Multiple elevator coordination in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of multiple elevator coordination?

To efficiently manage multiple elevators so that they serve passenger requests quickly and fairly, minimizing wait and travel times.

beginner

Name one common strategy used in multiple elevator coordination.

The 'Nearest Car' algorithm, which assigns the elevator closest to the requested floor to serve the call.

intermediate

Why is load balancing important in multiple elevator systems?

Load balancing prevents some elevators from being overused while others stay idle, improving overall system efficiency and reducing passenger wait times.

intermediate

Explain the role of a centralized controller in multiple elevator coordination.

A centralized controller collects all elevator and request data, then decides which elevator should respond to each call to optimize performance.

advanced

What challenges arise when coordinating multiple elevators in a tall building?

Challenges include handling many simultaneous requests, minimizing wait times, avoiding elevator conflicts, and managing different elevator speeds and capacities.

Which algorithm assigns the closest elevator to a floor request?

A Random Assignment

B Nearest Car

C Round Robin

D First Come First Serve

The Nearest Car algorithm chooses the elevator closest to the requested floor.

What is the main benefit of load balancing in elevator coordination?

A Increases elevator speed

B Reduces building height

C Prevents elevator overuse and reduces wait times

D Increases elevator capacity

Load balancing helps distribute requests evenly to reduce wait times and avoid overusing some elevators.

What does a centralized controller do in multiple elevator systems?

A Manages all elevators and assigns requests

B Controls only one elevator

C Repairs elevators

D Monitors weather conditions

A centralized controller manages all elevators and decides which elevator serves each request.

Which factor is NOT a challenge in coordinating multiple elevators?

A Managing elevator colors

B Avoiding elevator conflicts

C Minimizing passenger wait times

D Handling many simultaneous requests

Elevator colors do not affect coordination; the other options are key challenges.

Why might elevators have different speeds in a tall building?

A To save energy

B To confuse passengers

C Because of weather

D Due to design differences and mechanical limits

Elevators may have different speeds based on their design and mechanical capabilities.

Describe how multiple elevators can be coordinated to minimize passenger wait times.

Think about how to decide which elevator should answer a call.

List the main challenges faced when designing a multiple elevator coordination system for a tall building.

Consider what makes tall buildings complex for elevator systems.

Practice

1. What is the main goal of multiple elevator coordination in a building?

easy

A. To reduce wait and travel times for passengers

B. To increase the number of elevators in the building

C. To make elevators move randomly

D. To keep all elevators idle at the ground floor

Solution

1. Step 1: Understand elevator coordination purpose

   Multiple elevator coordination aims to improve efficiency by reducing passenger wait and travel times.

2. Step 2: Evaluate options based on goal

   Options B, C, and D do not focus on improving passenger experience or efficiency.

3. Final Answer:

   To reduce wait and travel times for passengers -> Option A

4. Quick Check:

   Goal of coordination = reduce wait/travel times [OK]

Hint: Focus on passenger experience improvement goals [OK]

Common Mistakes:

• Confusing coordination with adding more elevators
• Thinking elevators should stay idle
• Assuming random movement improves service

2. Which of the following is a correct way to assign an elevator to a new request in a multiple elevator system?

easy

A. Always assign the elevator on the ground floor

B. Assign the elevator farthest from the request floor regardless of direction

C. Assign the elevator closest to the request floor moving in the same direction

D. Assign elevators randomly to balance usage

Solution

1. Step 1: Understand assignment criteria

   Elevators should be assigned based on proximity and direction to minimize wait time.

2. Step 2: Analyze options

   Assign the elevator closest to the request floor moving in the same direction matches this logic. Options A, B, and C ignore direction or proximity, causing inefficiency.

3. Final Answer:

   Assign the elevator closest to the request floor moving in the same direction -> Option C

4. Quick Check:

   Closest elevator + direction match = correct assignment [OK]

Hint: Match elevator direction and proximity for assignment [OK]

Common Mistakes:

• Ignoring elevator direction when assigning
• Choosing elevators randomly
• Always picking ground floor elevator

3. Consider a system with 2 elevators: Elevator A at floor 3 moving up with destinations [5, 7], Elevator B at floor 6 moving down with destinations [4, 2]. A request comes from floor 4 to go up. Which elevator should be assigned?

medium

A. Neither elevator

B. Elevator B

C. Either elevator

D. Elevator A

Solution

1. Step 1: Analyze elevator positions and directions

   Elevator A is at floor 3 going up; Elevator B is at floor 6 going down.

2. Step 2: Match request direction and elevator direction

   Request is at floor 4 going up. Elevator A is below floor 4 and moving up, so it can pick up on the way. Elevator B is above floor 4 but moving down, so it cannot pick up going up.

3. Final Answer:

   Elevator A -> Option D

4. Quick Check:

   Elevator moving towards request floor in same direction = Elevator A [OK]

Hint: Pick elevator moving towards request floor in same direction [OK]

Common Mistakes:

• Choosing elevator moving away from request
• Ignoring elevator direction
• Assuming either elevator works

4. In a multiple elevator system, the controller assigns requests but sometimes an elevator gets stuck and does not update its position. What is the likely problem and how to fix it?

medium

A. Elevator state not updated; add regular position updates and health checks

B. Elevator hardware failure; replace elevator immediately

C. Controller assigns requests randomly; fix assignment logic

D. Elevator doors stuck open; fix door sensors

Solution

1. Step 1: Identify cause of stuck elevator in system

   If elevator position is not updated, controller cannot assign requests properly.

2. Step 2: Determine fix

   Adding regular position updates and health checks ensures controller has current elevator status to assign requests correctly.

3. Final Answer:

   Elevator state not updated; add regular position updates and health checks -> Option A

4. Quick Check:

   Missing updates cause stuck state; fix with health checks [OK]

Hint: Ensure elevator regularly reports position to controller [OK]

Common Mistakes:

• Assuming hardware failure without checking software updates
• Blaming random assignment logic
• Ignoring elevator state updates

5. You are designing a multiple elevator coordination system for a 20-floor building with 4 elevators. To minimize average wait time during peak hours, which strategy is best?

hard

A. Assign elevators randomly to requests to balance load

B. Divide floors into zones and assign elevators to zones dynamically

C. Let all elevators serve all floors equally without zoning

D. Keep all elevators idle at ground floor until called

Solution

1. Step 1: Understand peak hour challenges

   High traffic causes many requests; serving all floors equally can cause delays and conflicts.

2. Step 2: Evaluate zoning strategy

   Dividing floors into zones and assigning elevators reduces travel distance and wait time by localizing service.

3. Step 3: Compare other options

   Random assignment or no zoning causes inefficiency; keeping elevators idle wastes capacity.

4. Final Answer:

   Divide floors into zones and assign elevators to zones dynamically -> Option B

5. Quick Check:

   Zoning elevators reduces wait time in tall buildings [OK]

Hint: Use zoning to reduce travel distance and wait time [OK]

Common Mistakes:

• Ignoring zoning benefits in tall buildings
• Assuming random assignment balances load
• Keeping elevators idle wastes capacity