State management (idle, moving up, moving down) in LLD - System Design Exercise

Design: Elevator State Management System

Design focuses on the state management logic and transitions for a single elevator. Hardware control and multi-elevator coordination are out of scope.

Functional Requirements

FR1: Track elevator states: idle, moving up, moving down

FR2: Allow state transitions based on commands and current state

FR3: Handle requests to move elevator up or down

FR4: Prevent invalid state transitions (e.g., moving up when already moving up)

FR5: Provide current state information on demand

Non-Functional Requirements

NFR1: System should respond to state change requests within 100ms

NFR2: Support up to 100 concurrent elevator state requests

NFR3: Ensure state consistency at all times

NFR4: System availability target: 99.9% uptime

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

State machine to manage elevator states

Command handler to process move requests

State storage to keep current state

Event logger for state transitions

API or interface to query current state

Design Patterns

Finite State Machine (FSM) pattern

Observer pattern for state change notifications

Command pattern for encapsulating requests

Singleton pattern for centralized state management

Reference Architecture

 +---------------------+
 | Elevator Controller |
 +----------+----------+
            |
            v
 +---------------------+
 |   State Machine      |
 | (Idle, Moving Up,    |
 |  Moving Down)        |
 +----------+----------+
            |
            v
 +---------------------+
 | State Storage       |
 +---------------------+
            |
            v
 +---------------------+
 | Event Logger        |
 +---------------------+

Components

Elevator Controller

Custom logic module

Receives commands and interacts with the state machine

State Machine

Finite State Machine implementation

Manages elevator states and valid transitions

State Storage

In-memory store or lightweight database

Stores current elevator state persistently

Event Logger

Logging system

Records state changes for auditing and debugging

Request Flow

1. User sends a move up or move down command to Elevator Controller.

2. Elevator Controller forwards the command to the State Machine.

3. State Machine checks current state and validates transition.

4. If valid, State Machine updates the state to moving up or moving down.

5. State Storage is updated with the new state.

6. Event Logger records the state transition event.

7. Elevator Controller confirms the state change back to the user.

8. When movement completes, a command to set state to idle is processed similarly.

Database Schema

Entity: ElevatorState - id (primary key) - current_state (enum: idle, moving_up, moving_down) - last_updated_timestamp No complex relationships needed as this is a single entity tracking current state.

Scaling Discussion

Bottlenecks

High frequency of state change requests causing race conditions

State storage becoming a single point of failure

Event Logger performance degradation with large logs

Handling multiple elevators with isolated states

Solutions

Implement locking or atomic operations in state machine to prevent race conditions

Use distributed cache or replicated storage for state persistence

Archive old logs and use asynchronous logging to improve performance

Extend design to support multiple elevator instances with separate state machines

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying assumptions, 20 minutes designing the state machine and components, 10 minutes discussing scaling and edge cases, 5 minutes summarizing.

Explain the importance of clear state definitions and valid transitions

Describe how the finite state machine ensures consistency

Discuss how to handle concurrent requests safely

Mention persistence and logging for reliability and debugging

Outline scaling strategies for multiple elevators or high request volumes