Interview round · in depth

System design interview questions: make every trade-off explicit

Practice system design interviews by making your assumptions explicit: requirements, rough capacity, data model and APIs, then the bottleneck you would fix first and why.

Practice this interview Sample questions

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Prep sheetSystem design

01Requirements and constraints
02Capacity estimates
03Data models and APIs
04Failure and consistency

Next repTechnical interview lab

01Common prompts

Sample system design interview questions

These system design interview questions are practice prompts written or adapted by InterviewWare, not questions reported by any employer. Each comes with the signals an interviewer listens for and the follow-up that usually comes next.

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Question	What the interviewer listens for	Likely follow-up
R2 · Role depth & case
Q01Experienced Design a rate limiter for a distributed API.	Algorithms · consistency · failure behavior	What happens when the counter store is unavailable?
Q02Experienced Design a link-sharing service that creates short links and reports click counts. Start from requirements.	Requirements · data modeling · scalability	How would you prevent collisions and abuse?
Q03Senior / lead Design a feature-flag system that lets teams roll out changes gradually and roll back quickly.	Rollout safety · consistency · operability	How should flag checks behave when the configuration service is down?
Q04 Design a notification service that retries failed sends without delivering duplicates.	An idempotency key per message, at-least-once delivery made safe by de-duplication, and a dead-letter path for messages that keep failing.	The email provider accepted the message but your acknowledgment was lost. What happens next?
Q05 How would you scale a read-heavy product catalog?	The read/write ratio stated first; caching and read replicas with an explicit freshness budget; search served from a separate index.	A price change must appear everywhere within a minute. What does that do to your cache design?
Q06 Design a scheduler that runs millions of delayed jobs, such as “remind me in three days”.	How jobs are stored and found when due (time-bucketed tables or a sorted set), how workers claim a job exactly once, and what happens to jobs that fall due during an outage.	Ten thousand jobs fall due in the same second. How do you avoid a thundering herd?
Q07 Design a real-time leaderboard for a mobile game with a large daily player base.	A sorted structure for the top N, an approximate or batched approach for exact ranks far down the list, and a clear write path for score updates.	How would you run weekly leaderboards that reset without downtime?
Q08 Design a pipeline that collects events from many services and feeds a dashboard at most a minute behind.	A durable queue as the buffer, windowed aggregation, late and duplicate events, and what the dashboard shows when the pipeline lags.	A bug double-counted events for two hours. How do you backfill the correct numbers?
Q09 Design message storage and read receipts for group chats of up to a few hundred people.	Ordering within a conversation, a partition key that keeps a chat’s messages together, and read receipts stored as a per-member watermark instead of a row per message.	A user opens the app after a week offline. How do they catch up efficiently?

02Try one question

Answer a system design interview question

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Practice question · System design

Design a rate limiter for a distributed API.

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03Worked example

Design a “recently viewed items” list for an online shop.

Senior-level outline of the first ten minutes of a 45-minute round. Illustrative sample written by InterviewWare, not a real candidate’s answer.

01
Requirements
Each signed-in shopper sees their last 20 viewed items on every device, updated within a few seconds.
02
Estimate
If views outnumber purchases about ten to one, writes dominate, so I would keep the write path cheap.
03
Design
View events go onto a queue; a worker keeps a capped list per user in a key-value store, trimmed to 20.
04
Trade-off
I’m accepting a few seconds of delay to protect page latency; if it had to be instant, I would write the list synchronously and pay a higher write cost.

04Coaching notes

How to answer system design interview questions

What works

T1Spend the first five minutes on requirements: who uses it, the main read and write paths, and what “fast” and “correct” mean here.
T2Do the back-of-the-envelope maths out loud — requests per second, storage per year — and round generously.
T3Draw one end-to-end path that works before adding caches, queues or shards.
T4For every component you add, say what happens when it fails.
T5Close by naming the trade-off you made and the load at which you would revisit it.

Common mistakes in system design interviews

M1Drawing a microservice diagram before asking a single requirement question.
M2Designing for a billion users when the prompt implied a few thousand.
M3Adding a cache without saying how stale data gets invalidated.
M4Saying “use Kafka” or “use Redis” without the property you need from it.
M5Never mentioning monitoring, so nobody would know the design is failing.

05Tools that help

Tools for system design interview prep

Technical interview lab
Sketch components and connections on the system-design whiteboard, then save the board to talk through.
JS · PythonPublic tests
AI voice mock interview
Set a 45 or 60-minute voice session and talk a full design round through out loud, interruptions included.
Realtime15–60 min
Mock interview feedback
Read the reasoning line of your IW-1 review: it quotes the sentence where you stated, or skipped, a trade-off.
IW-14 × 0–4

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