Free course · updated 2026
System Design, from building blocks to full designs
10 phases and 72 lessons: high-level design, low-level design (OOP, SOLID and design patterns) and AI/ML system design, ending with 22 full interview questions worked through step by step. Every lesson has a diagram, an animation, a spoken interview answer and 4 exercises with answers.
Phases
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Phase 0
How system design interviews work
Learn the step-by-step way to answer any design question before you learn the parts. Without a method, even good knowledge comes out messy under pressure.
4 lessons · 4 must know -
Phase 1
Networking and APIs
Understand how a request travels from a phone or browser to your servers and back, and how to design the API it calls.
5 lessons · 3 must know -
Phase 2
Core concepts: scale, reliability and speed
The ten ideas every high-level design uses: scalability, availability, fault tolerance, load balancing, caching, consistency and CAP, rate limiting, asynchronous processing, and observability.
9 lessons · 9 must know -
Phase 3
Data: databases, storage and scaling data
Choose the right database, keep data correct with transactions, make queries fast with indexes, and scale data with replication and sharding.
9 lessons · 6 must know -
Phase 4
Distributed systems and security
How services split up and stay correct across a network, how machines agree, and how to keep users and data secure.
4 lessons · 2 must know -
Phase 5
Low-level design: OOP and SOLID
The building blocks of good class design: the four pillars of OOP, composition over inheritance, the five SOLID principles, class diagrams, and the concurrency basics LLD questions expect.
9 lessons · 7 must know -
Phase 6
Design patterns
Ten patterns that come up again and again in LLD interviews and real code: three for creating objects, three for structuring them, and four for behaviour.
10 lessons · 7 must know -
Phase 7
Low-level design questions
Practise the classic LLD questions end to end: requirements, classes, patterns, the key code and concurrency.
6 lessons · 3 must know -
Phase 8
High-level design questions
Practise the classic HLD questions with the full framework: requirements, estimates, API, data model, design, deep dives and trade-offs.
11 lessons · 7 must know -
Phase 9
AI and ML system design
The design questions AI and ML engineer interviews ask: RAG assistants, recommendations, fraud detection, LLM serving, and the pipeline that trains and ships models.
5 lessons · 3 must know
The four checks
- 1Explain
Say it in plain words in under a minute, with no notes.
- 2Draw
Sketch the diagram (or class diagram) from memory.
- 3Apply
Use it in a real design question and say exactly where it fits.
- 4Trade-offs
Say what it costs, when not to use it, and the alternative.
A topic is finished when you pass all four checks, not when you finish a video. Be strict: if you needed your notes, it is not ticked yet.
Your progress
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