Software Engineering Advice from Building Large-Scale Distributed Systems - Jeff Dean

Ideas you will meet

Distributed Systems

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

What we already know about this

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

• Low-Level Design Practice · 11 minParking lotVehicles, spots, tickets and fees: a classic exercise in allocation, pricing rules and concurrent entry gates.Read hereClose

  The problem

  Design a parking lot with several levels. It accepts motorcycles, cars and buses; assigns the nearest suitable free spot on entry; tracks occupancy; and charges a fee on exit by duration and vehicle type. Several entry and exit gates work at once.

  Entities

  Class	Responsibility
  ParkingLot	Owns the levels and gates; entry point for park and unpark
  Level	A floor holding spots; answers "which free spot fits this vehicle"
  ParkingSpot	Has a type and occupied state; can be reserved and freed
  Vehicle	Base type with subclasses or a type field (motorcycle, car, bus)
  Ticket	Created on entry: vehicle, spot, entry time
  PricingStrategy	Turns a stay (duration, vehicle type) into a fee

  100%

  Figure SDA-LLD-02-1 · Parking lot class structure · © forcsguy.com

  The entry and exit flow

  100%

  Figure SDA-LLD-02-2 · Entry and exit sequence · © forcsguy.com

  Key code

  type SpotType = "MOTORCYCLE" | "COMPACT" | "LARGE";
  const fits: Record<string, SpotType[]> = {
    MOTORCYCLE: ["MOTORCYCLE", "COMPACT", "LARGE"],
    CAR: ["COMPACT", "LARGE"],
    BUS: ["LARGE"],
  };
  
  class ParkingSpot {
    occupied = false;
    constructor(readonly id: string, readonly type: SpotType) {}
    tryReserve(): boolean {        // atomic in a real system (lock or compare-and-set)
      if (this.occupied) return false;
      this.occupied = true; return true;
    }
    free() { this.occupied = false; }
  }
  
  class Level {
    constructor(readonly spots: ParkingSpot[]) {}
    findAndReserve(vehicleType: string): ParkingSpot | null {
      for (const s of this.spots)
        if (fits[vehicleType].includes(s.type) && s.tryReserve()) return s;
      return null;
    }
  }
  

  The important details are tryReserve (one winner per spot) and the fits table (compatibility in one place).

  Pricing as a strategy

  interface PricingStrategy { fee(vehicleType: string, minutes: number): number }
  const hourly: PricingStrategy = {
    fee: (t, m) => Math.ceil(m / 60) * ({ MOTORCYCLE: 1, CAR: 2, BUS: 5 } as any)[t],
  };
  

  Swap in a daily cap, a weekend rate or a subscriber discount without touching ParkingLot.

  Before: pricing hard-coded in the ticket

  100%

  After: pricing as a strategy

  100%

  What changed

  • A new fee rule is a new class, not an edit to old code
  • Each pricing rule can be tested alone
  • The ticket no longer knows how fees are calculated

  Edge cases and extensions

  • Lot full: park returns "no spot" cleanly rather than throwing.
  • Lost ticket: look the vehicle up by plate; charge the maximum rate.
  • Nearest spot: keep free spots in a sorted structure per level (a heap or a set per type) so allocation is not a linear scan at scale.
  • Display boards that show free counts are observers of spot changes.
  • Persistence: if the app restarts, rebuild occupancy from stored tickets.

  Real-world reading

  Related reading

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  • Building Real Time Infrastructure at Facebook - Jeff Barber and Shie Erlich, Software Engineer at FacebookFacebook engineers · FacebookCase Studies

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  Open this lesson with its quiz and exercise