Level 4 · Master Whole systems

The first three levels built a toolbox. Level 4 is about judgement: combining those tools into complete systems, weighing cost and risk, and communicating the result to other engineers — in a design review or an interview.

The four case studies are reasoning exercises, not reports of how any particular company built its product. Each describes a plausible generic design derived from requirements, and each makes its assumptions explicit. Where the lesson mentions publicly documented industry practice, it is hedged as such; internal architectures change and are rarely fully public.

Modules

1. Case Study: Video Streaming — upload, transcoding pipelines, adaptive bitrate, and CDN-heavy delivery
2. Case Study: Ride-Hailing Dispatch — geospatial indexing, location ingestion, matching, and trip state
3. Case Study: Distributed File Storage — chunking, dedup, metadata, sync, and conflict handling
4. Case Study: Payment System — ledgers, idempotency, external processors, and reconciliation
5. Multi-Region & Disaster Recovery — RPO/RTO, active-passive vs active-active, and failover drills
6. Cost-Aware Design — where money goes, unit economics, and trade-offs that save it
7. Security Architecture — authentication, authorization, secrets, encryption, and threat modeling
8. The Interview Framework & Common Mistakes — a minute-by-minute plan for a 45-minute round
9. Writing Design Docs & Defending Trade-offs — structure, alternatives, and running a review
10. Capstone — Your Own Design Doc — a full design of a system you choose, with a scoring rubric

What you need before starting

• Levels 1–3. The case studies use vocabulary from every earlier lesson without re-explaining it.
• A notebook or document for your own designs. Level 4 has more writing than code; the skill being practised is explaining a design clearly enough that someone else could build it and critique it.

Finish with the capstone. It is the best evidence — for yourself or for a hiring panel — that you can do this work.