System design interview prep

System Design Interview Questions: A Framework That Holds Up

System design rounds give an open problem, like a URL shortener or chat app, and judge how you clarify requirements, estimate scale, sketch components, choose data stores, and discuss bottlenecks and trade-offs. A repeatable 6-step structure matters more than memorised designs.

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6-step framework
15 questions answered
HLD focus
2-8 years level

6-step system design answer: URL shortener example

45-60 minutes: about 5 min requirements, 5 estimates, 15 design, 15 deep dive, 5 wrap-up

1. Requirements: Shorten a URL, redirect fast, optional custom alias and expiry. Non-functional: high availability, low redirect latency, reads far more than writes.

2. Estimates: Say 100 million new URLs a month and a 100:1 read ratio. That is roughly 40 writes and 4,000 reads per second on average, and about 6TB of storage over five years at 1KB per record.

3. API: POST /urls returns a short code; GET /{code} returns a 301 or 302 redirect.

4. High-level design: Load balancer, stateless app servers, a key-value or relational store mapping code to URL, and a cache like Redis in front for hot codes.

5. Deep dive: Generate codes with base62 from a unique ID generator to avoid collisions; cache popular redirects; shard by code; use 302 if you need click analytics.

6. Trade-offs and wrap-up: Consistency is relaxed for analytics but strict for code creation; discuss abuse prevention, rate limiting and monitoring.

Requirement clarity
Asking the right questions before drawing boxes is the first thing interviewers score.
Back-of-envelope math
Requests per second, storage and bandwidth estimates justify every later decision.
Component choices
Caches, queues, databases and load balancers chosen for a reason, not by habit.
Trade-off reasoning
Consistency vs availability, latency vs cost, and what fails first under load.

Classic design prompts

Design a URL shortener.
Design a chat application.
Design a news feed.
Design a rate limiter.
Design a file storage service.

Concept questions

What is horizontal vs vertical scaling?
When do you add a cache?
SQL or NoSQL for this workload?
What is a message queue for?
What is consistent hashing?

Trade-off questions

Strong or eventual consistency here?
Push or pull for notifications?
Where would the system fail first at 10x traffic?
How do you keep the cache and database in sync?
How do you handle a hot key?

Clarify before designingSpend the first five minutes on requirements and scale. It changes every later choice.
Do the numbers out loudRough estimates show engineering judgment, even if the numbers are approximate.
Start simple, then scaleDraw the single-server version first, then add caches, replicas and sharding as the numbers demand.
Name the trade-off each timeFor every component, say what you gain and what you give up.
Practise speaking through designsBrainUp AI Interview Prep can run design-style mock questions so you rehearse structure and pacing.

What is a system design interview?

An open-ended interview where you design a large-scale system, such as a chat app or feed, and explain requirements, architecture, data choices and trade-offs.

How should I structure a system design answer?

Clarify requirements, estimate scale, define APIs, draw a high-level design, deep-dive into key components, then discuss bottlenecks and trade-offs.

Do freshers get system design questions?

Usually only basic ones, such as designing a parking lot class model or simple APIs. Full high-level design rounds are common from about two years of experience.

What is horizontal vs vertical scaling?

Vertical scaling adds more power to one machine. Horizontal scaling adds more machines behind a load balancer, which scales further but needs stateless services or data partitioning.

What is a load balancer?

A component that spreads incoming requests across servers using strategies like round-robin or least connections, and removes unhealthy servers from rotation.

When should you use a cache?

When data is read far more often than it changes and slightly stale results are acceptable. Common patterns are cache-aside and write-through, with a TTL or invalidation strategy.

SQL vs NoSQL: how do you choose?

Choose SQL for relational data, transactions and complex queries. Choose NoSQL for flexible schemas, very high write throughput or simple key-based access at large scale.

What is database sharding?

Splitting data across multiple databases by a shard key, such as user ID, so each holds part of the data and load. Choosing a good key avoids hot spots.

What is the CAP theorem?

During a network partition, a distributed system must choose between consistency and availability. Real designs pick per feature which one matters more.

What is a message queue used for?

Queues like Kafka or RabbitMQ decouple producers and consumers, absorb traffic spikes, and enable asynchronous processing and retries.

What is consistent hashing?

A hashing scheme that maps keys and servers onto a ring so adding or removing a server moves only a small fraction of keys, useful for caches and sharding.

How does a rate limiter work?

It caps requests per user or key over time using algorithms like token bucket or sliding window, usually backed by a fast shared store such as Redis.

What is a CDN?

A content delivery network caches static content on servers close to users, reducing latency and load on the origin.

What is the difference between HLD and LLD?

High-level design covers components, data flow and infrastructure. Low-level design covers classes, interfaces, schemas and detailed logic within a component.

How should I prepare for system design interviews?

Learn core building blocks, practise 10-15 classic problems with the same framework, do estimates out loud, and relate designs to systems you have actually worked on.

Practise system design answers with structure

BrainUp AI Interview Prep runs mock rounds and gives feedback on clarity and depth.

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System Design Interview Questions: A Framework That Holds Up

Use the same six steps for any design question.

6-step system design answer: URL shortener example

Built for scanners, humans and hiring intent.

Requirement clarity

Back-of-envelope math

Component choices

Trade-off reasoning

Copy the structure, not the wording.

Classic design prompts

Concept questions

Trade-off questions

Small edits that lift response rates.

Common questions, direct answers.

Practise system design answers with structure

System Design Interview Questions: A Framework That Holds Up

Use the same six steps for any design question.

6-step system design answer: URL shortener example

Built for scanners, humans and hiring intent.

Requirement clarity

Back-of-envelope math

Component choices

Trade-off reasoning

Copy the structure, not the wording.

Classic design prompts

Concept questions

Trade-off questions

Small edits that lift response rates.

Common questions, direct answers.

Practise system design answers with structure