Let's design an elevator system. We want to focus on the core functionalities of moving people efficiently between floors in a building.

Here are some key use cases to consider:

• A user on a specific floor needs to go to another floor.
• The system should handle multiple simultaneous requests from different users.
• The system should optimize the movement of elevators to minimize waiting times.
• The system should handle edge cases like power failures and emergency stops.

Elevator System Design

Let's design an elevator system. I've worked on similar distributed systems at Amazon, so I'm drawing from that experience.

1. Requirements

• Functional Requirements:

  • Users can request an elevator to their current floor.
  • Users can select a destination floor inside the elevator.
  • The system efficiently manages multiple elevator requests.
  • The system prevents overloading elevators.
  • The system handles errors gracefully (e.g., power outage, door malfunction).
  • Elevators should move to the requested floors.

• Non-Functional Requirements:

  • Low latency for elevator requests.
  • High availability.
  • Scalability to support many elevators and users.
  • Fault tolerance.
  • Real-time monitoring and diagnostics.

2. High-Level Design

We'll use a centralized architecture with the following components:

1. Elevator Controller: This is the central brain of the system. It receives requests from users, schedules elevators, and monitors their status. It makes the decisions about which elevator to dispatch to which floor.
2. Elevator: Each elevator has a local controller that communicates with the central Elevator Controller. It controls the motor, doors, and floor buttons.
3. Request Manager: This component receives elevator requests from users (via floor panels or a mobile app) and passes them to the Elevator Controller.
4. Monitoring System: Collects metrics from all components for monitoring and diagnostics.

Communication:

• The Elevator Controller and elevators communicate using a reliable message queue (e.g., Kafka or RabbitMQ). This allows asynchronous communication and resilience.
• Users interact with the Request Manager via HTTP/REST APIs or WebSockets for a more real-time feel.

3. Data Model

Table	Columns	Data Type(s)	Description
Elevators	elevator_id (PK), current_floor, status (idle, moving), capacity, current_load, direction (up, down, stationary)	INT, INT, ENUM, INT, INT, ENUM	Information about each elevator
Requests	request_id (PK), elevator_id (FK), pickup_floor, destination_floor, request_time, status (pending, assigned, completed)	INT, INT, INT, INT, TIMESTAMP, ENUM	Requests for elevator service
FloorButtons	floor_number (PK), up_button_status (active, inactive), down_button_status (active, inactive)	INT, ENUM, ENUM	Represents the state of the up and down buttons on each floor

4. API Design

Request Manager APIs:

• POST /request: Submits a new elevator request.
  • Request body: {floor: INT, direction: ENUM ('up', 'down')}
  • Response: {request_id: INT}

Elevator Controller APIs:

• GET /elevators: Returns the status of all elevators (for monitoring).
  • Response: [{elevator_id: INT, current_floor: INT, status: ENUM, ...}]
• POST /elevators/{elevator_id}/move: Instructs an elevator to move to a specific floor.
  • Request body: {floor: INT}
  • Response: OK

Elevator APIs (internal):

• POST /elevator/{elevator_id}/update: Updates the status of the elevator (e.g., current floor, status).
  • Request body: {current_floor: INT, status: ENUM}
  • Response: OK

5. Tradeoffs

• Centralized vs. Distributed Elevator Controller: A centralized controller is simpler to implement and manage, but it can be a single point of failure. A distributed controller is more complex but provides higher availability and scalability. For the initial design, I chose centralized, which can be expanded later.
• Message Queue: Using a message queue adds complexity but makes the system more resilient to failures. The tradeoff is development time vs. uptime.
• Elevator Assignment Algorithm: A simple algorithm could assign the closest idle elevator. A more sophisticated algorithm could consider factors like elevator direction, capacity, and the number of waiting passengers. Optimizing this for minimal wait time is critical and could be a future focus.

6. Alternative Approaches

• Peer-to-Peer Communication: Each elevator could communicate directly with other elevators to coordinate movements. This removes the central controller but increases complexity of the elevator software.
• Finite State Machines (FSM) for Elevator Logic: Representing the elevator's behavior using FSMs can make the code more maintainable and easier to reason about, especially when handling complex scenarios.

7. Edge Cases

• Elevator Overload: The system should prevent elevators from exceeding their capacity. If an elevator is full, it should not accept new requests.
• Door Malfunction: If the elevator doors fail to open or close, the system should take the elevator out of service and notify maintenance. An alarm should trigger within the elevator as well.
• Power Outage: The system should have a backup power supply. Elevators should move to the nearest floor and open their doors. Users should be notified via an audible alarm and visual display.
• Simultaneous Requests: The system should handle a large number of simultaneous requests efficiently. Load balancing and request prioritization are important.
• Out-of-Order Buttons: If a floor button malfunctions, the system should detect this and disable the button. A message should be displayed indicating the button is out of service.
• Invalid Floor Request: If the request floor doesn't exist, this should return an error to the user.

8. Future Considerations

• Smart Elevator Assignment: Implement a more intelligent elevator assignment algorithm based on machine learning to optimize wait times and energy efficiency. This could take into account traffic patterns throughout the day.
• Predictive Maintenance: Use sensor data from the elevators to predict potential maintenance issues and schedule repairs proactively.
• Integration with Building Management System: Integrate the elevator system with other building systems, such as security and access control.
• Mobile App Integration: Develop a mobile app that allows users to request elevators remotely and receive real-time updates on elevator status.
• Voice Control: Add voice control functionality to allow users to request elevators using voice commands.
• Energy Efficiency: Implement strategies to reduce energy consumption, such as regenerative braking and optimized elevator scheduling.