System Design Interview Playbook
System Design means planning a large software system — how all the pieces fit together to serve many users. In this lesson we practice, step by step, how to approach an interview question on the topic. We will learn to define requirements (what the system must do), scope (what is and isn't included)
System Design is like planning a whole city: roads that carry the traffic, warehouses that store things, traffic lights that keep the flow orderly, and maintenance crews that fix problems — all so the city keeps running smoothly even at rush hour, when everyone is out at once.
- Requirements and scope
- The core skill of this lesson: before designing a system, you clarify what it must do (requirements), what is and isn't included (scope), what assumptions you're making, and how to structure a clear answer. It's like taking the full order at a restaurant before you start cooking.
- Trade-off
- A conscious choice between two options, where each has an upside but also a price. There's no perfect answer — you pick what fits and explain to the interviewer what you gain and what you give up. Like choosing between a fast, pricey route and a cheap, slow one.
- Operational metric
- A number you can measure to tell whether a decision actually works once the system is live and serving real users (production). For example: latency (how long a response takes to come back), error rate (the share of requests that fail), queue lag (how many tasks are waiting in line), cache hit ratio (how often we found the answer ready in a fast temporary store), and more.
