Design Vending Machine (LLD)

1. Requirements

• Functional

  • Display slot inventory; accept coins and notes; dispense product and change.
  • Reject invalid selections when empty; refund on cancel before vend.
  • Track cash box inventory for operator reconciliation.

• Non-Functional

  • Coin insertion and vending steps transactional from customer perspective.
  • Extensible payment (cash now; card later via strategy).

• Assumptions / Out of Scope

  • Temperature-controlled compartments omitted unless modeled as slot metadata.

2. Core Entities

3. Class Diagram

4. State / Sequence Diagram (where relevant)

5. Design Patterns Applied

• State — Customer interaction states (Idle, AcceptingPayment, Dispensing). State pattern.
• Strategy — Payment mode cash vs card. Strategy pattern.
• Chain of Responsibility — Coin routing through validators. Chain of Responsibility pattern.

6. Implementation

Go

package vending

type Product struct {
    SKU, Name string
}

type Slot struct {
    Code         string
    PriceCents   int
    Quantity     int
    Product      Product
}

type VendingMachine struct {
    Slots       map[string]*Slot
    Inserted    int
    Selected    string
    CashBox     *CashInventory
}

func (machine *VendingMachine) InsertCoin(cents int) { /* ... */ }
func (machine *VendingMachine) Select(code string) error { /* ... */ }
func (machine *VendingMachine) Vend() error { /* decrement, change */ }

JavaScript

class VendingMachine {
  constructor({ slots, cashInventory }) {
    this.slotsByCode = new Map(slots);
    this.insertedCents = 0;
    this.selectedCode = null;
    this.cashInventory = cashInventory;
  }
  insertCoin({ cents }) { /* ... */ }
  selectSlot({ code }) { /* ... */ }
  vend() { /* ... */ }
  cancel() { /* refund */ }
}

7. Concurrency / Thread Safety

• Collisions: Operator restocking while purchase in flight; two customers on networked twins (out of scope for single unit).
• Granularity: Mutex around inventory updates during Vend.
• Go: sync.Mutex on VendingMachine public methods.

8. Extensibility & Followups

• Age-restricted slots requiring attendant unlock.
• Telemetry for jam detection and temperature alarms.
• Edge cases: exact change impossible with current float; display warning.