System Design Interview

The building blocks, the trade-offs and the classic problems, taught with one repeatable framework for the 45-minute round.

by the endYou can run a 45-minute system design round: clarify requirements, estimate scale, draw the architecture, go deep on one or two components, and defend your trade-offs.

Before you start: You have built or maintained a backend service and know what a database and an API are.

1. Module 0 — How the round works

   0 · ~0 min · optional

   Optional. The framework interviewers expect and the arithmetic you need at the whiteboard.

   you canYou know the steps of the round and can size a system on the back of an envelope.

   1. 1The system design interview frameworkcoming soonLesson
   2. 2Back-of-envelope estimation primercoming soonLesson
   3. 3Common system design interview mistakescoming soonOptional

2. Module 1 — Building blocks

   7 · ~37 min

   The components every design is assembled from, and when each one earns its place.

   you canYou can pick and justify each component rather than name-drop it.

   1. 1System Design Building Blocks: A Decision Map, Not a Parts List (2026)Lesson · 6 min
   2. 2Load Balancers Explained: Health Checks Do the Real Work (2026)Lesson · 5 min
   3. 3Caching Strategies: Why Invalidation Is the Hard Part (2026)Lesson · 7 min
   4. 4CDNs and Object Storage: Store at the Origin, Serve at the Edge (2026)Lesson · 5 min
   5. 5API Gateway Explained: It’s Not a Load Balancer (2026)Lesson · 4 min
   6. 6Rate Limiting Explained: Token Bucket, Leaky Bucket, and When Each Wins (2026)Lesson · 5 min
   7. 7Horizontal vs Vertical Scaling: Scale Up Until You Can’t (2026)Optional · 5 min

3. Module 2 — Data and consistency

   2 · ~10 min

   Choosing a datastore, spreading data across machines, and what you give up when you do.

   you canYou can choose a datastore, explain replication and sharding, and state the trade-off.

   1. 1SQL vs NoSQL: It’s About Access Patterns, Not Scale (2026)Lesson · 5 min
   2. 2CAP Theorem Explained: It Was Never ‘Pick Two’ (2026)Lesson · 5 min
   3. 3Database sharding and replication explainedcoming soonLesson
   4. 4Consistent hashing explainedcoming soonLesson
   5. 5Indexing and query performance primercoming soonOptional

4. Module 3 — Async and messaging

   3 · ~19 min

   Decoupling services with events, queues and streams.

   you canYou can decouple services with queues and streams and explain delivery guarantees.

   1. 1Monolith vs Microservices: It’s About Team Size, Not Traffic (2026)Lesson · 7 min
   2. 2Event-Driven Architecture: Trading Control for Decoupling (2026)Lesson · 5 min
   3. 3Kafka vs RabbitMQ: A Log vs a Queue, Not Fast vs Slow (2026)Lesson · 7 min
   4. 4Message delivery guarantees and idempotencycoming soonLesson

5. Module 4 — Classic problems

   0 · ~0 min

   Ten full designs, each walked with the same template: requirements, estimates, API, data model, diagram, deep dive, bottlenecks.

   you canYou can walk a full design from a one-line prompt using the same framework each time.

   1. 1Design a URL shortenercoming soonID generation, hashing, read-heavy cachingLesson
   2. 2Design a rate limitercoming soonToken bucket, distributed countersLesson
   3. 3Design a news feedcoming soonFan-out on write vs read, rankingLesson
   4. 4Design a chat systemcoming soonWebSockets, ordering, presenceLesson
   5. 5Design a notification servicecoming soonQueues, retries, dedupeLesson
   6. 6Design a file storage servicecoming soonChunking, sync, object storageLesson
   7. 7Design a web crawlercoming soonFrontier queue, politeness, dedupeLesson
   8. 8Design a video streaming servicecoming soonCDN, transcoding pipelineLesson
   9. 9Design a ride-sharing servicecoming soonGeospatial indexing, matchingLesson
   10. 10Design a typeahead searchcoming soonTries, caching, rankingLesson

6. Module 5 — Reliability and operations

   0 · ~0 min

   What interviewers ask once the happy path is drawn.

   you canYou can discuss failure, observability and security when the interviewer pushes.

   1. 1Failure modes and resilience patternscoming soonLesson
   2. 2Observability for system design interviewscoming soonLesson
   3. 3Authentication and authorization in system designcoming soonOptional

7. Module 6 — Modern and AI systems

   3 · ~24 min

   The AI-flavoured design questions that general interview sites cover thinly.

   you canYou can handle LLM, gateway and agent design questions.

   1. 1LLM System Design Interview: The Five-Layer Answer (2026)Corrected 16 OctInterview · 11 min
   2. 2AI Gateway Design: One Front Door for Every Model (2026)Lesson · 8 min
   3. 3Agent Orchestration Patterns: A 2026 GuideLesson · 5 min
   4. 4Design a RAG search systemcoming soonLesson

8. Module 7 — Finish

   0 · ~0 min

   Practice under time pressure and plan the last weeks.

   you canYou have a drill routine, a review rubric and a 2–4 week schedule.

   1. 1Mock interview practice guidecoming soonCapstone
   2. 2System design interview prep plancoming soonCapstone
   3. 3Communication in the system design roundcoming soonOptional