System design interview preparation: practise out loud from day one
You can read every system design book and still struggle in the round. The interview is a 45-minute conversation where you lead, draw, estimate and defend choices while someone interrupts you. That's a speaking skill as much as a knowledge skill, and the only way to build it is to practise out loud. A system design mock interview with an engineer from a top tech company shows you exactly where your answers fall short of the level you want.
Book a system design mock interview
Senior or staff? Book a deep-dive design mock
What the system design round tests at each level
[[table]]
Level | Experience | Expectation
Junior | 0 to 2 yrs | Rarely asked
Mid-level | 2 to 5 yrs | Clean, working design
Senior | 5 to 8 yrs | Drives it, goes deep
Staff+ | 8+ yrs | Trade-offs, cost, rollout
[[/table]]
At mid-level, interviewers want a sensible design that works and covers the basics. At senior level, you're expected to lead the whole conversation and go deep on the hardest part without being prompted. At staff level and above, the conversation shifts to cost, migration, failure handling across regions, and how several teams would build and run the system.
This is also the round that most often decides your level. At Google, Meta and Amazon, a weak design round is the most common reason experienced candidates get an offer one level lower than they applied for.
[[mock-interview-banner]]
The 6-step system design interview framework
Most candidates who struggle in this round know enough. What trips them up is time. They spend 20 minutes on boxes and arrows, and never get to the part that shows depth. A fixed structure stops that from happening.
1. Requirements (5 minutes). Ask who the users are, what the system must do, and what's out of scope. Write down two or three non-functional needs, like low latency or high availability.
2. Scale estimates (5 minutes). Rough numbers for daily users, requests per second, reads versus writes, and storage over five years. Round aggressively. The point is to know whether you need one database or a hundred.
3. API and data model (5 to 10 minutes). List the main endpoints and the tables or documents behind them. Pick your primary keys carefully, since they drive sharding later.
4. High-level design (10 minutes). Draw clients, load balancers, services, databases, caches and queues. Walk one request through the whole system.
5. Deep dive (15 minutes). Pick the one or two hardest parts and go deep. For a news feed, that's fan-out. For a chat app, it's message ordering and delivery. This is where senior candidates earn their level.
6. Wrap-up (5 minutes). Name the bottlenecks, explain what happens when a component fails, and say what you'd build next.
Concepts you need to know
You don't need to know every database ever built. You do need to be comfortable explaining these:
- Load balancing and how traffic gets spread across servers.
- Caching, including where to cache, how to invalidate, and what happens with hot keys.
- Databases, SQL vs NoSQL, indexes, replication and sharding.
- Consistency, strong vs eventual, and when each one is acceptable.
- Message queues for decoupling services and handling spikes.
- Rate limiting and protecting services from overload.
- Idempotency and retries, so the same request twice doesn't charge a customer twice.
- CDNs and blob storage for images, video and static files.
- Monitoring and failure handling, including timeouts, circuit breakers and graceful degradation.
[[mock-interview-banner]]
Top 15 system design interview questions
These come up again and again at FAANG and Indian product companies. They're common practice prompts, not leaked questions.
[[table]]
Design | Key challenge | Level
URL shortener | ID generation, reads | Mid
Rate limiter | Distributed counters | Mid
Key-value store | Partitioning, replication | Senior
News feed | Fan-out at scale | Senior
Chat app | Ordering, delivery | Senior
Notification system | Retries, priorities | Mid
Video streaming | Encoding, CDN | Senior
Ride matching | Location updates | Senior
Search autocomplete | Tries, ranking | Mid
Web crawler | Politeness, dedup | Senior
Payment system | Idempotency | Senior
Hotel booking | Double-booking | Mid
File storage | Sync, chunking | Senior
Top K leaderboard | Streaming counts | Mid
Distributed job scheduler | Failures, exactly-once | Staff
[[/table]]
A few notes on what interviewers dig into.
For the URL shortener, the interesting part is generating short IDs without collisions across many servers, and handling a read-heavy load with caching.
For the news feed, expect to discuss push vs pull fan-out, and what to do about users with millions of followers.
For the chat app, interviewers care about message ordering, offline delivery and how you'd show read receipts.
For the payment system, idempotency is the whole game. A retry must never charge a customer twice.
For the distributed job scheduler, staff-level candidates are expected to talk about leader election, retries, and what "exactly once" really means in practice.
Practise any of these in a live system design mock
System design mock interviews: how they work
A system design mock is a full 45 to 60-minute practice round with an engineer who plays the interviewer. They give you a prompt from the list above (or one matched to the company you're targeting), interrupt with follow-ups, and push on weak spots the way a real interviewer would.
After the round, you get a written scorecard. A good one covers:
- How well you gathered requirements and estimated scale
- Whether your high-level design was complete and sensible
- How deep you went on the hard parts
- How clearly you explained trade-offs
- Which level your performance matched (for example, mid-level or senior)
That last point matters most for experienced engineers. Hearing "this was a solid mid-level design" two weeks before a senior loop gives you time to fix it.
[[table]]
Self-practice | Mock interview
No interruptions | Real follow-ups
You judge yourself | Engineer scores you
No level signal | Clear level call
Free | Paid
[[/table]]
Self-practice is still useful for learning concepts. Use mocks to test whether you can put them together under pressure.
A 4-week system design prep plan
Week 1. Learn the concepts list above. Watch explainers and read about one real system each day.
Week 2. Practise five questions from the table using the 6-step framework. Time yourself and talk out loud.
Week 3. Take your first mock interview. Spend the rest of the week on whatever the scorecard flagged.
Week 4. Practise the senior and staff questions, then take a second mock to check your progress.
If you have eight or more years of experience, add cost, migration and multi-region failure to every practice question. Staff-level interviewers will ask.
[[mock-interview-banner]]
FAQs
How long does it take to prepare for a system design interview? Four to six weeks for most engineers with a few years of experience, at about an hour a day.
Do freshers get system design interviews? Rarely at FAANG. Some Indian product companies ask basic design or LLD questions for SDE1 roles.
What's the difference between HLD and LLD? High-level design covers services, databases and how they connect. Low-level design covers classes, interfaces and code structure for one component.
How do I practise system design alone? Pick a question, set a 45-minute timer, follow the framework out loud, and record yourself. Then compare your answer with a good write-up.
Is a system design mock interview worth it? For mid-level and senior engineers, yes. It's very hard to judge your own design round, and the level call alone can change how you prepare.
Book a system design mock interview
Get the Level You're Aiming For: Take a System Design Mock Interview
System design decides more senior offers than any other round. Practise it with an engineer from a top tech company, get a scorecard with a clear level call, and walk into your real interview knowing where you stand.
Book your system design mock interview
