System design interview practice sheet

The 45-minute plan

1. 5 min

   Clarify the problem

   Agree on users, requirements, quality targets and the assumptions that shape the design.

2. 5 min

   Estimate the load

   Calculate average and peak requests, data growth and the read-to-write ratio.

3. 10 min

   Draw the main paths

   Show services, stores, caches and queues. Trace one request and one write.

4. 15 min

   Choose one hard part

   Compare two plausible designs for the hardest constraint and name the cost you accept.

5. 5 min

   Break and change it

   Trace a crash, duplicate, timeout or spike. Then adapt to a changed requirement.

6. 5 min

   State the guarantees

   Explain what the design promises, where it can fail and what you would improve next.

Quick sizing formulas

• Average requests per second = requests per day divided by 86,400.
• Peak requests per second = average requests per second multiplied by the peak factor.
• Raw storage per year = writes per day multiplied by bytes per record and 365.

For example, 10 million requests a day is about 116 requests per second on average. At a five-times peak, plan for about 580 requests per second.

Five questions to practise

• 1.

  Design a URL shortener. Identifiers, read-heavy traffic, cache invalidation and analytics.

• 2.

  Design an API rate limiter. Burst handling, shared counters and failure policy.

• 3.

  Design a news feed. Fan-out, celebrity accounts, inactive readers and deletes.

• 4.

  Design a payment system. Retries, idempotency and out-of-order provider events.

• 5.

  Design a distributed job queue. Durable work, duplicate delivery, worker crashes and backlogs.

Work through each as an interview, then compare your decisions with a complete explanation. The exercises are free and work without an account.