Elevator, Floor, Request classes in LLD - Architecture Diagram

System Overview - Elevator, Floor, Request classes

This system models a building elevator control system. It manages elevator movements between floors based on user requests. Key requirements include handling multiple floor requests, moving elevators efficiently, and responding to user inputs promptly.

Architecture Diagram

User
  |
  v
Floor Panel
  |
  v
Request Handler
  |
  v
Elevator Controller
  |
  v
Elevator
  |
  v
Floor Sensors

Components

User

actor

Person who requests elevator service from floors or inside elevator

Floor Panel

input_interface

Interface on each floor to send elevator requests

Request Handler

service

Receives and queues requests from floor panels and elevator buttons

Elevator Controller

service

Decides elevator movement based on queued requests

Elevator

service

Physical elevator that moves between floors

Floor Sensors

sensor

Detects elevator position at floors

Request Flow - 8 Hops

User → Floor Panel

Floor Panel → Request Handler

Request Handler → Elevator Controller

Elevator Controller → Elevator

Elevator → Floor Sensors

Floor Sensors → Elevator Controller

Elevator Controller → Request Handler

Elevator Controller → Elevator

Failure Scenario

Component Fails:Request Handler

Impact:New elevator requests are not queued, causing elevator to ignore user calls

Mitigation:Implement retry logic and fallback queue storage; alert maintenance team for manual intervention

Architecture Quiz - 3 Questions

Which component decides the elevator's next move based on requests?

A Request Handler

B Elevator Controller

C Floor Panel

D Floor Sensors

Design Principle

This design separates concerns by using distinct components for input handling, request management, decision making, and physical elevator control. It ensures scalability and maintainability by decoupling user interface, logic, and hardware interaction.

Practice

1. What is the primary role of the Request class in an elevator system?

easy

A. To store the floor number and direction of a user's request

B. To move the elevator between floors

C. To open and close the elevator doors

D. To track the number of elevators in the building

Solution

1. Step 1: Understand the purpose of Request class

   The Request class holds information about where a user wants to go and in which direction.

2. Step 2: Compare roles of other classes

   Elevator moves and Floor represents building levels, but Request stores user input details.

3. Final Answer:

   To store the floor number and direction of a user's request -> Option A

4. Quick Check:

   Request = user floor and direction [OK]

Hint: Request class holds user floor and direction info [OK]

Common Mistakes:

• Confusing Request with Elevator movement
• Thinking Request controls doors
• Mixing Request with Floor class responsibilities

2. Which of the following is the correct way to define a Request class constructor in Python to store floor and direction?

easy

A. def Request(floor, direction): self.floor = floor; self.direction = direction

B. def __init__(self, floor, direction): self.floor = floor; self.direction = direction

C. def __init__(floor, direction): self.floor = floor; self.direction = direction

D. def __init__(self): floor = None; direction = None

Solution

1. Step 1: Recall Python constructor syntax

   Python constructors use def __init__(self, ...) and assign attributes with self.attribute = value.

2. Step 2: Check each option

   def __init__(self, floor, direction): self.floor = floor; self.direction = direction correctly uses self and assigns floor and direction. Others miss self or parameters.

3. Final Answer:

   def __init__(self, floor, direction): self.floor = floor; self.direction = direction -> Option B

4. Quick Check:

   Constructor with self and attributes = def __init__(self, floor, direction): self.floor = floor; self.direction = direction [OK]

Hint: Python constructors need self parameter and attribute assignment [OK]

Common Mistakes:

• Omitting self parameter
• Using class name as constructor
• Not assigning attributes to self

3. Given this Python snippet, what will be printed?

class Request:
    def __init__(self, floor, direction):
        self.floor = floor
        self.direction = direction

r = Request(5, 'up')
print(r.floor, r.direction)

medium

A. Error: missing self

B. floor direction

C. 5 up

D. None None

Solution

1. Step 1: Understand object creation and attribute assignment

   The Request object r is created with floor=5 and direction='up'. These are stored in attributes.

2. Step 2: Print attributes

   Printing r.floor and r.direction outputs 5 and 'up' respectively.

3. Final Answer:

   5 up -> Option C

4. Quick Check:

   Attributes print as assigned = 5 up [OK]

Hint: Print object attributes to see stored values [OK]

Common Mistakes:

• Expecting attribute names instead of values
• Confusing class variables with instance variables
• Assuming error without checking code carefully

4. Identify the error in this Elevator class snippet:

class Elevator:
    def __init__(self, current_floor):
        self.current_floor = current_floor

    def move_to(self, floor):
        current_floor = floor

medium

A. Indentation error in move_to method

B. Missing return statement in move_to method

C. Constructor should not have parameters

D. The move_to method updates a local variable, not the elevator's floor

Solution

1. Step 1: Analyze move_to method

   The method assigns current_floor = floor without self., so it changes a local variable only.

2. Step 2: Understand instance variable update

   To update the elevator's floor, it should be self.current_floor = floor.

3. Final Answer:

   The move_to method updates a local variable, not the elevator's floor -> Option D

4. Quick Check:

   Missing self. means local variable used [OK]

Hint: Use self. to update instance variables inside methods [OK]

Common Mistakes:

• Forgetting self. prefix
• Thinking return is needed to update state
• Assuming indentation is wrong without checking

5. In designing an elevator system with Elevator, Floor, and Request classes, which approach best handles multiple simultaneous requests efficiently?

hard

A. Use a priority queue in Elevator to process requests by nearest floor and direction

B. Process requests in the order they arrive without sorting

C. Assign each request to a random elevator immediately

D. Ignore direction and always move elevators to the highest requested floor first

Solution

1. Step 1: Understand multiple request handling

   Efficient elevator systems prioritize requests to minimize travel and wait time.

2. Step 2: Evaluate options for request processing

   Using a priority queue to pick nearest floors and matching direction optimizes movement. Others cause inefficiency or randomness.

3. Final Answer:

   Use a priority queue in Elevator to process requests by nearest floor and direction -> Option A

4. Quick Check:

   Priority queue for nearest requests = Use a priority queue in Elevator to process requests by nearest floor and direction [OK]

Hint: Prioritize nearest requests with direction for efficient elevator movement [OK]

Common Mistakes:

• Ignoring direction leads to inefficient routes
• Random assignment causes delays
• Processing requests strictly by arrival order wastes time