PricingNet: Pricing Modelling with Neural Networks at Skyscanner

Ideas you will meet

Distributed Systems

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

Machine Learning Systems

Recommendations, ranking, feature stores and serving models at scale.

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

  • Scaling Infrastructure at Twitter - Yao Yue, Staff Software Engineer at TwitterTwitterYouTube
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  • Software Engineering DailySoftware Engineering DailyPodcasts & Newsletters
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  • Software Engineering Advice from Building Large-Scale Distributed Systems - Jeff DeanGooglePapers
  • Software Engineering RadioIEEE Computer Society · SE RadioPodcasts & Newsletters

  Open this lesson with its quiz and exercise

• Low-Level Design Practice · 10 minShopping cartCarts, inventory and pricing rules: adding items, applying discounts, checking stock and turning a cart into an order safely.Read hereClose

  The problem

  Shoppers add and remove products, apply coupons, see a total, and check out. The system must check stock, compute the price correctly and create an order.

  Entities

  100%

  Figure SDA-LLD-11-1 · Shopping cart classes · © forcsguy.com

  Cart versus order

  A cart is mutable and may be stale. An order is a frozen record of what was bought, at what price, with which discounts. Checkout converts one into the other.

  100%

  Figure SDA-LLD-11-2 · Checkout flow · © forcsguy.com

  Pricing as ordered rules

  type Line = { sku: string; qty: number; unit: number };
  interface PricingRule { apply(lines: Line[]): { reason: string; cents: number }[] }
  
  const buy2get1: PricingRule = {
    apply: (lines) => lines.filter(l => l.sku === "TSHIRT" && l.qty >= 3)
      .map(l => ({ reason: "Buy 2 get 1", cents: -Math.floor(l.qty / 3) * l.unit })),
  };
  const coupon10: PricingRule = {
    apply: (lines) => [{ reason: "10% coupon", cents: -Math.round(lines.reduce((s, l) => s + l.qty * l.unit, 0) * 0.1) }],
  };
  
  function total(lines: Line[], rules: PricingRule[]) {
    const sub = lines.reduce((s, l) => s + l.qty * l.unit, 0);
    const adj = rules.flatMap(r => r.apply(lines));
    return { subtotal: sub, adjustments: adj, total: sub + adj.reduce((s, a) => s + a.cents, 0) };
  }
  

  Decide and document precedence (do percentage discounts apply before or after fixed ones?) and whether discounts stack. Keep a record of which rules applied so the invoice can explain the price.

  Inventory safely

  // conditional update: succeeds only if enough stock remains
  function reserve(sku: string, qty: number): boolean {
    const row = stock.get(sku)!;
    if (row.available < qty) return false;
    row.available -= qty; row.reserved += qty; return true;   // atomic in a real database: UPDATE ... WHERE available >= qty
  }
  

  Design notes

  • Guest and signed-in carts: merge a guest cart into the user's cart on login.
  • Abandoned carts: stock is not reserved by a cart, only by checkout; reservations expire.
  • Idempotent checkout: a double click must not create two orders (use a checkout key).
  • Money is integer minor units; taxes and shipping are further rules or services.
  • Extensions: wishlists, saved-for-later, per-region pricing, bundles.

  Real-world reading

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  Open this lesson with its quiz and exercise