How to Explain System Design in English

The six-beat spoken outline

1. 1. Requirements

   Who the user is, and what must work on day one.

   Clarify identity, write vs read, and one thing you are not building yet.

2. 2. Scale

   One rough number you can say out loud.

   QPS, storage, or “millions of keys.” The number can be rough. The sentence cannot be missing.

3. 3. API

   Name the two or three calls.

   “I would expose allow(userId) and a 429 when they are over the limit.”

4. 4. Data

   Where the state lives.

   Cache, database, or log. Say why that store, not a tool dump.

5. 5. Bottleneck

   What breaks first when traffic grows.

   Hot key, single leader, clock skew. Invite a follow-up.

6. 6. Trade-off

   What you gain and what you give up.

   Accuracy vs speed, consistency vs availability. One pair is enough.

Practise a prompt

Pick a classic prompt, talk for 60–90 seconds, and get a scorecard on that outline. You do not need to draw.

• Spoken outline

  Rate limiter

  A rate limiter is a “who, how many, what happens at the limit” story. Clarify identity, pick token bucket or a window, place it before expensive work, then name fairness.

• Spoken outline

  URL shortener

  A URL shortener is two paths: write a long URL, redirect a short code. Name how you mint the ID, where you store the map, and what you do on a collision.

• Spoken outline

  Unique ID generator

  Unique IDs are a “unique, roughly ordered, and cheap to mint” story. Name a Snowflake-style split, then clock skew — that is the follow-up they will ask.