Background Sync, Push & Silent Notifications — Mobile API Best Practices

Mobile OSes limit background work. To keep data fresh: send push notifications (APNs/FCM) for user-visible events, silent/data pushes to hint "something changed — sync" (throttled and not guaranteed), and schedule periodic background tasks (WorkManager, BGTaskScheduler) the OS runs opportunistically. Always do a sync on app foreground. Push payloads should be small hints, not data, and never contain secrets.

Freshness mechanisms

Mechanism	Delivery	Use for	Limits
Visible push (APNs/FCM)	Best effort, usually seconds	Messages, alerts	User permission required
Silent / data push	Throttled, may be dropped	'Sync now' hints	OS budgets; app killed by user → none (iOS)
Periodic background task	OS-scheduled	Prefetch, cleanup	Minimum intervals; not exact
Foreground sync	When opened	Guaranteed freshness	Only while in use
Persistent socket	Real-time	Active chat/calls	Foreground only (except VoIP/special)

POST https://fcm.googleapis.com/v1/projects/my-app/messages:send
{
  "message": {
    "token": "<device registration token>",
    "data": {"kind": "sync", "scope": "tasks", "hint_seq": "124487"},
    "android": {"priority": "normal", "collapse_key": "sync-tasks"},
    "apns": {"headers": {"apns-push-type": "background", "apns-priority": "5"},
             "payload": {"aps": {"content-available": 1}}}
  }
}

async def send_push(device: Device, message: dict) -> None:
    try:
        await fcm.send(device.token, message)
    except UnregisteredTokenError:
        await devices.delete(device.id)          # app uninstalled or token rotated
    except QuotaExceededError:
        await requeue_with_backoff(device, message)

Common pitfalls

Relying on silent push for correctness.
Sensitive data in push payloads (shown on lock screens, stored by providers).
Never pruning dead device tokens.

Key points

Push = hint, sync = truth
Sync on foreground
Prune invalid tokens

Further reading: Firebase Cloud Messaging: message types

Practice questions

Why shouldn't you rely on silent pushes to keep data correct?

Answer

OSes throttle or drop them (battery budgets, force-quit apps), so they're only hints; correctness comes from sync on launch/foreground and periodic sync.

What should a server do when FCM/APNs reports a token as unregistered?

Answer

Delete or deactivate that device token so you stop sending to it and keep delivery metrics meaningful.