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Distributed Systems

Consensus, time, failure and the fundamentals of scale-out.

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Our own lessons on these ideas. Read them here before you start, so it makes sense.

• Low-Level Design Practice · 10 minATM machineA machine that behaves differently in each state: the State pattern, card and PIN checks, and cash dispensing with limits.Read hereClose

  The problem

  An ATM accepts a card, checks the PIN, then lets the customer check balance, withdraw cash or deposit, and returns the card. It talks to a bank to authorise operations and holds a limited amount of cash.

  States

  100%

  Figure SDA-LLD-06-1 · ATM states · © forcsguy.com

  In Idle, "enter PIN" makes no sense; in Authenticated, "insert card" makes no sense. The State pattern puts those rules inside state objects instead of if (state === ...) chains.

  interface AtmState {
    insertCard(atm: Atm, card: Card): void;
    enterPin(atm: Atm, pin: string): void;
    withdraw(atm: Atm, cents: number): void;
  }
  
  class Idle implements AtmState {
    insertCard(atm, card) { atm.card = card; atm.setState(new HasCard()); }
    enterPin() { throw new Error("insert a card first"); }
    withdraw() { throw new Error("insert a card first"); }
  }
  class HasCard implements AtmState {
    tries = 0;
    insertCard() { throw new Error("card already inserted"); }
    enterPin(atm, pin) {
      if (atm.bank.checkPin(atm.card!, pin)) atm.setState(new Authenticated());
      else if (++this.tries >= 3) { atm.retainOrEject(); atm.setState(new Idle()); }
    }
    withdraw() { throw new Error("enter the PIN first"); }
  }
  

  Who is responsible for what

  100%

  Figure SDA-LLD-06-2 · Withdrawal sequence · © forcsguy.com

  • Bank owns the balance and account rules. ATM owns hardware, cash on hand and the session.
  • Cash dispenser holds counts per denomination and decides which notes to give (greedy from the largest, with a fallback).
  • Every step is logged so a failure between "dispensed" and "confirmed" can be reconciled later (a mismatch report, or an automatic reversal).

  Design notes

  • Treat PINs as secrets: never store or log them; the bank verifies, the ATM only forwards an encrypted PIN block.
  • Idempotency: a retried authorise call must not debit twice; use a transaction id.
  • Timeouts: an idle session returns to Idle and ejects the card.
  • Extensions: deposits (count and hold until verified), fund transfers, receipts, and multiple card types.

  Real-world reading

  Related reading

  • Apple Machine Learning BlogApple engineers · AppleBlogs
  • AWS Machine Learning BlogAmazon Web Services engineers · Amazon Web ServicesBlogs
  • Optimizing Payments with Machine Learning at DropboxDropbox engineers · DropboxCase Studies
  • Machine Learning Platform at YelpYelp engineers · YelpCase Studies

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  • Secret Detector: Preventing Secrets in Source Code at YelpYelp engineers · YelpCase Studies
  • Machine Learning at Jane StreetJane Street engineers · Jane StreetCase Studies
  • Waymo BlogWaymo engineers · WaymoBlogs

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