Elevator System Design

Question

Design the operation logic for an elevator system. Start with basic scheduling for a single building: how do you handle up/down call requests from floors, how does the elevator decide its direction and stops? Then extend to multi-elevator allocation — how do you assign a request to one of N elevators efficiently? Follow-up: the building has many floors (high-rise scenario). How do you partition floors across elevators? Discuss the sky lobby concept (express elevators to a mid-level transfer floor, then local elevators for upper zones) and its tradeoffs.

AceOffer · Accepted Answer

Design the operation logic for an elevator system. Start with basic scheduling for a single building: how do you handle up/down call requests from floors, how does the elevator decide its direction and stops? Then extend to multi-elevator allocation — how do you assign a request to one of N elevators efficiently? Follow-up: the building has many floors (high-rise scenario). How do you partition floors across elevators? Discuss the sky lobby concept (express elevators to a mid-level transfer floor, then local elevators for upper zones) and its tradeoffs.

Reported follow-ups:
1. The building has 100+ floors. How do you partition floors across elevator banks? What is a sky lobby and when would you use it? (when: Candidate handles basic single/multi elevator well)
2. How do you handle an elevator going out of service? How does the system adapt? (when: After zone partitioning discussion)

**Common mistakes:** High-rise partitioning answer becoming disorganized/scattered without a clear framework; Not defining a concrete cost function for multi-elevator assignment; Forgetting direction compatibility when assigning requests to moving elevators

**What passers do:** Progressively structured answer: single elevator → multi elevator → high-rise; Articulating a concrete scheduling algorithm (LOOK/scan) with reasoning; Proactively mentioning sky lobby and zone tradeoffs before being prompted

**Why people fail:** High-rise section answered in a scattered, unstructured way with no clear framework; Staying only at a conceptual level without concrete data structures or algorithm choices

**Edge cases probed:** High-rise with very many floors requiring zone partitioning; Sky lobby design and transfer floor placement; Multiple elevators — how to avoid all elevators clustering at same floor

**Alternative approaches:** FCFS (First-Come-First-Served) (Simple to implement but leads to high average wait times and erratic elevator movement; not realistic for production.); Destination Dispatch (Group Control) (Passengers enter destination floor at lobby; system groups passengers going to same/nearby floors into one car. Reduces stops and travel time but requires special input hardware and more complex allocation logic.); Predictive / ML-based scheduling (Uses historical demand patterns (rush hours, lunch peaks) to pre-position elevators. Higher throughput at scale but complex to build and validate.)

Elevator System Design

Understanding the Problem

Functional Requirements

Non-Functional Requirements

The Set Up

Defining the Core Entities

The API

High-Level Design

Potential Deep Dives

1)The building has 100+ floors. How do you partition floors across elevator banks? What is a sky lobby and when would you use it? (when: Candidate handles basic single/multi elevator well)

2)How do you handle an elevator going out of service? How does the system adapt? (when: After zone partitioning discussion)

What is Expected at Each Level?

Insider Notes