Went in thinking I'd cover the basics and move on but they kept pushing on scale, delivery guarantees, and how you handle things like rate limiting per user or per channel.
Start by clarifying requirements and scale, then design a high-level architecture covering ingestion, processing, and delivery. Dive into key components like message queues, fan-out, and delivery guarantees, and discuss trade-offs such as consistency vs. availability and push vs. pull.
Pro tip: Emphasize idempotency and deduplication to handle retries and ensure exactly-once delivery, and discuss how to prioritize notifications (e.g., critical vs. promotional) to avoid overwhelming users.
Ask about scale (users, notifications per second), types (email, push, SMS), latency, delivery guarantees, and priorities. Define functional and non-functional requirements.
Sketch components: API gateway, notification service, message queue, workers, and external providers. Explain data flow from ingestion to delivery.
Detail how to handle fan-out (e.g., for a user with many followers), use queues for decoupling, and manage retries with exponential backoff. Discuss storage for user preferences and notification history.
Discuss trade-offs: push vs. pull, synchronous vs. asynchronous, at-least-once vs. exactly-once. Explain scaling strategies like sharding, partitioning, and rate limiting.
Mention monitoring, alerting, and metrics. Suggest extensions like A/B testing, personalization, or analytics.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.