Skip to the lesson
Little Builders system design, explained small

System design for curious kids

Big computer systems, explained small.

How apps share the work, remember things and keep going when something breaks, told with lunch lines, toy boxes and cookie jars. Every drawing answers your pointer.

Start lesson oneSee all twenty
A rack of server blades. Move your pointer up and down it.

Lessons

14 building blocks

  1. Capacity EstimationGuessing how big things will get before you build them, using easy round numbers.
  2. API Design and NetworkingHow computers pass notes: the menu of what you may ask, the roads notes travel on, and the helpers at the door.
  3. Data Storage FundamentalsWhere apps keep their stuff, and the clever guide lists that help them find it fast.
  4. Caching StrategiesKeeping copies of popular things close by, and keeping those copies honest.
  5. Asynchronous CommunicationLeaving notes instead of waiting on the phone: queues, announcements, and promises about delivery.
  6. Distributed Systems CoreHow a team of computers agrees, picks a leader, and shares one lock, even when messages get lost.
  7. Advanced Database ArchitecturesSplitting data across many databases, keeping copies in step, and handling copies that fall behind.
  8. Data Consistency and TransactionsKeeping data correct when a job has many steps: a diary, a class vote, undo plans, and an outbox.
  9. High-Throughput StreamingMoving a never-ending river of notes through many helpers at once, while keeping order where it matters.
  10. System ResiliencyStaying up when parts break: retry politely, stop calling broken things, keep trouble in one room, and keep a backup ready.
  11. Observability and TelemetrySeeing what is going on inside a running system from the clues it leaves, and keeping score of how healthy it is.
  12. Cross-System ArchitectureDrawing the borders between the parts of a big system, so each part speaks its own clear language.
  13. Complex Migrations and EvolutionChanging a big system while people keep using it, one small step at a time, always with a way back.
  14. Operational Scale and CostSpending money on computers wisely: seeing who spends what, planning for busy days, and saving the fancy treatment for what matters.

Real world stories

6 case studies

  1. Flash Sale Inventory ManagementE-CommerceSelling 100 toys to a million clicks in one second, without falling over and without ever selling the same toy twice.
  2. Distributed Order FulfillmentE-CommerceGetting one order from click to doorstep across many teams, with a clear plan, clear states, patient retries, and a pile for problems.
  3. Real-Time Live LeaderboardStreamingShowing a million players the top 10 and their own place, moments after every score changes.
  4. Global Video Delivery NetworkStreamingTurning one uploaded movie into many sizes and getting it to every screen on Earth without a pause.
  5. Distributed Payment Processing EngineFinanceMoving money so that none is ever lost, made up, or taken twice, even when computers fail.
  6. Real-Time Fraud DetectionFinanceDeciding in the blink of an eye whether a card swipe is really you.