The Cracked EngineerRead lesson 1 free
New lessonWhy Docker on your Mac is secretly a VM

Learn what's underneath.Get cracked.

The Cracked Engineer teaches backend engineers how machines, networks, databases and money systems actually work. Real commands, every step shown, one layer at a time.

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  • 80 lessons, 12 modules
  • Lesson 1 free, no account
  • New lessons every week
POST /v1/transfersLayer 1 of 5

Money systems

Two ledger entries that sum to zero

Debit Alice $500. Credit Bob $500. If the two sides don't cancel out, money was created or lost.

entries: alice −500.00, bob +500.00, sum 0

Play with the lessons before you pay.

Every tile below is a working piece of a real lesson. Replay a bank webhook, break a TLA+ spec, watch a TLS handshake open. This is what learning here feels like.

Formal methods with TLA+

Catch a double credit before writing code

\* The bank can deliver one webhook twice
Deliver(id) ==
  \* no idempotency check
  /\ balance' = [balance EXCEPT
        !.alice = @ - 500,
        !.bob = @ + 500]
  /\ applied' = applied \union {id}

NoDoubleCredit ==
  balance.bob <= 500
Error: Invariant NoDoubleCredit is violated.

State 1: <Initial predicate>
  bob = 0
State 2: <Deliver("t81")>
  bob = 500
State 3: <Deliver("t81")>
  bob = 1000

TLC tries every order of events, so the duplicate webhook is found before any code exists.

Exchange systems

Read an order book

BTC/USDPriceSize
  1. 97,415.00.500
  2. 97,414.51.400
  3. 97,414.02.300
  4. 97,413.50.900
  5. 97,413.01.800
  6. 97,412.50.400
97,412.0spread 1.0
  1. 97,411.50.600
  2. 97,411.02.100
  3. 97,410.51.700
  4. 97,410.01.300
  5. 97,409.50.900
  6. 97,409.02.400

Simulated feed. In the module you build the engine that matches these orders.

How computers work

Catch the hidden VM

Two kernels answer on one laptop. Lesson 1 explains why.

Ledgers and payments

Replay a bank webhook

AccountBalance
Alice$1,500
Bob$500
  1. webhook t81: applied
How computers work

Feel how far away the disk is

  • Read from L1 cache1 ns
  • Read from RAM100 ns
  • Read 4 KB from an NVMe SSD20 µs
  • Round trip inside one data center500 µs
  • Round trip from New York to London70 ms
Log scale. Typical numbers, yours will vary.
Networks

Watch a connection open

Phone in San FranciscoServer in Virginia
  1. SYNTCP
  2. SYN ACKTCP
  3. ACK + ClientHelloTLS 1.3
  4. ServerHello + FinishedTLS 1.3
  5. GET /v1/ratesHTTP
  6. 200 OKHTTP
0 round trips, about 0 ms

Two round trips before your first byte of data. That is why connection reuse matters.

Access to AI is not intelligence. Anyone can ask a model. Few can tell when it's wrong. Keep the knowledge in your own head.

What cracked looks like.

Not trivia. The habits that make other engineers come to you when something breaks.

You find the bug in the right layer

A container dies from memory pressure. A request hangs for 30 seconds. You know where to look first, and why.

Your payment code can't pay twice

Ledgers, idempotent transfers and webhooks that arrive twice, checked with TLA+ so a duplicate charge dies in the design, not in production.

You stop bluffing in system design rounds

Isolation levels, consensus, idempotency and matching engines, taught the way senior interviewers probe them.

Your knowledge outlives the next framework

Frameworks change every year. Kernels, TCP and write ahead logs don't. Learn them once, by running them on your own laptop.

Five layers. Twelve modules. One stack.

Start anywhere. Each layer explains the one above it. 2 lessons are live today, and a new one ships every week.

Money systems

How does a balance stay right when every message can fail?

Ledgers and paymentsExchange systemsSecurity for money systems
21 lessons

Distributed systems

What does “exactly once” really mean?

Distributed systems
7 lessons

Storage

When is a write actually saved?

How databases keep data
7 lessons

Network

What happens between curl and the response?

Networks
8 lessons

Machine

Why is Docker on your Mac a VM?

How computers workLive
10 lessons

Proof

Can this design ever lose money?

Formal methods with TLA+
9 lessons

Build on it

What do you build once the layers make sense?

Building with AI modelsStablecoins and on-chain financeCompliance engineeringFrom engineer to founder
18 lessons

How it works.

  1. Read lesson 1 free

    No account, no card. Run the commands on your own laptop and answer the checks.

  2. Follow a path

    Foundations, payments or exchanges. Each path puts the modules in the right order for that job.

  3. Go Pro when it clicks

    Unlock every lesson, project and TLA+ spec, plus new lessons the week they ship.

Pick the path for the job you want.

Read first. Pay when you're convinced.

The first three lessons of every module are free. If they change how you see your own code, Pro is the rest of the stack.

Free

Start today. No account, no card.

$0forever

  • The first three lessons of every module
  • Every trace, check and recap in those lessons
  • The weekly build log
Read lesson 1 free

ProSave 27%

Every lesson, every layer, as each one ships.

$79a year

  • All 80 lessons across 12 modules
  • New lessons the week they ship
  • TLA+ specs with model checker runs for every money design
  • Build projects: a ledger, a matching engine, a container from scratch
  • Progress saved across your devices
Join Pro when it opens

Cancel anytime from your account.

Questions, answered.

Anything else? Reply to the weekly build log post and it gets answered there.

Who is this for?

Backend engineers with a year or more of experience who use Docker, Postgres and queues every day and want to know what happens underneath. It fits best if you're aiming at payments, fintech, exchange or infrastructure roles.

How is this different from videos or docs?

Videos explain. Docs describe. Each lesson here follows one real question down through the layers, with commands you run yourself, checks that explain every wrong answer and, for money systems, a TLA+ spec that proves the design.

How much math do I need?

Almost none for the systems modules. The TLA+ module uses basic logic and sets, and teaches both as it goes.

What does free include?

The first three lessons of every module, in full, with no account. You can read lesson 1 right now.

Are all 80 lessons written?

Not yet, and the course map shows exactly which ones are. Lessons ship every week, and Pro members get each one the week it's published.

Does it work on a phone?

Reading, traces, diagrams and checks all work on a phone. The exercises marked “try it on your machine” need a laptop with a terminal.

Can I cancel?

Yes. Cancel from your account at any time and you keep access until the end of the period you paid for.

Will this help me get hired?

It covers what systems interviews probe: idempotency, isolation levels, consensus, matching engines and ledgers. It won't replace practice problems, but you'll understand the systems you're asked to design instead of reciting them.

Can you explain why Docker on your Mac runs a VM?

If not, that's lesson 1. It takes 18 minutes, and containers will never look the same again.

Read lesson 1 free