Advanced~4h

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.

The pitch

What you practise, and what you leave with

A sized envelope for a system nobody walked you through: six figures, the assumption beside each, and the clause that would move 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

6 modules, 24 items

Lessons explain one idea. Drills repeat it until it sticks. Build steps make something that exists afterwards.

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