A step by step framework
Interviewers value structure. Following a predictable sequence keeps you from getting lost and lets the interviewer redirect you easily.
- Requirements: what the system must do and constraints like latency or availability
- Estimates: users, requests per second, data size, read to write ratio
- API design: main endpoints and their inputs and outputs
- Data model: entities, relationships and database choice
- High level design: clients, load balancer, services, storage, cache, queues
- Deep dive: one or two components in detail
- Trade offs and failure scenarios
Core concepts to know
You do not need to know every technology, but you should be comfortable explaining these building blocks and when to use them.
- Load balancing and horizontal scaling
- Caching layers and cache invalidation
- SQL versus NoSQL and when each fits
- Replication, partitioning and sharding
- Message queues and asynchronous processing
- Consistency, availability and the CAP trade off
- Rate limiting, idempotency and retries
- Monitoring, logging and alerting
Worked example: a URL shortener
Requirements: create a short link, redirect to the original URL, optional expiry. Reads far exceed writes. Design: a write API generates a unique short key using a counter or hash and stores it in a key value store. A read API looks up the key, with a cache in front because popular links are read often. Discuss key collisions, expiry cleanup, analytics through an asynchronous queue, and how to scale reads with replicas and caches.
Common mistakes
Most candidates who struggle jump to a diagram without understanding the problem or go deep on one detail and run out of time.
- Skipping requirements and estimates
- Naming technologies without explaining why
- Ignoring failure cases and data consistency
- Not checking in with the interviewer
How to practise
Practise common problems such as a chat app, news feed, rate limiter, file storage and payment system, out loud with a timer and a whiteboard or drawing tool. InterviewPlan AI mock interviews and the Interview Copilot for technical interviews can help you rehearse explaining designs clearly.