System design interview question · medium · fan-out · queues
Notification service
Design a service that sends push, email and in-app notifications when events happen, such as a reply to a shared conversation, a completed task, or a billing alert.
Practice it as a 50-minute mock interview
- Requirements & scope (7 min): Think like a product manager, translate design question to technical requirements, narrow the scope and write the non-functional requirements as numbers.
- Estimation (3 min): Size the system: estimate the scale it must handle, find the number that drives the design, and name the first bottleneck.
- API & data model (5 min): Write concrete endpoints and entities, and pick stores from access patterns.
- High-level architecture & Data flow (10 min): Start with the simplest design that works and trace a request end to end.
- Deep dives (20 min): Go deep on the 2–3 hardest parts, with alternatives and a reasoned choice.
- Failures, ops & evolution (5 min): Walk each component: what if it dies, slows down, or is overloaded?