Design Elevator System
What is an Elevator System?
An elevator system is a combination of mechanical components and software logic used in multi-story buildings to transport people or goods vertically between floors.
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Modern elevator systems typically consist of one or more elevator cars (also called lifts), each controlled by an embedded software system that manages a range of operations, including:
- Movement control (moving the elevator up or down)
- Door operations (opening and closing doors safely and efficiently)
- User request handling (both from inside and outside the elevator)
- Scheduling logic to decide which elevator responds to which request, in what order, and in which direction
In this chapter, we will explore the low-level design of an elevator system in detail.
Lets start by clarifying the requirements:
1. Clarifying Requirements
Before starting the design, it's important to ask thoughtful questions to uncover hidden assumptions and better define the scope of the system.
Here is an example of how a conversation between the candidate and the interviewer might unfold:
Candidate: Should we assume a single elevator or support multiple elevators operating in the same building?
Interviewer: Let’s design for a system with multiple elevators in the same building.
Candidate: Should the system handle both internal requests (floor buttons inside the elevator) and external requests (hall calls from each floor)?
Interviewer: Yes, the system should support both internal and external requests.
Candidate: Should we follow a specific elevator scheduling algorithm, like SCAN or LOOK, or is a basic first-come-first-serve strategy sufficient?
Interviewer: For this version, use a simple scheduling strategy like nearest elevator first. However, the design should be extensible enough to support pluggable scheduling algorithms in the future.
Candidate: Should the elevators display their current floor and movement direction (up/down)?
Interviewer: Yes, each elevator should display its current floor and movement direction in real-time.
After gathering the details, we can summarize the key system requirements.
1.1 Functional Requirements
- Support multiple elevators operating within the same building
- Handle both internal (cabin) and external (hall) floor requests
- Assign requests to the most appropriate elevator, based on proximity and direction
- Simulate basic elevator behaviors like moving between floors and stopping at requested floors
- Display real-time updates for each elevator’s current floor and movement direction (UP/DOWN)
1.2 Non-Functional Requirements
- Modularity: The system should follow object-oriented principles with clearly defined components
- Scalability: The design should be scalable to support buildings with many elevators and floors
- Concurrency Handling: The system must handle simultaneous floor requests without conflicts or race conditions
- Responsiveness: The system should respond quickly to user input and reflect changes (e.g., floor updates, direction changes) in real-time
- Maintainability: The design should be clean, modular, and easy to test, debug, or enhance
After the requirements are clear, lets identify the core entities/objects we will have in our system.