Elevator System — Low Level Design

Problem Statement

Design a low-level elevator system that manages multiple elevators across a multi-floor building, handling passenger requests efficiently.

Diagram

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Requirements

Functional

• Passengers can press a button on any floor (UP / DOWN) to request an elevator
• Passengers can select a destination floor inside the elevator
• Elevator moves to requested floors and opens/closes doors
• System dispatches the most appropriate elevator per request
• Elevator respects floor limits (min/max floor)

Non-Functional

• Responsive: elevator dispatched within seconds
• Extensible: pluggable dispatch strategy (SCAN, LOOK, nearest)
• Thread-safe: concurrent requests from multiple floors/passengers

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Core Entities

Enums

enum Direction {
  UP,
  DOWN,
  IDLE,
}
enum DoorState {
  OPEN,
  CLOSED,
}
enum ElevatorState {
  MOVING,
  IDLE,
  STOPPED,
}

Floor

class Floor {
  floorNumber: number;
  upButton: boolean; // external request panel
  downButton: boolean;
}

ElevatorCar

class ElevatorCar {
  id: string;
  currentFloor: number;
  direction: Direction;
  state: ElevatorState;
  doorState: DoorState;
  destinationQueue: Set<number>; // internal button presses

  move(): void; // advance one floor toward next destination
  openDoor(): void;
  closeDoor(): void;
  addDestination(floor: number): void;
  hasDestination(): boolean;
}

ElevatorController (Dispatcher)

class ElevatorController {
  elevators: ElevatorCar[];
  strategy: DispatchStrategy;

  requestElevator(floor: number, direction: Direction): void;
  selectFloor(elevatorId: string, floor: number): void;
  tick(): void; // advance simulation one step
}

DispatchStrategy (Interface)

interface DispatchStrategy {
  assign(request: Request, elevators: ElevatorCar[]): ElevatorCar;
}

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Dispatch Strategies

Nearest Idle

Assign the elevator that is idle and closest to the requested floor.

cost(elevator) = |elevator.currentFloor - requestedFloor|
pick: min(cost) where state === IDLE

SCAN (Elevator Algorithm)

Elevator sweeps in one direction, picks up all requests, then reverses.

If elevator moving UP  → serve all floors above current first
If elevator moving DOWN → serve all floors below current first

LOOK (Optimized SCAN)

Same as SCAN but reverses direction when no more requests exist in current direction — avoids unnecessary travel to extremes.

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Class Diagram

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Request Flow

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State Machine — Elevator

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Edge Cases & Discussion Points

Scenario	Handling
Two passengers request same floor	Only one elevator dispatched; first confirmed wins
Elevator at max floor, UP pressed	Ignore UP, elevator switches direction DOWN
Elevator full (weight sensor)	Skip floor assignment; dispatch next available elevator
Power outage	Elevator moves to nearest safe floor and opens door
Multiple elevators same distance	Prefer the one already moving in the same direction
Door obstruction	Retry close after timeout; raise alert after N retries

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Trade-offs

Decision	Option A	Option B
Queue type	Min-heap (priority)	Sorted set
Dispatch	Centralized controller	Each elevator self-manages
Threading	Single event loop (tick)	Per-elevator thread
Strategy	Hardcoded SCAN	Strategy pattern (pluggable)

Recommended: Centralized controller + Strategy pattern + priority queue per elevator.

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Follow-up Questions

• How would you handle a 100-floor skyscraper with 20 elevators? (zone partitioning)
• How do you ensure thread safety when multiple requests arrive simultaneously?
• How would you add VIP floors or express elevators?
• How do you optimize for peak hour traffic (morning lobby rush)?