System Design: Building Blocks

What each box costs, and what forces it

Caches, queues, replicas and shards are easy to name and hard to size. This is the arithmetic behind them: a peak rate from a population, a year of storage, the memory a hot set needs, the instances a lost zone costs — and then, given one stated constraint, which box it forces and which it rules out.

Start this project See the modules

Modules: 6
Drills: 9
Build steps: 2
Time: ~4h

Walk out able to

Take a product sentence you have never seen and put six numbers under it, with the assumption written beside each one.

What you work out

Rates, storage, memory

A population in, requests, gigabytes and a hot set out.

Instances and headroom

What a peak needs, and what losing a zone adds to it.

Caches and queues, costed

Reads still reaching the store; a backlog and its drain.

Replicas, shards, or neither

The clause that decides, and the one that rules a box out.

What you get

Skills that stick

Every project mixes short lessons, focused drills and real build steps, so you finish with working code and the judgement behind it.

You will practise

Peak rate, storage per year, hot-set memory
Instances for a peak, headroom for a lost zone
Cache load, two-speed latency, backlog and drain
Replica, shard, queue or none of them

Afterwards you can

Turn a product sentence into a peak rate
Size a fleet that survives a lost zone
Cost a cache and a queue in real units
Name the block a constraint forces

Modules

Step by step

Each module builds on the last. Read a short lesson, drill it until it's second nature, then put it to work.

1
Put a number on the load
~46 min
Turn a product sentence into a rate, a year of storage and a memory figure.
- LessonWhy put a number on it at all4 min
- LessonTurn a product sentence into a rate9 min
- LessonName the numbers you cannot invent10 min
- DrillSize a year of storage12 min
- DrillSize the memory a hot set needs11 min
2
The box in front
~35 min
Count the instances a peak needs, and the ones a lost zone costs.
- LessonCount the instances a peak needs9 min
- LessonQuestion a per-instance ceiling6 min
- DrillProvision for the zone you lose11 min
- DrillSay what share a backend takes9 min
3
The box beside
~26 min
Say what a cache removes from the store, and what the path then costs.
- LessonSay what a cache actually removes9 min
- LessonSay what a better hit ratio buys7 min
- DrillSpend a latency budget on two hops10 min
4
The box between
~27 min
Compute a backlog and its drain, and say whether the work may wait at all.
- LessonSay how deep the backlog gets10 min
- LessonSay why depth is only a symptom7 min
- DrillSay whether the work may wait10 min
5
The box behind
~37 min
Name the half that is out of room, and count the shards a ceiling forces.
- LessonTell a standby from a read replica5 min
- LessonName the half that is out of room11 min
- DrillCount the shards a ceiling forces11 min
- DrillSay where the data belongs10 min
6
Let the requirement decide
~43 min
Name the block a constraint forces, then size a system nobody walked you through.
- DrillName the block the constraint forces11 min
- Build stepPut the traffic under a product brief8 min
- Build stepSize the fleet the zones force8 min
- LessonTrace a changed clause to the figures8 min
- LessonName the block a clause adds8 min