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System Design Interview Guide: A Practical Approach

A system design interview asks you to design a service, such as a URL shortener or a chat app, in about an hour. There is no single right answer. Interviewers look at how you structure the problem and reason about trade-offs.

Step 1: Clarify requirements

Ask what the system must do (functional requirements) and how well it must do it (availability, latency, consistency, scale). Agree on what is out of scope so you can focus.

Step 2: Estimate the scale

Make rough, labeled assumptions about users, requests per second and data size. The goal is not precision. It is to learn whether you need one database or many, and whether reads or writes dominate.

Step 3: Sketch the high-level design

Draw the main components: clients, load balancer, application servers, database, cache, queue and storage as needed. Walk through one or two main requests from start to finish.

Step 4: Go deeper where it matters

Pick the hardest parts and discuss them: data model, indexing, partitioning, caching strategy, consistency, handling failures. Explain why you chose one option over another.

Step 5: Discuss trade-offs and growth

Name the weaknesses of your design and how you would monitor and improve it. Saying what you would change at ten times the load shows maturity.

How to practice

Study a handful of classic designs, then practice out loud with a friend or a timer. Read about how real systems handle caching, queues and databases. Related guides cover high-level design and low-level design in more detail.

Frequently asked questions

Do I need to have built large systems?

No. Reason from fundamentals and from systems you have worked on, and be honest about what you have and have not done.

Is drawing required?

A simple diagram helps you and the interviewer follow the design. Keep it clear rather than detailed.

Put this into practice.

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