Elevator system

The problem

A building has several elevators. People press a button on a floor (a hall call with direction) and a floor button inside the car (a car request). The system moves cars, opens doors and decides which car should answer each call.

Entities

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Figure SDA-LLD-07-1 · Elevator classes · © forcsguy.com

One elevator as a state machine

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Figure SDA-LLD-07-2 · Elevator car states · © forcsguy.com

A simple and effective scheduling rule is the elevator (scan) algorithm: keep two sorted sets of stops (above and below); continue in the current direction, serving stops in order; reverse only when none are left in that direction.

class Elevator {
  floor = 0; dir: "UP" | "DOWN" | "IDLE" = "IDLE";
  up = new SortedSet<number>(); down = new SortedSet<number>();   // pending stops

  addStop(f: number) { (f >= this.floor ? this.up : this.down).add(f); if (this.dir === "IDLE") this.pick(); }
  private pick() { this.dir = this.up.size ? "UP" : this.down.size ? "DOWN" : "IDLE"; }
  step() {                                   // called once per tick
    if (this.dir === "UP") { this.floor++; if (this.up.deleteIfPresent(this.floor)) this.openDoors(); if (!this.up.size) this.pick(); }
    if (this.dir === "DOWN") { this.floor--; if (this.down.deleteIfPresent(this.floor)) this.openDoors(); if (!this.down.size) this.pick(); }
  }
}

Many cars: the dispatcher

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Figure SDA-LLD-07-3 · Assigning a hall call · © forcsguy.com

Make the dispatcher a strategy so you can swap "nearest car" for "least loaded" or a smarter estimate without touching Elevator.

Design notes

• Concurrency: calls arrive while cars move. Use one event loop per elevator, or a queue per car, so state changes happen one at a time.
• Safety: doors must not close on a blocked sensor; weight limits stop a car from moving.
• Peak modes: morning (send idle cars to the lobby) and evening patterns are policies.
• Extensions: express cars, destination dispatch (people enter their destination at the hall), emergency and maintenance modes.

Real-world reading