Start HereBeginner1 of 19 in the curriculum

Start Here: How Software Systems Work

Zero assumptions. What a client, a server, an API, and a database actually are, where data lives, the vocabulary every design discussion uses, and how one app grows from 1 user to a million.

If words like 'load balancer', 'cache', or 'replica' feel fuzzy, start here. Each topic explains one idea with an everyday analogy first, then shows the real thing. By the end of this phase you'll be able to read any system design diagram and follow the rest of the course.

Already comfortable with web apps and databases? Skim Topic S.4 (the scaling story) and S.5 (vocabulary), then jump to Phase 0.

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5 topics ~37 min 6 code blocks & diagrams

Start with the first topic

S.1

Clients, Servers & the Life of a Request

Every system is programs talking to other programs over a network. A client asks, a server answers. Follow one click from a browser to a database and back, and every later topic is a variation of this trip.

8 min 1 diagram 1 code practice

S.2

APIs, JSON & Contracts

An API is the menu a server offers: which requests it accepts and what it returns. Agreeing on that contract, usually HTTP + JSON, is what lets teams, apps, and services work together without reading each other's code.

7 min 1 code practice

S.3

Where Data Lives: Memory, Disk, Databases & Caches

Data sits in different places that trade speed for size and safety: CPU cache, RAM, SSD, databases, remote services. Knowing roughly how fast each is explains why caches exist and why every design tries to avoid slow trips.

7 min 1 diagram practice

S.4

From 1 User to 1 Million: The Scaling Story

The single most useful story in system design: start with one server, then add each building block only when a real problem appears. Every later phase is a zoom-in on one step of this journey.

8 min 1 diagram practice

S.5

The Vocabulary: Reading Any Design Diagram

Fifteen words cover most of what you'll hear in design discussions: latency, throughput, availability, consistency, scalability, SPOF, replication, sharding, cache, queue, idempotency, and more. Learn them once with plain definitions.