Design an elevator control system

Q: Design an elevator control system

This question evaluates system design skills including architectural modeling of components, scheduling and concurrency control, state-machine design, and fault-tolerance for real-time control systems, and falls under the System Design domain.

Q: How do I approach System Design interview questions?

System Design questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master system design interviews.

Q: What difficulty level is this interview question?

This is a hard difficulty System Design question, commonly asked during Onsite rounds at Google.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Google during technical interviews.

Question

Elevator Control System Design (N floors, M elevators)

Context

You are designing a multi-elevator control system for a building with N floors (1..N) and M elevators. Users generate two types of requests:

Hall calls: from a floor, pressing Up or Down to request pickup.
Car calls: inside an elevator, pressing a destination floor for dropoff.

Assume each elevator has sensors for door state, weight/load, current floor/position, and fault signals.

Requirements

Model core classes and entities:
- Elevator, Door, Floor, Request (HallCall, CarCall), Controller, Scheduler.
Define key methods and interfaces:
- requestPickup(floor, direction)
- requestDropoff(floor)
- step()/tick()
- getStatus()
Choose and justify a scheduling strategy (e.g., nearest-car, SCAN/LOOK/collective-control, hybrid).
Describe state machines and transitions for elevator and door.
Handle concurrency and multiple simultaneous requests.
Address priority and fairness (avoid starvation, balance load).
Error handling and safety (overload, door obstruction, stuck elevator, out-of-service).
Extensibility and service modes (e.g., VIP, fire service, maintenance).
Provide class interfaces or language-agnostic pseudocode for main operations.

Design an elevator control system

Quick Overview

Elevator Control System Design (N floors, M elevators)

Context

Requirements

Solution

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