Why elevator design tests state machines in LLD - Design It to Understand It

Design: Elevator Control System

Design the elevator control logic focusing on state transitions and request handling. Out of scope: physical elevator mechanics and hardware details.

Functional Requirements

FR1: Manage elevator movements between floors

FR2: Handle multiple requests from different floors and inside the elevator

FR3: Ensure safe and efficient operation

FR4: Respond correctly to door open/close commands

FR5: Handle emergency stop and maintenance modes

Non-Functional Requirements

NFR1: System must respond to requests within 200ms

NFR2: Support up to 10 floors and 4 elevators

NFR3: Ensure 99.9% uptime for elevator control

NFR4: Must prevent conflicting commands that cause unsafe states

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

State machine to represent elevator states

Request queue management

Event handler for button presses and sensors

Safety and emergency modules

Design Patterns

Finite State Machine (FSM)

Event-driven architecture

Command pattern for request handling

Observer pattern for sensor updates

Reference Architecture

  +---------------------+
  | Elevator Controller |
  +----------+----------+
             |
   +---------v---------+
   |   State Machine   |
   +---------+---------+
             |
   +---------v---------+
   | Request Queue     |
   +---------+---------+
             |
   +---------v---------+
   | Motor & Door Ctrl |
   +-------------------+

Components

Elevator Controller

Custom logic module

Coordinates elevator operations and manages state transitions

State Machine

Finite State Machine implementation

Represents elevator states like Idle, MovingUp, MovingDown, DoorOpen, DoorClosed, Emergency

Request Queue

Priority queue data structure

Stores and prioritizes floor requests from users

Motor & Door Control

Hardware interface module

Executes commands to move elevator and open/close doors

Request Flow

1. User presses floor button inside elevator or on a floor panel

2. Request is sent to Elevator Controller

3. Controller adds request to Request Queue

4. State Machine checks current state and decides next action

5. If idle, State Machine transitions to MovingUp or MovingDown based on request

6. Motor & Door Control receives commands to move elevator or open/close doors

7. Sensors update State Machine on arrival at floors or door status

8. State Machine transitions to DoorOpen state to allow passenger movement

9. After door closes, State Machine checks for next request or returns to Idle

10. Emergency or maintenance events trigger special states to halt operations safely

Database Schema

Entities: - Elevator: id, current_floor, current_state - Request: id, floor_number, direction, timestamp - State: name, allowed_transitions Relationships: - Elevator has many Requests - State defines allowed transitions for Elevator states

Scaling Discussion

Bottlenecks

Handling many simultaneous requests causing queue congestion

State machine complexity increasing with more floors and elevators

Latency in sensor updates affecting state accuracy

Ensuring safety under hardware failures or emergency conditions

Solutions

Implement request batching and prioritization to reduce queue size

Modularize state machine per elevator to reduce complexity

Use real-time sensor data streaming with fallback checks

Add redundant safety checks and fail-safe states in state machine

Interview Tips

Time: 10 minutes to clarify requirements and constraints, 20 minutes to design state machine and data flow, 10 minutes to discuss scaling and safety considerations, 5 minutes for questions

Explain why elevator behavior fits well with state machines

Describe key states and transitions clearly

Show how requests are managed and prioritized

Highlight safety and emergency handling in design

Discuss scaling challenges and practical solutions