How to Prepare for a System Design Interview (Free Plan)

A practical, free plan to learn system design and prepare for system design interviews, plus weekly live practice sessions with volunteer engineers.

System design interviews are open ended on purpose. There is rarely one right answer. Interviewers want to see how you clarify a vague problem, make tradeoffs and explain them clearly. The good news: that is a learnable skill, and you do not need a paid course to learn it.

Start with our most-watched session (38,000+ views): the key concepts you need for system design interviews.

1. Learn the building blocks first

Before practicing full designs, get comfortable with the pieces most designs are made of:

  • Load balancers and how traffic is spread across servers
  • Caching (where to cache, what to cache, cache invalidation)
  • Databases: SQL vs NoSQL, indexes, replication, sharding and partitioning
  • Queues and streams for asynchronous work
  • CDNs and object storage for static and large content
  • Consistency vs availability tradeoffs (CAP, eventual consistency)
  • Rate limiting, idempotency and retries

For each one, be able to say what problem it solves, what it costs and when you would not use it.

2. Use a repeatable structure in every answer

A simple flow keeps you from getting lost in a 45-minute interview:

  1. Clarify requirements. Functional (what it does) and non-functional (scale, latency, availability).
  2. Estimate scale. Users, requests per second, storage. Rough numbers are fine.
  3. Define the API and the core data model.
  4. Draw the high-level design. Clients, services, data stores, and how requests flow.
  5. Deep dive into one or two parts the interviewer cares about.
  6. Discuss bottlenecks and tradeoffs. What breaks first at 10x scale, and what would you change?

3. Practice classic problems out loud

Work through common prompts such as a URL shortener, a news feed, a chat system, a rate limiter, a notification service and a file storage service. Practice talking, not just drawing. Explaining tradeoffs clearly is most of the score.

Watch a real mock interview

A full system design mock interview from an SDE Skills session: designing an API rate limiter, followed by feedback from the interviewer and other engineers.

4. Get feedback from other engineers

Reading about design only gets you so far. The fastest improvement comes from presenting a design and having experienced engineers poke holes in it.

SDE Skills, a volunteer-run 501(c)(3) nonprofit, hosts a free system design session every Saturday at 9:00 AM Pacific on Zoom. Engineers discuss real design problems and run mock interviews together. See the full schedule.

Join Saturday's session on Zoom Join our free Discord

A simple 4-week plan

Week Focus
1 Building blocks: caching, databases, load balancing, queues
2 The 6-step answer structure on 2 easy prompts (URL shortener, rate limiter)
3 Bigger prompts (news feed, chat) and estimating scale
4 Mock interviews with feedback; fix your weakest step

Keep practicing algorithms alongside design. See our LeetCode practice guide.