Elevator, Floor, Request classes in LLD - System Design Exercise

Design: Elevator Control System

Design the core classes and their interactions for elevator, floor, and request management. Exclude hardware control and detailed UI design.

Functional Requirements

FR1: Manage multiple elevators in a building

FR2: Handle requests from floors to go up or down

FR3: Handle requests from inside elevators to select destination floors

FR4: Optimize elevator movement to reduce wait and travel time

FR5: Track elevator current floor and direction

FR6: Support at least 10 floors and 4 elevators

Non-Functional Requirements

NFR1: Response time for request assignment should be under 200ms

NFR2: System should handle up to 100 concurrent requests

NFR3: Elevator state updates must be consistent and thread-safe

NFR4: Availability target: 99.9% uptime

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Elevator class to track state and handle movement

Floor class to represent each floor and its requests

Request class to represent a pickup or dropoff request

Scheduler or controller to assign requests to elevators

Design Patterns

Observer pattern for elevator state updates

Command pattern for requests

Singleton for central controller

State pattern for elevator movement states

Reference Architecture

  +----------------+       +----------------+       +----------------+
  |     Floor      |<----->|   Controller   |<----->|    Elevator    |
  +----------------+       +----------------+       +----------------+
         ^                         ^   ^                      ^
         |                         |   |                      |
  Floor requests             Assign requests           Elevator state
  (up/down buttons)          (pickup/dropoff)          (current floor,
                                                       direction, requests)

Components

Elevator

Class with attributes and methods

Represents an elevator with current floor, direction, and list of destination requests

Floor

Class with attributes

Represents a floor with up/down request buttons and pending requests

Request

Class

Represents a request either from a floor (pickup) or inside elevator (dropoff)

Controller

Singleton class

Receives requests and assigns them to elevators based on current state

Request Flow

1. User presses up/down button on a Floor, creating a pickup Request

2. Floor notifies Controller of the new Request

3. Controller evaluates all elevators and assigns the Request to the best Elevator

4. Elevator adds the Request to its destination queue

5. Elevator moves floor by floor, updating current floor and direction

6. When Elevator reaches a requested floor, it opens doors and removes the Request

7. Inside Elevator, user selects destination floor, creating a dropoff Request

8. Elevator adds dropoff Request to its queue

Database Schema

Entities: - Elevator(id, current_floor, direction, status, destination_queue) - Floor(number, up_button_pressed, down_button_pressed) - Request(id, type [pickup/dropoff], floor_number, direction [up/down/null], elevator_id [nullable], status) Relationships: - Elevator has many Requests - Floor can have multiple pickup Requests - Requests link to Elevator when assigned

Scaling Discussion

Bottlenecks

Controller becomes a bottleneck when handling many concurrent requests

Elevator state updates may cause race conditions if not synchronized

Request queue management can become complex with many elevators and floors

Solutions

Use concurrent data structures and locks to ensure thread safety

Distribute controller logic or shard by floors or elevator groups

Implement priority queues and efficient scheduling algorithms

Use event-driven architecture to decouple components

Interview Tips

Time: Spend 10 minutes clarifying requirements and constraints, 20 minutes designing classes and interactions, 10 minutes discussing scaling and optimizations, 5 minutes summarizing

Clearly define responsibilities of Elevator, Floor, and Request classes

Explain how requests flow through the system and get assigned

Discuss thread safety and state consistency

Mention how to optimize elevator scheduling

Address scaling challenges and solutions