System Design Interview Practice: A Structure That Works

System design interviews are open-ended on purpose. There is no single correct architecture, so the interviewer is judging how you reason: what you ask, what you prioritise, and whether you can explain the cost of each decision. That makes them hard to practise by reading alone. You need to say your reasoning out loud and have someone push back.

A structure for a 45-minute design interview

1. Clarify requirements (5 minutes). Who uses the system, what are the core actions, and which non-functional requirements matter most: latency, consistency, availability or cost?
2. Estimate scale (3 minutes). Rough numbers for users, requests per second and data size. The point is to decide whether one database is enough, not to be exact.
3. Sketch the high-level design (10 minutes). Clients, API, services, storage and any queues or caches. Name the main data entities and the API for the core actions.
4. Go deep on one or two parts (15 minutes). Pick the hardest part, such as the write path, the data model or a consistency problem, and work through it in detail.
5. Discuss failure and growth (7 minutes). What happens when a component fails, what breaks first at ten times the load, and how you would monitor it.
6. Summarise (2 minutes). Restate the design and the main trade-offs you accepted.

Common mistakes

• Drawing before asking. Jumping to boxes and arrows without requirements leads to designing the wrong system.
• Naming technologies without reasons. "We use Kafka" is not an answer. "We need durable, ordered events per user, so a partitioned log fits" is.
• Staying shallow everywhere. Interviewers learn more from one component discussed in depth than from ten listed.
• Ignoring failure. Retries, duplicate messages and partial writes are where real systems break and where strong candidates stand out.
• Not checking in. Ask "Should I go deeper on storage or on the read path?" so the interviewer can steer you to what they want to assess.

Prompts to practise

• Design a URL shortener with analytics.
• Design a rate limiter for a public API.
• Design a notification service that sends email and push messages.
• Design the backend for a ride-sharing app's trip matching.
• Design a file storage and sharing service.
• Design a news feed for a social app.

For each prompt, practise the full structure above out loud with a timer. Then pick one part and prepare to answer three follow-up questions on it.

Connect design questions to your own work

Many interviewers ask design questions about systems on your resume rather than textbook prompts: "How would you redesign the service you built if traffic grew ten times?" Prepare this for your main project. It is often easier to show depth on something you actually built.

Practise with follow-ups

Reading design guides helps you learn patterns; it does not teach you to answer under questioning. In a MockInterview AI interview for a software role, you get a system design question based on your target job, follow-up questions on your trade-offs, and a report that quotes your answers. See also common software engineer interview questions and the backend engineer practice page.