM, web push).

Device registration

On launch, the app requests a push token from the OS (APNs token on iOS, FCM token on Android). The app uploads the token + user ID + device metadata to your backend.

Tokens rotate. Always re-upload on app launch. Server should de-dupe and update the token-to-user mapping.

Fanout pipeline

When a notification needs to be sent (e.g., user A gets a new follower), the producer publishes to Kafka. A fanout worker resolves user A → list of devices → APNs/FCM payloads → delivery service.

Fanout is the bottleneck for broadcasts. Pre-compute target user lists where possible.

Delivery service

Maintains long-lived HTTP/2 connections to APNs and FCM. Throttles per-token to respect platform limits. Handles failures: invalid tokens are removed, rate-limit responses are retried with backoff, server errors are queued.

Deduplication

Each notification has a server-generated ID. APNs supports apns-collapse-id to coalesce. The client also dedupes on receive using a recent-IDs cache.

Silent push for background sync

iOS: content-available: 1 in the payload, app receives it via application(_:didReceiveRemoteNotification:fetchCompletionHandler:) and has ~30 seconds to do background work.

Android: data-only FCM messages route to the FirebaseMessagingService. Same ~30-second budget unless using high-priority data messages.

Correctness concerns

• Order: APNs and FCM do not guarantee ordering. If order matters, embed a sequence number.
• Dedupe: client and server both must dedupe. Network retries can deliver the same push twice.
• Battery drain: excessive silent pushes will get your app throttled by the OS.

Quiet hours and rate limits

Server-side per-user policy: do not send between 10pm and 7am local time. Server-side rate limit: at most N notifications per user per hour to avoid spamming.

Frequently Asked Questions

What happens if the user is offline?

APNs and FCM hold the most recent notification (with collapse ID) and deliver when the device reconnects. Anything older with the same collapse ID is dropped.

How do I get a delivery receipt?

APNs offers feedback for invalid tokens but no per-message receipt. FCM has limited support. For real receipts, you need the app to confirm receipt back to your server.

Can I send a notification to a user without their consent?

No. Both iOS and Android require user permission for visible notifications. Silent pushes do not require permission but are subject to OS budgeting.