Design elevator scheduling algorithm

Q: Design elevator scheduling algorithm

This question evaluates understanding of real-time scheduling, control policies, queueing and optimization trade-offs that affect passenger waiting and in-car travel time for a single-elevator system.

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Question

System Design: Single-Elevator Control for a 4-Stop Building

Context

You are designing the control policy for a single elevator serving four stops: Basement (B), Floor 1, Floor 2, and Floor 3. The goal is to minimize passenger waiting time (from pressing a hall button to doors opening) and travel time (time inside the car until destination).

Assumptions:

One car; no mechanical constraints beyond standard door times and acceleration.
Hall calls have direction (Up/Down); passengers select destination inside the car.
Ignore failures; assume typical door dwell and floor-to-floor travel times.

Task

Design the elevator’s decision policy for choosing its next stop. Describe:

How the car decides its next stop while moving.
How it reverses direction.
How it handles opposite-direction hall calls.
How it behaves when idle (no requests).
Any tie-breakers and fairness rules to minimize waiting and travel time.

Design elevator scheduling algorithm

System Design: Single-Elevator Control for a 4-Stop Building

Context

Task

Solution

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