How would you design a notification system for a mobile app?

Components: notification service (create/schedule), delivery workers (by channel), device token management, preference service. Channels: push (FCM/APNS), email, SMS, in-app. Design: event triggers notification, queue for processing, fan-out to channels, track delivery/opens. Challenges: high volume (batch, rate limit), device token management (refresh, multiple devices), preferences (user opt-outs), templating, scheduling. Scale: partition by user, horizontal workers, prioritize real-time vs batch. Analytics: delivery rates, open rates, engagement.